# Exploring the low-lying electronic states of C4H6OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone

S. Kumar, Denis Duflot, N.C. Jones, S.V. Hoffmann, P. Limão-Vieira

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S. Kumar, Denis Duflot, N.C. Jones, S.V. Hoffmann, P. Limão-Vieira. Exploring the low-lying electronic states of C4H6OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone. Journal of Quantitative Spectroscopy and Radiative Transfer, 2024, 317, pp.108926. 10.1016/j.jqsrt.2024.108926. hal-04609962

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# Exploring the low-lying electronic states of C₄H₆OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone

S. Kumar<sup>a,b</sup>, D. Duflot<sup>c,d,*</sup>, N.C. Jones<sup>e</sup>, S.V. Hoffmann<sup>e</sup>, P. Limão-Vieira<sup>a,*,**</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

### Keywords:

- Dihydro-2(3H)-thiophenone

- Dihydro-3(2H)-thiophenone

- Photoabsorption

- VUV

- Cross section

- Rydberg states

## ABSTRACT

Results of a detailed study on the electronic state spectroscopy of C₄H₆OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, have been obtained from high-resolution vacuum ultraviolet photoabsorption experiments together with quantum chemical calculations. The absolute photoabsorption cross-sections in the 3.7-10.7 eV energy range were obtained at the AU-UV beam line, ASTRID2 synchrotron radiation facility. The absorption spectra exhibit features due to transitions into valence and Rydberg states, superimposed on vibrational fine structure which appear much weaker in the photoabsorption spectrum of dihydro-3(2H)-thiophenone. Assignments have been proposed for some of the absorption bands with the aid of *ab initio* calculations at the equation-of-motion coupled-cluster singles and doubles level (EOM-CCSD) providing vertical excitation energies and oscillator strengths. The nature of the transitions was assessed by visual inspection of the natural orbitals for each transition and the average values from the electronic radial spatial extents of the electronic cloud. A comparison between the vibrational structure observed in the experimental spectra suggests relevant C=O stretching excitations for both molecules, with important CH₂ twisting and rocking modes for dihydro-2(3H)-thiophenone and ring stretching modes for dihydro-3(2H)-thiophenone. Photolysis lifetimes from 0 up to 50 km altitude in the Earth's atmosphere for both chemical compounds have been estimated from the absolute photoabsorption cross-sections.

## 1. Introduction

The electronic state spectroscopy of 2(5H)-thiophenone, C₄H₄OS, has been recently investigated [1], given its major role as a potential material component of organic electronic devices used in the synthesis of conjugated π-systems. The absence of unsaturated bonds in the five-membered ring has triggered the interest of the international scientific community to assess the origin and role of the low-frequency out-of-plane ring modes. Such information has been pivotal in determining non-planar ring equilibrium conformations for the low-lying ring puckering vibrations (see e.g. [2–6] and references therein). In this study we present for the first time a detailed study of the electronic structure of two isomers, dihydro-2(3H)-thiophenone and dihydro-3

(2H)-thiophenone, C₄H₆OS, also known as γ-thiobutyrolactone and dihydrothiophen-3-one. Within the wide photon energy range from 3.7 eV up to 10.7 eV, high-resolution vacuum ultraviolet (VUV) photoabsorption experiments together with quantum chemical calculations for the lowest-lying neutral and ionic states, have been performed.

Investigating the low-lying electronic states of these molecular compounds, and thus the assignment of the major vibronic transitions, requires detailed information from other spectroscopic studies. A literature survey reveals only valence shell photoelectron spectroscopy [7], laser induced fluorescence excitation [8,9] and photodissociation dynamics [10] studies. Furthermore, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone have also been investigated by theoretical methods on the geometrical and electronic structure of the neutral and

<footnote><sup>a</sup> Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal</footnote>

<footnote><sup>b</sup> Chemical Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, 94720, CA, USA</footnote>

<footnote><sup>c</sup> UMR 8523 - Physique des Lasers Atomes et Molécules, Univ. Lille, F-59000 Lille, France</footnote>

<footnote><sup>d</sup> CNRS, UMR 8523, F-59000 Lille, France</footnote>

<footnote><sup>e</sup> ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000, Aarhus C, Denmark</footnote>

<footnote>* Corresponding author at: UMR 8523 - Physique des Lasers Atomes et Molécules, Univ. Lille, F-59000 Lille, France.</footnote>

<footnote>** Corresponding author at: Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal.</footnote>

<footnote>E-mail addresses: denis.duflot@univ-lille.fr (D. Duflot), plimaovieira@fct.unl.pt (P. Limão-Vieira).</footnote>
<figure><img src="image_2.png" /></figure>

# Exploring the low-lying electronic states of C₄H₆OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone

S. Kumar<sup>a,b</sup>, D. Duflot<sup>c,d,*</sup>, N.C. Jones<sup>e</sup>, S.V. Hoffmann<sup>e</sup>, P. Limão-Vieira<sup>a,*,**</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Dihydro-2(3H)-thiophenone

Dihydro-3(2H)-thiophenone

Photoabsorption

VUV

Cross section

Rydberg states

## ABSTRACT

Results of a detailed study on the electronic state spectroscopy of C₄H₆OS isomers, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, have been obtained from high-resolution vacuum ultraviolet photoabsorption experiments together with quantum chemical calculations. The absolute photoabsorption cross-sections in the 3.7–10.7 eV energy range were obtained at the AU-UV beam line, ASTRID2 synchrotron radiation facility. The absorption spectra exhibit features due to transitions into valence and Rydberg states, superimposed on vibrational fine structure which appear much weaker in the photoabsorption spectrum of dihydro-3(2H)-thiophenone. Assignments have been proposed for some of the absorption bands with the aid of *ab initio* calculations at the equation-of-motion coupled-cluster singles and doubles level (EOM-CCSD) providing vertical excitation energies and oscillator strengths. The nature of the transitions was assessed by visual inspection of the natural orbitals for each transition and the average values from the electronic radial spatial extents of the electronic cloud. A comparison between the vibrational structure observed in the experimental spectra suggests relevant C=O stretching excitations for both molecules, with important CH₂ twisting and rocking modes for dihydro-2(3H)-thiophenone and ring stretching modes for dihydro-3(2H)-thiophenone. Photolysis lifetimes from 0 up to 50 km altitude in the Earth's atmosphere for both chemical compounds have been estimated from the absolute photoabsorption cross-sections.

## 1. Introduction

The electronic state spectroscopy of 2(5H)-thiophenone, C₄H₄OS, has been recently investigated [1], given its major role as a potential material component of organic electronic devices used in the synthesis of conjugated π-systems. The absence of unsaturated bonds in the five-membered ring has triggered the interest of the international scientific community to assess the origin and role of the low-frequency out-of-plane ring modes. Such information has been pivotal in determining non-planar ring equilibrium conformations for the low-lying ring puckering vibrations (see e.g. [2–6] and references therein). In this study we present for the first time a detailed study of the electronic structure of two isomers, dihydro-2(3H)-thiophenone and dihydro-3

(2H)-thiophenone, C₄H₆OS, also known as γ-thiobutyrolactone and dihydrothiophen-3-one. Within the wide photon energy range from 3.7 eV up to 10.7 eV, high-resolution vacuum ultraviolet (VUV) photoabsorption experiments together with quantum chemical calculations for the lowest-lying neutral and ionic states, have been performed.

Investigating the low-lying electronic states of these molecular compounds, and thus the assignment of the major vibronic transitions, requires detailed information from other spectroscopic studies. A literature survey reveals only valence shell photoelectron spectroscopy [7], laser induced fluorescence excitation [8,9] and photodissociation dynamics [10] studies. Furthermore, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone have also been investigated by theoretical methods on the geometrical and electronic structure of the neutral and

<footnote><sup>a</sup> Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal</footnote>

<footnote><sup>b</sup> Chemical Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, 94720, CA, USA</footnote>

<footnote><sup>c</sup> UMR 8523 - Physique des Lasers Atomes et Molécules, Univ. Lille, F-59000 Lille, France</footnote>

<footnote><sup>d</sup> CNRS, UMR 8523, F-59000 Lille, France</footnote>

<footnote><sup>e</sup> ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000, Aarhus C, Denmark</footnote>

<footnote>* Corresponding author at: UMR 8523 - Physique des Lasers Atomes et Molécules, Univ. Lille, F-59000 Lille, France.</footnote>

<footnote>** Corresponding author at: Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal.</footnote>

<footnote>E-mail addresses: denis.duflot@univ-lille.fr (D. Duflot), plimaovieira@fct.unl.pt (P. Limão-Vieira).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** The VUV photoabsorption cross-section in the 3.7–10.7 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone. Inset shows the structure of the molecules.</figcaption></figure>

ionic molecules [2,10]. In Section 2, we describe the structure and properties of dihydro-2(3H)-thiophenone and dihydro-3 (2H)-thiophenone, while in Section 3 our theoretical and experimental methods. The results are presented and discussed in Section 4 and, our conclusions from this work are summarized in Section 5.

## 2. Structure and properties of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone

The dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone molecules, (C₄H₆OS), have C₁ symmetry (A) in their ground electronic states $\tilde{X}^1$A, and the calculated outermost valence electronic configurations are: ... (11a)² (12a)² (13a)² (14a)² (15a)² (16a)² (17a)² (18a)² (19a)² (20a)² (21a)² (22a)² (23a)² (24a)² (25a)² (26a)² (27a)². Due to ring puckering there is no longer a molecular plane, the molecular orbitals termed in-plane and out-of-plane are referred from now on as “in-plane” and “out-of-plane”. In the Supplementary Material (SM) we include the MOs’ character showing that the highest occupied molecular orbital (HOMO), 27a, and the (HOMO-1), 26a, are the sulphur ($n_S$) “in-plane” and oxygen ($\tilde{n}_O$) “out-of-plane” lone pairs. Other molecular orbitals, (HOMO-2), 25a, and (HOMO-3), 24a, have ($\tilde{n}_S$), $\sigma_S$ and $\pi_{CO}$ characters. The lowest unoccupied molecular orbital (LUMO), 28a, and the (LUMO+1), 29a, are mainly $\pi^*(C=O)$ and $\sigma^*(C-S)$. A close inspection of the C₄H₆OS isomers photoabsorption spectral features (Figs. 1–4), and their assignments, reveal that the electronic excitations are mainly classified as valence and Rydberg character orbitals. For the calculated dominant excitation energies and oscillator strengths, see Table 1.

The fine structure visible in the spectra has been assigned to the main fundamental vibrational modes from the available infrared absorption and Raman data [9,10]. Assignments were possible from the available energies (and wavenumbers) in the ground electronic state to: a) dihydro-2(3H)-thiophenone, 0.211 eV (1702 cm⁻¹) for C=O stretching, $\nu_{C=O}$, 0.152 eV (1223 cm⁻¹) for CH₂ twisting, $\tau_{CH₂}$, 0.102 eV (825 cm⁻¹) for CH₂ rocking, $\rho_{CH₂}$, 0.078 eV (632 cm⁻¹) for S–C(O) stretching, $\nu_{C-S(O)}$ and 0.050 eV (404 cm⁻¹) for CO rocking, $\rho_{CO}$ [10]; b) dihydro-3 (2H)-thiophenone, 0.218 eV (1760 cm⁻¹) for C=O stretching, $\nu'_4(a)$, 0.120 eV (970 cm⁻¹), 0.089 eV (721 cm⁻¹) and 0.084 eV (679 cm⁻¹) for

<caption>**Table 1a**</caption>

The calculated dominant vertical excitation energies (EOM-CCSD/aug-cc-pV(T+d)Z+R) and oscillator strengths of dihydro-2(3H)-thiophenone (energies in eV). See text for details.

<table><caption>Dihydro-2(3H)-thiophenone</caption><thead><tr><th>State</th><th>E (eV)</th><th>f</th><th>&lt;r<sup>2</sup>&gt;<sup>a</sup></th><th>Dominant excitations</th><th>E<sub>exp.</sub> (eV)<sup>b</sup></th><th>Cross-section (Mb)</th></tr></thead><tbody><tr><td>X&#773;<sup>1</sup>A</td><td></td><td></td><td>104</td><td></td><td></td><td></td></tr><tr><td>2<sup>1</sup>A</td><td>4.733</td><td>0.00004</td><td>102</td><td>&pi;&#773;<sub>CO</sub>(28a) &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td>4.562</td><td>0.08</td></tr><tr><td>3<sup>1</sup>A</td><td>5.812</td><td>0.01791</td><td>111</td><td>&sigma;<sup>*</sup>(29a) &larr; n<sub>S</sub>(27a)</td><td>5.438</td><td>8.60</td></tr><tr><td>4<sup>1</sup>A</td><td>6.012</td><td>0.05335</td><td>110</td><td>&pi;&#773;<sub>CO</sub>(28a) &larr; n<sub>S</sub>(27a)</td><td>5.438</td><td>8.60</td></tr><tr><td>5<sup>1</sup>A</td><td>6.587</td><td>0.02233</td><td>138</td><td>4s &larr; n<sub>S</sub>(27a)</td><td>6.44(4)</td><td>18.03</td></tr><tr><td>6<sup>1</sup>A</td><td>6.800</td><td>0.05716</td><td>133</td><td>4s &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td>6.44(4)</td><td>18.03</td></tr><tr><td>7<sup>1</sup>A</td><td>6.935</td><td>0.01848</td><td>131</td><td>4p &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td>7.009</td><td>13.30</td></tr><tr><td>8<sup>1</sup>A</td><td>7.175</td><td>0.02306</td><td>153</td><td>4p &larr; n<sub>S</sub>(27a)</td><td>6.75(3)</td><td>10.10</td></tr><tr><td>9<sup>1</sup>A</td><td>7.221</td><td>0.00864</td><td>152</td><td>4p' &larr; n<sub>S</sub>(27a)</td><td>7.341</td><td>49.73</td></tr><tr><td>10<sup>1</sup>A</td><td>7.371</td><td>0.00293</td><td>168</td><td>4p'' &larr; n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>11<sup>1</sup>A</td><td>7.431</td><td>0.00303</td><td>156</td><td>4p' &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td></td><td></td></tr><tr><td>12<sup>1</sup>A</td><td>7.588</td><td>0.03581</td><td>165</td><td>4p' &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td>7.60(6)</td><td>20.03</td></tr><tr><td>13<sup>1</sup>A</td><td>7.608</td><td>0.08128</td><td>161</td><td>3d &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td>7.92(7)</td><td>20.14</td></tr><tr><td>14<sup>1</sup>A</td><td>7.700</td><td>0.01390</td><td>169</td><td>3d &larr; n<sub>S</sub>(27a)</td><td>7.788</td><td>29.93</td></tr><tr><td>15<sup>1</sup>A</td><td>7.943</td><td>0.03474</td><td>180</td><td>3d &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td>8.02(5)</td><td>19.48</td></tr><tr><td>16<sup>1</sup>A</td><td>8.006</td><td>0.00383</td><td>205</td><td>3d' &larr; n<sub>S</sub>(27a)</td><td>7.92(7)</td><td>20.05</td></tr><tr><td>17<sup>1</sup>A</td><td>8.033</td><td>0.01057</td><td>215</td><td>3d' &larr; &nbar;&#773;<sub>O</sub>(26a)</td><td></td><td></td></tr></tbody></table>

<footnote>a Mean value of r² (electronic radial spatial extents).</footnote>

<footnote>b the last decimal of the energy value is given in brackets for these less-resolved features.</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** The VUV photoabsorption cross-section in the 3.7–10.7 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone. Inset shows the structure of the molecules.</figcaption></figure>

ionic molecules [2,10]. In Section 2, we describe the structure and properties of dihydro-2(3H)-thiophenone and dihydro-3 (2H)-thiophenone, while in Section 3 our theoretical and experimental methods. The results are presented and discussed in Section 4 and, our conclusions from this work are summarized in Section 5.

## 2. Structure and properties of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone

The dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone molecules, (C₄H₆OS), have C₁ symmetry (A) in their ground electronic states $\bar{X}^1$A, and the calculated outermost valence electronic configurations are: ... (11a)² (12a)² (13a)² (14a)² (15a)² (16a)² (17a)² (18a)² (19a)² (20a)² (21a)² (22a)² (23a)² (24a)² (25a)² (26a)² (27a)². Due to ring puckering there is no longer a molecular plane, the molecular orbitals termed in-plane and out-of-plane are referred from now on as “in-plane” and “out-of-plane”. In the Supplementary Material (SM) we include the MOs’ character showing that the highest occupied molecular orbital (HOMO), 27a, and the (HOMO-1), 26a, are the sulphur ($n_S$) “in-plane” and oxygen ($\bar{n}_O$) “out-of-plane” lone pairs. Other molecular orbitals, (HOMO-2), 25a, and (HOMO-3), 24a, have ($\bar{n}_S$), $\sigma_S$ and $\pi_{CO}$ characters. The lowest unoccupied molecular orbital (LUMO), 28a, and the (LUMO+1), 29a, are mainly $\pi^*(C=O)$ and $\sigma^*(C-S)$. A close inspection of the C₄H₆OS isomers photoabsorption spectral features (Figs. 1–4), and their assignments, reveal that the electronic excitations are mainly classified as valence and Rydberg character orbitals. For the calculated dominant excitation energies and oscillator strengths, see Table 1.

The fine structure visible in the spectra has been assigned to the main fundamental vibrational modes from the available infrared absorption and Raman data [9,10]. Assignments were possible from the available energies (and wavenumbers) in the ground electronic state to: a) dihydro-2(3H)-thiophenone, 0.211 eV (1702 cm⁻¹) for C=O stretching, $\nu_{C=O}$, 0.152 eV (1223 cm⁻¹) for CH₂ twisting, $\tau_{CH₂}$, 0.102 eV (825 cm⁻¹) for CH₂ rocking, $\rho_{CH₂}$, 0.078 eV (632 cm⁻¹) for S–C(O) stretching, $\nu_{C-S(O)}$ and 0.050 eV (404 cm⁻¹) for CO rocking, $\rho_{CO}$ [10]; b) dihydro-3 (2H)-thiophenone, 0.218 eV (1760 cm⁻¹) for C=O stretching, $\nu'_4(a)$, 0.120 eV (970 cm⁻¹), 0.089 eV (721 cm⁻¹) and 0.084 eV (679 cm⁻¹) for

<caption>**Table 1a**</caption>

The calculated dominant vertical excitation energies (EOM-CCSD/aug-cc-pV(T+d)Z+R) and oscillator strengths of dihydro-2(3H)-thiophenone (energies in eV). See text for details.

<table><caption>Dihydro-2(3H)-thiophenone</caption><thead><tr><th>State</th><th>E (eV)</th><th>f</th><th>&lt;r<sup>2</sup>&gt;<sup>a</sup></th><th>Dominant excitations</th><th>E<sub>exp.</sub> (eV)<sup>b</sup></th><th>Cross-section (Mb)</th></tr></thead><tbody><tr><td>X&#773;<sup>1</sup>A</td><td></td><td></td><td>104</td><td></td><td></td><td></td></tr><tr><td>2<sup>1</sup>A</td><td>4.733</td><td>0.00004</td><td>102</td><td>&pi;&#773;<sub>CO</sub>(28a)&larr;&nolimits;<sub>O</sub>(26a)</td><td>4.562</td><td>0.08</td></tr><tr><td>3<sup>1</sup>A</td><td>5.812</td><td>0.01791</td><td>111</td><td>&sigma;<sup>*</sup>(29a)&larr;n<sub>S</sub>(27a)</td><td>5.438</td><td>8.60</td></tr><tr><td>4<sup>1</sup>A</td><td>6.012</td><td>0.05335</td><td>110</td><td>&pi;&#773;<sub>CO</sub>(28a)&larr;n<sub>S</sub>(27a)</td><td>5.438</td><td>8.60</td></tr><tr><td>5<sup>1</sup>A</td><td>6.587</td><td>0.02233</td><td>138</td><td>4s&larr;n<sub>S</sub>(27a)</td><td>6.44(4)</td><td>18.03</td></tr><tr><td>6<sup>1</sup>A</td><td>6.800</td><td>0.05716</td><td>133</td><td>4s&larr;&nolimits;<sub>O</sub>(26a)</td><td>6.44(4)</td><td>18.03</td></tr><tr><td>7<sup>1</sup>A</td><td>6.935</td><td>0.01848</td><td>131</td><td>4p&larr;&nolimits;<sub>O</sub>(26a)</td><td>7.009</td><td>13.30</td></tr><tr><td>8<sup>1</sup>A</td><td>7.175</td><td>0.02306</td><td>153</td><td>4p&larr;n<sub>S</sub>(27a)</td><td>6.75(3)</td><td>10.10</td></tr><tr><td>9<sup>1</sup>A</td><td>7.221</td><td>0.00864</td><td>152</td><td>4p'&larr;n<sub>S</sub>(27a)</td><td>7.341</td><td>49.73</td></tr><tr><td>10<sup>1</sup>A</td><td>7.371</td><td>0.00293</td><td>168</td><td>4p''&larr;n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>11<sup>1</sup>A</td><td>7.431</td><td>0.00303</td><td>156</td><td>4p'&larr;&nolimits;<sub>O</sub>(26a)</td><td></td><td></td></tr><tr><td>12<sup>1</sup>A</td><td>7.588</td><td>0.03581</td><td>165</td><td>4p'&larr;&nolimits;<sub>O</sub>(26a)</td><td>7.60(6)</td><td>20.03</td></tr><tr><td>13<sup>1</sup>A</td><td>7.608</td><td>0.08128</td><td>161</td><td>3d&larr;&nolimits;<sub>O</sub>(26a)</td><td>7.92(7)</td><td>20.14</td></tr><tr><td>14<sup>1</sup>A</td><td>7.700</td><td>0.01390</td><td>169</td><td>3d&larr;n<sub>S</sub>(27a)</td><td>7.788</td><td>29.93</td></tr><tr><td>15<sup>1</sup>A</td><td>7.943</td><td>0.03474</td><td>180</td><td>3d&larr;&nolimits;<sub>O</sub>(26a)</td><td>8.02(5)</td><td>19.48</td></tr><tr><td>16<sup>1</sup>A</td><td>8.006</td><td>0.00383</td><td>205</td><td>3d'&larr;n<sub>S</sub>(27a)</td><td>7.92(7)</td><td>20.05</td></tr><tr><td>17<sup>1</sup>A</td><td>8.033</td><td>0.01057</td><td>215</td><td>3d'&larr;&nolimits;<sub>O</sub>(26a)</td><td></td><td></td></tr></tbody></table>

<footnote>a Mean value of r² (electronic radial spatial extents).</footnote>

<footnote>b the last decimal of the energy value is given in brackets for these less-resolved features.</footnote>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** The VUV photoabsorption cross-section in the 3.7–5.2 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone.</figcaption></figure>

ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and $v'_{15}(a')$, 0.060 eV (483 cm$^{-1}$) for C=O “in-plane” wagging, $v'_{17}(a')$, 0.053 eV (427 cm$^{-1}$) for C=O “out-of-plane” wagging, $v'_{28}(a')$ and 0.008 eV (67 cm$^{-1}$) for ring bending, $v'_{30}(a')$ [8,9]. We also note the available data for the excited state of dihydro-3

<caption>**Table 2a**</caption>

Proposed vibrational assignments of dihydro-2(3H)-thiophenone absorption bands in the photon energy range 3.7 – 5.2 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(ρ<sub>co</sub>)</th><th>ΔE(τ<sub>CH2</sub>)</th></tr></thead><tbody><tr><td>π<sub>CO</sub>(28a) ← n<sub>O</sub>(26a), (2<sup>1</sup>A ← X<sup>1</sup>A)</td><td></td><td></td><td></td></tr><tr><td>0<sup>0</sup><sub>0</sub></td><td>4.16(1)(b)</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO</sub><sup>1</sup></td><td>4.20(0)(s)</td><td>0.039</td><td>—</td></tr><tr><td>τ<sub>CH2</sub><sup>1</sup></td><td>4.31(4)(b)</td><td>—</td><td>0.153</td></tr><tr><td>τ<sub>CH2</sub><sup>0</sup>ρ<sub>CO</sub><sup>1</sup></td><td>4.34(4)(s)</td><td>0.030</td><td>—</td></tr><tr><td>τ<sub>CH2</sub><sup>2</sup></td><td>4.43(4)(b)</td><td>—</td><td>0.120</td></tr><tr><td>τ<sub>CH2</sub><sup>3</sup></td><td>4.57(5)(b)</td><td>—</td><td>0.138</td></tr><tr><td>τ<sub>CH2</sub><sup>4</sup></td><td>4.70(4)(b)</td><td>—</td><td>0.129</td></tr><tr><td>ΔE</td><td></td><td>0.035</td><td>0.135</td></tr></tbody></table>

<sup>a</sup> (b) broad structure; (s) shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features);.

