# Rotationally-resolved VUV-photoelectron spectroscopy of silylidyne radical

Hai Linh Le, Ilan Audoin, Ugo Jacovella, Séverine Boyé-Péronne, Bérenger Gans

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Hai Linh Le, Ilan Audoin, Ugo Jacovella, Séverine Boyé-Péronne, Bérenger Gans. Rotationally-resolved VUV-photoelectron spectroscopy of silylidyne radical. Molecular Physics, inPress, 10.1080/00268976.2025.2550573. hal-05302929

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ARTICLE TEMPLATE

Rotationally-resolved VUV-photoelectron spectroscopy of silylidyne
radical

Hai Linh Le,ª Ilan Audoin,ª Ugo Jacovella,ª Séverine Boyé-Péronne,ª and Bérenger Gansª

ª Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France

ARTICLE HISTORY

Compiled July 24, 2025

ABSTRACT

Pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of silylidyne rad-
ical (SiH) has been recorded in the vicinity of its first ionization threshold at high
and medium spectral resolutions using a laser-based experiment. The obtained
rotationally-resolved spectrum of the X<sup>+1</sup>Σ<sup>+</sup> &larr; X<sup>2</sup>&Pi;<sub>&Omega;</sub> (&Omega; = 1/2 and 3/2) pho-
toionizing transitions allowed a precise determination of the corresponding adiabatic
ionization energy which refines the recent results obtained by Chen *et al*. J. Chem.
Phys. **157**, 014303 (2022) using a synchrotron-based experiment at lower spectral
resolution.

KEYWORDS

PFI ZEKE PES, radicals, electric discharge, photoelectron, SiH, VUV laser.

1. Introduction

Photoelectron spectroscopy is a well-established and powerful tool to address questions
in gas-phase physical chemistry and related fields [1, 2]. Being extremely sensitive, it is
a method of choice to study reactive intermediates that have short lifetimes and that
are usually present at very low concentrations such as free radical species. Photoion-
ization of free radicals has proven to be challenging due to the difficulty in producing
them in a cold and stable manner. A pioneering method to address this challenge is
flash vacuum pyrolysis [3, 4], in which radicals are generated *via* thermal decomposi-
tion of a precursor by heating a silicon carbide (SiC) tube. This technique typically
yields high concentrations of radicals and can be made selective by carefully choosing
the appropriate precursor. The main drawback of this method is that the radicals
are typically produced with rotational temperatures of several hundred kelvins, and
even higher vibrational temperatures, resulting in spectral congestion and reduced
sensitivity due to the population of many different quantum states. An alternative
approach has been proposed to generate radicals selectively, while maintaining low
rotational and vibrational temperatures, by employing laser photolysis in a quartz
capillary mounted at the exit of a pulsed valve [5]. This method has been successfully
used to obtain high-resolution photoelectron spectra of various radicals [6–8]. How-
ever, when the radical product is highly reactive, a significant fraction can be lost due to recombination in the capillary, even when photolysis is performed as close to the exit as possible. To overcome this issue, an alternative approach involves carrying out the photolysis directly in the extraction region, after the beam has been skimmed [9].

Another way of producing neutral radicals is by using an electric discharge coupled with a molecular beam [10]. However, this method is typically non-selective and generates a myriad of different species, which often limits its applicability in photoelectron spectroscopy. Nevertheless, it has been successfully employed by the Merkt group to study the photoelectron spectroscopy from metastable atomic states (see, for instance, Ref. [11]).

The challenges associated with photoionization studies of free radicals using tabletop laser systems have motivated the scientific community to increasingly rely on the ease of use and fast tunability of synchrotron radiation sources, thereby expanding the range of radicals studied. As a result, radical sources have been extensively optimized on synchrotron beamlines to align with specific operating modes, such as flow-tube reactors that produce radicals *via* chemical reactions [12], or pyrolysis sources [13]. As is often the case, these advantages come with trade-offs, most notably, the limited spectral resolution of experiments using monochromatized VUV synchrotron light, which remains lower than what can be achieved with tabletop laser systems. For example, VUV laser systems can attain spectral resolutions as high as 0.008 cm⁻¹ [14], whereas even the most advanced synchrotron beamlines, such as DESIRS at the SOLEIL synchrotron, are typically limited to around 0.5 cm⁻¹ [15].

