# Valence and Rydberg excitations of 4-fluorotoluene in the 4.3–10.8 eV photoabsorption energy region

L.V.S. Dalagnol, S. Kumar, A.I. Lozano, M.H.F. Bettega, N.C. Jones, S.V.

Hoffmann, A. Souza Barbosa, P. Limão-Vieira

► To cite this version:

L.V.S. Dalagnol, S. Kumar, A.I. Lozano, M.H.F. Bettega, N.C. Jones, et al., Valence and Rydberg excitations of 4-fluorotoluene in the 4.3–10.8 eV photoabsorption energy region. Journal of Quantitative Spectroscopy and Radiative Transfer, 2024, 327, pp.109125. 10.1016/j.jqsrt.2024.109125. hal-04886006

HAL Id: hal-04886006

https://hal.science/hal-04886006v1

Submitted on 16 Jan 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Valence and Rydberg excitations of 4-fluorotoluene in the 4.3–10.8 eV photoabsorption energy region

L.V.S. Dalagnol<sup>a</sup>, S. Kumar<sup>b,1</sup>, A.I. Lozano<sup>b,2</sup>, M.H.F. Bettega<sup>a</sup>, N.C. Jones<sup>c</sup>, S.V. Hoffmann<sup>c</sup>, A. Souza Barbosa<sup>a, *,</sup> P. Limão-Vieira<sup>a, b, *</sup>

## ARTICLE INFO

Keywords:

4-Fluorotoluene

4-C<sub>7</sub>H<sub>7</sub>F

Electronic excitation

Vibrational excitation

Rydberg series

Ab initio calculations

Photoabsorption

Synchrotron radiation

## ABSTRACT

This work reports on the first measurements of the lowest-lying singlet states as studied by photon spectroscopy for para-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F. Here we present the high-resolution vacuum ultraviolet photoabsorption spectrum in the 4.3–10.8 eV energy-range, with assignments supported by ab initio calculations (vertical excitation energies and oscillator strengths) at two different levels of theory, equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) and time-dependent density functional theory (TD-DFT). The electronic state spectroscopy of 4-C<sub>7</sub>H<sub>7</sub>F reveals the main character of the transitions to be valence, mixed valence-Rydberg and Rydberg, with the associated vibronic series. The absolute cross-section measurements were obtained and used to estimate the photolysis lifetime of 4-fluorotoluene from the sea level up to limit of the stratopause (50 km) in the Earth’s atmosphere.

## 1. Introduction

In this work we report results from our last contribution to the electronic state spectroscopy of fluorotoluene isomers, i.e., *para*-fluorotoluene (4-C<sub>7</sub>H<sub>7</sub>F), while we have recently investigated the electronic excitation of *ortho*-fluorotoluene (2-C<sub>7</sub>H<sub>7</sub>F) [1] and *meta*-fluorotoluene (3-C<sub>7</sub>H<sub>7</sub>F) [2], using experimental and theoretical methods. Other comprehensive studies on the singlet excitations by photoabsorption experiments and *ab initio* calculations have also included toluene [3] and difluorotluene [4]. To the best of our knowledge there are no previous experimental measurements on the vacuum ultraviolet absorption for the 4-C<sub>7</sub>H<sub>7</sub>F molecule in the energy range covered in the present work (4.3–10.8 eV). The structure of 4-C<sub>7</sub>H<sub>7</sub>F has been studied previously because of relevant vibrational energy distribution via methyl group torsion in the S<sub>1</sub> state, the intramolecular dynamics dictating the barrier to internal CH<sub>3</sub> rotation, and the photodissociation and photoisomerization at 193 and 248 nm [5–7].

The near and far ultraviolet absorption spectrum of 4-C<sub>7</sub>H<sub>7</sub>F has been

investigated by Cave and Thompson [8] and Hammond et al. [9] in the energy range 4.4–5.2 eV (282–238 nm) and 5.6–9.5 eV (220–130 nm), respectively, the former suggesting the $D_0^0$ transition of the lowest-lying band at 4.572 eV (36,876 cm<sup>-1</sup>), with these studies including no assignments of the electronic transitions. Previous experimental work on 4-C<sub>7</sub>H<sub>7</sub>F include the ionisation energies of the lowest-lying ionic states by He(I) photoelectron spectroscopy [10,11] and charge-transfer equilibrium constant measurements [12]. Additionally, a combination of resonance-enhanced multiphoton ionisation and zero electron kinetic energy spectroscopy has been reported on the S<sub>1</sub> ← S<sub>0</sub> excitation, showing the adiabatic ionisation energy to be 70,946 cm<sup>-1</sup> (8.796 eV) [13], as well as a complete assignment of vibration and vibration-torsion levels of the S<sub>1</sub> and $D_0^0$ electronic states [14,15].

Infrared and Raman spectra have been obtained to provide a complete assignment of fundamental vibration frequencies [16,17], whereas nano and picosecond time-resolved photoelectron spectroscopy in the S<sub>1</sub> state provide relevant information on the frequencies of several

<footnote><sup>a</sup> Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-980 Curitiba, Paraná, Brazil</footnote>

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

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: alessandra@fisica.ufpr.br (A. Souza Barbosa), plimaovieira@fct.unl.pt (P. Limão-Vieira).</footnote>

<footnote n="1">¹ Current address: Chemical Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, 94720, California, USA.</footnote>

<footnote n="2">² Current address: Institut de Recherche en Astrophysique et Planétologie (IRAP), Université Toulouse III - Paul Sabatier, CNRS, CNES, 9 Avenue du Colonel Roche, 31028 Toulouse, France.</footnote>
<caption>**Table 1a**</caption>

Calculated vertical excitation energies (EOM-CCSD level at the aug-cc-pVDZ basis set) (eV) and oscillator strengths (singlet states) of the main electronic states of eclipsed 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F compared with experimental (energies in eV) (details in text).

<table><thead><tr><th>State</th><th>E</th><th>f<sub>L</sub></th><th>HOMO<br>3π/n<sub>F</sub> (5a<sup>+</sup>)</th><th>HOMO-1<br>2π (4a<sup>+</sup>)</th><th>Exp.</th><th>Cross section (Mb)</th></tr></thead><tbody><tr><td>X<sup>-</sup> <sup>1</sup>A'</td><td>-</td><td>-</td><td></td><td>2π<sup>-</sup>(8a<sup>+</sup>) (8%)</td><td>4.568</td><td>12.52</td></tr><tr><td>2 <sup>1</sup>A'</td><td>5.165</td><td>0.01183</td><td>1π<sup>-</sup>(7a<sup>+</sup>) (28%)</td><td></td><td></td><td></td></tr><tr><td>1 <sup>1</sup>A''</td><td>5.769</td><td>0.00010</td><td>3s(25a') (37%)</td><td></td><td></td><td></td></tr><tr><td>2 <sup>1</sup>A''</td><td>6.293</td><td>0.00001</td><td>3p(26a') (40%)</td><td></td><td></td><td></td></tr><tr><td>3 <sup>1</sup>A''</td><td>6.396</td><td>0.00480</td><td>3p(27a') (34%)</td><td></td><td></td><td></td></tr><tr><td>3 <sup>1</sup>A'</td><td>6.540</td><td>0.00680</td><td>3p<sup>-</sup>(6a<sup>+</sup>) (28%) + 2π<sup>-</sup>(8a<sup>+</sup>) (11%)</td><td>1π<sup>-</sup>(7a<sup>+</sup>) (5%)</td><td></td><td></td></tr><tr><td>4 <sup>1</sup>A''</td><td>6.799</td><td>0.03225</td><td>3d<sup>-</sup>(29a') (11%)</td><td>3p(26a') (19%)</td><td>5.95(5)</td><td>18.15</td></tr><tr><td>5 <sup>1</sup>A''</td><td>7.041</td><td>0.00003</td><td>3d(28a') (39%)</td><td></td><td></td><td></td></tr><tr><td>4 <sup>1</sup>A'</td><td>7.086</td><td>0.20091</td><td>1π<sup>-</sup>(7a<sup>+</sup>) (6%) + 3p<sup>-</sup>(6a<sup>+</sup>) (5%)</td><td>3p<sup>-</sup>(6a<sup>+</sup>) (16%) + 2π<sup>-</sup>(8a<sup>+</sup>) (9%) + 1π<sup>-</sup>(7a<sup>+</sup>) (5%)</td><td>6.13(5)</td><td>19.55</td></tr><tr><td>6 <sup>1</sup>A''</td><td>7.097</td><td>0.00001</td><td></td><td>3p<sup>-</sup>(27a') (32%) + 3s(25a') (4%)</td><td></td><td></td></tr><tr><td>7 <sup>1</sup>A''</td><td>7.244</td><td>0.00705</td><td>3d<sup>-</sup>(29a') (13%) + 3d<sup>-</sup>(30a') (12%) + 4s(31a') (8%)</td><td></td><td></td><td></td></tr><tr><td>8 <sup>1</sup>A'</td><td>7.390</td><td>0.00000</td><td></td><td>3d<sup>-</sup>(29a') (16%) + 4s(31a') (14%)</td><td></td><td></td></tr><tr><td>5 <sup>1</sup>A'</td><td>7.658</td><td>0.48329</td><td>4π<sup>-</sup>/3p(10a') (31%) + 3π<sup>-</sup>/3p(9a') (5%)</td><td>1π<sup>-</sup>(7a<sup>+</sup>) (4%)</td><td>6.842</td><td>187.48</td></tr><tr><td>6 <sup>1</sup>A'</td><td>7.730</td><td>0.23800</td><td>2π<sup>-</sup>(8a<sup>+</sup>) (26%)</td><td>1π<sup>-</sup>(7a<sup>+</sup>) (11%)</td><td></td><td></td></tr><tr><td>9 <sup>1</sup>A''</td><td>7.852</td><td>0.00045</td><td>4d<sup>-</sup>(30a') (18%) + 4s(31a') (18%)</td><td></td><td></td><td></td></tr><tr><td>7 <sup>1</sup>A'</td><td>7.941</td><td>0.11042</td><td>3π<sup>-</sup>/3p(9a') (11%) + 4π<sup>-</sup>/4p(10a') (6%)</td><td>3p<sup>-</sup>(6a<sup>+</sup>) (11%) + 2π<sup>-</sup>(8a<sup>+</sup>) (7%)</td><td>7.682</td><td>26.72</td></tr></tbody></table>

vibrational modes [18,19]. Chiba and co-workers [20] reported direct observation of π*-σ* hyperconjugation in the S<sub>1</sub> state of isomeric fluorotluenes by double-resonance infrared spectroscopy. Finally, laser-induced fluorescence and zero electron kinetic energy spectroscopy in the S<sub>1</sub> state, allowed the direct observation of vibrational energy dissipation via CH<sub>3</sub> torsion [6,21–26], while Ha et al. [27] reported vibronic assignments of the S<sub>1</sub> → S<sub>0</sub> emission in jet-cooled para-fluorotoluene.