(2H)-thiophenone of 0.155 eV (1249 cm$^{-1}$) for C=O stretching, $v'_{4}(a')$, 0.107 eV (864 cm$^{-1}$), 0.079 eV (638 cm$^{-1}$) and 0.076 eV (616 cm$^{-1}$) for ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and $v'_{15}(a')$, 0.041 eV (329 cm$^{-1}$) for C=O “in-plane” wagging, $v'_{17}(a')$, 0.043 eV (346 cm$^{-1}$) for C=O “out-of-plane” wagging, $v'_{28}(a')$ and 0.007 eV (58 cm$^{-1}$) for ring bending, $v'_{30}(a')$ [8,9]. For the different vibronic structures (X) assigned in the spectra, we use the notation $X_m^n$, with m and n denoting the initial and final vibrational states being excited.

Rydberg assignments have been performed from the calculated quantum defects for the four lowest experimental vertical ionisation energies taken from the work of Chin et al. [7] of dihydro-2 (3H)-thiophenone to be 9.49 eV (27a)$^{-1}$, 9.66 eV (26a)$^{-1}$, 11.80 eV (25a)$^{-1}$ and 12.29 eV (24a)$^{-1}$, respectively, whereas for dihydro-3 (2H)-thiophenone to be 8.92 eV (27a)$^{-1}$, 9.85 eV (26a)$^{-1}$, 11.63 eV (25a)$^{-1}$ and 12.57 eV (24a)$^{-1}$, respectively.

### 3. Theoretical and experimental methods

All the calculations for ground and excited states of neutral molecules were performed using the Molpro 2019.1 code [11,12]. The geometry was optimized at the CCSD level with the aug-cc-pVTZ basis set from Dunning [13,14] for H, C and O. For S, the modified aug-cc-p(V+d) Z from Dunning et al. [13] was employed. Harmonic frequencies were also obtained at the same level. For the description of Rydberg excited

<caption>**Table 1b**</caption>

The calculated dominant vertical excitation energies (EOM-CCSD/aug-cc-pV(T+d)Z+R) and oscillator strengths of dihydro-3(2H)-thiophenone (energies in eV). See text for details.

<table><caption>dihydro-3(2H)-thiophenone</caption><thead><tr><th>State</th><th>E (eV)</th><th>f</th><th>&lt;r<sup>2</sup>&gt;<sup>a</sup></th><th>Dominant excitations</th><th>E<sub>exp.</sub> (eV)<sup>b</sup></th><th>Cross-section (Mb)</th></tr></thead><tbody><tr><td>X&#773;<sup>1</sup>A</td><td></td><td></td><td>102</td><td></td><td></td><td></td></tr><tr><td>2<sup>1</sup>A</td><td>4.369</td><td>0.00054</td><td>100</td><td>π<sub>CO</sub>(28a) ← n&#773;<sub>O</sub>(26a)</td><td>4.131</td><td>0.24</td></tr><tr><td>3<sup>1</sup>A</td><td>5.315</td><td>0.00038</td><td>110</td><td>σ<sup>*</sup>(29a) ← n<sub>S</sub>(27a)</td><td>5.08(9)</td><td>0.49</td></tr><tr><td>4<sup>1</sup>A</td><td>5.942</td><td>0.00817</td><td>117</td><td>π<sub>CO</sub>(28a) ← n<sub>S</sub>(27a)</td><td>5.56(5)</td><td>1.44</td></tr><tr><td>5<sup>1</sup>A</td><td>5.986</td><td>0.01614</td><td>131</td><td>4 s ← n<sub>S</sub>(27a)</td><td>5.57(5)</td><td>1.46</td></tr><tr><td>6<sup>1</sup>A</td><td>6.553</td><td>0.02372</td><td>147</td><td>4p ← n<sub>S</sub>(27a)</td><td>6.368</td><td>8.70</td></tr><tr><td>7<sup>1</sup>A</td><td>6.736</td><td>0.02445</td><td>151</td><td>4p' ← n<sub>S</sub>(27a)</td><td>6.570</td><td>17.74</td></tr><tr><td>8<sup>1</sup>A</td><td>6.909</td><td>0.01085</td><td>149</td><td>4p' ← n&#773;<sub>O</sub>(26a) + 4p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>9<sup>1</sup>A</td><td>6.982</td><td>0.00349</td><td>161</td><td>4p' ← n<sub>S</sub>(27a) + 4s' ← n&#773;<sub>O</sub>(26a)</td><td></td><td></td></tr><tr><td>10<sup>1</sup>A</td><td>7.279</td><td>0.01019</td><td>181</td><td>3d' ← n<sub>S</sub>(27a)</td><td>7.16(3)</td><td>9.82</td></tr><tr><td>11<sup>1</sup>A</td><td>7.357</td><td>0.00671</td><td>143</td><td>3p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>12<sup>1</sup>A</td><td>7.406</td><td>0.00742</td><td>185</td><td>3d' ← n<sub>S</sub>(27a)</td><td>7.363</td><td>13.59</td></tr><tr><td>13<sup>1</sup>A</td><td>7.443</td><td>0.00085</td><td>193</td><td>3d'' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>14<sup>1</sup>A</td><td>7.503</td><td>0.00407</td><td>183</td><td>4p' ← n&#773;<sub>O</sub>(26a) + 5p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>15<sup>1</sup>A</td><td>7.623</td><td>0.00371</td><td>237</td><td>5 s ← n<sub>S</sub>(27a)</td><td>7.43(3)</td><td>16.82</td></tr><tr><td>16<sup>1</sup>A</td><td>7.709</td><td>0.00718</td><td>232</td><td>5p' ← n&#773;<sub>O</sub>(26a) + 5p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>17<sup>1</sup>A</td><td>7.732</td><td>0.01043</td><td>229</td><td>3d' ← n<sub>S</sub>(27a) + 4p' ← n&#773;<sub>O</sub>(26a)</td><td></td><td></td></tr></tbody></table>

<footnote><sup>a</sup> Mean value of r<sup>2</sup> (electronic radial spatial extents).</footnote>

<footnote><sup>b</sup> the last decimal of the energy value is given in brackets for these less-resolved features.</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** The VUV photoabsorption cross-section in the 3.7–5.2 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone.</figcaption></figure>

ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and $v'_{15}(a')$, 0.060 eV (483 cm$^{-1}$) for C=O “in-plane” wagging, $v'_{17}(a')$, 0.053 eV (427 cm$^{-1}$) for C=O “out-of-plane” wagging, $v'_{28}(a')$ and 0.008 eV (67 cm$^{-1}$) for ring bending, $v'_{30}(a')$ [8,9]. We also note the available data for the excited state of dihydro-3

<caption>**Table 2a**</caption>

Proposed vibrational assignments of dihydro-2(3H)-thiophenone absorption bands in the photon energy range 3.7 – 5.2 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(ρ<sub>co</sub>)</th><th>ΔE(τ<sub>CH2</sub>)</th></tr></thead><tbody><tr><td>π<sub>CO</sub>(28a) ← n<sub>O</sub>(26a), (2<sup>1</sup>A ← X<sup>1</sup>A)</td><td></td><td></td><td></td></tr><tr><td>0<sup>0</sup><sub>0</sub></td><td>4.16(1)(b)</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO</sub><sup>1</sup></td><td>4.20(0)(s)</td><td>0.039</td><td>—</td></tr><tr><td>τ<sub>CH2</sub><sup>1</sup></td><td>4.31(4)(b)</td><td>—</td><td>0.153</td></tr><tr><td>τ<sub>CH2</sub><sup>0</sup>ρ<sub>CO</sub><sup>1</sup></td><td>4.34(4)(s)</td><td>0.030</td><td>—</td></tr><tr><td>τ<sub>CH2</sub><sup>2</sup></td><td>4.43(4)(b)</td><td>—</td><td>0.120</td></tr><tr><td>τ<sub>CH2</sub><sup>3</sup></td><td>4.57(5)(b)</td><td>—</td><td>0.138</td></tr><tr><td>τ<sub>CH2</sub><sup>4</sup></td><td>4.70(4)(b)</td><td>—</td><td>0.129</td></tr><tr><td>ΔE</td><td></td><td>0.035</td><td>0.135</td></tr></tbody></table>

<sup>a</sup> (b) broad structure; (s) shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features);

(2H)-thiophenone of 0.155 eV (1249 cm$^{-1}$) for C=O stretching, $v'_{4}(a')$, 0.107 eV (864 cm$^{-1}$), 0.079 eV (638 cm$^{-1}$) and 0.076 eV (616 cm$^{-1}$) for ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and $v'_{15}(a')$, 0.041 eV (329 cm$^{-1}$) for C=O “in-plane” wagging, $v'_{17}(a')$, 0.043 eV (346 cm$^{-1}$) for C=O “out-of-plane” wagging, $v'_{28}(a')$ and 0.007 eV (58 cm$^{-1}$) for ring bending, $v'_{30}(a')$ [8,9]. For the different vibronic structures (X) assigned in the spectra, we use the notation $X_m^n$, with m and n denoting the initial and final vibrational states being excited.

Rydberg assignments have been performed from the calculated quantum defects for the four lowest experimental vertical ionisation energies taken from the work of Chin et al. [7] of dihydro-2 (3H)-thiophenone to be 9.49 eV (27a)$^{-1}$, 9.66 eV (26a)$^{-1}$, 11.80 eV (25a)$^{-1}$ and 12.29 eV (24a)$^{-1}$, respectively, whereas for dihydro-3 (2H)-thiophenone to be 8.92 eV (27a)$^{-1}$, 9.85 eV (26a)$^{-1}$, 11.63 eV (25a)$^{-1}$ and 12.57 eV (24a)$^{-1}$, respectively.

### 3. Theoretical and experimental methods

All the calculations for ground and excited states of neutral molecules were performed using the Molpro 2019.1 code [11,12]. The geometry was optimized at the CCSD level with the aug-cc-pVTZ basis set from Dunning [13,14] for H, C and O. For S, the modified aug-cc-p(V+d) Z from Dunning et al. [13] was employed. Harmonic frequencies were also obtained at the same level. For the description of Rydberg excited

<caption>**Table 1b**</caption>

The calculated dominant vertical excitation energies (EOM-CCSD/aug-cc-pV(T+d)Z+R) and oscillator strengths of dihydro-3(2H)-thiophenone (energies in eV). See text for details.

<table><thead><tr><th colspan="5">dihydro-3(2H)-thiophenone</th><th rowspan="2">E<sub>exp.</sub> (eV)<sup>b</sup></th><th rowspan="2">Cross-section (Mb)</th></tr><tr><th>State</th><th>E (eV)</th><th>f</th><th>&lt;r<sup>2</sup>&gt;<sup>a</sup></th><th>Dominant excitations</th></tr></thead><tbody><tr><td>X&#773;<sup>1</sup>A</td><td></td><td></td><td>102</td><td></td><td></td><td></td></tr><tr><td>2<sup>1</sup>A</td><td>4.369</td><td>0.00054</td><td>100</td><td>π<sub>CO</sub>(28a) ← n&#773;<sub>O</sub>(26a)</td><td>4.131</td><td>0.24</td></tr><tr><td>3<sup>1</sup>A</td><td>5.315</td><td>0.00038</td><td>110</td><td>σ<sup>*</sup>(29a) ← n<sub>S</sub>(27a)</td><td>5.08(9)</td><td>0.49</td></tr><tr><td>4<sup>1</sup>A</td><td>5.942</td><td>0.00817</td><td>117</td><td>π<sub>CO</sub>(28a) ← n<sub>S</sub>(27a)</td><td>5.56(5)</td><td>1.44</td></tr><tr><td>5<sup>1</sup>A</td><td>5.986</td><td>0.01614</td><td>131</td><td>4 s ← n<sub>S</sub>(27a)</td><td>5.57(5)</td><td>1.46</td></tr><tr><td>6<sup>1</sup>A</td><td>6.553</td><td>0.02372</td><td>147</td><td>4p ← n<sub>S</sub>(27a)</td><td>6.368</td><td>8.70</td></tr><tr><td>7<sup>1</sup>A</td><td>6.736</td><td>0.02445</td><td>151</td><td>4p' ← n<sub>S</sub>(27a)</td><td>6.570</td><td>17.74</td></tr><tr><td>8<sup>1</sup>A</td><td>6.909</td><td>0.01085</td><td>149</td><td>4p' ← n&#773;<sub>O</sub>(26a) + 4p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>9<sup>1</sup>A</td><td>6.982</td><td>0.00349</td><td>161</td><td>4p' ← n<sub>S</sub>(27a) + 4s' ← n&#773;<sub>O</sub>(26a)</td><td></td><td></td></tr><tr><td>10<sup>1</sup>A</td><td>7.279</td><td>0.01019</td><td>181</td><td>3d' ← n<sub>S</sub>(27a)</td><td>7.16(3)</td><td>9.82</td></tr><tr><td>11<sup>1</sup>A</td><td>7.357</td><td>0.00671</td><td>143</td><td>3p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>12<sup>1</sup>A</td><td>7.406</td><td>0.00742</td><td>185</td><td>3d' ← n<sub>S</sub>(27a)</td><td>7.363</td><td>13.59</td></tr><tr><td>13<sup>1</sup>A</td><td>7.443</td><td>0.00085</td><td>193</td><td>3d'' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>14<sup>1</sup>A</td><td>7.503</td><td>0.00407</td><td>183</td><td>4p' ← n&#773;<sub>O</sub>(26a) + 5p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>15<sup>1</sup>A</td><td>7.623</td><td>0.00371</td><td>237</td><td>5 s ← n<sub>S</sub>(27a)</td><td>7.43(3)</td><td>16.82</td></tr><tr><td>16<sup>1</sup>A</td><td>7.709</td><td>0.00718</td><td>232</td><td>5p' ← n&#773;<sub>O</sub>(26a) + 5p' ← n<sub>S</sub>(27a)</td><td></td><td></td></tr><tr><td>17<sup>1</sup>A</td><td>7.732</td><td>0.01043</td><td>229</td><td>3d' ← n<sub>S</sub>(27a) + 4p' ← n&#773;<sub>O</sub>(26a)</td><td></td><td></td></tr></tbody></table>

<footnote><sup>a</sup> Mean value of r<sup>2</sup> (electronic radial spatial extents).</footnote>

<footnote><sup>b</sup> the last decimal of the energy value is given in brackets for these less-resolved features.</footnote>
<caption>**Table 2b**</caption>

Proposed vibrational assignments of dihydro-3(2H)-thiophenone absorption bands in the photon energy range 3.7 – 5.2 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(v<sub>4</sub>)</th><th>ΔE(v<sub>13</sub>)</th><th>ΔE(v<sub>14</sub>)</th><th>ΔE(v<sub>15</sub>)</th><th>ΔE(v<sub>17</sub>)</th><th>ΔE(2v<sub>17</sub>)</th><th>ΔE(v<sub>28</sub>)</th><th>ΔE(v<sub>30</sub>)</th><th>Ref. [9]</th></tr></thead><tbody><tr><td>π<sub>CO</sub>(28a) + n&#773;<sub>O</sub>(26a), (2<sup>1</sup>A &harr; X&#771;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0<sub>0</sub><sup>1</sup>, 28<sub>0</sub><sup>1</sup>*</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.723</td></tr><tr><td>28<sub>0</sub><sup>2</sup>*</td><td>3.767</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.044</td><td>-</td><td>3.766</td></tr><tr><td>28<sub>0</sub><sup>3</sup>*</td><td>3.772</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.049</td><td>-</td><td>3.771</td></tr><tr><td>28<sub>0</sub><sup>4</sup>30<sub>0</sub><sup>2</sup>*</td><td>3.785</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.013</td><td>3.785</td></tr><tr><td>28<sub>0</sub><sup>4</sup>30<sub>1</sub><sup>1</sup>*</td><td>3.788</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.788</td></tr><tr><td>15<sub>0</sub><sup>1</sup>*</td><td>3.799</td><td>-</td><td>-</td><td>-</td><td>0.076</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.799</td></tr><tr><td>17<sub>0</sub><sup>2</sup>*</td><td>3.804</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.081</td><td>-</td><td>-</td><td>3.804</td></tr><tr><td>17<sub>0</sub><sup>4</sup>28<sub>0</sub><sup>2</sup>30<sub>0</sub><sup>1</sup>*</td><td>3.813</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.040</td><td>-</td><td>0.044</td><td>0.006</td><td>3.813</td></tr><tr><td>13<sub>1</sub><sup>1</sup>30<sub>1</sub><sup>1</sup>*</td><td>3.822</td><td>-</td><td>0.099</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.008</td><td>3.822</td></tr><tr><td>13<sub>0</sub><sup>1</sup>*</td><td>3.830</td><td>-</td><td>0.107</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.830</td></tr><tr><td>14<sub>0</sub><sup>1</sup>17<sub>0</sub><sup>1</sup>*</td><td>3.843</td><td>-</td><td>-</td><td>0.080</td><td>-</td><td>0.040</td><td>-</td><td>-</td><td>-</td><td>3.844</td></tr><tr><td>28<sub>0</sub><sup>2</sup>17<sub>0</sub><sup>2</sup>*</td><td>3.850</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.083</td><td>-</td><td>-</td><td>3.849</td></tr><tr><td>?</td><td>3.859</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.859</td></tr><tr><td>?</td><td>3.86(2)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.863</td></tr><tr><td>4<sub>0</sub><sup>1</sup>30<sub>0</sub><sup>1</sup>*</td><td>3.883</td><td>0.154</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.006</td><td>3.884</td></tr><tr><td>28<sub>0</sub><sup>2</sup>*</td><td>3.90(5)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.905</td></tr><tr><td>4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>*/13<sub>0</sub><sup>2</sup></td><td>3.925</td><td>0.153</td><td>0.095</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.049</td><td>-</td><td>3.925</td></tr><tr><td>4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>1</sub><sup>1</sup>*</td><td>3.957</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.957</td></tr><tr><td>4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>2</sub><sup>2</sup>*/4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>17<sub>0</sub><sup>1</sup> /13<sub>0</sub><sup>2</sup>17<sub>0</sub><sup>1</sup></td><td>3.964</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.039</td><td>-</td><td>-</td><td>0.007</td><td>3.964</td></tr><tr><td>13<sub>0</sub><sup>1</sup>4<sub>0</sub><sup>1</sup></td><td>3.982</td><td>0.152</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>1</sub><sup>1</sup>7<sub>0</sub><sup>1</sup> /4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>17<sub>0</sub><sup>2</sup> /13<sub>0</sub><sup>2</sup>17<sub>0</sub><sup>2</sup></td><td>4.00(6)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.042</td><td>0.081</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup></td><td>4.01(8)(s)</td><td>-</td><td>0.093</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>2</sub><sup>1</sup>7<sub>0</sub><sup>2</sup> /4<sub>0</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>17<sub>3</sub><sup>1</sup> /13<sub>0</sub><sup>2</sup>17<sub>3</sub><sup>1</sup></td><td>4.04(8)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.042</td><td>0.084</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>2</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>1</sup></td><td>4.077</td><td>0.152</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>1</sup></td><td>4.11(6)(s)</td><td>-</td><td>0.098</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>1</sup>4<sub>0</sub><sup>2</sup></td><td>4.131</td><td>0.149</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup></td><td>4.21(0)(s)</td><td>-</td><td>0.094</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>3</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>2</sup></td><td>4.22(7)(b)</td><td>0.150</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup>4<sub>0</sub><sup>1</sup></td><td>4.280</td><td>0.149</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>5</sup></td><td>4.30(0)(s,w)</td><td>-</td><td>0.090</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>5</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>3</sup></td><td>4.37(5)(b)</td><td>0.148</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>7</sup></td><td>4.41(1)(s,b)</td><td>-</td><td>0.111</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup>4<sub>0</sub><sup>4</sup></td><td>4.422</td><td>0.142</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup></td><td>4.50(5)(s,b)</td><td>-</td><td>0.094</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>5</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>4</sup></td><td>4.52(2)(s,w)</td><td>0.147</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup>4<sub>0</sub><sup>3</sup></td><td>4.57(2)(b)</td><td>0.150</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>0</sub><sup>6</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>5</sup></td><td>4.67(5)(b,w)</td><td>0.153</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>ΔE</td><td>0.150</td><td>0.098</td><td>0.080</td><td>0.076</td><td>0.041</td><td>0.082</td><td>0.047</td><td>0.008</td><td></td><td></td></tr></tbody></table>

states, the basis set was augmented at the mass centre of the molecule by a set (6s6p4d) of diffuse orbitals, taken from [14,15] (aug-cc-pV(T+d) Z+R basis set). The nature of the transition was assessed by visual inspection of the natural orbitals for each transition and the average value $\langle r^2 \rangle$ of the electronic cloud. All the present calculations were performed at the CCSD optimized geometry of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (Supplementary Material (SM) Fig. S1). The excited states' vertical transition energies, together with dipolar oscillator strengths (length gauge) were obtained at the EOM-CCSD level [16]. In addition, the calculated vertical ionisation energies were obtained at the EOMIP-CCSD level [17] using the CFOUR code [18] as well as with P3+ calculations [19] performed with the Gaussian 16 code [20], with both the aug-cc-pV(T+d)Z basis set. The data are compared with the available experimental values in Table S1.