In photoelectron spectroscopy (PES), the techniques employed with both VUV radiation sources – such as pulsed field ionization zero-kinetic energy PES (PFI-ZEKE) [16], threshold (or slow) PES (TPES or SPES) [17], and pulsed-field-ionization photoelectron (PFI-PE) [18] – tend to compromise spectral resolution. Nonetheless, the highest resolutions are still achieved using laser-based systems, reaching down to 0.06 cm⁻¹ [16], compared to around 1 cm⁻¹ with synchrotron-based methods [19]. When studying free radicals, typically present at very low densities, achieving such high resolution is particularly challenging. However, low signal-to-noise ratio can be improved at the expense of the spectral resolution and laser-based photoelectron spectroscopy can still routinely deliver radical PES with spectral resolutions around 1 cm⁻¹. In contrast, synchrotron-based setups typically yield PES of radicals with more modest resolutions, on the order of 10 meV (80 cm⁻¹) [20, 21].

In this paper, we demonstrate the successful integration of a pulsed electric discharge radical source with a VUV laser-based setup for photoelectron spectroscopy. Although promising, this type of radical source has been seldom used in the context of free radical photoelectron spectroscopy, likely due to its lower selectivity in radical production compared to other methods, as well as the inherent lack of mass selectivity in photoelectron spectra obtained with tabletop laser systems. Note that the mass-selectivity issue can be overcome using mass-analyzed threshold ionization (MATI) [22]. Our setup offers two spectral resolution modes: a low-resolution (LR) mode, comparable to typical synchrotron-based VUV experiments, and a high-resolution (HR) mode that provides enhanced spectral detail. The performance of the system is demonstrated through measurements on the silylidyne radical (SiH).
## 2. Experimental details

The experimental setup “VULCAIM” has been already detailed elsewhere [23, 24]. Briefly, it consists in a pulsed broadly-tunable VUV laser with two spectral-resolution configurations (high: < 0.1 cm⁻¹ (HR) and low: ≈ 3 – 5 cm⁻¹ (LR) with estimated fluxes in the range of 10⁸–10¹⁰ ph per pulse) coupled with the PFI-ZEKE spectrometer and a pulsed molecular beam.

In the present work, the pulsed valve used to generate the molecular beam was equipped with an electric discharge to produce SiH from a silane/helium mixture (0.85%, Messer). Two types of pulsed valves were used: a solenoid valve (Parker, Series 9) and a piezo valve (MassSpecpecD BV) with typical opening times of 160 µs and 40 µs, respectively. In Refs. 23, 24, the PFI-ZEKE spectrometer was collinear with the molecular beam. In the present study, it has been rotated by 90° to avoid any saturation of the MCP by the light emitted by the electric discharge. In addition, to efficiently deflect the charged particles generated by the electric discharge, two skimmers (Ø = 6 and 2 mm) have been installed (separated by 10 cm) and a deflection plate has been inserted in between (with a typical voltage of 500 V).

To measure the total ion yield, an extraction pulse of 200 V/cm was applied 200 ns after the VUV laser photoionization. The PFI-ZEKE spectra were obtained using a pulse sequence consisting of a pre-pulse of 166 mV/cm (1 µs long) followed by a pulse of –830 mV/cm (20 µs long) applied 1 µs after the photoexcitation. This pulse sequence led to a spectral resolution of 1.2 cm⁻¹ and a Stark shift of 1.3 cm⁻¹ in high resolution measurement. The spectral resolution was measured on a single isolated line and the Stark shift by field free extrapolation.

The spectra were corrected from the VUV flux fluctuations by recording simultaneously the signal from an electron multiplier tube (R5150-10, Hamamatsu). Note that the measured fluxes were not corrected by the detector response.

## 3. Results and discussion

### 3.1. Radical production

Figure 1 displays time of flights recorded around 10 eV (below the first IE of SiH₄ [25]) using two different types of pulsed valves. This figure shows that the piezo valve, which generates denser gas pulses, appears to promote increased reactivity and consequently the formation of larger species, whereas the solenoid valve used in our setup favors the production of smaller radicals. As expected, from SiH₄, SiHₓ (x = 0 – 3) radicals are produced as well as Si₂Hₓ (x = 0 – 3) and Si₃Hₓ (x = 0 – 2) species by reactivity. Note that the silicon atom has three main stable natural isotopes (²⁸Si (92.2%), ²⁹Si (4.7%), and ³⁰Si (3.1%) [26]) that contribute to the mass distribution observed in Figure 1. Interestingly, due to an impurity in our gas sample, ion signals corresponding to nitrogen- and silicon-containing radicals are also observed.

Since this paper focuses on the SiH radical, all spectra presented below were recorded using the solenoid valve.