In the present study we report a joint experimental and theoretical methodology to investigate the electronic state spectroscopy of neutral 4-C<sub>7</sub>H<sub>7</sub>F by combining high-resolution vacuum ultraviolet (VUV) photoabsorption with calculations of the vertical excitation energies and oscillator strengths in the gauge length. In the next section we present the computational details used in the calculations, Section 3 provides a brief summary of the structure and properties of 4-fluorotoluene, while Section 4 is devoted to a brief discussion of the experimental details. Section 5 presents the results and discussion and the absolute photoabsorption cross sections used to calculate photolysis rates from 0 to 50 km altitude in the Earth’s atmosphere. Some conclusions that can be drawn from this study are given in Section 6. Additional details can be found in Supplementary Information (SI).

## 2. Computational details

The calculated vertical excitation energies and oscillator strengths for the major electronic transitions assigned in the 4-fluorotoluene absorption spectrum, have been obtained at two different levels of approximation, the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) [28–31] and the time-dependent density functional theory (TD-DFT) [32,33]. Likewise in ortho-fluorotoluene (2-C<sub>7</sub>H<sub>7</sub>F) and meta-fluorotoluene (3-C<sub>7</sub>H<sub>7</sub>F) [1,2], the calculated spectrum of para-fluorotoluene (4-C<sub>7</sub>H<sub>7</sub>F) at the EOM-CCSD/aug-cc-pVDZ level gives a better agreement with the experiment for the lowest-lying excited states up to ~8 eV (Table 1), while TD-DFT/CAMB3LYP/aug-cc-pVDZ level of theory [34] is in concurrence with higher excitation energies (Tables S1 and S2). Additional results for both conformers on the electronic configuration, most active molecular orbitals and vertical excitation energies and oscillator strengths at the CAMB3LYP/aug-cc-pVTZ level of theory are included in SI (Fig. S7, Tables S3 and S4, respectively). A close comparison of such results from CAMB3LYP/aug-cc-pVDZ shows no appreciable differences.

The optimised geometries of 4-fluorotoluene have been calculated at the DFT level of theory, with the CAMB3LYP functional and Dunning’s augmented correlation consistent valence double-ζ (aug-cc-pVDZ) basis set [35], as implemented in the GAMESS-US 2020 computational package

[36], with the only non-default grid options choice NRAD=190 due to the size of the molecule. Two equilibrium conformations of C<sub>s</sub> symmetry, eclipsed and staggered, the latter differing by a 60-degree rotation of the methyl group and 2.5 meV (without zero-point energy correction) above the former (2.9 meV after ZPE correction) are shown in Fig. 1 (see also Figs. S1 and S2 for bond lengths and bond angles). Thus, at room temperature both conformers can contribute to the absorption spectrum. We have also obtained at the TD-DFT/CAMB3LYP/aug-cc-pVDZ level of theory bond lengths and bond angles of the neutral first excited and cationic ground-state geometries of 4-C<sub>7</sub>H<sub>7</sub>F conformers (Figs. S3–S5). Note that only the eclipsed conformer results in a stable geometry in the first excited electronic state.

## 3. Structure and properties of 4-fluorotoluene

In the electronic ground-state, X<sup>-</sup> <sup>1</sup>A', 4-fluorotoluene has C<sub>s</sub> symmetry and the calculated outermost valence electronic configurations at the DFT/aug-cc-pVDZ and RHF-CCSD/aug-cc-pVDZ levels for the eclipsed conformer are: ... (16a')<sup>2</sup> (17a')<sup>2</sup> (18a')<sup>2</sup> (1a'')<sup>2</sup> (19a')<sup>2</sup> (20a')<sup>2</sup> (2a'')<sup>2</sup> (21a')<sup>2</sup> (22a')<sup>2</sup> (23a')<sup>2</sup> (3a'')<sup>2</sup> (24a')<sup>2</sup> (4a'')<sup>2</sup> (5a'')<sup>2</sup> and ... (16a')<sup>2</sup> (17a')<sup>2</sup> (1a'')<sup>2</sup> (18a')<sup>2</sup> (19a')<sup>2</sup> (20a')<sup>2</sup> (21a')<sup>2</sup> (2a'')<sup>2</sup> (22a')<sup>2</sup> (23a')<sup>2</sup> (24a')<sup>2</sup> (3a'')<sup>2</sup> (4a'')<sup>2</sup> (5a'')<sup>2</sup>, respectively, whereas for the staggered conformer are: ... (12a')<sup>2</sup> (5a'')<sup>2</sup> (13a')<sup>2</sup> (14a')<sup>2</sup> (15a')<sup>2</sup> (6a'')<sup>2</sup> (16a')<sup>2</sup> (7a'')<sup>2</sup> (8a'')<sup>2</sup> (17a')<sup>2</sup> (18a')<sup>2</sup> (9a'')<sup>2</sup> (10a')<sup>2</sup> (19a')<sup>2</sup> and ... (13a')<sup>2</sup> (4a'')<sup>2</sup> (14a')<sup>2</sup> (15a')<sup>2</sup> (16a')<sup>2</sup> (5a'')<sup>2</sup> (6a'')<sup>2</sup> (17a')<sup>2</sup> (7a'')<sup>2</sup> (18a')<sup>2</sup> (8a'')<sup>2</sup> (19a')<sup>2</sup> (9a'')<sup>2</sup> (20a')<sup>2</sup>, respectively. The Supplementary Information (SI) contains a comprehensive description of the electronic configurations of both conformers. At the TD-DFT/CAMB3LYP/aug-cc-pVDZ level the highest occupied molecular orbital (HOMO) 5a'' has 3π/n<sub>F</sub> character, while the second highest occupied molecular orbital (HOMO-1) 4a'' has 2π character (Fig. S6). The calculated vertical excitation energies in Tables 1a and 1b needed to help assign the VUV photoabsorption features (Figs. 2 to 7) are mainly due to the promotion of an electron from the HOMO and HOMO-1 to valence, mixed valence-Rydberg and Rydberg character orbitals (see also Tables S1 and S2 for the calculated dominant excitation energies and oscillator strengths at the TD-DFT/CAMB3LYP/aug-cc-pVDZ level of theory).

The photoabsorption spectrum shows extensive fine structures which have been assigned to fundamental vibrational modes (Tables 2 to 6) based on the energies (and wavenumbers) calculated at the DFT/CAMB3LYP/aug-cc-pVDZ level of theory compared with the available experimental data in the literature [13,14,37,15,21–27] (see Tables S5 to S7). As far as notation of such vibrational modes is concerned, we have adopted Gardner et al.’s [6,14,25] assignments.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Calculated structure of the two lowest-energy conformers of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F optimised at the DFT/CAMB3LYP/aug-cc-pVDZ level.</figcaption></figure>

The tentative assignment of the Rydberg series has been performed on the ionisation states energies and the application of quantum defect theory (Table 7). The five lowest-lying ionisation energies, either adiabatic (IE)<sub>ad</sub> or vertical (IE)<sub>v</sub>, used to obtain the quantum defects associated with each transition to a given Rydberg orbital (see Section 5.2), have been obtained experimentally to be (IE<sub>1</sub>)<sub>ad</sub> = 8.796 eV (5a")<sup>-1</sup> [13], (IE<sub>2</sub>)<sub>v</sub> = 9.60 eV (4a")<sup>-1</sup> [10], (IE<sub>3</sub>)<sub>v</sub> = 11.91 eV (24a")<sup>-1</sup> [10], (IE<sub>4</sub>)<sub>v</sub> = 12.68 eV (3a")<sup>-1</sup> [10], and (IE<sub>5</sub>)<sub>v</sub> = 13.18 eV (23a")<sup>-1</sup> [10].

# 4. Experimental details

## 4.1. 4-Fluorotoluene sample

The liquid sample used in the VUV measurements was purchased from Sigma-Aldrich, with a stated purity of 97 % (CAS: 352–32–9). The sample was degassed by repeated freeze–pump–thaw cycles before use in the measurements.

## 4.2. VUV photoabsorption

The high-resolution VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F (Fig. 2) was measured at the AU-UV beam line of the ASTRID2 synchrotron facility, Aarhus University, Denmark. The experimental configuration has been reported before [38,39] thus only a short description will be given here. Briefly, the monochromatized light with a resolution better than 0.08 nm (corresponding to 3 meV at the midpoint of the energy range studied) passes through a gas cell that is filled with 4-fluorotoluene vapour. Light transmitted through the cell passes through a MgF<sub>2</sub> window and is detected using a photomultiplier tube. The MgF<sub>2</sub> window sets the lower limit of detection at 115 nm. The pressure of the 4-fluorotoluene vapour in the gas cell is monitored using a 1 Torr full scale capacitance manometer (Chell model CDG100D). The spectrum was measured in small sections with a pressure appropriate for the absorption cross-section over the pressure range 0.02–1.26 mbar, with typical attenuations of less than 50 %, thus guaranteeing the data collected is free of any saturation effects.

Absolute photoabsorption cross sections (σ in units of Mb ≡ 10<sup>-18</sup> cm<sup>2</sup>) are obtained using the Beer-Lambert attenuation law, I<sub>t</sub> = I<sub>0</sub> exp (-Nσl). Here I<sub>t</sub> is the radiation intensity transmitted through the gas sample, I<sub>0</sub> is that through the evacuated cell, N the molecular number

density of the sample gas, σ the absolute photoabsorption cross section, and *l* the absorption path length (15.5 cm). The light intensity at each wavelength is kept quasi-constant through ASTRID2 operating in a “top-up” mode that compensates for the constant beam decay. To compensate for the slight intensity variations (~2–3 %), the incident flux is normalized to the accurately determined beam current in the storage ring. The accuracy of the cross section is estimated to be better than ± 5 %. Only when absorption by the sample is very weak (I<sub>0</sub> ≈ I<sub>t</sub>), does the error increase as a percentage of the measured cross section.

# 5. Results and discussion

The high-resolution VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F recorded at room temperature, is depicted in Fig. 2 extending from 4.2 to 10.8 eV. In Figs. 3–7 we show detailed sections of the different absorption features, while Tables 2 to 6 include a comprehensive assignment of the different vibronic transitions. The lowest-lying absorption band in the 4.2–5.6 eV photon energy range (Fig. 3) was recorded down to 3.875 eV (320 nm), where discernible structure has been found close to the onset at 4.568 eV. The high-resolution cross-section data associated with the major absorption features are assigned to (π* ← π/n<sub>F</sub>) valence, mixed valence-Rydberg and Rydberg characters. Different Rydberg series converging to the five lowest-lying ionisation energies, together with their quantum defects, are assigned in Table 7.

Vertical excitation energies and oscillator strengths have been obtained at the EOM-CCSD/aug-cc-pVDZ (Tables 1a and 1b) and TD-DFT/CAMB3LYP/aug-cc-pVDZ (Tables S1 and S2) and TD-DFT/CAMB3LYP/aug-cc-pVTZ (Tables S3 and S4) levels of calculation to help interpret the absorption spectrum. Note that the former is limited up to ~ 8 eV where it gives a better agreement with experiment, while the latter is chosen for excitation energies above 8.0 eV and extends up to ~10.9 eV due to computational costs. The calculated transition energy for the lowest-lying excited state at EOM-CCSD level is overestimated by ~0.5 eV relative to the experimental data, whilst at the TD-DFT/CAMB3LYP level it is by ~0.7 eV. A thorough discussion of the different transitions is given in the next sections.