The absolute photoabsorption cross-sections of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (Fig. 1a and b) were measured at room temperature using the AU-UV beam line, at the ASTRID2 synchrotron radiation facility, Aarhus University, Denmark. The description of the beamline has been reported on several occasions elsewhere [21–23], and so only a brief description is given here. The photon beam with a resolution of better than 0.08 nm passes through the sample in the absorption gas cell containing, either dihydro-2

(3H)-thiophenone or dihydro-3(2H)-thiophenone, vapour. The lower wavelength limit of detection (115 nm) is defined by the transmission windows (MgF<sub>2</sub>) used to enclose the absorption cell, and the transmitted UV light detected by a photomultiplier tube (PMT). The sample's absolute pressure value in the absorption gas cell is measured using a capacitance manometer (Chell CDG100D), while any saturation effects in the absorption cross-sections are avoided selecting a pressure appropriate for the cross-section in the measurement region to have and attenuation of 50 % or less. At room temperature the highest achievable pressure for dihydro-2(3H)-thiophenone was 0.55 mbar, while for dihydro-3(2H)-thiophenone this was 1.07 mbar. The Beer-Lambert attenuation law: $I_t = I_0e^{(-N\sigma l)}$, where $I_t$ is the light intensity transmitted through the gas sample, $I_0$ is that through the evacuated cell, N the molecular number density of C<sub>4</sub>H<sub>6</sub>OS, and l the absorption path length (15.5 cm), was used to obtain absolute photoabsorption cross-section values, $\sigma$, in units of megabarn (1Mb ≡ 10<sup>-18</sup> cm<sup>2</sup>).

The synchrotron ring current was monitored throughout the collection of each spectrum, and background scans were recorded with the cell evacuated. Accurate cross-section values are obtained by recording the VUV spectrum in small (5 or 10 nm) sections, allowing an overlap of at least 10 points between the adjoining sections. To compensate for beam decay in the storage ring, ASTRID2 operates in a “top-up” mode allowing

<footnote><sup>a</sup> (s) shoulder structure; (b) broad structure; (w) weak feature (the last decimal of the energy value is given in brackets for these less-resolved features);<sup>*</sup> assignment according to Sagear et al. [9]; ? feature remains unassigned.</footnote>
<caption>**Table 2b**</caption>

Proposed vibrational assignments of dihydro-3(2H)-thiophenone absorption bands in the photon energy range 3.7 – 5.2 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(v<sub>4</sub>)</th><th>ΔE(v<sub>13</sub>)</th><th>ΔE(v<sub>14</sub>)</th><th>ΔE(v<sub>15</sub>)</th><th>ΔE(v<sub>17</sub>)</th><th>ΔE(2v<sub>17</sub>)</th><th>ΔE(v<sub>28</sub>)</th><th>ΔE(v<sub>30</sub>)</th><th>Ref. [9]</th></tr></thead><tbody><tr><td>π<sub>CO</sub>(28a)+n&#773;<sub>O</sub>(26a), (2<sup>1</sup>A&#x2190;X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0<sub>0</sub><sup>1</sup>, 28<sub>0</sub><sup>1</sup>*</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.723</td></tr><tr><td>28<sub>0</sub><sup>2</sup>*</td><td>3.767</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.044</td><td>-</td><td>3.766</td></tr><tr><td>28<sub>0</sub><sup>3</sup>*</td><td>3.772</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.049</td><td>-</td><td>3.771</td></tr><tr><td>28<sub>0</sub><sup>4</sup>30<sub>0</sub><sup>2</sup>*</td><td>3.785</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.013</td><td>3.785</td></tr><tr><td>28<sub>0</sub><sup>4</sup>30<sub>1</sub><sup>1</sup>*</td><td>3.788</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.788</td></tr><tr><td>15<sub>0</sub><sup>1</sup>*</td><td>3.799</td><td>-</td><td>-</td><td>-</td><td>0.076</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.799</td></tr><tr><td>17<sub>0</sub><sup>2</sup>*</td><td>3.804</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.081</td><td>-</td><td>-</td><td>3.804</td></tr><tr><td>17<sub>0</sub><sup>4</sup>28<sub>0</sub><sup>2</sup>30<sub>0</sub><sup>1</sup>*</td><td>3.813</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.040</td><td>-</td><td>0.044</td><td>0.006</td><td>3.813</td></tr><tr><td>13<sub>1</sub><sup>1</sup>30<sub>1</sub><sup>1</sup>*</td><td>3.822</td><td>-</td><td>0.099</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.008</td><td>3.822</td></tr><tr><td>13<sub>0</sub><sup>1</sup>*</td><td>3.830</td><td>-</td><td>0.107</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.830</td></tr><tr><td>14<sub>0</sub><sup>1</sup>17<sub>0</sub><sup>1</sup>*</td><td>3.843</td><td>-</td><td>-</td><td>0.080</td><td>-</td><td>0.040</td><td>-</td><td>-</td><td>-</td><td>3.844</td></tr><tr><td>28<sub>0</sub><sup>2</sup>17<sub>0</sub><sup>2</sup>*</td><td>3.850</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.083</td><td>-</td><td>-</td><td>3.849</td></tr><tr><td>?</td><td>3.859</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.859</td></tr><tr><td>?</td><td>3.86(2)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.863</td></tr><tr><td>4<sub>1</sub><sup>1</sup>30<sub>0</sub><sup>1</sup>*</td><td>3.883</td><td>0.154</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.006</td><td>3.884</td></tr><tr><td>28<sub>0</sub><sup>2</sup>*</td><td>3.90(5)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.905</td></tr><tr><td>4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>*/13<sub>0</sub><sup>2</sup></td><td>3.925</td><td>0.153</td><td>0.095</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.049</td><td>-</td><td>3.925</td></tr><tr><td>4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>1</sub><sup>1</sup>*</td><td>3.957</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>3.957</td></tr><tr><td>4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>2</sub><sup>2</sup>*/4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>17<sub>0</sub><sup>1</sup> /13<sub>0</sub><sup>2</sup>17<sub>0</sub><sup>1</sup></td><td>3.964</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.039</td><td>-</td><td>-</td><td>0.007</td><td>3.964</td></tr><tr><td>13<sub>0</sub><sup>1</sup>4<sub>0</sub><sup>1</sup></td><td>3.982</td><td>0.152</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>1</sub><sup>1</sup>7<sub>0</sub><sup>1</sup> /4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>17<sub>0</sub><sup>2</sup> /13<sub>0</sub><sup>2</sup>17<sub>0</sub><sup>2</sup></td><td>4.00(6)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.042</td><td>0.081</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup></td><td>4.01(8)(s)</td><td>-</td><td>0.093</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>30<sub>2</sub><sup>1</sup>17<sub>2</sub><sup>1</sup> /4<sub>1</sub><sup>1</sup>28<sub>0</sub><sup>3</sup>17<sub>3</sub><sup>1</sup> /13<sub>0</sub><sup>2</sup>17<sub>3</sub><sup>1</sup></td><td>4.04(8)(s)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.042</td><td>0.084</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>2</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>1</sup></td><td>4.077</td><td>0.152</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>1</sup></td><td>4.11(6)(s)</td><td>-</td><td>0.098</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>1</sup>4<sub>0</sub><sup>2</sup></td><td>4.131</td><td>0.149</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup></td><td>4.21(0)(s)</td><td>-</td><td>0.094</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>3</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>2</sup></td><td>4.22(7)(b)</td><td>0.150</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup>4<sub>0</sub><sup>3</sup></td><td>4.280</td><td>0.149</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>5</sup></td><td>4.30(0)(s,w)</td><td>-</td><td>0.090</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>2</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>3</sup></td><td>4.37(5)(b)</td><td>0.148</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>7</sup></td><td>4.41(1)(s,b)</td><td>-</td><td>0.111</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup>4<sub>0</sub><sup>4</sup></td><td>4.422</td><td>0.142</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>0</sub><sup>3</sup></td><td>4.50(5)(s,b)</td><td>-</td><td>0.094</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>5</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>4</sup></td><td>4.52(2)(s,w)</td><td>0.147</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>13<sub>1</sub><sup>4</sup>4<sub>0</sub><sup>3</sup></td><td>4.57(2)(b)</td><td>0.150</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4<sub>1</sub><sup>6</sup>28<sub>0</sub><sup>3</sup> /13<sub>0</sub><sup>2</sup>4<sub>0</sub><sup>5</sup></td><td>4.67(5)(b,w)</td><td>0.153</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>ΔE</td><td>0.150</td><td>0.098</td><td>0.080</td><td>0.076</td><td>0.041</td><td>0.082</td><td>0.047</td><td>0.008</td><td>-</td><td>-</td></tr></tbody></table>

states, the basis set was augmented at the mass centre of the molecule by a set (6s6p4d) of diffuse orbitals, taken from [14,15] (aug-cc-pV(T+d) Z+R basis set). The nature of the transition was assessed by visual inspection of the natural orbitals for each transition and the average value $\langle r^2 \rangle$ of the electronic cloud. All the present calculations were performed at the CCSD optimized geometry of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (Supplementary Material (SM) Fig. S1). The excited states' vertical transition energies, together with dipolar oscillator strengths (length gauge) were obtained at the EOM-CCSD level [16]. In addition, the calculated vertical ionisation energies were obtained at the EOMIP-CCSD level [17] using the CFOUR code [18] as well as with P3+ calculations [19] performed with the Gaussian 16 code [20], with both the aug-cc-pV(T+d)Z basis set. The data are compared with the available experimental values in Table S1.

The absolute photoabsorption cross-sections of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (Fig. 1a and b) were measured at room temperature using the AU-UV beam line, at the ASTRID2 synchrotron radiation facility, Aarhus University, Denmark. The description of the beamline has been reported on several occasions elsewhere [21–23], and so only a brief description is given here. The photon beam with a resolution of better than 0.08 nm passes through the sample in the absorption gas cell containing, either dihydro-2

(3H)-thiophenone or dihydro-3(2H)-thiophenone, vapour. The lower wavelength limit of detection (115 nm) is defined by the transmission windows (MgF<sub>2</sub>) used to enclose the absorption cell, and the transmitted UV light detected by a photomultiplier tube (PMT). The sample's absolute pressure value in the absorption gas cell is measured using a capacitance manometer (Chell CDG100D), while any saturation effects in the absorption cross-sections are avoided selecting a pressure appropriate for the cross-section in the measurement region to have and attenuation of 50 % or less. At room temperature the highest achievable pressure for dihydro-2(3H)-thiophenone was 0.55 mbar, while for dihydro-3(2H)-thiophenone this was 1.07 mbar. The Beer-Lambert attenuation law: $I_t = I_0e^{(-N\sigma l)}$, where $I_t$ is the light intensity transmitted through the gas sample, $I_0$ is that through the evacuated cell, N the molecular number density of C<sub>4</sub>H<sub>6</sub>OS, and l the absorption path length (15.5 cm), was used to obtain absolute photoabsorption cross-section values, $\sigma$, in units of megabarn (1Mb ≡ $10^{-18}$ cm<sup>2</sup>).

The synchrotron ring current was monitored throughout the collection of each spectrum, and background scans were recorded with the cell evacuated. Accurate cross-section values are obtained by recording the VUV spectrum in small (5 or 10 nm) sections, allowing an overlap of at least 10 points between the adjoining sections. To compensate for beam decay in the storage ring, ASTRID2 operates in a “top-up” mode allowing

<footnote><sup>a</sup> (s) shoulder structure; (b) broad structure; (w) weak feature (the last decimal of the energy value is given in brackets for these less-resolved features);<sup>*</sup> assignment according to Sagear et al. [9]; ? feature remains unassigned.</footnote>
<caption>**Table 3a**</caption>

Proposed vibrational assignments of dihydro-2(3H)-thiophenone absorption bands in the photon energy range 6.0 – 8.0 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE<br>(ρ<sub>CO</sub>)</th><th>ΔE(v<sub>c</sub><br>s(o))</th><th>ΔE<br>(ρ<sub>CH2</sub>)</th><th>ΔE<br>(v<sub>c=0</sub>)</th></tr></thead><tbody><tr><td>4s←n<sub>S</sub>(27a), (5<sup>1</sup>A←X&#773;<sup>1</sup>A) /<br>4s←n&#773;<sub>O</sub>(26a), (6<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4s(27a)<sup>-1</sup>/4s(26a)<sup>-1</sup></td><td>6.44(4)<br>(b)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)<sub>0</sub></sub><sup>1</sup></td><td>6.51(5)<br>(b)</td><td>—</td><td>0.071</td><td>—</td><td>—</td></tr><tr><td>4p←n<sub>S</sub>(27a), (7<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4p(26a)<sup>-1</sup></td><td>7.009</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C=O<sub>0</sub></sub><sup>1</sup></td><td>7.20(0)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>0.191</td></tr><tr><td>4p'←n<sub>S</sub>(27a), (9<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4p'(27a)<sup>-1</sup></td><td>7.341</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)<sub>0</sub></sub><sup>1</sup></td><td>7.42(0)<br>(s)</td><td>—</td><td>0.079</td><td>—</td><td>—</td></tr><tr><td>4p'←n&#773;<sub>O</sub>(26a), (12<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4p'(26a)<sup>-1</sup></td><td>7.60(6)<br>(b)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CH2<sub>0</sub></sub><sup>1</sup></td><td>7.70(1)<br>(s)</td><td>—</td><td>—</td><td>0.095</td><td>—</td></tr><tr><td>ρ<sub>CH2<sub>0</sub></sub><sup>2</sup>/3d(27a)<sup>-1</sup></td><td>7.788</td><td>—</td><td>—</td><td>0.087</td><td>—</td></tr><tr><td>ρ<sub>CH2<sub>0</sub></sub><sup>3</sup></td><td>7.872</td><td>—</td><td>—</td><td>0.084</td><td>—</td></tr><tr><td>3d←n&#773;<sub>O</sub>(26a), (13<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>3d(26a)<sup>-1</sup></td><td>7.92(7)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub><sup>1</sup></td><td>7.95(8)<br>(s)</td><td>0.031</td><td>—</td><td>—</td><td>—</td></tr><tr><td></td><td>ΔE</td><td>0.031</td><td>0.075</td><td>0.089</td><td>0.191</td></tr></tbody></table>

<sup>a</sup> (b) broad feature; (s) shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features);

the light intensity to be kept quasi-constant. The small variations (~2–3 %) of the incident flux are normalized to the beam current in the storage ring. This methodology allows us to determine the accuracy of the photoabsorption cross-sections to within ± 5 %.

The liquid samples of dihydro-2(3H)-thiophenone (γ-thiobutyrolactone, CAS number 1003–10–7) and dihydro-3(2H)-thiophenone (dihydrothiophen-3-one, CAS number 1003–04–9) used in the VUV measurements were purchased from Sigma-Aldrich and Fluorochem, with stated purities of 98 % and 95 % respectively. The samples were degassed by repeated freeze–pump–thaw cycles before use.

## 4. Results and discussion

The room temperature high-resolution VUV photoabsorption spectra of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, in the photon energy range 3.7–10.7 eV, are shown in Fig. 1, with expanded sections of the recorded absolute cross-section values depicted in Fig. 2 to Fig. 4. The proposed assignments of the recorded absorption features are listed in Tables 2–5. The spectra were measured down to an energy of 3.875 and 3.757 eV, for dihydro-2(3H)-thiophenone and dihydro-3 (2H)-thiophenone, respectively, the former showing no absorption features before the onset to the first band above 4.1 eV, while for the latter the origin of the band has been identified at 3.723 eV (30,028 cm<sup>-1</sup>) [8, 9] from jet-cooled fluorescence excitation, with the $\tilde{0}_0^0$ transition not observed in the present work.

The main electronic transitions are assigned to the promotion of an electron from the $n_S$ (HOMO) and the $\tilde{n}_O$ (HOMO-1) to the lowest unoccupied molecular orbitals of valence and Rydberg (Section 4.4) character (Table 1, Figs. S2 and S3). In general, we observe a reasonably good level of agreement between ab initio results and experimental data (Table 1). The different absorption bands exhibit fine structures, which have been mainly assigned to C=O stretching, CH<sub>2</sub> rocking and ring

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** The VUV photoabsorption cross-section in the 5.0–8.0 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone. The vertical marks on the top of the spectrum are fine structure superimposed on the Rydberg series. See text for details.</figcaption></figure>

stretching modes, prevalent above 6.0 eV. The comprehensive assignment of the photoabsorption spectra in Fig. 1 has been divided into sections comprising the different photon energy ranges (see below); the major electronic transitions and their related fine structure have been assigned with the aid of the quantum chemical calculations. The photoabsorption features above 5.5 eV are largely due to the contribution and overlap of different Rydberg electronic states upon which relevant vibrational fine structures are superimposed, thus contributing to congested spectra (mainly above 8.0 eV). To avoid too many assignments in the different expanded sections of Figs. 2–4, complete descriptions of the assignments are listed in Tables 2–4, instead.

A close comparison of dihydro-2(3H)-thiophenone and dihydro-3 (2H)-thiophenone photoabsorption spectra (Fig. 1) shows, generally speaking, both molecules have similar bands, although in the former the features above 5 eV are red-shifted relative to the latter. This can be related to an inductive effect from the interaction of the carbonyl group and the ring heteroatom, which is more pronounced in case of dihydro-2 (3H)-thiophenone from the electron density of the HOMO-1 (see Fig. S3).

### 4.1. Region I (3.7–5.2 eV)

#### 4.1.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)

The calculations for the photoabsorption band in this region (Fig. 2a) suggest an electronic excitation from the (HOMO-1), 26a, to the (LUMO), 28a, assigned to the $\pi^*(28a)←\tilde{n}_O(26a)$, $(2^1A←\tilde{X}^1A)$ transi-
<caption>**Table 3a**</caption>

Proposed vibrational assignments of dihydro-2(3H)-thiophenone absorption bands in the photon energy range 6.0 – 8.0 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE<br>(ρ<sub>CO</sub>)</th><th>ΔE(v<sub>c</sub><br>s(o))</th><th>ΔE<br>(ρ<sub>CH2</sub>)</th><th>ΔE<br>(v<sub>c=0</sub>)</th></tr></thead><tbody><tr><td>4s←n<sub>S</sub>(27a), (5<sup>1</sup>A←X<sup>1</sup>A) /<br>4s←n&#773;<sub>O</sub>(26a), (6<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4s(27a)<sup>-1</sup>/4s(26a)<sup>-1</sup></td><td>6.44(4)<br>(b)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)O<sub>0</sub></sub><sup>1</sup></td><td>6.51(5)<br>(b)</td><td>—</td><td>0.071</td><td>—</td><td>—</td></tr><tr><td>4p←n<sub>S</sub>(27a), (7<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4p(26a)<sup>-1</sup></td><td>7.009</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C=O<sub>0</sub></sub><sup>1</sup></td><td>7.20(0)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>0.191</td></tr><tr><td>4p'←n<sub>S</sub>(27a), (9<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4p'(27a)<sup>-1</sup></td><td>7.341</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)O<sub>0</sub></sub><sup>1</sup></td><td>7.42(0)<br>(s)</td><td>—</td><td>0.079</td><td>—</td><td>—</td></tr><tr><td>4p'←n&#773;<sub>O</sub>(26a), (12<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4p'(26a)<sup>-1</sup></td><td>7.60(6)<br>(b)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CH2<sub>0</sub></sub><sup>1</sup></td><td>7.70(1)<br>(s)</td><td>—</td><td>—</td><td>0.095</td><td>—</td></tr><tr><td>ρ<sub>CH2<sub>0</sub></sub><sup>2</sup>/3d(27a)<sup>-1</sup></td><td>7.788</td><td>—</td><td>—</td><td>0.087</td><td>—</td></tr><tr><td>ρ<sub>CH2<sub>0</sub></sub><sup>3</sup></td><td>7.872</td><td>—</td><td>—</td><td>0.084</td><td>—</td></tr><tr><td>3d←n&#773;<sub>O</sub>(26a), (13<sup>1</sup>A←X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>3d(26a)<sup>-1</sup></td><td>7.92(7)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub><sup>1</sup></td><td>7.95(8)<br>(s)</td><td>0.031</td><td>—</td><td>—</td><td>—</td></tr><tr><td></td><td>ΔE</td><td>0.031</td><td>0.075</td><td>0.089</td><td>0.191</td></tr></tbody></table>

<sup>a</sup> (b) broad feature; (s) shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features);

the light intensity to be kept quasi-constant. The small variations (~2–3 %) of the incident flux are normalized to the beam current in the storage ring. This methodology allows us to determine the accuracy of the photoabsorption cross-sections to within ± 5 %.

The liquid samples of dihydro-2(3H)-thiophenone (γ-thiobutyrolactone, CAS number 1003–10–7) and dihydro-3(2H)-thiophenone (dihydrothiophen-3-one, CAS number 1003–04–9) used in the VUV measurements were purchased from Sigma-Aldrich and Fluorochem, with stated purities of 98 % and 95 % respectively. The samples were degassed by repeated freeze–pump–thaw cycles before use.

## 4. Results and discussion

The room temperature high-resolution VUV photoabsorption spectra of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, in the photon energy range 3.7–10.7 eV, are shown in Fig. 1, with expanded sections of the recorded absolute cross-section values depicted in Fig. 2 to Fig. 4. The proposed assignments of the recorded absorption features are listed in Tables 2–5. The spectra were measured down to an energy of 3.875 and 3.757 eV, for dihydro-2(3H)-thiophenone and dihydro-3 (2H)-thiophenone, respectively, the former showing no absorption features before the onset to the first band above 4.1 eV, while for the latter the origin of the band has been identified at 3.723 eV (30,028 cm<sup>-1</sup>) [8, 9] from jet-cooled fluorescence excitation, with the $\tilde{0}_0^0$ transition not observed in the present work.

The main electronic transitions are assigned to the promotion of an electron from the $n_S$ (HOMO) and the $\tilde{n}_O$ (HOMO-1) to the lowest unoccupied molecular orbitals of valence and Rydberg (Section 4.4) character (Table 1, Figs. S2 and S3). In general, we observe a reasonably good level of agreement between ab initio results and experimental data (Table 1). The different absorption bands exhibit fine structures, which have been mainly assigned to C=O stretching, CH<sub>2</sub> rocking and ring

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** The VUV photoabsorption cross-section in the 5.0–8.0 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone. The vertical marks on the top of the spectrum are fine structure superimposed on the Rydberg series. See text for details.</figcaption></figure>

stretching modes, prevalent above 6.0 eV. The comprehensive assignment of the photoabsorption spectra in Fig. 1 has been divided into sections comprising the different photon energy ranges (see below); the major electronic transitions and their related fine structure have been assigned with the aid of the quantum chemical calculations. The photoabsorption features above 5.5 eV are largely due to the contribution and overlap of different Rydberg electronic states upon which relevant vibrational fine structures are superimposed, thus contributing to congested spectra (mainly above 8.0 eV). To avoid too many assignments in the different expanded sections of Figs. 2–4, complete descriptions of the assignments are listed in Tables 2–4, instead.

A close comparison of dihydro-2(3H)-thiophenone and dihydro-3 (2H)-thiophenone photoabsorption spectra (Fig. 1) shows, generally speaking, both molecules have similar bands, although in the former the features above 5 eV are red-shifted relative to the latter. This can be related to an inductive effect from the interaction of the carbonyl group and the ring heteroatom, which is more pronounced in case of dihydro-2 (3H)-thiophenone from the electron density of the HOMO-1 (see Fig. S3).

### 4.1. Region I (3.7–5.2 eV)

#### 4.1.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)

The calculations for the photoabsorption band in this region (Fig. 2a) suggest an electronic excitation from the (HOMO-1), 26a, to the (LUMO), 28a, assigned to the $\pi^*(28a)←\tilde{n}_O(26a)$, $(2^1A←\tilde{X}^1A)$ transi-
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 4.** The VUV photoabsorption cross-section in the 8.0–10.7 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone. The vertical marks on the top of the spectrum are fine structure superimposed on the Rydberg series. See text for details.</figcaption></figure>

tion, with an oscillator strength of <0.0001 (Table 1a). The band $0_0^0$ origin is tentatively assigned at 4.16(1) eV, with weak vibrational pattern due to CH$_2$ twisting, $\tau_{CH_2'}$ and CO rocking, $\rho_{CO'}$ vibrational modes, with average vibrational spacings ($\Delta E$) of 0.135 and 0.035 eV (Table 2a).

### 4.1.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)

The broad absorption band (see Fig. 2b) is overlaid with a large number of vibrational features that are listed in Table 2b. The calculations in Table 1b assign the electronic transition to promotion of an electron from the (HOMO-1), 26a, to the (LUMO), 28a, $\pi_{CO'}(28a) \leftarrow \bar{n}_O(26a)$, $(2^1A \leftarrow \bar{X}^1A)$, with an oscillator strength of ~0.0005. The photoabsorption cross-section peaks at 4.131 eV with a magnitude of 0.24 Mb. The electronic band origin has been reported at 3.723 eV (30,028 cm$^{-1}$) from the jet-cooled fluorescence spectrum of Laane et al. [8]. Analysis of this region indicates combinations of seven vibrations involving C=O stretching, $v_4(a)$, ring stretching, $\dot{v}_{13}(a)$, $\dot{v}_{14}(a)$ and $\dot{v}_{15}(a)$, C=O “in-plane” wagging, $\dot{v}_{17}(a)$, C=O “out-of-plane” wagging, $v_{28}(a)$ and ring bending, $v_{30}(a)$ modes, with average value ($\Delta E$) of 0.150, 0.098, 0.080, 0.076, 0.041, 0.047 and 0.008 eV (Table 2b). Such values are in excellent agreement with the vibrational energies in the $S_1$ ($n, \pi^*$) electronic excited state from fluorescence excitation spectroscopy [8,9]. It is relevant to note that in the present spectrum the absorption band shows up to three combinations with the ring bending, $v_{30}(a)$ vibration in accord with Sagear et al. [9].

<caption>**Table 3b**</caption>

Proposed vibrational assignments of dihydro-3(2H)-thiophenone absorption bands in the photon energy range 6.0 – 8.0 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(v<sub>4</sub>)</th><th>ΔE(v<sub>13</sub>)</th><th>ΔE(v<sub>15</sub>)</th></tr></thead><tbody><tr><td>4s←n<sub>S</sub>(27a) (6<sup>1</sup>A←X<sup>1</sup>A)</td><td></td><td></td><td></td><td></td></tr><tr><td>0<sub>0</sub><sup>0</sup></td><td>6.368</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4s(26a)<sup>-1</sup>/4<sub>0</sub><sup>1</sup></td><td>6.512</td><td>0.144</td><td>—</td><td>—</td></tr><tr><td>4s(26a)<sup>-1</sup>4<sub>0</sub><sup>1</sup>/4<sub>0</sub><sup>2</sup></td><td>6.65(2)(s)</td><td>0.140</td><td>—</td><td>—</td></tr><tr><td>4s(26a)<sup>-1</sup>4<sub>0</sub><sup>1</sup>/4<sub>0</sub><sup>0</sup></td><td>6.80(5)(s,w)</td><td>0.153</td><td>—</td><td>—</td></tr><tr><td>4p'←n<sub>S</sub>(27a) (7<sup>1</sup>A←X<sup>1</sup>A)</td><td></td><td></td><td></td><td></td></tr><tr><td>0<sub>0</sub><sup>0</sup></td><td>6.567</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>1</sup></td><td>6.72(4)(s)</td><td>0.157</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>0</sup></td><td>6.88(8)(s,w)</td><td>0.164</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>3</sup></td><td>7.05(7)(s)</td><td>0.169</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>3</sup>13<sub>0</sub><sup>1</sup>/4p(26a)<sup>-1</sup></td><td>7.16(3)(s)</td><td>—</td><td>0.106</td><td>—</td></tr><tr><td>4p(26a)<sup>-1</sup></td><td>7.16(3)(s)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>13<sub>0</sub><sup>1</sup></td><td>7.26(3)(b)</td><td>—</td><td>0.100</td><td>—</td></tr><tr><td>13<sub>0</sub><sup>1</sup>4<sub>0</sub><sup>1</sup></td><td>7.42(9)(s)</td><td>0.166</td><td>—</td><td>—</td></tr><tr><td>3d←n<sub>S</sub>(27a) (10<sup>1</sup>A←X<sup>1</sup>A)</td><td></td><td></td><td></td><td></td></tr><tr><td>3d' (27a)<sup>-1</sup></td><td>7.363</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>1</sup></td><td>7.49(6)(s)</td><td>0.133</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>0</sup>/5p(27a)<sup>-1</sup></td><td>7.63(5)(b)</td><td>0.139</td><td>—</td><td>—</td></tr><tr><td>13<sub>0</sub><sup>1</sup>/4p'(26a)<sup>-1</sup></td><td>7.739</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>15<sub>0</sub><sup>1</sup>/5p(27a)<sup>-1</sup></td><td>7.817</td><td>—</td><td>—</td><td>0.078</td></tr><tr><td>ΔE</td><td>0.152</td><td>0.103</td><td>0.078</td><td></td></tr></tbody></table>

## 4.2. Region II (5.0–8.0 eV)

### 4.2.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)

This region of the absorption spectrum is shown in detail in Fig. 3a with the assignments of the spectral features in Table 3a. The two major transitions, with vertical values at 6.44(4) eV (calculated oscillator strength of $f \approx 0.0572$) and at 7.92(7) eV ($f \approx 0.0813$) and local maxima cross-sections of 18.03 and 20.14 Mb, have been assigned to $4s \leftarrow \bar{n}_O(26a)$, $(6^1A \leftarrow \bar{X}^1A)$ and $4s \leftarrow \bar{n}_O(26a)$, $(13^1A \leftarrow \bar{X}^1A)$. However, the most intense feature (49.73 Mb) peaks at 7.341 eV and has been assigned to a Rydberg excitation (4p' ← 27a), which will be discussed in Section 4.4. Within the absorption region, the proposed vibrational assignments are due to C=O stretching, $\nu_{C=O'}$, CH$_2$ rocking, $\rho_{CH_2'}$, S–C(O) stretching, $\nu_{C-S(O)}$ and CO rocking, $\rho_{CO'}$ modes (Table 3a), with average values ($\Delta E$) of 0.191 eV, 0.089 eV, 0.075 eV and 0.031 eV. With the exception of $\nu_{C-S(O)}$, these values are reduced from the ground-state of dihydro-2(3H)-thiophenone [10], suggesting a broadening of the potential energy surfaces involved in such electronic excitations.

Table 1a also lists a valence character electronic transition with a vertical value at 5.438 eV, assigned to $\pi_{CO'}^*(28a) \leftarrow n_S(27a)$, $(4^1A \leftarrow \bar{X}^1A)$ with a significant high oscillator strength value of $f \approx 0.0534$. The other electronic excitations are Rydberg in character, display significant vibrational structure (see Tables 3a and 4a) and will be comprehensively discussed in Section 4.4. The rather broad underlying contributions in the 5.0–8.0 eV absorption region enhancing the background signal, may originate from $\sigma^*$ dissociative character of the lowest unoccupied molecular orbitals along the C–C and C–S bonds, as noted in (LUMO+1), 29, (Table S2), as well as from other higher energy MOs.

### 4.2.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)

The valence electronic excitations in this energy band show transitions at 5.08(9) eV (0.49 Mb) and 5.56(5) eV (1.44 Mb), with calculated oscillator strengths $f \approx 0.0004$ and $f \approx 0.0082$. These transitions have been assigned to $\sigma^*(29a) \leftarrow n_S(27a)$, $(3^1A \leftarrow \bar{X}^1A)$ and to $\pi_{CO'}^*(28a) \leftarrow n_S(27a)$, $(4^1A \leftarrow \bar{X}^1A)$ (Table 1b), while above 5.8 eV other electronic

<footnote><sup>a</sup> (s) shoulder structure; (w) weak feature; (b) broad feature; (the last decimal of the energy value is given in brackets for these less-resolved features);.</footnote>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 4.** The VUV photoabsorption cross-section in the 8.0–10.7 eV range for: (a) dihydro-2(3H)-thiophenone; (b) dihydro-3(2H)-thiophenone. The vertical marks on the top of the spectrum are fine structure superimposed on the Rydberg series. See text for details.</figcaption></figure>

tion, with an oscillator strength of <0.0001 (Table 1a). The band $0_0^0$ origin is tentatively assigned at 4.16(1) eV, with weak vibrational pattern due to CH$_2$ twisting, $\tau_{CH_2'}$ and CO rocking, $\rho_{CO'}$ vibrational modes, with average vibrational spacings ($\Delta E$) of 0.135 and 0.035 eV (Table 2a).

### 4.1.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)

The broad absorption band (see Fig. 2b) is overlaid with a large number of vibrational features that are listed in Table 2b. The calculations in Table 1b assign the electronic transition to promotion of an electron from the (HOMO-1), 26a, to the (LUMO), 28a, $\pi_{CO'}(28a) \leftarrow \bar{n}_O(26a)$, $(2^1A \leftarrow \bar{X}^1A)$, with an oscillator strength of ~0.0005. The photoabsorption cross-section peaks at 4.131 eV with a magnitude of 0.24 Mb. The electronic band origin has been reported at 3.723 eV (30,028 cm$^{-1}$) from the jet-cooled fluorescence spectrum of Laane et al. [8]. Analysis of this region indicates combinations of seven vibrations involving C=O stretching, $v_4(a)$, ring stretching, $\dot{v}_{13}(a)$, $\dot{v}_{14}(a)$ and $\dot{v}_{15}(a)$, C=O “in-plane” wagging, $\dot{v}_{17}(a)$, C=O “out-of-plane” wagging, $v_{28}(a)$ and ring bending, $v_{30}(a)$ modes, with average value ($\Delta E$) of 0.150, 0.098, 0.080, 0.076, 0.041, 0.047 and 0.008 eV (Table 2b). Such values are in excellent agreement with the vibrational energies in the $S_1$ ($n, \pi^*$) electronic excited state from fluorescence excitation spectroscopy [8,9]. It is relevant to note that in the present spectrum the absorption band shows up to three combinations with the ring bending, $v_{30}(a)$ vibration in accord with Sagear et al. [9].

<caption>**Table 3b**</caption>

Proposed vibrational assignments of dihydro-3(2H)-thiophenone absorption bands in the photon energy range 6.0 – 8.0 eV<sup>a</sup>. Energies in eV. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(v<sub>4</sub>)</th><th>ΔE(v<sub>13</sub>)</th><th>ΔE(v<sub>15</sub>)</th></tr></thead><tbody><tr><td>4s←n<sub>S</sub>(27a) (6<sup>1</sup>A←X<sup>1</sup>A)</td><td></td><td></td><td></td><td></td></tr><tr><td>0<sub>0</sub><sup>0</sup></td><td>6.368</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4s(26a)<sup>-1</sup>/4<sub>0</sub><sup>1</sup></td><td>6.512</td><td>0.144</td><td>—</td><td>—</td></tr><tr><td>4s(26a)<sup>-1</sup>4<sub>0</sub><sup>1</sup>/4<sub>0</sub><sup>2</sup></td><td>6.65(2)(s)</td><td>0.140</td><td>—</td><td>—</td></tr><tr><td>4s(26a)<sup>-1</sup>4<sub>0</sub><sup>1</sup>/4<sub>0</sub><sup>0</sup></td><td>6.80(5)(s,w)</td><td>0.153</td><td>—</td><td>—</td></tr><tr><td>4p'←n<sub>S</sub>(27a) (7<sup>1</sup>A←X<sup>1</sup>A)</td><td></td><td></td><td></td><td></td></tr><tr><td>0<sub>0</sub><sup>0</sup></td><td>6.567</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>1</sup></td><td>6.72(4)(s)</td><td>0.157</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>0</sup></td><td>6.88(8)(s,w)</td><td>0.164</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>3</sup></td><td>7.05(7)(s)</td><td>0.169</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>3</sup>13<sub>0</sub><sup>1</sup>/4p(26a)<sup>-1</sup></td><td>7.16(3)(s)</td><td>—</td><td>0.106</td><td>—</td></tr><tr><td>4p(26a)<sup>-1</sup></td><td>7.16(3)(s)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>13<sub>0</sub><sup>1</sup></td><td>7.26(3)(b)</td><td>—</td><td>0.100</td><td>—</td></tr><tr><td>13<sub>0</sub><sup>1</sup>4<sub>0</sub><sup>1</sup></td><td>7.42(9)(s)</td><td>0.166</td><td>—</td><td>—</td></tr><tr><td>3d←n<sub>S</sub>(27a) (10<sup>1</sup>A←X<sup>1</sup>A)</td><td></td><td></td><td></td><td></td></tr><tr><td>3d' (27a)<sup>-1</sup></td><td>7.363</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>1</sup></td><td>7.49(6)(s)</td><td>0.133</td><td>—</td><td>—</td></tr><tr><td>4<sub>0</sub><sup>0</sup>/5p(27a)<sup>-1</sup></td><td>7.63(5)(b)</td><td>0.139</td><td>—</td><td>—</td></tr><tr><td>13<sub>0</sub><sup>1</sup>/4p'(26a)<sup>-1</sup></td><td>7.739</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>15<sub>0</sub><sup>1</sup>/5p(27a)<sup>-1</sup></td><td>7.817</td><td>—</td><td>—</td><td>0.078</td></tr><tr><td>ΔE</td><td>0.152</td><td>0.103</td><td>0.078</td><td></td></tr></tbody></table>

## 4.2. Region II (5.0–8.0 eV)

### 4.2.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)

This region of the absorption spectrum is shown in detail in Fig. 3a with the assignments of the spectral features in Table 3a. The two major transitions, with vertical values at 6.44(4) eV (calculated oscillator strength of $f \approx 0.0572$) and at 7.92(7) eV ($f \approx 0.0813$) and local maxima cross-sections of 18.03 and 20.14 Mb, have been assigned to $4s \leftarrow \bar{n}_O(26a)$, $(6^1A \leftarrow \bar{X}^1A)$ and $4s \leftarrow \bar{n}_O(26a)$, $(13^1A \leftarrow \bar{X}^1A)$. However, the most intense feature (49.73 Mb) peaks at 7.341 eV and has been assigned to a Rydberg excitation (4p' ← 27a), which will be discussed in Section 4.4. Within the absorption region, the proposed vibrational assignments are due to C=O stretching, $\nu_{C=O'}$, CH$_2$ rocking, $\rho_{CH_2'}$, S–C(O) stretching, $\nu_{C-S(O')}$, and CO rocking, $\rho_{CO'}$ modes (Table 3a), with average values ($\Delta E$) of 0.191 eV, 0.089 eV, 0.075 eV and 0.031 eV. With the exception of $\nu_{C-S(O)}$, these values are reduced from the ground-state of dihydro-2(3H)-thiophenone [10], suggesting a broadening of the potential energy surfaces involved in such electronic excitations.

Table 1a also lists a valence character electronic transition with a vertical value at 5.438 eV, assigned to $\pi_{CO'}^*(28a) \leftarrow n_S(27a)$, $(4^1A \leftarrow \bar{X}^1A)$ with a significant high oscillator strength value of $f \approx 0.0534$. The other electronic excitations are Rydberg in character, display significant vibrational structure (see Tables 3a and 4a) and will be comprehensively discussed in Section 4.4. The rather broad underlying contributions in the 5.0–8.0 eV absorption region enhancing the background signal, may originate from $\sigma^*$ dissociative character of the lowest unoccupied molecular orbitals along the C–C and C–S bonds, as noted in (LUMO+1), 29, (Table S2), as well as from other higher energy MOs.