## 5.1. Valence state spectroscopy of 4-fluorotoluene

The electronic state spectroscopy of 4-C<sub>7</sub>H<sub>7</sub>F results from a
<caption>**Table 1b**</caption>

Calculated vertical excitation energies (EOM-CCSD level at the aug-cc-pVDZ basis set) (eV) and oscillator strengths (singlet states) of the main electronic states of staggered 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F compared with experimental data (energies in eV) (details in text).

<table><thead><tr><th>State</th><th>E</th><th>f<sub>L</sub></th><th>HOMO<br>3π/n<sub>F</sub> (20a')</th><th>HOMO-1<br>2π (9a')</th><th>Exp.</th><th>Cross section (Mb)</th></tr></thead><tbody><tr><td>X&#773;<sup>1</sup>A'</td><td>-</td><td>-</td><td></td><td></td><td></td><td></td></tr><tr><td>1<sup>1</sup>A''</td><td>5.164</td><td>0.01098</td><td>3s(21a') (23 %)</td><td></td><td>4.568</td><td>12.52</td></tr><tr><td>2<sup>1</sup>A'</td><td>5.776</td><td>0.00010</td><td>3s(21a') (37 %)</td><td></td><td></td><td></td></tr><tr><td>2<sup>1</sup>A''</td><td>6.302</td><td>0.00009</td><td>3p(10a'') (39 %)</td><td></td><td></td><td></td></tr><tr><td>3<sup>1</sup>A'</td><td>6.402</td><td>0.00488</td><td>3p'(22a') (35 %)</td><td></td><td></td><td></td></tr><tr><td>4<sup>1</sup>A'</td><td>6.807</td><td>0.03898</td><td>3d'(24a') (4 %)</td><td>3p(10a') (33 %)</td><td>5.95(5)</td><td>18.15</td></tr><tr><td>5<sup>1</sup>A'</td><td>6.845</td><td>0.00088</td><td>3p'(23a') (17 %) + 3π*(26a') (12 %)</td><td>3p(10a') (6 %)</td><td></td><td></td></tr><tr><td>3<sup>1</sup>A''</td><td>7.045</td><td>0.03325</td><td>1π*/3d(12a'') (36 %)</td><td></td><td></td><td></td></tr><tr><td>4<sup>1</sup>A''</td><td>7.084</td><td>0.26467</td><td></td><td>3p''(23a') (19 %) + 5π*/4d(28a') (7 %) + 3π*(26a') (5 %)</td><td>6.13(5)</td><td>19.55</td></tr><tr><td>5<sup>1</sup>A''</td><td>7.096</td><td>0.01415</td><td></td><td>3p'(22a') (29 %) + 3s(21a') (4 %)</td><td></td><td></td></tr><tr><td>6<sup>1</sup>A'</td><td>7.115</td><td>0.03641</td><td>3p''(23a') (11 %)</td><td>3d(11a') (17 %) + 4d(14a'') (4 %)</td><td></td><td></td></tr><tr><td>7<sup>1</sup>A'</td><td>7.273</td><td>0.00673</td><td>3π*(26a') (19 %) + 3d'(24a') (13 %)</td><td></td><td></td><td></td></tr><tr><td>6<sup>1</sup>A''</td><td>7.401</td><td>0.00747</td><td></td><td>3d'(24a') (16 %) + 3π*(26a') (18 %)</td><td></td><td></td></tr><tr><td>8<sup>1</sup>A'</td><td>7.660</td><td>0.22756</td><td>3d'(25a') (10 %)</td><td>3d(11a') (27 %)</td><td></td><td></td></tr><tr><td>9<sup>1</sup>A'</td><td>7.666</td><td>0.43431</td><td>3d'(25a') (19 %)</td><td>3d(11a') (15 %)</td><td>6.842</td><td>187.48</td></tr><tr><td>7<sup>1</sup>A''</td><td>7.716</td><td>0.00065</td><td>4d(14a') (27 %) + 2π*(13a'') (5 %)</td><td></td><td></td><td></td></tr></tbody></table>

comprehensive analysis of the different absorption features (shape and relative intensities) with relevant information obtained from the calculated vertical excitation energies and oscillator strengths. A close inspection of Tables 1a and 1b shows both conformers with no appreciable differences in the calculated energies, thus we restrict the discussion for the most stable conformer, i.e., the eclipsed 4-fluorotoluene. The nature of different electronic excitations is also assessed by visual inspection of the natural molecular orbitals for such transitions (see Figs. S6–S8), while any discernible fine structure is tentatively assigned on the information provided by other spectroscopic methods available in the literature. Note that all MOs obtained from the calculations are virtual orbitals.

The 4-C<sub>7</sub>H<sub>7</sub>F molecule has been the subject of different spectroscopic investigations, yet restricted to either the S<sub>1</sub> ← S<sub>0</sub> excitation or S<sub>1</sub> → S<sub>0</sub> laser induced fluorescence within a few wavenumbers (0–1300 cm<sup>-1</sup>) from the 0<sub>0</sub><sup>0</sup> transition [6,13-15,18,19,22–27,37,40].

The results from EOM-CCSD calculations in Table 1a, allow the assignment of the absorption bands peaking at 4.568 eV to valence 1π*(7a'') ← 3π/n<sub>F</sub>(5a'') + 2π*(8a'') ← 2π(4a'') (2<sup>1</sup>A' ← X&#773;<sup>1</sup>A'), at 6.13(5),

6.842 and 7.682 eV to mixed valence-Rydberg {3p''(6a'') + 2π*(8a'') + 1π*(7a'')} ← 2π(4a'') + {1π*(7a'') + 3p''(6a'')} ← 3π/n<sub>F</sub>(5a'') (4<sup>1</sup>A' ← X&#773;<sup>1</sup>A'), 1π*(7a'') ← 2π(4a'') + {4π* /3p(10a'') + 3π* /3p(9a'')} ← π/n<sub>F</sub>(5a'') (5<sup>1</sup>A' ← X&#773;<sup>1</sup>A') and {3p''(6a'') + 2π*(8a'')} ← 2π(4a'') + {3π* /3p(9a'') + 4π* /4p(10a'')} ← 3π/n<sub>F</sub>(5a'') (7<sup>1</sup>A' ← X&#773;<sup>1</sup>A'), respectively, while feature at 5.95(5) eV to Rydberg character (see Section 5.2) and assigned to 3p(26a') ← 2π(4a'') + 3d'(29a') ← 3π/n<sub>F</sub>(5a'') (4<sup>1</sup>A' ← X&#773;<sup>1</sup>A'). For the absorption features peaking at 9.541 and 10.19(6) eV (Fig. 7), the calculations using TD-DFT/CAMB3LYP for the eclipsed conformer (Table S1) assign these features to 9d(36a') + 9d'(37a') ← π(4a'') (34<sup>1</sup>A'' ← X&#773;<sup>1</sup>A') and 3p(26a') ← σ(23a') (25<sup>1</sup>A' ← X&#773;<sup>1</sup>A') transitions, while the same characters are also noted for the staggered conformer calculated at 9.659 and 10.309 eV (see Table S2).

Finally, and as for the other two isomers of 4-C<sub>7</sub>H<sub>7</sub>F, ortho-fluorotoluene (2-C<sub>7</sub>H<sub>7</sub>F) [1] and meta-fluorotoluene (3-C<sub>7</sub>H<sub>7</sub>F) [2], of note are several spectral features that overlap with many different Rydberg electronic states and the superposition of several vibrational modes (see Figs. 4–7 and Tables 3–6). These contribute to relevant broadening and

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** High-resolution VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F in the 4.2–10.8 eV photon energy range.</figcaption></figure>
intensity enhancement of the absorption features which by their nature would not appear as intensely as they do.

### 5.1.1. Valence and vibronic excitation in the range 4.4–5.4 eV

A close comparison of the lowest-lying absorption band $0_0^0$ origin of para-fluorotoluene with its isomers, ortho-fluorotoluene and meta-fluorotoluene, shows that this transition energy is red shifted in the sequence 2-C$_7$H$_7$F (4.647 eV) > 3-C$_7$H$_7$F (4.636 eV) > 4-C$_7$H$_7$F (4.568 eV). Also interesting to note is the maximum absolute cross-section values of the transition, 12.52 Mb for 4-C$_7$H$_7$F (Table 1), 5.52 Mb for 3-C$_7$H$_7$F [2] and 4.22 Mb for 2-C$_7$H$_7$F [1]. The 4-C$_7$H$_7$F origin band is in good agreement with the values 4.570 eV (36,859.9 cm$^{-1}$) [14,22,24] and 4.572 eV (36, 872 cm$^{-1}$) [27] from fluorescence spectroscopy measurements. It has been assigned in Table 1a to the promotion of an electron from the HOMO to LUMO+7 and from the HOMO-1 to the LUMO+9 to $1\pi^+(7a'') \leftarrow 3\pi/n_F(5a'') + 2\pi^+(8a'') \leftarrow 2\pi(4a'') (2^1A' \leftarrow \tilde{X}^1A')$, with an oscillator strength $f_L \approx 0.012$. This electronic transition shows five progressions of the $\nu'_9(a')$ vibrational mode (ring breathing, see Table S3) that are detailed in Table 2 with an average vibrational spacing of 0.099 eV, 0.105 eV in the first excited state (S$_1$) Table S7, and 0.104 eV in the neutral ground-state (S$_0$), Tables S5 and S6. These energy values are in good agreement with 0.100 and 0.103 eV in the first excited and ground-state from Gardner et al. [6,14,25]. Since the current energy resolution does not allow to resolve the internal torsional/rotational contributions mainly from the methyl group, fluorescence spectroscopy experiments on the S$_1 \leftarrow S_0$ transition up to ~ 1300 cm$^{-1}$ internal excitation, report a comprehensive analysis of the internal routes for energy redistribution among the available degrees of freedom, i.e., vibrational and torsional (and rotational) motions [13–15]. Thus, additional assignments are proposed in Table 2 and are mainly due to ring breathing, $\nu'_5(a'_1)$ and $\nu'_11(a'_1)$ and ring bending, $\nu'_{28}(a'_1'')$ modes coupled with the boat deformation $\nu'_{20}(a'_2'')$ and C–F rocking $\nu'_{29}(a'_1'')$ modes. For further details see Ayles et al. [13] and Gardner et al. [14,25] (and references therein) for other assignments in D$_{3h}$ point group.

In the lower energy region of the $0_0^0$ origin (see Fig. 3) shoulder structures are visible and some of them have been assigned to hot-bands (Table 2), yet they can also be part of the unresolved internal rotational contribution of the methyl group.