### 4.2.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)

The valence electronic excitations in this energy band show transitions at 5.08(9) eV (0.49 Mb) and 5.56(5) eV (1.44 Mb), with calculated oscillator strengths $f \approx 0.0004$ and $f \approx 0.0082$. These transitions have been assigned to $\sigma^*(29a) \leftarrow n_S(27a)$, $(3^1A \leftarrow \bar{X}^1A)$ and to $\pi_{CO'}^*(28a) \leftarrow n_S(27a)$, $(4^1A \leftarrow \bar{X}^1A)$ (Table 1b), while above 5.8 eV other electronic

<footnote><sup>a</sup> (s) shoulder structure; (w) weak feature; (b) broad feature; (the last decimal of the energy value is given in brackets for these less-resolved features);.</footnote>
**Table 4a**

Proposed vibrational assignments of dihydro-2(3H)-thiophenone Rydberg series converging to the ionic electronic ground (27a)⁻¹, first (26a)⁻¹, second (25a)⁻¹ and third (24a)⁻¹ excited states in the photon energy range 8.0 – 10.7 eVᵃ. Energies in eV. See text for details. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE (ρ<sub>co</sub>)</th><th>ΔE(ν<sub>c</sub>, s<sub>(o)</sub>)</th><th>ΔE (ρ<sub>ch2</sub>)</th><th>ΔE (ν<sub>c=0</sub>)</th></tr></thead><tbody><tr><td>3d ← n&#773;<sub>O</sub>(26a), (17<sup>1</sup>A ← X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>5s(27a)<sup>-1</sup>/3d'(26a)<sup>-1</sup></td><td>8.02(5)<br>(b)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub><sup>1</sup></td><td>8.067</td><td>0.042</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>1</sup></td><td>8.146</td><td>—</td><td>0.079</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup></td><td>8.22(2)<br>(b)</td><td>—</td><td>0.076</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>3</sup></td><td>8.30(4)<br>(s,w)</td><td>—</td><td>0.082</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=00</sub><sup>1</sup></td><td>8.38(3)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>0.161</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=0<sup>2</sup></sub></td><td>8.55(1)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>0.168</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=0<sup>3</sup></sub></td><td>8.725</td><td>—</td><td>—</td><td>—</td><td>0.174</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=0<sup>4</sup></sub></td><td>8.90(4)<br>(w)</td><td>—</td><td>—</td><td>—</td><td>0.179</td></tr><tr><td>5p(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup></td><td>8.20(0)<br>(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>1</sup></td><td>8.30(4)<br>(s,w)</td><td>—</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>2</sup>/5p'(27a)<sup>-1</sup>/4s(25a)<sup>-1</sup></td><td>8.41(1)<br>(s,w)</td><td>—</td><td>—</td><td>0.107</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>3</sup></td><td>8.51(5)<br>(s)</td><td>—</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>4</sup>/4d(27a)<sup>-1</sup>/5p'(26a)<sup>-1</sup></td><td>8.616</td><td>—</td><td>—</td><td>0.101</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>5</sup>/4d(26a)<sup>-1</sup></td><td>8.725</td><td>—</td><td>—</td><td>0.109</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>6</sup>/6p(27a)<sup>-1</sup>/6s(26a)<sup>-1</sup></td><td>8.837</td><td>—</td><td>—</td><td>0.112</td><td>—</td></tr><tr><td>7d(26a)<sup>-1</sup></td><td>9.36(8)<br>(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)</sub><sup>1</sup></td><td>9.44(6)<br>(s,w)</td><td>—</td><td>0.078</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)</sub><sup>2</sup></td><td>9.52(3)<br>(s,w)</td><td>—</td><td>0.077</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)</sub><sup>3</sup></td><td>9.60(4)<br>(w)</td><td>—</td><td>0.081</td><td>—</td><td>—</td></tr><tr><td>ΔE</td><td></td><td>0.042</td><td>0.079</td><td>0.106</td><td>0.171</td></tr></tbody></table>

transitions are solely due to Rydberg excitations (Section 4.4) accompanied by vibrational fine structure (Tables 3b and 4b).

**4.3. Region III (8.0–10.7 eV)**

In this energy range, the photoabsorption spectra result from the contribution of members of the different Rydberg transitions converging to (27a)⁻¹ X&#773;<sup>2</sup>A, (26a)⁻¹ A&#773;<sup>2</sup>A, (25a)⁻¹ B<sup>2</sup>A and (24a)⁻¹ C&#773;<sup>2</sup>A ionic states of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (Section 4.4). The absorption features may result from the overlap of the many different Rydberg electronic states and the superposition of the different vibrational modes contributing to the spectra. As a consequence, we note a significant broadening of the absorption features, while others appear enhanced and/or manifest themselves as discernible. Although the quantum chemical calculations in Table 1 predict Rydberg excitations only, a close inspection of the photoabsorption spectra shows a significant underlying contribution responsible to increase the background intensity. It is also relevant to note that above ≈ 9.0 eV, photoionisation to the (26a)⁻¹, (25a)⁻¹ and (24a)⁻¹ states can contribute to the spectra, thus broadening the absorption features and diminishing their observable intensities. This is more relevant in the dihydro-3(2H)-thiophenone than in the dihydro-2(3H)-thiophenone absorption spectrum, which can be related to the inductive effect between the C=O group and the sulphur atom being more prevalent for the latter

molecule.

**4.3.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)**

The absorption cross-section for this band shows several features which have been assigned to Rydberg transitions (see Table 1a). Above 8.3 eV, the magnitude of the cross-section increases as the photon energy is increased, where broad structures above the underlying significant absorption signal are observed (Fig. 4a). In this region, the spectrum is dominated by Rydberg series converging to the four lowest-lying ionic states, also with contributions of C=O stretching, ν<sub>C=O</sub>, CH₂ rocking, ρ<sub>CH₂</sub>, S–C(O) stretching, ν<sub>C–S(O)</sub>, and CO rocking, ρ<sub>CO</sub> modes, with average spacings (ΔE) of 0.171, 0.106, 0.079 and 0.042 eV (see Section 4.4 and Table 4a).

**4.3.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)**

The electronic transitions are dominated by Rydberg series converging to the ionic electronic ground (27a)⁻¹, first (26a)⁻¹, second (25a)⁻¹ and third (24a)⁻¹ excited states. The quite diffuse absorption features are mainly accompanied by a few quanta of C=O stretching, v̇₄(a), and ring stretching, v̇₁₃(a), v̇₁₄(a) and v̇₁₅(a) modes (Table 4b).

**4.4. Rydberg series**

The VUV photoabsorption spectra of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone above 5.5 eV exhibit a relevant Rydberg character (Figs. 3 and 4), supported by the ab initio calculations (Table 1), with the experimental energies, tentative assignments and quantum defects given in Table 5. Regarding dihydro-2(3H)-thiophenone, although the calculations in Table 1b assign the dominant electronic transitions to promotion of an electron from the oxygen “out-of-plane” lone pair (HOMO-1), 26a, to different Rydberg orbitals, 4s ← n&#773;<sub>O</sub>(26a), (6<sup>1</sup>A ← X&#773;<sup>1</sup>A), 4p ← n&#773;<sub>O</sub>(26a), (7<sup>1</sup>A ← X&#773;<sup>1</sup>A), 4p'' ← n&#773;<sub>O</sub>(26a), (12<sup>1</sup>A ← X&#773;<sup>1</sup>A), 3d ← n&#773;<sub>O</sub>(26a), (13<sup>1</sup>A ← X&#773;<sup>1</sup>A) and 3d ← n&#773;<sub>O</sub>(26a), (15<sup>1</sup>A ← X&#773;<sup>1</sup>A), such is not consistent with the assignments based on the quantum defects obtained for the sulphur atom (see below). Nonetheless, the experimental energy values for these transitions in Table 1b are in accord with the information from Table 5b.

The calculated vertical ionisation energies at the EOMIP-CCSD/aug-cc-pV(T+d)Z, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, are in good agreement with the experimental values of the lowest-lying ionic states (Table S1). From the different theoretical methods, P3+ appears to be the more accurate, while the results provided by the Koopmans' theorem are not very accurate due to an imperfect cancellation of relaxation and correlation effects between the neutral and the ionised states. The P3+ and EOMIP-CCSD results are nevertheless very close to each other to within 0.1–0.2 eV.

Each absorption feature in the dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone spectra have been verified with the Rydberg formula: E<sub>n</sub> = IE - R/(n<sub>δ</sub>)<sup>2</sup>, where IE is the ionisation energy, n is the principal quantum number of the Rydberg orbital of energy E<sub>n</sub>, R is the Rydberg constant (13.61 eV), and δ is the quantum defect resulting from the penetration of the Rydberg orbital into the core. Following a procedure we have implemented before when assigning the different members of Rydberg series, the quantum defects expected for sulphur are: 1.9 < δ < 2.0 for ns, 1.4 < δ < 1.8 for np, and 0.1 < δ < 0.5 for nd [24]. To the best of our knowledge, no previous Rydberg features of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone have been reported in the literature.

A close comparison between the vibrational fine structure discernible in the photoabsorption spectra of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (see Tables 3 and 4), reveals that both molecular systems exhibit relevant C=O stretching excitations, the former with important CH₂ twisting and rocking modes, whereas for the latter ring stretching modes are prevalent.

<footnote>ᵃ (s) shoulder structure; (w) weak feature; (b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features);.</footnote>
**Table 4a**

Proposed vibrational assignments of dihydro-2(3H)-thiophenone Rydberg series converging to the ionic electronic ground (27a)⁻¹, first (26a)⁻¹, second (25a)⁻¹ and third (24a)⁻¹ excited states in the photon energy range 8.0 – 10.7 eVᵃ. Energies in eV. See text for details. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE (ρ<sub>co</sub>)</th><th>ΔE(ν<sub>c</sub>, s<sub>(o)</sub>)</th><th>ΔE (ρ<sub>CH2</sub>)</th><th>ΔE (ν<sub>c=0</sub>)</th></tr></thead><tbody><tr><td>3d ← n&#773;<sub>O</sub>(26a), (17<sup>1</sup>A ← X&#773;<sup>1</sup>A)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>5s(27a)<sup>-1</sup>/3d'(26a)<sup>-1</sup></td><td>8.02(5)<br>(b)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub><sup>1</sup></td><td>8.067</td><td>0.042</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>1</sup></td><td>8.146</td><td>—</td><td>0.079</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup></td><td>8.22(2)<br>(b)</td><td>—</td><td>0.076</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>3</sup></td><td>8.30(4)<br>(s,w)</td><td>—</td><td>0.082</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=00</sub><sup>1</sup></td><td>8.38(3)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>0.161</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=0<sup>2</sup></sub></td><td>8.55(1)<br>(s)</td><td>—</td><td>—</td><td>—</td><td>0.168</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=0<sup>3</sup></sub></td><td>8.725</td><td>—</td><td>—</td><td>—</td><td>0.174</td></tr><tr><td>ρ<sub>CO<sub>0</sub></sub>ν<sub>C-S(O)</sub><sup>2</sup>ν<sub>C=0<sup>4</sup></sub></td><td>8.90(4)<br>(w)</td><td>—</td><td>—</td><td>—</td><td>0.179</td></tr><tr><td>5p(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup></td><td>8.20(0)<br>(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>1</sup></td><td>8.30(4)<br>(s,w)</td><td>—</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>2</sup>/5p'(27a)<sup>-1</sup>/4s(25a)<sup>-1</sup></td><td>8.41(1)<br>(s,w)</td><td>—</td><td>—</td><td>0.107</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>3</sup></td><td>8.51(5)<br>(s)</td><td>—</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>4</sup>/4d(27a)<sup>-1</sup>/5p'(26a)<sup>-1</sup></td><td>8.616</td><td>—</td><td>—</td><td>0.101</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>5</sup>/4d(26a)<sup>-1</sup></td><td>8.725</td><td>—</td><td>—</td><td>0.109</td><td>—</td></tr><tr><td>ρ<sub>CH<sub>2</sub><sub>0</sub></sub><sup>6</sup>/6p(27a)<sup>-1</sup>/6s(26a)<sup>-1</sup></td><td>8.837</td><td>—</td><td>—</td><td>0.112</td><td>—</td></tr><tr><td>7d(26a)<sup>-1</sup></td><td>9.36(8)<br>(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)</sub><sup>1</sup></td><td>9.44(6)<br>(s,w)</td><td>—</td><td>0.078</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)</sub><sup>2</sup></td><td>9.52(3)<br>(s,w)</td><td>—</td><td>0.077</td><td>—</td><td>—</td></tr><tr><td>ν<sub>C-S(O)</sub><sup>3</sup></td><td>9.60(4)<br>(w)</td><td>—</td><td>0.081</td><td>—</td><td>—</td></tr><tr><td>ΔE</td><td></td><td>0.042</td><td>0.079</td><td>0.106</td><td>0.171</td></tr></tbody></table>

transitions are solely due to Rydberg excitations (Section 4.4) accompanied by vibrational fine structure (Tables 3b and 4b).

**4.3. Region III (8.0–10.7 eV)**

In this energy range, the photoabsorption spectra result from the contribution of members of the different Rydberg transitions converging to (27a)⁻¹ X̃²A, (26a)⁻¹ Ã²A, (25a)⁻¹ B²A and (24a)⁻¹ C̃²A ionic states of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (Section 4.4). The absorption features may result from the overlap of the many different Rydberg electronic states and the superposition of the different vibrational modes contributing to the spectra. As a consequence, we note a significant broadening of the absorption features, while others appear enhanced and/or manifest themselves as discernible. Although the quantum chemical calculations in Table 1 predict Rydberg excitations only, a close inspection of the photoabsorption spectra shows a significant underlying contribution responsible to increase the background intensity. It is also relevant to note that above ≈ 9.0 eV, photoionisation to the (26a)⁻¹, (25a)⁻¹ and (24a)⁻¹ states can contribute to the spectra, thus broadening the absorption features and diminishing their observable intensities. This is more relevant in the dihydro-3(2H)-thiophenone than in the dihydro-2(3H)-thiophenone absorption spectrum, which can be related to the inductive effect between the C=O group and the sulphur atom being more prevalent for the latter

molecule.

**4.3.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)**

The absorption cross-section for this band shows several features which have been assigned to Rydberg transitions (see Table 1a). Above 8.3 eV, the magnitude of the cross-section increases as the photon energy is increased, where broad structures above the underlying significant absorption signal are observed (Fig. 4a). In this region, the spectrum is dominated by Rydberg series converging to the four lowest-lying ionic states, also with contributions of C=O stretching, ν<sub>C=O</sub>, CH₂ rocking, ρ<sub>CH₂</sub>, S–C(O) stretching, ν<sub>C–S(O)</sub>, and CO rocking, ρ<sub>CO</sub> modes, with average spacings (ΔE) of 0.171, 0.106, 0.079 and 0.042 eV (see Section 4.4 and Table 4a).

**4.3.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)**

The electronic transitions are dominated by Rydberg series converging to the ionic electronic ground (27a)⁻¹, first (26a)⁻¹, second (25a)⁻¹ and third (24a)⁻¹ excited states. The quite diffuse absorption features are mainly accompanied by a few quanta of C=O stretching, v̇₄(a), and ring stretching, v̇₁₃(a), v̇₁₄(a) and v̇₁₅(a) modes (Table 4b).

**4.4. Rydberg series**

The VUV photoabsorption spectra of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone above 5.5 eV exhibit a relevant Rydberg character (Figs. 3 and 4), supported by the ab initio calculations (Table 1), with the experimental energies, tentative assignments and quantum defects given in Table 5. Regarding dihydro-2(3H)-thiophenone, although the calculations in Table 1b assign the dominant electronic transitions to promotion of an electron from the oxygen “out-of-plane” lone pair (HOMO-1), 26a, to different Rydberg orbitals, 4s ← n&#773;<sub>O</sub>(26a), (6<sup>1</sup>A ← X&#773;<sup>1</sup>A), 4p ← n&#773;<sub>O</sub>(26a), (7<sup>1</sup>A ← X&#773;<sup>1</sup>A), 4p'' ← n&#773;<sub>O</sub>(26a), (12<sup>1</sup>A ← X&#773;<sup>1</sup>A), 3d ← n&#773;<sub>O</sub>(26a), (13<sup>1</sup>A ← X&#773;<sup>1</sup>A) and 3d ← n&#773;<sub>O</sub>(26a), (15<sup>1</sup>A ← X&#773;<sup>1</sup>A), such is not consistent with the assignments based on the quantum defects obtained for the sulphur atom (see below). Nonetheless, the experimental energy values for these transitions in Table 1b are in accord with the information from Table 5b.

The calculated vertical ionisation energies at the EOMIP-CCSD/aug-cc-pV(T+d)Z, dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone, are in good agreement with the experimental values of the lowest-lying ionic states (Table S1). From the different theoretical methods, P3+ appears to be the more accurate, while the results provided by the Koopmans' theorem are not very accurate due to an imperfect cancellation of relaxation and correlation effects between the neutral and the ionised states. The P3+ and EOMIP-CCSD results are nevertheless very close to each other to within 0.1–0.2 eV.

Each absorption feature in the dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone spectra have been verified with the Rydberg formula: E<sub>n</sub> = IE - R/(n-δ)<sup>2</sup>, where IE is the ionisation energy, n is the principal quantum number of the Rydberg orbital of energy E<sub>n</sub>, R is the Rydberg constant (13.61 eV), and δ is the quantum defect resulting from the penetration of the Rydberg orbital into the core. Following a procedure we have implemented before when assigning the different members of Rydberg series, the quantum defects expected for sulphur are: 1.9 < δ < 2.0 for ns, 1.4 < δ < 1.8 for np, and 0.1 < δ < 0.5 for nd [24]. To the best of our knowledge, no previous Rydberg features of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone have been reported in the literature.

A close comparison between the vibrational fine structure discernible in the photoabsorption spectra of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone (see Tables 3 and 4), reveals that both molecular systems exhibit relevant C=O stretching excitations, the former with important CH₂ twisting and rocking modes, whereas for the latter ring stretching modes are prevalent.

<footnote>ᵃ (s) shoulder structure; (w) weak feature; (b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features);.</footnote>
<caption>**Table 4b**</caption>

Proposed vibrational assignments of dihydro-3(2H)-thiophenone Rydberg series converging to the ionic electronic ground (27a)⁻¹, first (26a)⁻¹, second (25a)⁻¹ and third (24a)⁻¹ excited states in the photon energy range 8.0 – 10.7 eVª. Energies in eV. See text for details. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(v<sub>4</sub>)</th><th>ΔE(v<sub>13</sub>)</th><th>ΔE(v<sub>14</sub>)</th><th>ΔE(v<sub>15</sub>)</th></tr></thead><tbody><tr><td>3d(26a)<sup>-1</sup></td><td>8.10(4)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4<sup>1</sup>/<sub>0</sub>4s(25a)<sup>-1</sup></td><td>8.26(0)(s,w)</td><td>0.156</td><td>—</td><td>—</td><td>—</td></tr><tr><td>6s(27a)<sup>-1</sup></td><td>8.114</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>3d(26a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>8.23(3)(s,w)</td><td>—</td><td>0.119</td><td>—</td><td>—</td></tr><tr><td>5d(27a)<sup>-1</sup>/13<sup>2</sup>/<sub>0</sub></td><td>8.34(9)(s)</td><td>—</td><td>0.116</td><td>—</td><td>—</td></tr><tr><td>6p(27a)<sup>-1</sup></td><td>8.17(8)(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>6p(27a)<sup>-1</sup>/4s(25a)<sup>-1</sup>/14<sup>1</sup>/<sub>0</sub></td><td>8.26(0)(s,w)</td><td>—</td><td>—</td><td>0.082</td><td>—</td></tr><tr><td>5d(27a)<sup>-1</sup>/14<sup>2</sup>/<sub>0</sub></td><td>8.34(9)(s)</td><td>—</td><td>—</td><td>0.089</td><td>—</td></tr><tr><td>6p'(27a)<sup>-1</sup></td><td>8.26(0)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>5d'(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup></td><td>8.36(6)(s)</td><td>—</td><td>0.106</td><td>—</td><td>—</td></tr><tr><td>7p(27a)<sup>-1</sup>/5d(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup>/15<sup>1</sup>/<sub>0</sub></td><td>8.41(7)(s)</td><td>—</td><td>—</td><td>—</td><td>0.051</td></tr><tr><td>6d(27a)<sup>-1</sup>/14<sup>1</sup>/<sub>0</sub></td><td>8.51(5)(w)</td><td>—</td><td>—</td><td>0.098</td><td>—</td></tr><tr><td>6d'(27a)<sup>-1</sup>/5p(26a)<sup>-1</sup></td><td>8.545</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9s(27a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>8.64(0)(b,w)</td><td>—</td><td>0.095</td><td>—</td><td>—</td></tr><tr><td>7p'(27a)<sup>-1</sup></td><td>8.46(3)(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>13<sup>1</sup>/<sub>0</sub>/8p(27a)<sup>-1</sup></td><td>8.58(0)(s,w)</td><td>—</td><td>0.117</td><td>—</td><td>—</td></tr><tr><td>13<sup>1</sup>/<sub>0</sub>14<sup>1</sup>/<sub>0</sub>5p(26a)<sup>-1</sup></td><td>8.76(2)(b)</td><td>0.182</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4d(26a)<sup>-1</sup>/4p(25a)<sup>-1</sup></td><td>8.88(8)(s)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>13<sup>1</sup>/<sub>0</sub></td><td>8.991</td><td>—</td><td>0.103</td><td>—</td><td>—</td></tr><tr><td>6p(26a)<sup>-1</sup>/14<sup>1</sup>/<sub>0</sub></td><td>9.08(3)(s)</td><td>—</td><td>—</td><td>0.092</td><td>—</td></tr><tr><td>5d(26a)<sup>-1</sup></td><td>9.26(6)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>7p(26a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>9.37(9)(b,w)</td><td>—</td><td>0.113</td><td>—</td><td>—</td></tr><tr><td>8s(26a)<sup>-1</sup>/13<sup>2</sup>/<sub>0</sub></td><td>9.48(6)(s)</td><td>—</td><td>0.107</td><td>—</td><td>—</td></tr><tr><td>7s(26a)<sup>-1</sup></td><td>9.33(6)(s)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>15<sup>1</sup>/<sub>0</sub></td><td>9.37(9)(b,w)</td><td>—</td><td>—</td><td>—</td><td>0.043</td></tr><tr><td>8s(26a)<sup>-1</sup></td><td>9.48(6)(s)</td><td>0.150</td><td>—</td><td>—</td><td>—</td></tr><tr><td>8s(26a)<sup>-1</sup>/15<sup>1</sup>/<sub>0</sub></td><td>9.53(0)(s,w)</td><td>—</td><td>—</td><td>—</td><td>0.044</td></tr><tr><td>4p'(25a)<sup>-1</sup></td><td>9.63(4)(s)</td><td>0.148</td><td>—</td><td>—</td><td>—</td></tr><tr><td>8p'(26a)<sup>-1</sup></td><td>9.53(0)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4p'(25a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>9.63(4)(s)</td><td>—</td><td>0.104</td><td>—</td><td>—</td></tr><tr><td>3d'(25a)<sup>-1</sup></td><td>10.06(4)(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>5s(25a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>10.17(1)(w)</td><td>—</td><td>0.107</td><td>—</td><td>—</td></tr><tr><td>ΔE</td><td></td><td>0.159</td><td>0.109</td><td>0.090</td><td>0.046</td></tr></tbody></table>

### 4.4.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)

The lowest-lying Rydberg transition is assigned to the 4s↔n<sub>S</sub>(27a), (5<sup>1</sup>A←X<sup>1</sup>A) excitation, with the first member (n = 4) at 6.44(4) eV and having a quantum defect δ = 1.87 (Table 5a). The absorption band that may contain the 4s member is also due to 4s(26a)<sup>-1</sup>. Other transitions to the higher-order Rydberg members up to n = 9, of the ns series, are also reported, while the features at 8.02(5) and 9.22(2) eV (n = 5 s and 9 s) have also been assigned to the contribution of other Rydberg members 3d'(26a)<sup>-1</sup> and 7p'(26a)<sup>-1</sup>. The first members of the (np ← 27a) and (np' ← 27a) series are associated with absorption features at 6.75(3) and 7.341 eV (δ = 1.77 and 1.48) (Table 5a). The 5p, 6p and 7p features at 8.20(0), 8.768 and 9.01(4) eV can be assigned to 5s(26a)<sup>-1</sup>, 4d'(26a)<sup>-1</sup> and 6p'(26a)<sup>-1</sup>, respectively, whereas the 5p', 6p' and 8p' features at 8.41 (1), 8.837 and 9.17(0) eV can also be assigned to 4s(25a)<sup>-1</sup>, 6s(26a)<sup>-1</sup> and 7p(26a)<sup>-1</sup>, respectively. Our assignments also report the presence of two (nd ← 27a) and (nd' ← 27a) series, with the n = 3 features at 7.788 and 7.92(7) eV (δ = 0.17 and 0.05) (Table 5a). Note that transitions to higher members of the nd' Rydberg series, up to n = 7, are also discernible. The features at 8.90(7), 7.92(7), 8.616 and 9.11(6) eV are also assigned to 6p(26a)<sup>-1</sup>/4s(24a)<sup>-1</sup>, 3d(26a)<sup>-1</sup>, 5p'(26a)<sup>-1</sup> and 5d'(26a)<sup>-1</sup>/4p(25a)<sup>-1</sup>, respectively. Due to the low magnitude of higher members of the Rydberg series in the absorption spectrum, no attempt was made to assign states for n > 7.

The assignment of the Rydberg series converging to the ionic electronic first excited state, (26a)<sup>-1</sup> A<sup>2</sup>A, are listed in Table 5a. The first members of the ns, np, np', nd and nd' series are associated with features at 6.44(4) eV (δ = 1.94), 7.009 eV (δ = 1.73), 7.60(6) eV (δ = 1.42), 7.92 (7) eV (δ = 0.20) and 8.02(5) eV (δ = 0.11) (Table 5a).

The Rydberg series converging to the ionic electronic second excited state, (25a)<sup>-1</sup> B<sup>2</sup>A, have been assigned to the (ns, np ← 25a) transitions. The first and only discernible members are associated with features at 8.41(1) eV (δ = 1.99) and 9.11(6) eV (δ = 1.75) (Table 5a). A single Rydberg series converging to the ionic electronic third excited state, (24a)<sup>-1</sup> C<sup>2</sup>A, has been assigned to the 4s ← n<sub>S</sub>(24a) transition at 8.90(7) eV (δ = 1.99) (Table 5a). The tentative assignment of this series has only been made for n = 4, since higher members lie outside the photon energy range investigated.

Across the entire photon energy range where Rydberg transitions converging to the (27a)<sup>-1</sup> X̃<sup>2</sup>A, (26a)<sup>-1</sup> Ã<sup>2</sup>A and (25a)<sup>-1</sup> B̃<sup>2</sup>A ionic states are the prevalent electronic excitations, Tables 3a and 4a contain a comprehensive assignment of the vibrational excitation superimposed on such transitions. The fine structure is mainly due to C=O stretching, ν<sub>C=O</sub>, CH<sub>2</sub> rocking, ρ<sub>CH<sub>2</sub></sub>, S-C(O) stretching, ν<sub>C-S(O)</sub>, and CO rocking, ρ<sub>CO</sub> modes. Combination bands of these modes have also been assigned.

### 4.4.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)

The first member of ns Rydberg transition is assigned to 4s↔n<sub>S</sub>(27a), (5<sup>1</sup>A←X̃<sup>1</sup>A) at 5.57(5) eV with a quantum defect δ = 1.98, while other transitions to the higher-order Rydberg members n = 9 are also reported in Table 5b. The slightly larger value of the quantum defect for the 9 s member can be due to the rather broad and weak nature of such feature. The absorption spectrum containing the 7s and 8s members have also been assigned to other Rydberg excitations, 5d'(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup> and 6d'(27a)<sup>-1</sup>/5p(26a)<sup>-1</sup>. The lowest-lying members of the two (np ← 27a) and (np' ← 27a) series are associated with absorption features at 6.368

<footnote>ª (s) shoulder structure; (w) weak feature; (b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features);</footnote>
<caption>**Table 4b**</caption>

Proposed vibrational assignments of dihydro-3(2H)-thiophenone Rydberg series converging to the ionic electronic ground (27a)⁻¹, first (26a)⁻¹, second (25a)⁻¹ and third (24a)⁻¹ excited states in the photon energy range 8.0 – 10.7 eVª. Energies in eV. See text for details. See text for details.

<table><thead><tr><th>Assignment</th><th>Energy</th><th>ΔE(v<sub>4</sub>)</th><th>ΔE(v<sub>13</sub>)</th><th>ΔE(v<sub>14</sub>)</th><th>ΔE(v<sub>15</sub>)</th></tr></thead><tbody><tr><td>3d(26a)<sup>-1</sup></td><td>8.10(4)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4<sup>1</sup>/<sub>0</sub>4s(25a)<sup>-1</sup></td><td>8.26(0)(s,w)</td><td>0.156</td><td>—</td><td>—</td><td>—</td></tr><tr><td>6s(27a)<sup>-1</sup></td><td>8.114</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>3d(26a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>8.23(3)(s,w)</td><td>—</td><td>0.119</td><td>—</td><td>—</td></tr><tr><td>5d(27a)<sup>-1</sup>/13<sup>2</sup>/<sub>0</sub></td><td>8.34(9)(s)</td><td>—</td><td>0.116</td><td>—</td><td>—</td></tr><tr><td>6p(27a)<sup>-1</sup></td><td>8.17(8)(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>6p(27a)<sup>-1</sup>/4s(25a)<sup>-1</sup>/14<sup>1</sup>/<sub>0</sub></td><td>8.26(0)(s,w)</td><td>—</td><td>—</td><td>0.082</td><td>—</td></tr><tr><td>5d(27a)<sup>-1</sup>/14<sup>2</sup>/<sub>0</sub></td><td>8.34(9)(s)</td><td>—</td><td>—</td><td>0.089</td><td>—</td></tr><tr><td>6p'(27a)<sup>-1</sup></td><td>8.26(0)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>5d'(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup></td><td>8.36(6)(s)</td><td>—</td><td>0.106</td><td>—</td><td>—</td></tr><tr><td>7p(27a)<sup>-1</sup>/5d(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup>/15<sup>1</sup>/<sub>0</sub></td><td>8.41(7)(s)</td><td>—</td><td>—</td><td>—</td><td>0.051</td></tr><tr><td>6d(27a)<sup>-1</sup>/14<sup>1</sup>/<sub>0</sub></td><td>8.51(5)(w)</td><td>—</td><td>—</td><td>0.098</td><td>—</td></tr><tr><td>6d'(27a)<sup>-1</sup>/5p(26a)<sup>-1</sup></td><td>8.545</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9s(27a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>8.64(0)(b,w)</td><td>—</td><td>0.095</td><td>—</td><td>—</td></tr><tr><td>7p'(27a)<sup>-1</sup></td><td>8.46(3)(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>13<sup>1</sup>/<sub>0</sub>/8p(27a)<sup>-1</sup></td><td>8.58(0)(s,w)</td><td>—</td><td>0.117</td><td>—</td><td>—</td></tr><tr><td>13<sup>1</sup>/<sub>0</sub>14<sup>1</sup>/<sub>0</sub>5p(26a)<sup>-1</sup></td><td>8.76(2)(b)</td><td>0.182</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4d(26a)<sup>-1</sup>/4p(25a)<sup>-1</sup></td><td>8.88(8)(s)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>13<sup>1</sup>/<sub>0</sub></td><td>8.991</td><td>—</td><td>0.103</td><td>—</td><td>—</td></tr><tr><td>6p(26a)<sup>-1</sup>/14<sup>1</sup>/<sub>0</sub></td><td>9.08(3)(s)</td><td>—</td><td>—</td><td>0.092</td><td>—</td></tr><tr><td>5d(26a)<sup>-1</sup></td><td>9.26(6)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>7p(26a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>9.37(9)(b,w)</td><td>—</td><td>0.113</td><td>—</td><td>—</td></tr><tr><td>8s(26a)<sup>-1</sup>/13<sup>2</sup>/<sub>0</sub></td><td>9.48(6)(s)</td><td>—</td><td>0.107</td><td>—</td><td>—</td></tr><tr><td>7s(26a)<sup>-1</sup></td><td>9.33(6)(s)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>15<sup>1</sup>/<sub>0</sub></td><td>9.37(9)(b,w)</td><td>—</td><td>—</td><td>—</td><td>0.043</td></tr><tr><td>8s(26a)<sup>-1</sup></td><td>9.48(6)(s)</td><td>0.150</td><td>—</td><td>—</td><td>—</td></tr><tr><td>8s(26a)<sup>-1</sup>/15<sup>1</sup>/<sub>0</sub></td><td>9.53(0)(s,w)</td><td>—</td><td>—</td><td>—</td><td>0.044</td></tr><tr><td>4p'(25a)<sup>-1</sup></td><td>9.63(4)(s)</td><td>0.148</td><td>—</td><td>—</td><td>—</td></tr><tr><td>8p'(26a)<sup>-1</sup></td><td>9.53(0)(s,w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>4p'(25a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>9.63(4)(s)</td><td>—</td><td>0.104</td><td>—</td><td>—</td></tr><tr><td>3d'(25a)<sup>-1</sup></td><td>10.06(4)(w)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>5s(25a)<sup>-1</sup>/13<sup>1</sup>/<sub>0</sub></td><td>10.17(1)(w)</td><td>—</td><td>0.107</td><td>—</td><td>—</td></tr><tr><td>ΔE</td><td></td><td>0.159</td><td>0.109</td><td>0.090</td><td>0.046</td></tr></tbody></table>

### 4.4.1. Dihydro-2(3H)-thiophenone (γ-thiobutyrolactone)

The lowest-lying Rydberg transition is assigned to the 4s↔n<sub>S</sub>(27a), (5<sup>1</sup>A←X<sup>1</sup>A) excitation, with the first member (n = 4) at 6.44(4) eV and having a quantum defect δ = 1.87 (Table 5a). The absorption band that may contain the 4s member is also due to 4s(26a)<sup>-1</sup>. Other transitions to the higher-order Rydberg members up to n = 9, of the ns series, are also reported, while the features at 8.02(5) and 9.22(2) eV (n = 5 s and 9 s) have also been assigned to the contribution of other Rydberg members 3d'(26a)<sup>-1</sup> and 7p'(26a)<sup>-1</sup>. The first members of the (np ← 27a) and (np' ← 27a) series are associated with absorption features at 6.75(3) and 7.341 eV (δ = 1.77 and 1.48) (Table 5a). The 5p, 6p and 7p features at 8.20(0), 8.768 and 9.01(4) eV can be assigned to 5s(26a)<sup>-1</sup>, 4d'(26a)<sup>-1</sup> and 6p'(26a)<sup>-1</sup>, respectively, whereas the 5p', 6p' and 8p' features at 8.41 (1), 8.837 and 9.17(0) eV can also be assigned to 4s(25a)<sup>-1</sup>, 6s(26a)<sup>-1</sup> and 7p(26a)<sup>-1</sup>, respectively. Our assignments also report the presence of two (nd ← 27a) and (nd' ← 27a) series, with the n = 3 features at 7.788 and 7.92(7) eV (δ = 0.17 and 0.05) (Table 5a). Note that transitions to higher members of the nd' Rydberg series, up to n = 7, are also discernible. The features at 8.90(7), 7.92(7), 8.616 and 9.11(6) eV are also assigned to 6p(26a)<sup>-1</sup>/4s(24a)<sup>-1</sup>, 3d(26a)<sup>-1</sup>, 5p'(26a)<sup>-1</sup> and 5d'(26a)<sup>-1</sup>/4p(25a)<sup>-1</sup>, respectively. Due to the low magnitude of higher members of the Rydberg series in the absorption spectrum, no attempt was made to assign states for n > 7.

The assignment of the Rydberg series converging to the ionic electronic first excited state, (26a)<sup>-1</sup> A<sup>2</sup>A, are listed in Table 5a. The first members of the ns, np, np', nd and nd' series are associated with features at 6.44(4) eV (δ = 1.94), 7.009 eV (δ = 1.73), 7.60(6) eV (δ = 1.42), 7.92 (7) eV (δ = 0.20) and 8.02(5) eV (δ = 0.11) (Table 5a).

The Rydberg series converging to the ionic electronic second excited state, (25a)<sup>-1</sup> B<sup>2</sup>A, have been assigned to the (ns, np ← 25a) transitions. The first and only discernible members are associated with features at 8.41(1) eV (δ = 1.99) and 9.11(6) eV (δ = 1.75) (Table 5a). A single Rydberg series converging to the ionic electronic third excited state, (24a)<sup>-1</sup> C<sup>2</sup>A, has been assigned to the 4s ← n<sub>S</sub>(24a) transition at 8.90(7) eV (δ = 1.99) (Table 5a). The tentative assignment of this series has only been made for n = 4, since higher members lie outside the photon energy range investigated.

Across the entire photon energy range where Rydberg transitions converging to the (27a)<sup>-1</sup> X̃<sup>2</sup>A, (26a)<sup>-1</sup> Ã<sup>2</sup>A and (25a)<sup>-1</sup> B̃<sup>2</sup>A ionic states are the prevalent electronic excitations, Tables 3a and 4a contain a comprehensive assignment of the vibrational excitation superimposed on such transitions. The fine structure is mainly due to C=O stretching, ν<sub>C=O</sub>, CH<sub>2</sub> rocking, ρ<sub>CH<sub>2</sub></sub>, S-C(O) stretching, ν<sub>C-S(O)</sub>, and CO rocking, ρ<sub>CO</sub> modes. Combination bands of these modes have also been assigned.

### 4.4.2. Dihydro-3(2H)-thiophenone (dihydrothiophen-3-one)

The first member of ns Rydberg transition is assigned to 4s↔n<sub>S</sub>(27a), (5<sup>1</sup>A←X̃<sup>1</sup>A) at 5.57(5) eV with a quantum defect δ = 1.98, while other transitions to the higher-order Rydberg members n = 9 are also reported in Table 5b. The slightly larger value of the quantum defect for the 9 s member can be due to the rather broad and weak nature of such feature. The absorption spectrum containing the 7s and 8s members have also been assigned to other Rydberg excitations, 5d'(27a)<sup>-1</sup>/5s(26a)<sup>-1</sup> and 6d'(27a)<sup>-1</sup>/5p(26a)<sup>-1</sup>. The lowest-lying members of the two (np ← 27a) and (np' ← 27a) series are associated with absorption features at 6.368

<footnote>ª (s) shoulder structure; (w) weak feature; (b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features);</footnote>
<caption>**Table 5a**</caption>

Energy values (eV), quantum defects ($\delta$) and assignments of the Rydberg series converging to $(27a)^{-1} \tilde{X}^2A$, $(26a)^{-1} \tilde{A}^2A$, $(25a)^{-1} \tilde{B}^2A$ and $(24a)^{-1} \tilde{C}^2A$ ionic states of dihydro-2(3H)-thiophenone. See text for details.

<table><thead><tr><th>E<sub>n</sub><br>(IE<sub>1</sub>)<sub>v</sub> = 9.49 eV (27a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>2</sub>)<sub>v</sub> = 9.66 eV (26a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>3</sub>)<sub>v</sub> = 11.80 eV (25a)<sup>-1</sup></th><th>δ</th><th>Assignment</th></tr></thead><tbody><tr><td>(ns ← 27a)</td><td></td><td></td><td>(ns ← 26a)</td><td></td><td></td><td>(ns ← 25a)</td><td></td><td></td></tr><tr><td>6.44(4)(b)</td><td>1.87</td><td>4s</td><td>6.44(4)(b)</td><td>1.94</td><td>4s</td><td>8.41(1)(s,w)</td><td>1.99</td><td>4s</td></tr><tr><td>8.02(5)(b)</td><td>1.95</td><td>5s</td><td>8.20(0)(s,w)</td><td>1.95</td><td>5s</td><td>(np ← 25a)</td><td></td><td></td></tr><tr><td>8.67(0)(b,w)</td><td>1.93</td><td>6s</td><td>8.837</td><td>1.93</td><td>6s</td><td>9.11(6)(s)</td><td>1.75</td><td>4p</td></tr><tr><td>8.962</td><td>1.92</td><td>7s</td><td>9.13(0)(s)</td><td>1.93</td><td>7s</td><td>(IE<sub>4</sub>)<sub>v</sub> = 12.29 eV (24a)<sup>-1</sup></td><td></td><td></td></tr><tr><td>–</td><td>–</td><td>8s</td><td>9.29(4)(s)</td><td>1.91</td><td>8s</td><td>(ns ← 24a)</td><td></td><td></td></tr><tr><td>9.22(2)(s)</td><td>1.87</td><td>9s</td><td>(np ← 26a)</td><td></td><td></td><td>8.90(7)(b,w)</td><td>1.99</td><td>4s</td></tr><tr><td>(np ← 27a)</td><td></td><td></td><td>7.009</td><td>1.73</td><td>4p</td><td></td><td></td><td></td></tr><tr><td>6.75(3)(s,w)</td><td>1.77</td><td>4p</td><td>8.37(7)(s,w)</td><td>1.74</td><td>5p</td><td></td><td></td><td></td></tr><tr><td>8.20(0)(s,w)</td><td>1.75</td><td>5p</td><td>8.90(7)(b,w)</td><td>1.75</td><td>6p</td><td></td><td></td><td></td></tr><tr><td>8.768</td><td>1.66</td><td>6p</td><td>9.17(0)(w)</td><td>1.73</td><td>7p</td><td></td><td></td><td></td></tr><tr><td><b>9.01(4)(w)</b></td><td>1.65</td><td>7p</td><td>9.32(2)(s)</td><td>1.65</td><td>8p</td><td></td><td></td><td></td></tr><tr><td>9.147</td><td>1.70</td><td>8p</td><td>9.40(3)(s)</td><td>1.72</td><td>9p</td><td></td><td></td><td></td></tr><tr><td>(np' ← 27a)</td><td></td><td></td><td>(np' ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7.341</td><td>1.48</td><td>4p'</td><td>7.60(6)(b)</td><td>1.42</td><td>4p'</td><td></td><td></td><td></td></tr><tr><td>8.41(1)(s,w)</td><td>1.45</td><td>5p'</td><td>8.616</td><td>1.39</td><td>5p'</td><td></td><td></td><td></td></tr><tr><td>8.837</td><td>1.43</td><td>6p'</td><td><b>9.01(4)(w)</b></td><td>1.41</td><td>6p'</td><td></td><td></td><td></td></tr><tr><td>9.05(3)(s,w)</td><td>1.42</td><td>7p'</td><td>9.22(2)(s)</td><td>1.42</td><td>7p'</td><td></td><td></td><td></td></tr><tr><td>9.17(0)(w)</td><td>1.48</td><td>8p'</td><td>(nd ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>(nd ← 27a)</td><td></td><td></td><td>7.92(7)(s)</td><td>0.20</td><td>3d</td><td></td><td></td><td></td></tr><tr><td>7.788</td><td>0.17</td><td>3d</td><td>8.725</td><td>0.18</td><td>4d</td><td></td><td></td><td></td></tr><tr><td>8.55(1)(s)</td><td>0.19</td><td>4d</td><td>9.086</td><td>0.13</td><td>5d</td><td></td><td></td><td></td></tr><tr><td>8.90(7)(b,w)</td><td>0.17</td><td>5d</td><td>9.26(3)(b)</td><td>0.14</td><td>6d</td><td></td><td></td><td></td></tr><tr><td>9.086</td><td>0.19</td><td>6d</td><td>9.36(8)(w)</td><td>0.17</td><td>7d</td><td></td><td></td><td></td></tr><tr><td>(nd' ← 27a)</td><td></td><td></td><td>(nd' ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7.92(7)(s)</td><td>0.05</td><td>3d'</td><td>8.02(5)(b)</td><td>0.11</td><td>3d'</td><td></td><td></td><td></td></tr><tr><td>8.616</td><td>0.05</td><td>4d'</td><td>8.768</td><td>0.09</td><td>4d'</td><td></td><td></td><td></td></tr><tr><td>8.93(9)(s)</td><td>0.03</td><td>5d'</td><td>9.11(6)(s)</td><td>0.00</td><td>5d'</td><td></td><td></td><td></td></tr><tr><td>9.11(6)(s)</td><td>-0.03</td><td>6d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9.20(8)(s)</td><td>0.05</td><td>7d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

(b) broad feature; (w) weak structure; (s) shoulder structure (the last decimal on the energy value is given in brackets for these less-resolved features).

<caption>**Table 5b**</caption>

Energy values (eV), quantum defects ($\delta$) and assignments of the Rydberg series converging to $(27a)^{-1} \tilde{X}^2A$, $(26a)^{-1} \tilde{A}^2A$, $(25a)^{-1} \tilde{B}^2A$ and $(24a)^{-1} \tilde{C}^2A$ ionic states of dihydro-3(2H)-thiophenone. See text for details.