Likewise, for the other isomers of 4-fluorotoluene, its geometry in the first excited state has been calculated at the TD-DFT/CAMB3LYP/aug-cc-pVDZ level of theory (see Fig. S3 for the eclipsed conformer) with bond lengths in Å and bond angles in (°). The lowest-lying electronic excitation from the ground-state geometry results in CH$_3$ group rotation with the possible role of hyperconjugation [20,22] between the ring $\pi^*$ and the methyl CH $\sigma^*$ antibonding molecular orbitals. According Chiba et al. [20] such intramolecular process results in methyl CH bond strengths being reduced by the partial electron transfer process that can occur from the ring $\pi^*$ to the CH $\sigma^*$ MOs. A close inspection of C7–H13 and C7–H12 bond lengths in the ground-state (1.098 Å and 1.096 Å, Fig. S1) and excited-state (1.102 Å and 1.103 Å, Fig. S3) of 4–C$_7$H$_7$F, show no appreciable changes (< 1 %), which is also observed in the cationic ground-state (1.098 Å and 1.096 Å, Fig. S5). Such evidence has been reported before for the two other isomers, 3–C$_7$H$_7$F [2] and 2–C$_7$H$_7$F [1], while H14 pointing out-of-plane, hence rendering a favourable electron transfer process from the ring $\pi^*$ to the CH $\sigma^*$ molecular orbital. This is certainly possible for a change in geometry as long as the two different potential energy surfaces diabatically cross within the framework of the electronic excited (in the S$_1$ state) 4–C$_7$H$_7$F molecule. Finally, we note the other methyl C–H bonds remain almost within the ring plane, this meaning that hyperconjugation may be spatially prohibited.

### 5.1.2. Valence and vibronic excitation in the range 5.5–10.8 eV

A close comparison between the three isomers' photoabsorption spectra shows identical shapes with increasing red shift for the sequence

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

Proposed vibrational assignments in the 4.2–5.6 eV absorption band of 4-fluorotoluene, 4-C$_7$H$_7$F. $\Delta E$ values in meV.

<table><thead><tr><th>Assignment</th><th>Energy (eV)</th><th>ΔE (ν'<sub>9</sub>)</th><th colspan="3">Assignment</th></tr><tr><th></th><th></th><th></th><th>Ref. [13]</th><th>Ref. [14]</th><th>Ref. [25]</th></tr></thead><tbody><tr><td>9<sup>0</sup><sub>2</sub></td><td>4.36(6)(w)</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>0</sup><sub>1</sub></td><td>4.39(1)(s,w)</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>1</sub></td><td>4.415</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>0</sup><sub>1</sub></td><td>4.437</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>0</sup><sub>1</sub></td><td>4.453</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>1</sub></td><td>4.464</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>0</sup><sub>0</sub></td><td>4.489</td><td>0.0 (–79)</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>0</sub></td><td>4.512</td><td>0.0 (–56)</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>0</sup><sub>0</sub></td><td>4.542</td><td>0.0 (–26)</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>0</sup><sub>0</sub></td><td>4.557</td><td>0.0 (–11)</td><td>–</td><td>–</td><td>–</td></tr><tr><td>0<sup>0</sup><sub>0</sub></td><td><strong>4.568</strong></td><td><strong>0.0</strong></td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>5</sub>9<sup>0</sup><sub>0</sub></td><td>4.584</td><td>0.0 (+16)</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>0</sup><sub>1</sub></td><td>4.591</td><td>102</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>1</sub></td><td>4.617</td><td>105</td><td>11<sup>1</sup></td><td>–</td><td>14<sup>2</sup>,29<sup>1</sup>,11<sup>1</sup></td></tr><tr><td>p<sub>3</sub>9<sup>0</sup><sub>1</sub></td><td>4.640</td><td>98</td><td>–</td><td>28<sup>1</sup></td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>0</sup><sub>1</sub></td><td>4.656</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>1</sub></td><td>4.667</td><td>99</td><td>9<sup>1</sup></td><td>–</td><td>9<sup>1</sup>,29<sup>2</sup></td></tr><tr><td>p<sub>1</sub>9<sup>2</sup><sub>0</sub></td><td>4.690</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>5</sub>9<sup>0</sup><sub>0</sub></td><td>4.694</td><td>110</td><td>–</td><td>–</td><td>9<sup>1</sup>,20<sup>2</sup></td></tr><tr><td>p<sub>2</sub>9<sup>2</sup><sub>0</sub></td><td>4.716</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>2</sup><sub>0</sub></td><td>4.739</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>2</sup><sub>0</sub></td><td>4.754</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>0</sub></td><td>4.766</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>0</sup><sub>0</sub></td><td>4.789</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>5</sub>9<sup>2</sup><sub>0</sub></td><td>4.791</td><td>97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>0</sub></td><td>4.815</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>3</sup><sub>0</sub></td><td>4.836</td><td>97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>4</sup><sub>0</sub></td><td>4.85(5)(s)</td><td>101</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>3</sup><sub>0</sub></td><td>4.863</td><td>97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>4</sup><sub>0</sub></td><td>4.887</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>5</sub>9<sup>3</sup><sub>0</sub></td><td>4.891</td><td>100</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>0</sub></td><td>4.913</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>4</sup><sub>0</sub></td><td>4.939</td><td>103</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>4</sup><sub>0</sub></td><td>4.95(7)(s)</td><td>102</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>1</sub></td><td>4.961</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>5</sup><sub>0</sub></td><td>4.984</td><td>101</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>5</sub>9<sup>0</sup><sub>0</sub></td><td>4.988</td><td>97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>5</sup><sub>0</sub></td><td>5.011</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>0</sup><sub>0</sub></td><td>5.038</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>5</sup><sub>0</sub></td><td>5.04(9)(s)</td><td>92</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>0</sub></td><td>5.060</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>0</sup><sub>0</sub></td><td>5.087</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>5</sub>9<sup>5</sup><sub>0</sub></td><td>5.088</td><td>100</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>0</sub></td><td>5.110</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>6</sup><sub>0</sub></td><td>5.136</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>4</sub>9<sup>0</sup><sub>0</sub></td><td>5.15(1)(s)</td><td>102</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>0</sub></td><td>5.157</td><td>97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>7</sup><sub>0</sub>/p<sub>5</sub>9<sup>6</sup><sub>0</sub></td><td>5.18(5)(w,b)</td><td>98/97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>7</sup><sub>0</sub></td><td>5.206</td><td>96</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>0</sup><sub>0</sub></td><td>5.23(4)(w)</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>0</sub></td><td>5.25(6)(w)</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>8</sup><sub>0</sub>/p<sub>5</sub>9<sup>7</sup><sub>0</sub></td><td>5.27(9)(w,b)</td><td>94</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>0</sub></td><td>5.304</td><td>98</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>3</sub>9<sup>8</sup><sub>0</sub></td><td>5.33(0)(w)</td><td>96</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>0</sub></td><td>5.35(0)(w)</td><td>94</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>1</sub>9<sup>0</sup><sub>0</sub></td><td>5.37(4)(w,b)</td><td>95</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>0</sup><sub>0</sub></td><td>5.39(9)(w)</td><td>95</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>0</sub></td><td>5.44(9)(w)</td><td>99</td><td>–</td><td>–</td><td>–</td></tr><tr><td>p<sub>2</sub>9<sup>10</sup><sub>0</sub></td><td>5.49(6)(w)</td><td>97</td><td>–</td><td>–</td><td>–</td></tr><tr><td>9<sup>0</sup><sub>10</sub></td><td>5.54(9)(w)</td><td>100</td><td>–</td><td>–</td><td>–</td></tr><tr><td>ΔE</td><td></td><td>99</td><td>–</td><td>–</td><td>–</td></tr></tbody></table>

(w) weak structure; (s) shoulder structure; (b) broad structure; (?) unassigned feature (the last decimal of the energy value is given in brackets for these less-resolved features).
<figure><img src="image_5.png" /><figcaption>Fig. 3. The VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F in the 4.2–5.6 eV photon energy range. See text for details on the assignments.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F in the 5.6–6.6 eV photon energy range.</figcaption></figure>

2–C<sub>7</sub>H<sub>7</sub>F < 3–C<sub>7</sub>H<sub>7</sub>F < 4–C<sub>7</sub>H<sub>7</sub>F. We note a general enhancement of fine structure and the number of features from 3–C<sub>7</sub>H<sub>7</sub>F to 2–C<sub>7</sub>H<sub>7</sub>F to 4–C<sub>7</sub>H<sub>7</sub>F, yet with no relevant isomer effect. Also noteworthy is the prevalent π–π* transition at ~ 6.8 eV with increasing magnitude 4–C<sub>7</sub>H<sub>7</sub>F < 3–C<sub>7</sub>H<sub>7</sub>F < 2–C<sub>7</sub>H<sub>7</sub>F, while for the lowest-lying absorption band centred at ~ 4.6 eV the magnitude increases for the sequence 2–C<sub>7</sub>H<sub>7</sub>F < 3–C<sub>7</sub>H<sub>7</sub>F < 4–C<sub>7</sub>H<sub>7</sub>F.

The VUV spectrum reveals a second absorption band that is composed of two electronic transitions centred at 5.95(5) eV and 6.13(5) eV with cross-sections of 18.15 and 19.55 Mb and assigned mainly to Rydberg and mixed valence-Rydberg 3p(26a') ← 2π(4a'') + 3d'(29a') ← 3π/n<sub>F</sub>(5a'')⟨4¹A' ← ˜X¹A'⟩ and {3p''(6a'') + 2π' (8a'') + 1π' (7a'')} ← 2π(4a'') + {1π' (7a'') + 3p''(6a'')} ← 3π/n<sub>F</sub>(5a'')⟨4¹A' ← ˜X¹A'⟩ transitions with f<sub>L</sub> ≈ 0.0323 and f<sub>L</sub> ≈ 0.2009, respectively (Table 1a). However, at the TD-DFT/CAMB3LYP/aug-cc-pvDZ level of calculation (Tables S1 and S2),

the absorption band is solely due to a valence π*(7a'') ← π/n<sub>F</sub>(5a'') + π*(6a'') ← π(4a'')⟨3¹A' ← ˜X¹A'⟩ transition with an oscillator strength f<sub>L</sub> = 0.0147. The 0<sup>0</sup><sub>0</sub> origin is assigned at 5.84(6) eV (Table 3) and a close inspection of the absorption band in Fig. 4 reveals a rather broad nature of the features which is reminiscent of a valence-like character. The main vibrational modes are due to a combination of a few quanta of ring breathing, ν'<sub>9</sub>(a') and ν'<sub>11</sub>(a') modes (Table 3), with mean energy values of 0.102 and 0.057 eV (Table 3), with contributions from the lowest-lying 3s and 3p Rydberg members converging to (4a'')<sup>-1</sup> and (5a'')<sup>-1</sup>. For further details see Section 5.2.