<table><thead><tr><th>E<sub>n</sub><br>(IE<sub>1</sub>)<sub>v</sub> = 8.92 eV (27a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>2</sub>)<sub>v</sub> = 9.85 eV (26a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>3</sub>)<sub>v</sub> = 11.63 eV (25a)<sup>-1</sup></th><th>δ</th><th>Assignment</th></tr></thead><tbody><tr><td>(ns ← 27a)</td><td></td><td></td><td>(ns ← 26a)</td><td></td><td></td><td>(ns ← 25a)</td><td></td><td></td></tr><tr><td>5.57(5)(b)</td><td>1.98</td><td>4s</td><td>6.512</td><td>1.98</td><td>4s</td><td>8.26(0)(s,w)</td><td>1.99</td><td>4s</td></tr><tr><td>7.43(3)(s,w)</td><td>1.97</td><td>5s</td><td>8.36(6)(s)</td><td>1.97</td><td>5s</td><td>10.17(1)(w)</td><td>1.95</td><td>5s</td></tr><tr><td>8.114</td><td>1.89</td><td>6s</td><td>8.991</td><td>2.02</td><td>6s</td><td>(np ← 25a)</td><td></td><td></td></tr><tr><td>8.36(6)(s)</td><td>2.04</td><td>7s</td><td>9.33(6)(s)</td><td>1.85</td><td>7s</td><td>8.88(8)(s)</td><td>1.77</td><td>4p</td></tr><tr><td>8.545</td><td>1.98</td><td>8s</td><td>9.48(6)(s)</td><td>1.89</td><td>8s</td><td>10.35(8)(s,w)</td><td>1.73</td><td>5p</td></tr><tr><td>8.64(0)(b,w)</td><td>2.03</td><td>9s</td><td>(np ← 26a)</td><td></td><td></td><td>(np ← 25a)</td><td></td><td></td></tr><tr><td>(np ← 27a)</td><td></td><td></td><td>7.16(3)(s)</td><td>1.75</td><td>4p</td><td>9.63(4)(s)</td><td>1.39</td><td>4p'</td></tr><tr><td>6.368</td><td>1.69</td><td>4p</td><td>8.545</td><td>1.77</td><td>5p</td><td>10.57(9)(s)</td><td>1.40</td><td>5p'</td></tr><tr><td>7.63(5)(b)</td><td>1.75</td><td>5p</td><td>9.08(3)(s)</td><td>1.79</td><td>6p</td><td>(nd ← 25a)</td><td></td><td></td></tr><tr><td>8.17(8)(w)</td><td>1.72</td><td>6p</td><td>9.37(9)(b,w)</td><td>1.62</td><td>7p</td><td>9.82(4)(w)</td><td>0.25</td><td>3d</td></tr><tr><td>8.41(7)(s)</td><td>1.80</td><td>7p</td><td>(np' ← 26a)</td><td></td><td></td><td>10.65(2)(w)</td><td>0.27</td><td>4d</td></tr><tr><td>8.58(0)(s,w)</td><td>1.67</td><td>8p</td><td>7.739</td><td>1.46</td><td>4p'</td><td>(nd' ← 25a)</td><td></td><td></td></tr><tr><td>8.67(0)(s,w)</td><td>1.62</td><td>9p</td><td>8.76(2)(b)</td><td>1.46</td><td>5p'</td><td>10.06(4)(w)</td><td>0.05</td><td>3d'</td></tr><tr><td>(np' ← 27a)</td><td></td><td></td><td>9.19(8)(b)</td><td>1.43</td><td>6p'</td><td>(IE<sub>4</sub>)<sub>v</sub> = 12.57 eV (24a)<sup>-1</sup></td><td></td><td></td></tr><tr><td>6.570</td><td>1.59</td><td>4p'</td><td>–</td><td></td><td>7p'</td><td>(ns ← 24a)</td><td></td><td></td></tr><tr><td>7.817</td><td>1.49</td><td>5p'</td><td>9.53(0)(s,w)</td><td>1.48</td><td>8p'</td><td>9.19(8)(b)</td><td>1.99</td><td>4s</td></tr><tr><td>8.26(0)(s,w)</td><td>1.46</td><td>6p'</td><td>(nd ← 26a)</td><td></td><td></td><td>(np ← 24a)</td><td></td><td></td></tr><tr><td>8.46(3)(w)</td><td>1.33</td><td>7p'</td><td>8.10(4)(s,w)</td><td>0.21</td><td>3d</td><td>9.88(7)(w,b)</td><td>1.74</td><td>4p</td></tr><tr><td>(nd ← 27a)</td><td></td><td></td><td>8.88(8)(s)</td><td>0.24</td><td>4d</td><td>(np' ← 24a)</td><td></td><td></td></tr><tr><td>7.16(3)(s)</td><td>0.22</td><td>3d</td><td>9.26(6)(s,w)</td><td>0.17</td><td>5d</td><td>10.57(9)(s)</td><td>1.39</td><td>4p'</td></tr><tr><td>7.90(2)(s,w)</td><td>0.34</td><td>4d</td><td>(nd' ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.34(9)(s)</td><td>0.12</td><td>5d</td><td>8.23(3)(s,w)</td><td>0.09</td><td>3d'</td><td></td><td></td><td></td></tr><tr><td>8.51(5)(w)</td><td>0.20</td><td>6d</td><td>8.551</td><td>0.11</td><td>4d'</td><td></td><td></td><td></td></tr><tr><td>(nd' ← 27a)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7.363</td><td>0.04</td><td>3d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.05(1)(b)</td><td>0.04</td><td>4d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.36(6)(s)</td><td>0.04</td><td>5d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.545</td><td>-0.02</td><td>6d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

(b) broad feature; (s) shoulder structure; (w) weak structure (the last decimal on the energy value is given in brackets for these less-resolved features).
<caption>**Table 5a**</caption>

Energy values (eV), quantum defects ($\delta$) and assignments of the Rydberg series converging to $(27a)^{-1}\tilde{X}^2A$, $(26a)^{-1}\tilde{A}^2A$, $(25a)^{-1}\tilde{B}^2A$ and $(24a)^{-1}\tilde{C}^2A$ ionic states of dihydro-2(3H)-thiophenone. See text for details.

<table><thead><tr><th>E<sub>n</sub><br>(IE<sub>1</sub>)<sub>v</sub> = 9.49 eV (27a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>2</sub>)<sub>v</sub> = 9.66 eV (26a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>3</sub>)<sub>v</sub> = 11.80 eV (25a)<sup>-1</sup></th><th>δ</th><th>Assignment</th></tr></thead><tbody><tr><td>(ns ← 27a)</td><td></td><td></td><td>(ns ← 26a)</td><td></td><td></td><td>(ns ← 25a)</td><td></td><td></td></tr><tr><td>6.44(4)(b)</td><td>1.87</td><td>4s</td><td>6.44(4)(b)</td><td>1.94</td><td>4s</td><td>8.41(1)(s,w)</td><td>1.99</td><td>4s</td></tr><tr><td>8.02(5)(b)</td><td>1.95</td><td>5s</td><td>8.20(0)(s,w)</td><td>1.95</td><td>5s</td><td>(np ← 25a)</td><td></td><td></td></tr><tr><td>8.67(0)(b,w)</td><td>1.93</td><td>6s</td><td>8.837</td><td>1.93</td><td>6s</td><td>9.11(6)(s)</td><td>1.75</td><td>4p</td></tr><tr><td>8.962</td><td>1.92</td><td>7s</td><td>9.13(0)(s)</td><td>1.93</td><td>7s</td><td>(IE<sub>4</sub>)<sub>v</sub> = 12.29 eV (24a)<sup>-1</sup></td><td></td><td></td></tr><tr><td>–</td><td>–</td><td>8s</td><td>9.29(4)(s)</td><td>1.91</td><td>8s</td><td>(ns ← 24a)</td><td></td><td></td></tr><tr><td>9.22(2)(s)</td><td>1.87</td><td>9s</td><td>(np ← 26a)</td><td></td><td></td><td>8.90(7)(b,w)</td><td>1.99</td><td>4s</td></tr><tr><td>(np ← 27a)</td><td></td><td></td><td>7.009</td><td>1.73</td><td>4p</td><td></td><td></td><td></td></tr><tr><td>6.75(3)(s,w)</td><td>1.77</td><td>4p</td><td>8.37(7)(s,w)</td><td>1.74</td><td>5p</td><td></td><td></td><td></td></tr><tr><td>8.20(0)(s,w)</td><td>1.75</td><td>5p</td><td>8.90(7)(b,w)</td><td>1.75</td><td>6p</td><td></td><td></td><td></td></tr><tr><td>8.768</td><td>1.66</td><td>6p</td><td>9.17(0)(w)</td><td>1.73</td><td>7p</td><td></td><td></td><td></td></tr><tr><td><b>9.01(4)(w)</b></td><td>1.65</td><td>7p</td><td>9.32(2)(s)</td><td>1.65</td><td>8p</td><td></td><td></td><td></td></tr><tr><td>9.147</td><td>1.70</td><td>8p</td><td>9.40(3)(s)</td><td>1.72</td><td>9p</td><td></td><td></td><td></td></tr><tr><td>(np' ← 27a)</td><td></td><td></td><td>(np' ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7.341</td><td>1.48</td><td>4p'</td><td>7.60(6)(b)</td><td>1.42</td><td>4p'</td><td></td><td></td><td></td></tr><tr><td>8.41(1)(s,w)</td><td>1.45</td><td>5p'</td><td>8.616</td><td>1.39</td><td>5p'</td><td></td><td></td><td></td></tr><tr><td>8.837</td><td>1.43</td><td>6p'</td><td><b>9.01(4)(w)</b></td><td>1.41</td><td>6p'</td><td></td><td></td><td></td></tr><tr><td>9.05(3)(s,w)</td><td>1.42</td><td>7p'</td><td>9.22(2)(s)</td><td>1.42</td><td>7p'</td><td></td><td></td><td></td></tr><tr><td>9.17(0)(w)</td><td>1.48</td><td>8p'</td><td>(nd ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>(nd ← 27a)</td><td></td><td></td><td>7.92(7)(s)</td><td>0.20</td><td>3d</td><td></td><td></td><td></td></tr><tr><td>7.788</td><td>0.17</td><td>3d</td><td>8.725</td><td>0.18</td><td>4d</td><td></td><td></td><td></td></tr><tr><td>8.55(1)(s)</td><td>0.19</td><td>4d</td><td>9.086</td><td>0.13</td><td>5d</td><td></td><td></td><td></td></tr><tr><td>8.90(7)(b,w)</td><td>0.17</td><td>5d</td><td>9.26(3)(b)</td><td>0.14</td><td>6d</td><td></td><td></td><td></td></tr><tr><td>9.086</td><td>0.19</td><td>6d</td><td>9.36(8)(w)</td><td>0.17</td><td>7d</td><td></td><td></td><td></td></tr><tr><td>(nd' ← 27a)</td><td></td><td></td><td>(nd' ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7.92(7)(s)</td><td>0.05</td><td>3d'</td><td>8.02(5)(b)</td><td>0.11</td><td>3d'</td><td></td><td></td><td></td></tr><tr><td>8.616</td><td>0.05</td><td>4d'</td><td>8.768</td><td>0.09</td><td>4d'</td><td></td><td></td><td></td></tr><tr><td>8.93(9)(s)</td><td>0.03</td><td>5d'</td><td>9.11(6)(s)</td><td>0.00</td><td>5d'</td><td></td><td></td><td></td></tr><tr><td>9.11(6)(s)</td><td>-0.03</td><td>6d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9.20(8)(s)</td><td>0.05</td><td>7d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

(b) broad feature; (w) weak structure; (s) shoulder structure (the last decimal on the energy value is given in brackets for these less-resolved features).

<caption>**Table 5b**</caption>

Energy values (eV), quantum defects ($\delta$) and assignments of the Rydberg series converging to $(27a)^{-1}\tilde{X}^2A$, $(26a)^{-1}\tilde{A}^2A$, $(25a)^{-1}\tilde{B}^2A$ and $(24a)^{-1}\tilde{C}^2A$ ionic states of dihydro-3(2H)-thiophenone. See text for details.

<table><thead><tr><th>E<sub>n</sub><br>(IE<sub>1</sub>)<sub>v</sub> = 8.92 eV (27a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>2</sub>)<sub>v</sub> = 9.85 eV (26a)<sup>-1</sup></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub><br>(IE<sub>3</sub>)<sub>v</sub> = 11.63 eV (25a)<sup>-1</sup></th><th>δ</th><th>Assignment</th></tr></thead><tbody><tr><td>(ns ← 27a)</td><td></td><td></td><td>(ns ← 26a)</td><td></td><td></td><td>(ns ← 25a)</td><td></td><td></td></tr><tr><td>5.57(5)(b)</td><td>1.98</td><td>4s</td><td>6.512</td><td>1.98</td><td>4s</td><td>8.26(0)(s,w)</td><td>1.99</td><td>4s</td></tr><tr><td>7.43(3)(s,w)</td><td>1.97</td><td>5s</td><td>8.36(6)(s)</td><td>1.97</td><td>5s</td><td>10.17(1)(w)</td><td>1.95</td><td>5s</td></tr><tr><td>8.114</td><td>1.89</td><td>6s</td><td>8.991</td><td>2.02</td><td>6s</td><td>(np ← 25a)</td><td></td><td></td></tr><tr><td>8.36(6)(s)</td><td>2.04</td><td>7s</td><td>9.33(6)(s)</td><td>1.85</td><td>7s</td><td>8.88(8)(s)</td><td>1.77</td><td>4p</td></tr><tr><td>8.545</td><td>1.98</td><td>8s</td><td>9.48(6)(s)</td><td>1.89</td><td>8s</td><td>10.35(8)(s,w)</td><td>1.73</td><td>5p</td></tr><tr><td>8.64(0)(b,w)</td><td>2.03</td><td>9s</td><td>(np ← 26a)</td><td></td><td></td><td>(np ← 25a)</td><td></td><td></td></tr><tr><td>(np ← 27a)</td><td></td><td></td><td>7.16(3)(s)</td><td>1.75</td><td>4p</td><td>9.63(4)(s)</td><td>1.39</td><td>4p'</td></tr><tr><td>6.368</td><td>1.69</td><td>4p</td><td>8.545</td><td>1.77</td><td>5p</td><td>10.57(9)(s)</td><td>1.40</td><td>5p'</td></tr><tr><td>7.63(5)(b)</td><td>1.75</td><td>5p</td><td>9.08(3)(s)</td><td>1.79</td><td>6p</td><td>(nd ← 25a)</td><td></td><td></td></tr><tr><td>8.17(8)(w)</td><td>1.72</td><td>6p</td><td>9.37(9)(b,w)</td><td>1.62</td><td>7p</td><td>9.82(4)(w)</td><td>0.25</td><td>3d</td></tr><tr><td>8.41(7)(s)</td><td>1.80</td><td>7p</td><td>(np' ← 26a)</td><td></td><td></td><td>10.65(2)(w)</td><td>0.27</td><td>4d</td></tr><tr><td>8.58(0)(s,w)</td><td>1.67</td><td>8p</td><td>7.739</td><td>1.46</td><td>4p'</td><td>(nd' ← 25a)</td><td></td><td></td></tr><tr><td>8.67(0)(s,w)</td><td>1.62</td><td>9p</td><td>8.76(2)(b)</td><td>1.46</td><td>5p'</td><td>10.06(4)(w)</td><td>0.05</td><td>3d'</td></tr><tr><td>(np' ← 27a)</td><td></td><td></td><td>9.19(8)(b)</td><td>1.43</td><td>6p'</td><td>(IE<sub>4</sub>)<sub>v</sub> = 12.57 eV (24a)<sup>-1</sup></td><td></td><td></td></tr><tr><td>6.570</td><td>1.59</td><td>4p'</td><td>–</td><td>–</td><td>7p'</td><td>(ns ← 24a)</td><td></td><td></td></tr><tr><td>7.817</td><td>1.49</td><td>5p'</td><td>9.53(0)(s,w)</td><td>1.48</td><td>8p'</td><td>9.19(8)(b)</td><td>1.99</td><td>4s</td></tr><tr><td>8.26(0)(s,w)</td><td>1.46</td><td>6p'</td><td>(nd ← 26a)</td><td></td><td></td><td>(np ← 24a)</td><td></td><td></td></tr><tr><td>8.46(3)(w)</td><td>1.33</td><td>7p'</td><td>8.10(4)(s,w)</td><td>0.21</td><td>3d</td><td>9.88(7)(w,b)</td><td>1.74</td><td>4p</td></tr><tr><td>(nd ← 27a)</td><td></td><td></td><td>8.88(8)(s)</td><td>0.24</td><td>4d</td><td>(np' ← 24a)</td><td></td><td></td></tr><tr><td>7.16(3)(s)</td><td>0.22</td><td>3d</td><td>9.26(6)(s,w)</td><td>0.17</td><td>5d</td><td>10.57(9)(s)</td><td>1.39</td><td>4p'</td></tr><tr><td>7.90(2)(s,w)</td><td>0.34</td><td>4d</td><td>(nd' ← 26a)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.34(9)(s)</td><td>0.12</td><td>5d</td><td>8.23(3)(s,w)</td><td>0.09</td><td>3d'</td><td></td><td></td><td></td></tr><tr><td>8.51(5)(w)</td><td>0.20</td><td>6d</td><td>8.551</td><td>0.11</td><td>4d'</td><td></td><td></td><td></td></tr><tr><td>(nd' ← 27a)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7.363</td><td>0.04</td><td>3d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.05(1)(b)</td><td>0.04</td><td>4d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.36(6)(s)</td><td>0.04</td><td>5d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8.545</td><td>-0.02</td><td>6d'</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

(b) broad feature; (s) shoulder structure; (w) weak structure (the last decimal on the energy value is given in brackets for these less-resolved features).
eV and 6.570 eV ($\delta = 1.69$ and 1.59) (Table 5b). The relatively high value of the quantum defect for the 7p Rydberg series member ($\delta = 1.80$) is attributed to the uncertainty in its vertical position from the shape of the absorption feature. The 6p' feature at 8.26(0) eV can also be assigned to $4s(25a)^{-1}$. The tentative assignments also report two ($nd \leftarrow 27a$) and ($nd' \leftarrow 27a$) series, with their first members $n = 3$ at 7.16(3) and 7.363 eV ($\delta = 0.22$ and 0.04).

Five Rydberg series converging to the ionic electronic first excited state, are listed in Table 5b and have been assigned to the ($ns, np, np', nd, nd' \leftarrow 26a$) transitions. The first members of these series have features at 6.512 eV ($\delta = 1.98$), 7.16(3) eV ($\delta = 1.75$), 7.739 eV ($\delta = 1.46$), 8.10(4) eV ($\delta = 0.21$) and 8.23(3) eV ($\delta = 0.09$). The features at 8.88(8) and 9.19 (8) eV are also assigned to $4p(25a)^{-1}$ and $4s(24a)^{-1}$.

The Rydberg series converging to the ionic electronic second excited state have been assigned to the ($ns, np, np', nd, nd' \leftarrow 25a$) transitions. The first members of the $ns, np, np', nd$ and $nd'$ series are associated with features at 8.26(0) eV ($\delta = 1.99$), 8.88(8) eV ($\delta = 1.77$), 9.63(4) eV ($\delta = 1.39$), 9.82(4) eV ($\delta = 0.25$) and 10.06(4) eV ($\delta = 0.05$) (Table 5a). The feature at 10.57(9) eV is also assigned to $4p'(24a)^{-1}$.

The Rydberg series converging to the ionic electronic third excited state have been assigned to the ($ns, np, np' \leftarrow 24a$) transitions. The first members have features at 9.19(8) eV ($\delta = 1.99$), 9.88(7) eV ($\delta = 1.74$) and 10.57(9) eV ($\delta = 1.39$) (Table 5a). Tentative assignments of these series have only been made for $n = 4$ since higher-energy members lie outside the photon energy range investigated.

The vibrational assignments involving the Rydberg series converging to the $(27a)^{-1} \tilde{X}^2A$, $(26a)^{-1} \tilde{A}^2A$ and $(25a)^{-1} \tilde{B}^2A$ ionic states are included in Tables 3b and 4b, and have not been included in Figs. 3b and 4b to avoid congestion. The $0_0^0$ origin band of the $4s \leftarrow n_S(27a)$ $(6^1A \leftarrow \tilde{X}^1A)$ transition is suggested at 6.44(4) eV and is mainly accompanied by excitation of up to three quanta of the C=O stretching, $v'_4(a')$ mode (Table 3b). The other fine structure has been assigned ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and $v'_{15}(a)$ modes. Combination bands mainly involve the ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and C=O stretching, $v'_4(a')$ modes.

## 4.5. Absolute photoabsorption cross-sections and atmospheric photolysis

The absolute cross-section values of the most representative electronic transitions, in units of Mb, are listed in Table 1 for dihydro-2(3H)-thiophenone (γ-thiobutyrolactone) and dihydro-3(2H)-thiophenone (dihydrothiophen-3-one). As far as the authors are aware no previous studies are available in the literature to compare with the present high-resolution VUV photoabsorption data in the wavelength region 116–330 nm (3.7–10.7 eV) investigated. From the absolute photoabsorption cross-sections together with the available solar actinic flux data in the Earth’s atmosphere from ground level up to the limit of the stratopause (50 km) [25], we have estimated the photolysis rates of these molecules. This information is indicative of a molecular compound local lifetime when emitted to the atmosphere and subject to the interaction of solar ultraviolet radiation [26]. For further details on this methodology see Limão-Vieira and co-workers [27]. The quantum yield for dissociation is taken to be 1.0 due to lack of any information in the literature.

Computed photolysis lifetimes of less than 1 sunlit day were calculated at altitudes above 10 km for dihydro-2(3H)-thiophenone and above ground level for dihydro-3(2H)-thiophenone, the former reaching less than 2 days at sea-level. Notwithstanding, in the troposphere these molecules can be efficiently broken up by UV absorption. A thorough literature survey shows that there are no studies of the gas-phase kinetics for $C_4H_6OS$ reactions with e.g. *OH, Cl and NO$_3$ radicals, to assess the role of such processes as likely main atmospheric sink mechanism.