The prevalent absorption band in the VUV spectrum of 4-fluorotoluene with a maximum at 6.842 eV and a cross-section of 187.48 Mb (Fig. 2) is assigned to a 1π*(7a'') ← 2π(4a'') + {4π*/3p(10a'') + 3π*/3p(9a'')} ← π/n<sub>F</sub>(5a'')⟨5¹A' ← ˜X¹A'⟩ transition from the EOM-CCSD/
<figure><img src="image_7.png" /><figcaption>Fig. 5. VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F in the 6.4–7.6 eV photon energy range.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F in the 6.4–8.9 eV photon energy range.</figcaption></figure>

aug-cc-pvDZ level of theory (Table 1a) with $f_L \approx 0.4833$. The TD-DFT/CAMB3LYP/aug-cc-pvDZ calculation in Table S1 shows the same character for the electronic transition. The $0_0^0$ origin band is tentatively assigned at 6.57(6) eV and shows several quanta of the ring breathing, $\nu'_9(a')$ mode, with mean energy value of 0.103 eV (Table 4), in combination with CH<sub>3</sub> rocking, $\nu'_{30}(a'')$, ring breathing, $\nu'_{11}(a')$ and C-CH<sub>3</sub> stretching, $\nu'_{10}(a')$ modes. At higher energy, the next mixed valence-Rydberg transition $\{3p''(6a'') + 2\pi^*(8a'')\} \leftarrow 2\pi(4a'') + \{3\pi^*/3p(9a'') + 4\pi^*/4p(10a'')\} \leftarrow 3\pi/n_F(5a'')(7\uparrow A' \leftarrow \tilde{X}^1A')$ peaking at 7.682 eV ($f_L \approx 0.1104$), with its $0_0^0$ origin band at 7.62(5) eV, is accompanied by two quanta of ring breathing, $\nu'_9(a')$ and $\nu'_{11}(a')$ modes, with mean energy values of 0.109 and 0.054 eV (Table 5).

The next two absorption bands with $0_0^0$ origins at 9.42(8) and 9.95(5) eV have been assigned to $9d(36a') + 9d'(37a') \leftarrow \pi(4a'')(34^1A'' \leftarrow \tilde{X}^1A')$ and $3p(26a') \leftarrow \sigma(23a')(25^1A' \leftarrow \tilde{X}^1A')$ Rydberg transitions (see Section

5.2), with calculated oscillator strengths $f_L = 0.0817$ and $f_L = 0.0757$ (Table S1). Note for the latter, the tentative assignment is not in agreement with the theoretical calculation character of the transition (Table 6). A band maximum at 10.525 eV is assigned to $ns/np \leftarrow \pi/n_F(5a'')(50^1A'' \leftarrow \tilde{X}^1A')$ Rydberg transition (Table S1); we are not able to unambiguously assign the members of such Rydberg series from the calculations, while the experimental feature in such band can be due to $4s(24a')^{-1}$ and $3p'(3a'')^{-1}$ together with fine structure from the ring breathing, $\nu'_9(a')$ and $\nu'_{11}(a')$ modes (Table 6). Features at 10.670 and 10.721 eV (Table 6) remain unassigned but can be part of the fine structure related to the ring breathing mode $\nu'_{11}(a_1')$.

Finally, as the photon energy increases, the features' relative intensity tend to decrease, while we note a relevant overlap of the different vibronic transitions (mixed valence-Rydberg and Rydberg in character), making the assignments more difficult to perform unambiguously.
<figure><img src="image_9.png" /><figcaption>Fig. 7. VUV photoabsorption spectrum of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F in the 8.8–10.8 eV photon energy range.</figcaption></figure>

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

Proposed vibrational assignments in the 5.6–6.6 eV absorption band of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F.

<table><thead><tr><th>Energy (eV)</th><th>Assignment</th><th>ΔE (ν<sub>9</sub>)</th><th>ΔE (ν<sub>11</sub>)</th></tr></thead><tbody><tr><td>5.84(6)(b)</td><td>0<sup>0</sup><sub>0</sub></td><td>–</td><td>–</td></tr><tr><td>5.89(6)(s,w)</td><td>11<sup>1</sup><sub>0</sub></td><td>–</td><td>0.050</td></tr><tr><td>5.95(5)(s,w)</td><td>9<sup>1</sup><sub>0</sub></td><td>0.109</td><td>–</td></tr><tr><td>5.99(8)(b)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub>/3p(5a<sup>+</sup>)<sup>-1</sup></td><td>0.102</td><td>–</td></tr><tr><td>6.06(1)(b)</td><td>9<sup>2</sup><sub>0</sub></td><td>0.106</td><td>–</td></tr><tr><td>6.10(0)(b,w)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub>/3p(5a<sup>+</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>0.102</td><td>–</td></tr><tr><td>6.13(5)(b)</td><td>0<sup>0</sup><sub>0</sub></td><td>–</td><td>–</td></tr><tr><td>6.19(8)(b)</td><td>11<sup>1</sup><sub>0</sub>/3s(4a<sup>+</sup>)<sup>-1</sup></td><td>–</td><td>0.063</td></tr><tr><td>6.24(1)(b,w)</td><td>9<sup>1</sup><sub>0</sub></td><td>0.091</td><td>–</td></tr><tr><td>6.28(9)(s)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub>/3s(4a<sup>+</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>0.094</td><td>–</td></tr><tr><td>6.34(8)(s)</td><td>9<sup>2</sup><sub>0</sub></td><td>0.107</td><td>–</td></tr><tr><td></td><td>ΔE</td><td>0.102</td><td>0.057</td></tr></tbody></table>

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

## 5.2. Rydberg transitions

The photoabsorption spectrum displays features above ~ 6.0 eV (Figs. 4–7) which have been assigned to members of different Rydberg series converging to the lowest-lying (5a")<sup>-1</sup> X̃<sup>2</sup>A", (4a")<sup>-1</sup> Ã<sup>2</sup>A", (24a")<sup>-1</sup> B̃<sup>2</sup>A', (23a")<sup>-1</sup> C̃<sup>2</sup>A' and (3a")<sup>-1</sup> D̃<sup>2</sup>A" ionic states of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F (Table 7). The experimental adiabatic and/or vertical ionisation energies needed to estimate the different Rydberg series have been obtained from Ayles et al. [13] and Palmer and co-workers [10] (see Section 3), with most of the proposed Rydberg structures labelled in Figs. 4–7 and listed in Table 7.

The peak positions have been obtained from the Rydberg formula: E<sub>n</sub> = E<sub>i</sub> - R / (n - δ)<sup>2</sup>, where E<sub>i</sub> 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 the quantum defect resulting from the penetration of the Rydberg orbital into the core. A thorough literature survey shows no previous Rydberg assignments of 4-fluorotoluene to compare with the present data. The Rydberg features appear in the major absorption regions as broad peaks (and in some cases too intense) due to contribution of other overlapping Rydberg members and vibrational structure (Tables 5 and 6), a common behaviour which was also found in the other isomers, ortho-fluorotoluene (2-C<sub>7</sub>H<sub>7</sub>F) [1] and

meta-fluorotoluene (3-C<sub>7</sub>H<sub>7</sub>F) [2].

The first member of the ns ← 5a" Rydberg transition converging to the ionic electronic ground-state (5a")<sup>-1</sup> X̃<sup>2</sup>A" is barely visible from the absorption spectrum at 5.55(5) eV, with a quantum defect δ = 0.95 (Table 7). Higher members up to n = 12 are listed in Table 7 while those at 8.409 eV (n = 7 s), 8.512 eV (n = 8 s), 8.568 eV (n = 9 s), 8.622 eV (n = 10 s), and 8.66(1) eV (n = 11 s) have also been assigned to 6d'(5a")<sup>-1</sup>, 7d'(5a")<sup>-1</sup>/3s(24a")<sup>-1</sup>, 8d(5a")<sup>-1</sup>, 9d'(5a")<sup>-1</sup> and 4d(4a")<sup>-1</sup> Rydberg members, respectively. The n = 3 members of the np and np' series are associated with the peaks at 5.99(8) and 6.73(3) eV with δ = 0.79 and δ = 0.43 and extend up to n = 7. The feature at 8.31(0) (n = 6p) is also assigned to 4p(4a")<sup>-1</sup> (Table 7). The lowest-lying members of the nd series are at 6.94(6) and 7.84(0) eV, with quantum defects of 0.29 and 0.23, the former also assigned to 3p(4a")<sup>-1</sup>, the latter to 3d(4a")<sup>-1</sup>, whereas n = 3d' series is related to the peak at 7.14(8) eV (δ = 0.13). The vibronic excitations related to the Rydberg series converging to the ionic electronic ground-state have been assigned in Table 5 to the ring breathing ν<sub>9</sub>(a') and ν<sub>11</sub>(a') modes, with average values of 0.109 and 0.054 eV (0.105 and 0.056 eV in the ground-state).

Further to a similar methodology adopted in 2-C<sub>7</sub>H<sub>7</sub>F [1] and 3-C<sub>7</sub>H<sub>7</sub>F [2], we have obtained the cationic ground state geometry of eclipsed 4-C<sub>7</sub>H<sub>7</sub>F at the DFT/CAMB3LYP/aug-cc-pVDZ level of theory,
<table><caption>**Table 4**
Proposed vibrational assignments in the 6.4–7.6 eV absorption band of 4-fluorotoluene, 4-C<sub>7</sub>H<sub>7</sub>F.</caption><thead><tr><th>Energy (eV)</th><th>Assignment</th><th>ΔE (ν'<sub>30</sub>)</th><th>ΔE (ν'<sub>11</sub>)</th><th>ΔE (ν'<sub>10</sub>)</th><th>ΔE (ν'<sub>9</sub>)</th></tr></thead><tbody><tr><td>6.57(6)(s,w)</td><td>0<sup>0</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>6.61(1)(s,w)</td><td>30<sup>1</sup><sub>0</sub></td><td>0.035</td><td>—</td><td>—</td><td>—</td></tr><tr><td>6.63(2)(s,w)</td><td>11<sup>1</sup><sub>0</sub></td><td>—</td><td>0.056</td><td>—</td><td>—</td></tr><tr><td>6.65(7)(s)</td><td>10<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.081</td><td>—</td></tr><tr><td>6.68(0)(s,w)</td><td>9<sup>0</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.104</td></tr><tr><td>6.71(5)(b)</td><td>30<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.104</td></tr><tr><td>6.75(8)(b)</td><td>10<sup>1</sup><sub>0</sub> + 9<sup>0</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.101</td></tr><tr><td>6.73(3)(b)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub>/3p'(5a')<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>0.101</td></tr><tr><td>6.79(2)(s)</td><td>9<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.112</td></tr><tr><td>6.80(9)(b,w)</td><td>30<sup>1</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.094</td></tr><tr><td>6.839</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.106</td></tr><tr><td>6.86(3)(s,w)</td><td>10<sup>1</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.105</td></tr><tr><td>6.89(2)(s)</td><td>9<sup>3</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.100</td></tr><tr><td>6.94(6)(s)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>3</sup><sub>0</sub>/3d'(5a')<sup>-1</sup>/3p'(4a')<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>0.107</td></tr><tr><td>6.99(7)(b)</td><td>9<sup>4</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.105</td></tr><tr><td>7.04(5)(s)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>4</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.099</td></tr><tr><td>7.09(7)(s)</td><td>9<sup>5</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.100</td></tr><tr><td>7.14(8)(w)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>5</sup><sub>0</sub>/3d'(5a')<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>0.103</td></tr><tr><td>7.19(8)(s)</td><td>9<sup>6</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.101</td></tr><tr><td>7.24(6)(s,w)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>6</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.098</td></tr><tr><td>7.30(4)(s,w)</td><td>9<sup>7</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.106</td></tr><tr><td>ΔE</td><td></td><td>0.035</td><td>0.056</td><td>0.081</td><td>0.103</td></tr></tbody></table>

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

with bond lengths in Å and bond angles in (°) listed in Fig. S5. A careful inspection of Figs. S1 and S5 shows that the ring breathing $ν'_9(a')$ mode, is modestly reduced by 5% from its value in the electronic ground-state (Tables S5 and S7), which agrees with the increase of C4–C5 bond length, just to give an example. In the cation, the bond angle $∠C1–C2–C3$ is 120.88° whereas in the neutral ground-state is 121.45°, which may provide a weakening of the C–C bond strength. This agrees with the proposed assignments in Tables 3 and 4 for the Rydberg members converging to the ionic electronic ground-state (5a'')⁻¹ $\tilde{X}^2A''$.

The first members of the *ns*, *np*, *np'*, *nd* and *nd'* Rydberg transitions converging to the ionic electronic first excited state (4a'')⁻¹ $\tilde{A}^2A''$ are associated with the peaks at 6.19(8) eV ($δ = 1.00$), 6.94(6) eV ($δ = 0.74$), 7.45(5) eV ($δ = 0.48$), 7.84(0) eV ($δ = 0.22$) and 8.007 eV ($δ = 0.08$), respectively (Table 7). The features at 9.204 and 9.291 eV can also be assigned to 6d(4a'')⁻¹ and 3s(3a'')⁻¹. The vibrational structure discernible in the Rydberg transitions (Figs. 6 and 7) can be due to excitation of CH rocking $ν'_7(a')$ and ring breathing $ν'_9(a')$ and $ν'_{11}(a')$ modes (Tables 5 and 6).

The Rydberg series converging to the ionic electronic second excited state (24a'')⁻¹ $\tilde{B}^2A'$, *n* = 3 for *ns*, *np*, *np'*, *nd* and *nd'* are anticipated at 8.512, 9.15(4), 9.73(6), 10.12(9) and 10.31(5) eV, respectively, (Table 7) with quantum defects 1.00, 0.78, 0.50, 0.24 and 0.08, respectively. The fine structure has been assigned to vibrational excitation involving CH rocking $ν'_7(a')$ mode (see Table 6).

The *n* = 3 members of the *ns*, *np* and *np'* Rydberg transitions converging to the ionic electronic third excited state (23a'')⁻¹ $\tilde{C}^2A'$ have been assigned at 9.291 eV ($δ = 1.00$), 10.00(3) eV ($δ = 0.75$) and 10.58 (3) eV ($δ = 0.45$), respectively (Table 7). Finally, the feature at 9.74(3) eV is assigned to 3s(23a'')⁻¹ ($δ = 1.01$) member of the Rydberg series converging to the ionic electronic fourth excited state (Table 7). No further assignments have been performed to higher members of the Rydberg series.

### 5.3. Absolute photoabsorption cross sections and atmospheric photolysis

We list in Table 1 the major electronic transitions and their related absolute photoabsorption cross-section values in units of Mb. As far as

authors are aware, there are no previous absolute absorption studies to compare with the present data in the photon energy region 4.3–10.8 eV. The absolute photoabsorption cross-section values for energies < 6.9 eV (> 180 nm) can be used in combination with solar actinic flux measurements [41] to obtain the photolysis rate of a given molecular compound in the Earth’s atmosphere from 0 up to 50 km altitude. Briefly, the solar actinic flux as a function of altitude (z) and wavelength (λ), $F(z, λ)$, lists fluxes measured from ground-level up to the limit of the stratopause (50 km) in 10 km steps [41]. The photolysis rate $J(z, λ)$ is given by the product of $F(z, λ)$, the absolute photoabsorption cross-section, $σ(λ)$, and the quantum yield for dissociation, $φ(λ)$. Due to a lack of any information in the literature, and as the $π^*$ MO in these transitions has C=C anti-bonding character, we shall assume that $φ(λ)$ following absorption is one. The reciprocal of the photolysis rate at a given altitude yields the local photolysis lifetime. For a comprehensive description see Ref. [42] and references therein.

Photolysis lifetimes of less than 4 sunlit days were calculated at altitudes above 20 km with less than a day above 24 km, thus indicating that 4-fluorotoluene molecules can be broken up quite efficiently by UV absorption at these altitudes. However, we are aware of a single work on the reactions of 4-fluorotoluene with OH• radical [43], with a rate coefficient of ≈ (9.9 ± 1.7) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, which may provide one of the main reactive sink mechanisms in the Earth’s atmosphere.

## 6. Conclusions

The present work provides a first complete joint experimental and theoretical investigation on the electronic state spectroscopy of 4-fluorotoluene in the photon energy range from 4.3 to 10.8 eV. The prevalent absorption features have been assigned to electronic excitations from the ground-state to valence, mixed valence-Rydberg and Rydberg states, with the aid of theoretical calculations on the vertical excitation energies and oscillator strengths at the equation-of-motion coupled cluster method restricted to the single and double excitations (EOM-CCSD) and time-dependent density functional theory (TD-DFT). The discernible fine structure has been mainly assigned to ring breathing, $ν'_9(a')$ and $ν'_{11}(a')$, in combination with CH₃ rocking, $ν'_{30}(a'')$, C-CH₃ stretching, $ν'_{10}(a')$ and CH rocking $ν'_7(a')$ modes. The photolysis lifetimes in the
<table><caption>**Table 5**
Proposed vibrational assignments in the 7.4–9.1 eV absorption band of 4-fluoro-toluene, 4-C<sub>7</sub>H<sub>7</sub>F.</caption><thead><tr><th>Energy (eV)</th><th>Assignment</th><th>ΔE (ν'<sub>11</sub>)</th><th>ΔE (ν'<sub>9</sub>)</th><th>ΔE (ν'<sub>7</sub>)</th></tr></thead><tbody><tr><td>7.62(5) (s,w)</td><td>0<sup>0</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td></tr><tr><td>7.682</td><td>11<sup>1</sup><sub>0</sub></td><td>0.057</td><td>—</td><td>—</td></tr><tr><td>7.737</td><td>11<sup>2</sup><sub>0</sub></td><td>0.055</td><td>—</td><td>—</td></tr><tr><td>7.79(0) (s)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.108</td><td>—</td></tr><tr><td>7.900</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub>/4d'(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.110</td><td>—</td></tr><tr><td>7.737</td><td>4p'(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>7.84(0) (s)</td><td>4d'(5a<sup>"</sup>)<sup>-1</sup>/9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.103</td><td>—</td></tr><tr><td>7.953</td><td>5s(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.007</td><td>5s(5a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub></td><td>0.054</td><td>—</td><td>—</td></tr><tr><td>8.059</td><td>5s(5a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub>/5s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/5p(5a<sup>"</sup>)<sup>-1</sup></td><td>0.052</td><td>0.106</td><td>—</td></tr><tr><td>8.16(8) (s)</td><td>5s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/5p(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/5d(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.109</td><td>—</td></tr><tr><td>8.290</td><td>5s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub> + 7<sup>1</sup><sub>0</sub>/5p(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub> + 7<sup>1</sup><sub>0</sub>/5d(5a<sup>"</sup>)<sup>-1</sup> + 7<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.122</td></tr><tr><td>8.007</td><td>3d'(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.059</td><td>3d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub></td><td>0.052</td><td>—</td><td>—</td></tr><tr><td>8.109</td><td>3d'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.102</td><td>—</td></tr><tr><td>8.235</td><td>3d'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub> + 7<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.126</td></tr><tr><td>8.241</td><td>6s(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.290</td><td>6s(5a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub></td><td>0.049</td><td>—</td><td>—</td></tr><tr><td>8.355</td><td>6s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.114</td><td>—</td></tr><tr><td>8.346</td><td>6s(5a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub>/5p'(5a<sup>"</sup>)<sup>-1</sup></td><td>0.056</td><td>—</td><td>—</td></tr><tr><td>8.457</td><td>5p'(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/7p(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.111</td><td>—</td></tr><tr><td>8.568</td><td>5p'(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/7p(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/9s(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.111</td><td>—</td></tr><tr><td>8.409</td><td>7s(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.457</td><td>7s(5a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub></td><td>0.048</td><td>—</td><td>—</td></tr><tr><td>8.512</td><td>7s(5a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub>/7s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/8s(5a<sup>"</sup>)<sup>-1</sup>/7d'(5a<sup>"</sup>)<sup>-1</sup>/3s(24a<sup>"</sup>)<sup>-1</sup></td><td>0.055</td><td>0.103</td><td>—</td></tr><tr><td>8.622</td><td>7s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/8s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/7d'(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/3s(24a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/10s(5a<sup>"</sup>)<sup>-1</sup>/9d'(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.110</td><td>—</td></tr><tr><td>8.734</td><td>7s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>3</sup><sub>0</sub>/8s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/7d'(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/3s(24a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/10s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/9d'(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.112</td><td>—</td></tr><tr><td>8.66(1) (s)</td><td>4d(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.716</td><td>4d(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub></td><td>0.055</td><td>—</td><td>—</td></tr><tr><td>8.771</td><td>4d(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub>/4d(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/5s(4a<sup>"</sup>)<sup>-1</sup></td><td>0.055</td><td>0.110</td><td>—</td></tr><tr><td>8.885</td><td>4d(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/5s(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.114</td><td>—</td></tr><tr><td>8.682</td><td>10s(5a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.784</td><td>10s(5a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.102</td><td>—</td></tr><tr><td>8.716</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td></tr><tr><td>8.771</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub></td><td>0.055</td><td>—</td><td>—</td></tr><tr><td>8.828</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub>/4d'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>0.057</td><td>0.112</td><td>—</td></tr><tr><td>8.881</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>0.110</td><td>—</td></tr><tr><td>8.936</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub>/4d'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>2</sup><sub>0</sub>/5p'(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.108</td><td>—</td></tr><tr><td>9.01(0) (w)</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub> + 7<sup>1</sup><sub>0</sub>/5d(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>0.129</td></tr><tr><td>9.043</td><td>4d'(4a<sup>"</sup>)<sup>-1</sup> + 11<sup>2</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub>/4d'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>3</sup><sub>0</sub>/5p'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/6s(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>0.107</td><td>—</td></tr><tr><td></td><td>ΔE</td><td>0.054</td><td>0.109</td><td>0.126</td></tr></tbody></table>