## 5. Conclusions

The AU-UV beam line, at the ASTRID2 synchrotron radiation facility,

Aarhus University, Denmark has been used to measure the absolute photoabsorption cross-sections of dihydro-2(3H)-thiophenone (γ-thiobutyrolactone) and dihydro-3(2H)-thiophenone (dihydrothiophen-3-one) from 3.7 up to 10.7 eV. A close comparison of both absorption spectra shows that the proximity between the S atom and the C=O bond plays a major role in the shape and form of the spectra which is more noticeable in case of dihydro-2(3H)-thiophenone. The spectra show features due to transitions into valence and Rydberg states, superimposed on vibrational fine structure which appear much weaker in the photoabsorption spectrum of dihydro-3(2H)-thiophenone. Assignments of the major electronic transitions have been proposed from the information provided by the calculated vertical excitation energies and oscillator strengths with EOM-CCSD ab initio calculations. The electronic state spectroscopy of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone has been investigated for the first-time, showing that transitions into Rydberg states dominate the energy range covered by the present absorption spectra. A close comparison between the vibrational structure of the thiophenone molecules shows C=O stretching excitations, with relevant CH$_2$ twisting and rocking modes for dihydro-2(3H)-thiophenone and ring stretching modes for dihydro-3(2H)-thiophenone. The photolysis lifetimes were also estimated from ground-level up to 50 km altitude, showing that these molecules can be efficiently broken up by UV absorption in the troposphere. Due to a lack of any comprehensive information on gas-phase kinetics for these molecules and reactions with relevant radicals present in the Earth’s atmosphere, it stands to be proven what the main atmospheric sink mechanism is.

### CRediT authorship contribution statement

**S. Kumar:** Investigation. **D. Duflot:** Funding acquisition, Software, Writing – original draft, Writing – review & editing. **N.C. Jones:** Data curation, Methodology, Writing – review & editing. **S.V. Hoffmann:** Data curation, Funding acquisition, Methodology, Writing – review & editing. **P. Limão-Vieira:** Conceptualization, Methodology, Supervision, Funding acquisition, Writing – original draft, Writing – review & editing.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

SK and PLV acknowledges the Portuguese National Funding Agency (FCT) through research grant CEFITEC (UIDB/00068/2020). PLV also acknowledges his visiting professor position at Federal University of Paraná, Curitiba, Brazil. The authors wish to acknowledge the beam time at the ISA synchrotron, Aarhus University, Denmark. The research leading to this result has been supported by the project CALIPSO plus under the grant agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. This contribution is also based upon work from the COST Action CA18212-Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology). DD acknowledges support from the CaPPA project (Chemical and Physical Properties of the Atmosphere) funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract no. ANR-10-LABX-005; the Région Hauts de France and the Ministère de l'Enseignement
eV and 6.570 eV ($\delta = 1.69$ and 1.59) (Table 5b). The relatively high value of the quantum defect for the 7p Rydberg series member ($\delta = 1.80$) is attributed to the uncertainty in its vertical position from the shape of the absorption feature. The 6p' feature at 8.26(0) eV can also be assigned to $4s(25a)^{-1}$. The tentative assignments also report two ($nd \leftarrow 27a$) and ($nd' \leftarrow 27a$) series, with their first members $n = 3$ at 7.16(3) and 7.363 eV ($\delta = 0.22$ and 0.04).

Five Rydberg series converging to the ionic electronic first excited state, are listed in Table 5b and have been assigned to the ($ns, np, np', nd, nd' \leftarrow 26a$) transitions. The first members of these series have features at 6.512 eV ($\delta = 1.98$), 7.16(3) eV ($\delta = 1.75$), 7.739 eV ($\delta = 1.46$), 8.10(4) eV ($\delta = 0.21$) and 8.23(3) eV ($\delta = 0.09$). The features at 8.88(8) and 9.19 (8) eV are also assigned to $4p(25a)^{-1}$ and $4s(24a)^{-1}$.

The Rydberg series converging to the ionic electronic second excited state have been assigned to the ($ns, np, np', nd, nd' \leftarrow 25a$) transitions. The first members of the $ns, np, np', nd$ and $nd'$ series are associated with features at 8.26(0) eV ($\delta = 1.99$), 8.88(8) eV ($\delta = 1.77$), 9.63(4) eV ($\delta = 1.39$), 9.82(4) eV ($\delta = 0.25$) and 10.06(4) eV ($\delta = 0.05$) (Table 5a). The feature at 10.57(9) eV is also assigned to $4p'(24a)^{-1}$.

The Rydberg series converging to the ionic electronic third excited state have been assigned to the ($ns, np, np' \leftarrow 24a$) transitions. The first members have features at 9.19(8) eV ($\delta = 1.99$), 9.88(7) eV ($\delta = 1.74$) and 10.57(9) eV ($\delta = 1.39$) (Table 5a). Tentative assignments of these series have only been made for $n = 4$ since higher-energy members lie outside the photon energy range investigated.

The vibrational assignments involving the Rydberg series converging to the $(27a)^{-1} \tilde{X}^2A$, $(26a)^{-1} \tilde{A}^2A$ and $(25a)^{-1} \tilde{B}^2A$ ionic states are included in Tables 3b and 4b, and have not been included in Figs. 3b and 4b to avoid congestion. The $0_0^0$ origin band of the $4s \leftarrow n_S(27a)$ $(6^1A \leftarrow \tilde{X}^1A)$ transition is suggested at 6.44(4) eV and is mainly accompanied by excitation of up to three quanta of the C=O stretching, $v'_4(a')$ mode (Table 3b). The other fine structure has been assigned ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and $v'_{15}(a)$ modes. Combination bands mainly involve the ring stretching, $v'_{13}(a')$, $v'_{14}(a')$ and C=O stretching, $v'_4(a')$ modes.

## 4.5. Absolute photoabsorption cross-sections and atmospheric photolysis

The absolute cross-section values of the most representative electronic transitions, in units of Mb, are listed in Table 1 for dihydro-2(3H)-thiophenone (γ-thiobutyrolactone) and dihydro-3(2H)-thiophenone (dihydrothiophen-3-one). As far as the authors are aware no previous studies are available in the literature to compare with the present high-resolution VUV photoabsorption data in the wavelength region 116–330 nm (3.7–10.7 eV) investigated. From the absolute photoabsorption cross-sections together with the available solar actinic flux data in the Earth’s atmosphere from ground level up to the limit of the stratopause (50 km) [25], we have estimated the photolysis rates of these molecules. This information is indicative of a molecular compound local lifetime when emitted to the atmosphere and subject to the interaction of solar ultraviolet radiation [26]. For further details on this methodology see Limão-Vieira and co-workers [27]. The quantum yield for dissociation is taken to be 1.0 due to lack of any information in the literature.

Computed photolysis lifetimes of less than 1 sunlit day were calculated at altitudes above 10 km for dihydro-2(3H)-thiophenone and above ground level for dihydro-3(2H)-thiophenone, the former reaching less than 2 days at sea-level. Notwithstanding, in the troposphere these molecules can be efficiently broken up by UV absorption. A thorough literature survey shows that there are no studies of the gas-phase kinetics for $C_4H_6OS$ reactions with e.g. *OH, Cl and $NO_3$ radicals, to assess the role of such processes as likely main atmospheric sink mechanism.

## 5. Conclusions

The AU-UV beam line, at the ASTRID2 synchrotron radiation facility,

Aarhus University, Denmark has been used to measure the absolute photoabsorption cross-sections of dihydro-2(3H)-thiophenone (γ-thiobutyrolactone) and dihydro-3(2H)-thiophenone (dihydrothiophen-3-one) from 3.7 up to 10.7 eV. A close comparison of both absorption spectra shows that the proximity between the S atom and the C=O bond plays a major role in the shape and form of the spectra which is more noticeable in case of dihydro-2(3H)-thiophenone. The spectra show features due to transitions into valence and Rydberg states, superimposed on vibrational fine structure which appear much weaker in the photoabsorption spectrum of dihydro-3(2H)-thiophenone. Assignments of the major electronic transitions have been proposed from the information provided by the calculated vertical excitation energies and oscillator strengths with EOM-CCSD ab initio calculations. The electronic state spectroscopy of dihydro-2(3H)-thiophenone and dihydro-3(2H)-thiophenone has been investigated for the first-time, showing that transitions into Rydberg states dominate the energy range covered by the present absorption spectra. A close comparison between the vibrational structure of the thiophenone molecules shows C=O stretching excitations, with relevant $CH_2$ twisting and rocking modes for dihydro-2(3H)-thiophenone and ring stretching modes for dihydro-3(2H)-thiophenone. The photolysis lifetimes were also estimated from ground-level up to 50 km altitude, showing that these molecules can be efficiently broken up by UV absorption in the troposphere. Due to a lack of any comprehensive information on gas-phase kinetics for these molecules and reactions with relevant radicals present in the Earth’s atmosphere, it stands to be proven what the main atmospheric sink mechanism is.

### CRediT authorship contribution statement

S. Kumar: Investigation. D. Duflot: Funding acquisition, Software, Writing – original draft, Writing – review & editing. N.C. Jones: Data curation, Methodology, Writing – review & editing. S.V. Hoffmann: Data curation, Funding acquisition, Methodology, Writing – review & editing. P. Limão-Vieira: Conceptualization, Methodology, Supervision, Funding acquisition, Writing – original draft, Writing – review & editing.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

SK and PLV acknowledges the Portuguese National Funding Agency (FCT) through research grant CEFITEC (UIDB/00068/2020). PLV also acknowledges his visiting professor position at Federal University of Paraná, Curitiba, Brazil. The authors wish to acknowledge the beam time at the ISA synchrotron, Aarhus University, Denmark. The research leading to this result has been supported by the project CALIPSO plus under the grant agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. This contribution is also based upon work from the COST Action CA18212-Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology). DD acknowledges support from the CaPPA project (Chemical and Physical Properties of the Atmosphere) funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract no. ANR-10-LABX-005; the Région Hauts de France and the Ministère de l'Enseignement
Supérieur et de la Recherche (CPER ECRIN) and the European Fund for Regional Economic Development for their financial support. This work was performed using HPC resources from GENCI-TGCC (Grant No. 2023–A0130801859) and the Centre de Ressources Informatiques (CRI) of the Université de Lille.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jqsrt.2024.108926.

## References

[1] Kumar S, Duflot D, Jones NC, Hoffmann SV, García G, Limão-Vieira P. The electronic structure of 2(5H)-thiophenone investigated by vacuum ultraviolet synchrotron radiation and theoretical calculations. Eur Phys J D 2023;77:202.

[2] Torok CR, Lavrich RJ, Tubergen MJ. Spectroscopic and ab initio structure of the heterocyclic ring tetrahydrothiophen-3-one. J Mol Struct 2002;612:223-30.

[3] Margulès L, Eugenia Sanz M, Kassi S, Petitprez D, Wlodarczak G, López JC, et al. Rotational spectrum, ring-puckering vibration and ab initio calculations on tetrahydrothiophene. Chem Phys 2001;263:19-31.

[4] Harder H, Lesarri AG, Alonso JL, Dreizler H, Lopez JC. Determination of the ring-inversion barrier in γ-thiobutyrolactone by microwave Fourier transform spectroscopy. Z Fur Naturforsch - J Phys Sci 1992;47:588-90.

[5] Alonso JL, Lopez JC, Mata F. Microwave spectrum of gamma-thiobutyrolactone. Z Fur Naturforsch - J Phys Sci 1982;37:129-33.

[6] López JC, Villamañán RM, Alonso JL, Muñoz JM. Ring inversion in tetrahydrothiophen-3-one. A microwave study. J Chem Soc, Faraday Trans 1990; 86:5-10.

[7] Chin WS, Xu ZP, Mok CY, Huang HH, Mutoh H, Masuda S. He I and He II photoelectron spectra of thiophenones. J Electron Spectrosc Relat Phenom 1998; 88-91:97-101.

[8] Laane J, Sagear PA, Lee SN. Jet-cooled fluorescence spectra and carbonyl wagging potential energy functions of tetrahydrofuran-3-one and tetrahydrothiophen-3-one in their S1 (n,π*) excited states. J Mol Struct 1997;408/409:557-61.

[9] Sagear PA, Lee SN, Laane J. Laser induced fluorescence spectra and carbonyl wagging potential energy functions for the S1(n, π*) excited states of tetrahydrofuran-3-one and tetrahydrothiophen-3-one: correlation between inversion barrier and angle strain for cyclic ketones. J Chem Phys 1997;106: 3876-83.

[10] Dugarte NY, Erben MF, Romano RM, Ge MF, Li Y, Della Védova CO. Matrix photochemistry at low temperatures and spectroscopic properties of γ-butyrothiolactone. J Phys Chem A 2010;114:9462-70.

[11] Werner HJ, Knowles PJ, Knizia G, Manby FR, Schütz M, Celani P, et al. MOLPRO, version 2019.1, a package of ab initio programs. 2019.

[12] Werner HJ, Knowles PJ, Knizia G, Manby FR, Schütz M. Molpro: a general-purpose quantum chemistry program package. WIREs Comput Mol Sci 2012;2:242-53.

[13] Dunning J, Peterson KA, Wilson AK. Gaussian basis sets for use in correlated molecular calculations. X. The atoms aluminum through argon revisited. J Chem Phys 2001;114:9244-53.

[14] Woon DE, Dunning TH. Gaussian basis sets for use in correlated molecular calculations. III. The atoms aluminum through argon. J Chem Phys 1993;98: 1358-71.

[15] Kaufmann K, Baumeister W, Jungen M. Universal Gaussian basis sets for an optimum representation of Rydberg and continuum wavefunctions. J Phys B At Mol Opt Phys 1989;22:2223-40.

[16] Korona T, Werner HJ. Local treatment of electron excitations in the EOM-CCSD method. J Chem Phys 2003;118:3006-19.

[17] Stanton JF, Gauss J. Analytic energy derivatives for ionized states described by the equation-of-motion coupled cluster method. J Chem Phys 1994;101:8938-44.

[18] Matthews DA, Cheng L, Harding ME, Lipparini F, Stopkowicz S, Jagau TC, et al. Coupled-cluster techniques for computational chemistry: the CFOUR program package. J Chem Phys 2020;152:214108.

[19] Corzo HH, Galano A, Dolgounitcheva O, Zakrzewski VG, Ortiz JV. NR2 and P3+: accurate, efficient electron-propagator methods for calculating valence, vertical ionization energies of closed-shell molecules. J Phys Chem A 2015;119:8813-21.

[20] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 16 Rev. C.01. Wallingford, CT; 2016.

[21] Palmer MH, Ridley T, Hoffmann SV, Jones NC, Coreno M, De Simone M, et al. Interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations. J Chem Phys 2015;142:134302.

[22] Eden S, Limão-Vieira P, Hoffmann SV, Mason NJ. VUV photoabsorption in CF₃X (X = Cl, Br, I) fluoro-alkanes. Chem Phys 2006;323:313-33.

[23] Duflot D, Hoffmann SV, Jones NC, Limão-Vieira P. Synchrotron radiation UV-VUV photoabsorption of gas phase molecules. In: Pereira AS, Tavares P, Limão-Vieira P, editors. Radiat Bioanal Spectrosc Tech Theor Method 2019:43-81.

[24] Robin MB. Higher excited states of polyatomic molecules, volume iii. Academic Press; 1985.

[25] Chemical kinetics and photochemical data for use in stratospheric modelling, evaluation number 12 1997:97. 4January 15.

[26] Duflot D, Hoffmann SV, Jones NC, Limão-Vieira P. Synchrotron radiation UV-VUV photoabsorption of gas phase molecules. In: Pereira AS, Tavares P, Limão-Vieira P, editors. Radiat Bioanal Spectrosc Tech Theor Method 2019:43-81.

[27] Limão-Vieira P, Eden S, Kendall PA, Mason NJ, Hoffmann SV. VUV photo-absorption cross-section for CCl2F2. Chem Phys Lett 2002;364:535-41.
Supérieur et de la Recherche (CPER ECRIN) and the European Fund for Regional Economic Development for their financial support. This work was performed using HPC resources from GENCI-TGCC (Grant No. 2023–A0130801859) and the Centre de Ressources Informatiques (CRI) of the Université de Lille.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jqsrt.2024.108926.

## References

[1] Kumar S, Duflot D, Jones NC, Hoffmann SV, García G, Limão-Vieira P. The electronic structure of 2(5H)-thiophenone investigated by vacuum ultraviolet synchrotron radiation and theoretical calculations. Eur Phys J D 2023;77:202.

[2] Torok CR, Lavrich RJ, Tubergen MJ. Spectroscopic and ab initio structure of the heterocyclic ring tetrahydrothiophen-3-one. J Mol Struct 2002;612:223-30.

[3] Margulès L, Eugenia Sanz M, Kassi S, Petitprez D, Wlodarczak G, López JC, et al. Rotational spectrum, ring-puckering vibration and ab initio calculations on tetrahydrothiophene. Chem Phys 2001;263:19-31.

[4] Harder H, Lesarri AG, Alonso JL, Dreizler H, Lopez JC. Determination of the ring-inversion barrier in γ-thiobutyrolactone by microwave Fourier transform spectroscopy. Z Fur Naturforsch - J Phys Sci 1992;47:588-90.

[5] Alonso JL, Lopez JC, Mata F. Microwave spectrum of gamma-thiobutyrolactone. Z Fur Naturforsch - J Phys Sci 1982;37:129-33.

[6] López JC, Villamañán RM, Alonso JL, Muñoz JM. Ring inversion in tetrahydrothiophen-3-one. A microwave study. J Chem Soc, Faraday Trans 1990; 86:5-10.

[7] Chin WS, Xu ZP, Mok CY, Huang HH, Mutoh H, Masuda S. He I and He II photoelectron spectra of thiophenones. J Electron Spectrosc Relat Phenom 1998; 88-91:97-101.

[8] Laane J, Sagear PA, Lee SN. Jet-cooled fluorescence spectra and carbonyl wagging potential energy functions of tetrahydrofuran-3-one and tetrahydrothiophen-3-one in their S1 (n,π*) excited states. J Mol Struct 1997;408/409:557-61.

[9] Sagear PA, Lee SN, Laane J. Laser induced fluorescence spectra and carbonyl wagging potential energy functions for the S1(n, π*) excited states of tetrahydrofuran-3-one and tetrahydrothiophen-3-one: correlation between inversion barrier and angle strain for cyclic ketones. J Chem Phys 1997;106: 3876-83.

[10] Dugarte NY, Erben MF, Romano RM, Ge MF, Li Y, Della Védova CO. Matrix photochemistry at low temperatures and spectroscopic properties of γ-butyrothiolactone. J Phys Chem A 2010;114:9462-70.

[11] Werner HJ, Knowles PJ, Knizia G, Manby FR, Schütz M, Celani P, et al. MOLPRO, version 2019.1, a package of ab initio programs. 2019.

[12] Werner HJ, Knowles PJ, Knizia G, Manby FR, Schütz M. Molpro: a general-purpose quantum chemistry program package. WIREs Comput Mol Sci 2012;2:242-53.

[13] Dunning J, Peterson KA, Wilson AK. Gaussian basis sets for use in correlated molecular calculations. X. The atoms aluminum through argon revisited. J Chem Phys 2001;114:9244-53.

[14] Woon DE, Dunning TH. Gaussian basis sets for use in correlated molecular calculations. III. The atoms aluminum through argon. J Chem Phys 1993;98: 1358-71.

[15] Kaufmann K, Baumeister W, Jungen M. Universal Gaussian basis sets for an optimum representation of Rydberg and continuum wavefunctions. J Phys B At Mol Opt Phys 1989;22:2223-40.

[16] Korona T, Werner HJ. Local treatment of electron excitations in the EOM-CCSD method. J Chem Phys 2003;118:3006-19.

[17] Stanton JF, Gauss J. Analytic energy derivatives for ionized states described by the equation-of-motion coupled cluster method. J Chem Phys 1994;101:8938-44.

[18] Matthews DA, Cheng L, Harding ME, Lipparini F, Stopkowicz S, Jagau TC, et al. Coupled-cluster techniques for computational chemistry: the CFOUR program package. J Chem Phys 2020;152:214108.

[19] Corzo HH, Galano A, Dolgounitcheva O, Zakrzewski VG, Ortiz JV. NR2 and P3+: accurate, efficient electron-propagator methods for calculating valence, vertical ionization energies of closed-shell molecules. J Phys Chem A 2015;119:8813-21.

[20] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 16 Rev. C.01. Wallingford, CT; 2016.

[21] Palmer MH, Ridley T, Hoffmann SV, Jones NC, Coreno M, De Simone M, et al. Interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations. J Chem Phys 2015;142:134302.

[22] Eden S, Limão-Vieira P, Hoffmann SV, Mason NJ. VUV photoabsorption in CF₃X (X = Cl, Br, I) fluoro-alkanes. Chem Phys 2006;323:313-33.

[23] Duflot D, Hoffmann SV, Jones NC, Limão-Vieira P. Synchrotron radiation UV-VUV photoabsorption of gas phase molecules. In: Pereira AS, Tavares P, Limão-Vieira P, editors. Radiat Bioanal Spectrosc Tech Theor Method 2019:43-81.

[24] Robin MB. Higher excited states of polyatomic molecules, volume iii. Academic Press; 1985.

[25] Chemical kinetics and photochemical data for use in stratospheric modelling, evaluation number 12 1997:97. 4January 15.

[26] Duflot D, Hoffmann SV, Jones NC, Limão-Vieira P. Synchrotron radiation UV-VUV photoabsorption of gas phase molecules. In: Pereira AS, Tavares P, Limão-Vieira P, editors. Radiat Bioanal Spectrosc Tech Theor Method 2019:43-81.

[27] Limão-Vieira P, Eden S, Kendall PA, Mason NJ, Hoffmann SV. VUV photo-absorption cross-section for CCl2F2. Chem Phys Lett 2002;364:535-41.