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

<table><caption>**Table 6**
Proposed vibrational assignments in the 8.8–10.8 eV absorption band of 4-fluoro-toluene, 4-C<sub>7</sub>H<sub>7</sub>F.</caption><thead><tr><th>Energy (eV)</th><th>Assignment</th><th>ΔE (ν'<sub>11</sub>)</th><th>ΔE (ν'<sub>10</sub>)</th><th>ΔE (ν'<sub>9</sub>)</th><th>ΔE (ν'<sub>7</sub>)</th></tr></thead><tbody><tr><td>9.096</td><td>6p(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9.204</td><td>6p(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.108</td><td>—</td></tr><tr><td>9.15(4)(s)</td><td>6p'(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9.253</td><td>6p'(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/7p(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>0.099</td><td>—</td></tr><tr><td>9.291</td><td>3s(3a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9.400</td><td>3s(3a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub>/9p(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>0.109</td><td>—</td></tr><tr><td>9.340</td><td>8p(4a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9.45(4)(s, w)</td><td>8p(4a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.114</td><td>—</td></tr><tr><td>9d(36a<sup>"</sup>) + 9d'(37a<sup>"</sup>) ← π(4a<sup>"</sup>) (34<sup>1</sup>A<sup>"</sup> ← X̃<sup>1</sup>A<sup>"</sup>)</td><td>9d(36a<sup>"</sup>) + 9d'(37a<sup>"</sup>) ← π(4a<sup>"</sup>) (34<sup>1</sup>A<sup>"</sup> ← X̃<sup>1</sup>A<sup>"</sup>)</td><td>9d(36a<sup>"</sup>) + 9d'(37a<sup>"</sup>)</td><td>9d(36a<sup>"</sup>)</td><td>9d'(37a<sup>"</sup>)</td><td>9d(36a<sup>"</sup>)</td></tr><tr><td>9.42(8)(s)</td><td>0<sup>0</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>9.500</td><td>10<sup>1</sup><sub>0</sub></td><td>—</td><td>0.072</td><td>—</td><td>—</td></tr><tr><td>9.541</td><td>9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.113</td><td>—</td></tr><tr><td>9.66(0)(s, w)</td><td>9<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.119</td><td>—</td></tr><tr><td>9.593</td><td>10<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.093</td><td>—</td></tr><tr><td>3p(26a<sup>"</sup>) ← σ(23a<sup>"</sup>) (25<sup>1</sup>A<sup>"</sup> ← X̃<sup>1</sup>A<sup>"</sup>)</td><td>3p(26a<sup>"</sup>) ← σ(23a<sup>"</sup>) (25<sup>1</sup>A<sup>"</sup> ← X̃<sup>1</sup>A<sup>"</sup>)</td><td>3p(26a<sup>"</sup>)</td><td>3p(26a<sup>"</sup>)</td><td>3p(26a<sup>"</sup>)</td><td>3p(26a<sup>"</sup>)</td></tr><tr><td>9.95(5) (b,w)</td><td>0<sup>0</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>10.07(6) (s,w)</td><td>7<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.121</td></tr><tr><td>10.19(6) (b,w)</td><td>7<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>—</td><td>0.120</td></tr><tr><td>10.31(5) (b,w)</td><td>7<sup>3</sup><sub>0</sub>/3d(24a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>0.119</td></tr><tr><td>10.00(3) (w)</td><td>3p(3a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>10.12(9) (b,w)</td><td>7<sup>1</sup><sub>0</sub>/3d(24a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>0.126</td></tr><tr><td>10.25(9) (b,w)</td><td>7<sup>2</sup><sub>0</sub>/3d(24a<sup>"</sup>)<sup>-1</sup> + 7<sup>3</sup><sub>0</sub>/3d(24a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>0.130</td></tr><tr><td>10.37(1) (b,w)</td><td>7<sup>2</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub>/3d(24a<sup>"</sup>)<sup>-1</sup> + 7<sup>1</sup><sub>0</sub> + 9<sup>2</sup><sub>0</sub>/3d(24a<sup>"</sup>)<sup>-1</sup> + 9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.112</td><td>—</td></tr><tr><td>ns/np ← π/π<sub>F</sub>(5a<sup>"</sup>) (50<sup>1</sup>A<sup>"</sup> ← X̃<sup>1</sup>A<sup>"</sup>)</td><td>ns/np ← π/π<sub>F</sub>(5a<sup>"</sup>) (50<sup>1</sup>A<sup>"</sup> ← X̃<sup>1</sup>A<sup>"</sup>)</td><td>ns/np</td><td>π/π<sub>F</sub>(5a<sup>"</sup>)</td><td>π/π<sub>F</sub>(5a<sup>"</sup>)</td><td>π/π<sub>F</sub>(5a<sup>"</sup>)</td></tr><tr><td>10.410</td><td>4s(24a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>10.467</td><td>11<sup>1</sup><sub>0</sub></td><td>0.057</td><td>—</td><td>—</td><td>—</td></tr><tr><td>10.525</td><td>9<sup>1</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.115</td><td>—</td></tr><tr><td>10.58(3) (b)</td><td>11<sup>1</sup><sub>0</sub> + 9<sup>1</sup><sub>0</sub>/3p'(3a<sup>"</sup>)<sup>-1</sup></td><td>—</td><td>—</td><td>0.116</td><td>—</td></tr><tr><td>10.62(6) (w)</td><td>9<sup>2</sup><sub>0</sub></td><td>—</td><td>—</td><td>0.104</td><td>—</td></tr><tr><td>10.670</td><td>(?)</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>10.721</td><td>(?)</td><td>0.051</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ΔE</td><td></td><td>0.054</td><td>0.072</td><td>0.109</td><td>0.123</td></tr></tbody></table>

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

Earth's atmosphere were also obtained from ground-level up to the limit of the stratopause indicating that solar photolysis is expected to be a relevant sink at altitudes higher than 24 km.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at XXX

CRediT authorship contribution statement

L.V.S. Dalagnol: Data curation, Investigation, Methodology, Software. S. Kumar: Investigation. A.I. Lozano: Investigation. M.H.F. Bettega: Data curation, Investigation, Methodology, Software, Writing –
<caption>**Table 7**</caption>

Energy values (eV), quantum defects (δ) and assignments of the Rydberg series converging to (5a')⁻¹ X²A', (4a')⁻¹ ˜A²A', (24a')⁻¹ ˜B²A', (23a')⁻¹ ˜C²A' and (3a')⁻¹ ˜D²A' ionic states of 4-fluorotoluene, 4-C₇H₇F.

<table><thead><tr><th>E<sub>n</sub></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub></th><th>δ</th><th>Assignment</th><th>E<sub>n</sub></th><th>δ</th><th>Assignment</th></tr></thead><tbody><tr><td>(IE<sub>1</sub>)<sub>ad</sub> = 8.796 eV (5a')<sup>-1</sup><br>(ns &larr; 5a')</td><td></td><td></td><td>(IE<sub>1</sub>)<sub>ad</sub> = 8.796 eV (5a')<sup>-1</sup><br>(nd' &larr; 5a')</td><td></td><td></td><td>(IE<sub>2</sub>)<sub>v</sub> = 9.60 eV (4a')<sup>-1</sup><br>(nd' &larr; 4a')</td><td></td><td></td><td></td></tr><tr><td>5.55(5)(s,w)</td><td>0.95</td><td>3s</td><td>7.14(8)(w)</td><td>0.13</td><td>3d'</td><td>8.007</td><td>0.08</td><td>3d'</td><td></td></tr><tr><td>7.31(0)(b,w)</td><td>0.97</td><td>4s</td><td>7.900</td><td>0.10</td><td>4d'</td><td>8.716</td><td>0.08</td><td>4d'</td><td></td></tr><tr><td>7.953</td><td>0.98</td><td>5s</td><td>8.235</td><td>0.07</td><td>5d'</td><td>9.050</td><td>0.02</td><td>5d'</td><td></td></tr><tr><td>8.241</td><td>1.05</td><td>6s</td><td>8.409</td><td>0.07</td><td>6d'</td><td>(IE<sub>3</sub>)<sub>v</sub> = 11.91 eV (24a')<sup>-1</sup><br>(ns &larr; 24a')</td><td></td><td></td><td></td></tr><tr><td>8.409</td><td>1.07</td><td>7s</td><td>8.512</td><td>0.08</td><td>7d'</td><td>8.512</td><td>1.00</td><td>3s</td><td></td></tr><tr><td>8.512</td><td>1.08</td><td>8s</td><td>-</td><td>-</td><td>8d'</td><td>10.410</td><td>0.99</td><td>4s</td><td></td></tr><tr><td>8.568</td><td>1.27</td><td>9s</td><td>8.622</td><td>0.16</td><td>9d'</td><td>(np &larr; 24a')</td><td></td><td></td><td></td></tr><tr><td>8.622</td><td>1.16</td><td>10s</td><td>(IE<sub>2</sub>)<sub>v</sub> = 9.60 eV (4a')<sup>-1</sup><br>(ns &larr; 4a')</td><td></td><td></td><td>9.15(4)(s)</td><td>0.78</td><td>3p</td><td></td></tr><tr><td>8.66(1)(s)</td><td>0.96</td><td>11s</td><td>6.19(8)(b)</td><td>1.00</td><td>3s</td><td>10.59(2)(b)</td><td>0.79</td><td>4p</td><td></td></tr><tr><td>8.682</td><td>1.07</td><td>12s</td><td>8.098</td><td>0.99</td><td>4s</td><td>(np' &larr; 24a')</td><td></td><td></td><td></td></tr><tr><td>(np' &larr; 5a')</td><td></td><td></td><td>8.771</td><td>0.95</td><td>5s</td><td>9.73(6)(b,w)</td><td>0.50</td><td>3p'</td><td></td></tr><tr><td>5.99(8)(b)</td><td>0.79</td><td>3p</td><td>9.043</td><td>1.06</td><td>6s</td><td>(nd' &larr; 24a')</td><td></td><td></td><td></td></tr><tr><td>7.46(0)(b,w)</td><td>0.81</td><td>4p</td><td>9.204</td><td>1.14</td><td>7s</td><td>10.12(9)(b,w)</td><td>0.24</td><td>3d</td><td></td></tr><tr><td>8.059</td><td>0.70</td><td>5p</td><td>9.291</td><td>1.36</td><td>8s</td><td>(nd' &larr; 24a')</td><td></td><td></td><td></td></tr><tr><td>8.31(0)(s)</td><td>0.71</td><td>6p</td><td>(np' &larr; 4a')</td><td></td><td></td><td>10.31(5)(b,w)</td><td>0.08</td><td>3d'</td><td></td></tr><tr><td>8.457</td><td>0.66</td><td>7p</td><td>6.94(6)(s)</td><td>0.74</td><td>3p</td><td>(IE<sub>4</sub>)<sub>v</sub> = 12.68 eV (3a')<sup>-1</sup><br>(ns &larr; 3a')</td><td></td><td></td><td></td></tr><tr><td>(np' &larr; 5a')</td><td></td><td></td><td>8.31(0)(s)</td><td>0.75</td><td>4p</td><td>9.291</td><td>1.00</td><td>3s</td><td></td></tr><tr><td>6.73(3)(b)</td><td>0.43</td><td>3p'</td><td>8.86(6)(s)</td><td>0.69</td><td>5p</td><td>(np' &larr; 3a')</td><td></td><td></td><td></td></tr><tr><td>7.737</td><td>0.42</td><td>4p'</td><td>9.096</td><td>0.80</td><td>6p</td><td>10.00(3)(w)</td><td>0.75</td><td>3p</td><td></td></tr><tr><td>8.151</td><td>0.41</td><td>5p'</td><td>9.253</td><td>0.74</td><td>7p'</td><td>(np' &larr; 3a')</td><td></td><td></td><td></td></tr><tr><td>8.346</td><td>0.50</td><td>6p'</td><td>9.340</td><td>0.76</td><td>8p'</td><td>10.58(3)(b)</td><td>0.45</td><td>3p'</td><td></td></tr><tr><td>8.46(9)(s)</td><td>0.55</td><td>7p'</td><td>9.400</td><td>0.75</td><td>9p'</td><td>(IE<sub>5</sub>)<sub>v</sub> = 13.18 eV (23a')<sup>-1</sup><br>(ns &larr; 23a')</td><td></td><td></td><td></td></tr><tr><td>(nd' &larr; 5a')</td><td></td><td></td><td>(np' &larr; 4a')</td><td></td><td></td><td>9.74(3)(s,w)</td><td>1.01</td><td>3s</td><td></td></tr><tr><td>6.94(6)(s)</td><td>0.29</td><td>3d</td><td>7.45(5)(s,w)</td><td>0.48</td><td>3p'</td><td></td><td></td><td></td><td></td></tr><tr><td>7.84(0)(s)</td><td>0.23</td><td>4d</td><td>8.49(5)(s,w)</td><td>0.49</td><td>4p'</td><td></td><td></td><td></td><td></td></tr><tr><td>8.16(8)(s)</td><td>0.34</td><td>5d</td><td>8.936</td><td>0.47</td><td>5p'</td><td></td><td></td><td></td><td></td></tr><tr><td>8.38(6)(s,w)</td><td>0.24</td><td>6d</td><td>9.15(4)(s)</td><td>0.48</td><td>6p'</td><td></td><td></td><td></td><td></td></tr><tr><td>8.49(5)(s,w)</td><td>0.28</td><td>7d</td><td>9.291</td><td>0.36</td><td>7p'</td><td></td><td></td><td></td><td></td></tr><tr><td>8.568</td><td>0.27</td><td>8d</td><td>(nd' &larr; 4a')</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>7.84(0)(s)</td><td>0.22</td><td>3d</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>8.66(1)(s)</td><td>0.19</td><td>4d</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>9.01(0)(w)</td><td>0.20</td><td>5d</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>9.204</td><td>0.14</td><td>6d</td><td></td><td></td><td></td><td></td></tr></tbody></table>

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

review & editing. **N.C. Jones:** Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Supervision, Validation, Writing – review & editing. **S.V. Hoffmann:** Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Project administration, Supervision, Validation, Writing – review & editing. **A. Souza Barbosa:** Data curation, Investigation, Methodology, Software, Writing – review & editing. **P. Limão-Vieira:** Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Project administration, Supervision, Validation, 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

LVSD, ASB and MHFB acknowledge support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). LVSD, ASB and MHFB also acknowledge Prof.

Carlos A. M. de Carvalho for computational support at LFTC-DFis-UFPR and at LCPAD-UFPR. 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 CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. SK, AIL and PLV acknowledge the Portuguese National Funding Agency (FCT) through research grant CEFITEC UIDP/00068/2020, DOI: 10.54499/UIDP/00068/2020. PLV also acknowledges his visiting professor position at Federal University of Paraná, Curitiba, Brazil. 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).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jqsrt.2024.109125.

References

[1] Randi PAS, Kumar S, Lozano AI, Bettega MHF, Hoffmann SV, Jones NC, et al. Valence and Rydberg excitations of 2-fluorotoluene in the 4.4–10.8 eV photoabsorption energy region. J Quant Spectrosc Radiat Transf 2023;303:108597.

[2] Bandeira E, Kumar S, Lozano AI, Bettega MHF, Hoffmann SV, Jones NC, et al. Valence and Rydberg excitations of 3-fluorotoluene in the 4.4–10.8 eV photoabsorption energy region. J Quant Spectrosc Radiat Transf 2024;312:108796.

[3] Serralheiro C, Duflot D, Silva FF, Hoffmann SV, Jones NC, Mason NJ, et al. Toluene valence and Rydberg excitations as studied by ab initio calculations and vacuum ultraviolet (VUV) synchrotron radiation. J Phys Chem A 2015;119:9059–69.
[4] Barbosa AS, Silva FF, Rebelo A, Hoffmann SV, Bettega MHF, Limão-Vieira P. Valence and Rydberg excitations of 2,4- and 2,6-difluorotoluene as studied by vacuum ultraviolet synchrotron radiation and ab initio calculations. J Phys Chem A 2016;120:8998-9007.

[5] Huang CL, Jiang JC, Dyakov YA, Lin MF, Tseng CM, Lin SH, et al. Photodissociation and photoisomerization of α-fluorotoluene and 4-fluorotoluene in a molecular beam. J Chem Phys 2006;125:133305.

[6] Gardner AM, Tuttle WD, Whalley LE, Wright TG. Direct observation of vibrational energy dispersal: via methyl torsions. Chem Sci 2018;9:2270-83.

[7] Rottstegge J, Hartwig H, Dreizler H. The rotational spectrum, structure and barrier V6 to internal rotation of p-fluorotoluene. J Mol Struct 1999;478:37-47.

[8] Cave WT, Thompson HW. The ultra-violet absorption spectra of fluorinated toluenes. Discuss Farad Soc 1950;9:35-46.

[9] Hammond VJ, Price WC, Teegan JP, Walsh AD. The absorption spectra of some substituted benzenes and naphthalenes in the vacuum ultra-violet. Discuss Faraday Soc 1950;9:53-60.

[10] Palmer MH, Moyes W, Spiers M, Ridyard JNA. The electronic structure of substituted benzenes; Ab initio calculations and photoelectron spectra for the methyl- and fluoro-benzenes and fluorotoluenes. J Mol Struct 1978;49:105-23.

[11] Watanabe K, Nakayama T, Mottl J. Ionization potentials of some molecules. J Quant Spectrosc Radiat Transf 1962;2:369-82.

[12] Lias SG, Ausloss P. Ionization energies of organic compounds by equilibrium measurements. J Am Chem Soc 1978;100:6027-34.

[13] Ayles VL, Hammond CJ, Bergeron DE, Richards OJ, Wright TG. Zero electron kinetic energy spectroscopy of the para-fluorotoluene cation. J Chem Phys 2007; 126:244304.

[14] Gardner AM, Tuttle WD, Whalley L, Claydon A, Carter JH, Wright TG. Torsion and vibration-torsion levels of the S1 and ground cation electronic states of para -fluorotoluene. J Chem Phys 2016;145:124307.

[15] Tuttle WD, Gardner AM, Whalley LE, Wright TG. Vibration and vibration-torsion levels of the S1 state of para -fluorotoluene in the 580-830 cm-1 range: interactions and coincidences. J Chem Phys 2017;146:244310.

[16] Green JHS. Vibrational spectra of benzene derivatives-VI. p-Disubstituted compounds. Spectrochim Acta 1970;26A:1503-13.

[17] Furguson EE, Hudson RL, Nielsen JR, Smith DC. Vibrational spectra of fluorinated aromatics. IX. p-Fluorotoluene. J Chem Phys 1953;21:1736-40.

[18] Bell SM, Whiteside PT, Reid KL. Laser photoelectron spectroscopy and dynamics of S1 p-fluorotoluene. J Phys Chem A 2003;107:7373-9. https://doi.org/10.1021/jp030341+.

[19] Davies JA, Reid KL. Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states. J Chem Phys 2011;135:124305.

[20] Chiba T, Okuyama K, Fujii A. Observation of evidence for the π-σ* hyperconjugation in the S1 state of o-, m-, and p-fluorotoluenes by double-resonance infrared spectroscopy. J Phys Chem A 2016;120:5573-80.

[21] Stewart LD, Gascooke JR, Lawrance WD. A strong interaction between torsion and vibration in S 0 and S 1 m -fluorotoluene. J Chem Phys 2019;150:174303.

[22] Okuyama K, Mikami N, Ito M. Internal rotation of the methyl group in the electronically excited state: o-, m-, and p-fluorotoluene. J Phys Chem 1985;89: 5617-25.

[23] Koto E, Furuya K, Ogawa T. Direct observation of intramolecular vibrational energy redistribution under white light excitation. II. Fluorescence spectra of

fluorobenzene derivatives by controlled electron impact. Spectrochim Acta, Part A 2000;56:515-21.

[24] Gascooke JR, Stewart LD, Sibley PG, Lawrance WD. Pervasive interactions between methyl torsion and low frequency vibrations in S0 and S1 p-fluorotoluene. J Chem Phys 2018;149:074301.

[25] Gardner AM, Whalley LE, Kemp DJ, Tuttle WD, Wright TG. Identification of separate isoenergetic routes for vibrational energy flow in p-fluorotoluene. J Chem Phys 2019;151:154302.

[26] Kemp DJ, Whalley LE, Gardner AM, Tuttle WD, Warner LG, Wright TG. Complexity surrounding an apparently simple Fermi resonance in p -fluorotoluene revealed using two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy. J Chem Phys 2019;150:064306.

[27] Ha YM, Choi LS, Lee SK. Vibronic assignments of the S1 → S0 emission spectrum of the jet cooled p-Fluorotoluene. Bull Korean Chem Soc 1998;19:202-6.

[28] Emrich K. An extension of the coupled cluster formalism to excited states (I). Nucl Phys 1981;A351:379-96.

[29] Sekino H, Bartlett RJ. A linear response, coupled-cluster theory for excitation energy. Int J Quantum Chem 1984;26:255-65.

[30] Stanton JF, Bartlett RJ. The equation of motion coupled-cluster method. A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties. J Chem Phys 1993;98:7029-39.

[31] Bartlett RJ. Coupled-cluster theory and its equation-of-motion extensions. Wiley Interdiscip Rev Comput Mol Sci 2012;2:126-38.

[32] Bauernschmitt R, Ahlrichs R. Treatment of electronic excitations within the adiabatic approximation of time dependent density functional theory. Chem Phys Lett 1996;256:454-64.

[33] Casida ME. Time-dependent density-functional theory for molecules and molecular solids. J Mol Struct-Theochem 2009;914:3-18.

[34] Yanai T, Tew DP, Handy NC. A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP). Chem Phys Lett 2004;393:51-7.

[35] Kendall RA, Dunning TH, Harrison RJ. Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions. J Chem Phys 1992;96: 6796-806. https://doi.org/10.1063/1.462569.

[36] Barca GMJ, Bertoni C, Carrington L, Datta D, De Silva N, Deustua JE, et al. Recent developments in the general atomic and molecular electronic structure system. J Chem Phys 2020;152:154102.

[37] Davies AR, Kemp DJ, Wright TG. Unpicking vibration-vibration and vibration-torsion interactions in m-fluorotoluene. J Mol Spectrosc 2021;381:111522.

[38] 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.

[39] 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.

[40] Davies AR, Kemp DJ, Wright TG. Methyl-torsion-facilitated internal energy delocalization following electronic excitation in m -fluorotoluene: can meta and para substitution be directly compared? AIP Adv 2020;10:125206.

[41] Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling. Evaluation number 12 1997. Publication 97-4January 15.

[42] 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.

[43] Villata LS, Rosso JA, Gonzalez MC, Mártire DO. Kinetics of the reactions of O- and HO. with α,α,α-trifluorotoluene and 4-fluorotoluene. J Chem Res 1997;172.