### 3.2. Ion yield of SiH radical

The ion yield spectrum is obtained by integrating the peak at a given *m/z* (see Figure 1) as a function of the photon energy. In Figure 2, we compare our ion yield
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Mass spectra recorded with the piezo valve (blue line) and the solenoid valve (black line) coupled with a pulsed electric discharge. In both cases, the photon energy was about 10 eV. Note that the isotopologues due to the Si isotopes are not assigned, see text for details.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** SiH⁺ ion yield recorded with the DESIRS beamline at the SOLEIL synchrotron in ref. [21] (upper purple line) and with the VULCAIM setup with the low spectral resolution configuration (lower black line). The spectra have been vertically shifted for clarity.</figcaption></figure>

spectrum of SiH ($m/z$ = 29), recorded using the LR configuration, with the spectrum reported in Ref. [21] using a synchrotron beamline and a flow-tube reactor. The two spectra not only show excellent agreement, but also highlight a significant gain in efficiency: while the synchrotron measurement required 13 hours of acquisition time, our laser-based measurement was completed in less than 3 hours.

### 3.3. PFI-ZEKE PES of SiH

By inverting the polarities of the extraction pulses and the MCP detector, our spectrometer can be operated in a suitable mode for recording PFI-ZEKE photoelectron spectra [23, 24]. The main limitation of the PFI-ZEKE technique is its lack of mass selectivity. Indeed, to illustrate this issue, we have recorded a PFI-ZEKE spectrum with our LR configuration and reported it in Figure 3, in which the mass-selected SPES spectrum of SiH from ref. [21] is also reported. One can see that our PFI-ZEKE PES is dominated by the strong photoionizing transitions of atomic silicon and the signal from SiH is only observed for its origin band near 64000 cm⁻¹. These six photoionizing transitions of Si [27] are assigned to:

* $^2P_{1/2}^° \leftarrow ^3P_2$ at 65524.60 cm⁻¹,

* $^2P_{1/2}^° \leftarrow ^3P_1$ at 65670.64 cm⁻¹,

* $^2P_{1/2}^° \leftarrow ^3P_0$ at 65747.76 cm⁻¹,

* $^2P_{3/2}^° \leftarrow ^3P_2$ at 65811.84 cm⁻¹,
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Experimental PFI-ZEKE photoelectron spectrum recorded with the LR mode using an electric discharge and a SiH₄:He mixture (in black) compared with the mass-selected SiH slow photoelectron spectrum (SPES) of Ref. [21] (in purple).</figcaption></figure>

• $^2P_{3/2}^o \leftarrow {}^3P_1$ at 65957.88 cm⁻¹,

• $^2P_{3/2}^o \leftarrow {}^3P_0$ at 66035.00 cm⁻¹.

The issue of mass selectivity in PFI-ZEKE photoelectron spectroscopy can be avoided using high-resolution MATI [28]. Unfortunately, although we recently employed this technique when the extraction axis was still parallel to the propagation of the molecular beam, we were unable to use it in the present study. This is because our current spectrometer design has not yet been adapted to accommodate the 90-degree rotation of the extraction axis. Indeed, the perpendicular orientation of our spectrometer relative to the molecular beam, combined with its design, limits the available time for proper ion separation (see Ref. [28] for details).

Nevertheless, we performed a PFI-ZEKE PES measurement with our HR configuration which is presented in Figure 4. This spectrum exhibits narrow lines which are due to rotationally resolved photoionizing transitions from the two spin-orbit components of the electronic ground state ($X^2\Pi_\Omega$) of neutral SiH to the electronic ground state ($X^+^1\Sigma^+$) of SiH⁺. Using the well known molecular constants of the electronic ground states of SiH [29] and SiH⁺ [30] and the standard expressions for rotational levels in $^2\Pi$ and $^1\Sigma$ states, we modeled the $X^+^1\Sigma^+ \leftarrow X^2\Pi_\Omega$ photoionizing transition. Only $\Delta J = \pm 1/2$, $\pm 3/2$, and $\pm 5/2$ branches were included. The calculated intensities of the rotational lines were simply obtained using a rotational temperature of 60 K, a spin-orbit temperature of about 150 K and the following manually-adjusted weighting factors for the rotational branches: (0.6, 1, 0.5, and 0.3) for $\Delta J = -1/2, +1/2, \pm 3/2$, and $\pm 5/2$, respectively. Concerning the energy calcu-
<figure><img src="image_5.png" /><figcaption>**Figure 4.** High-resolution experimental photoelectron spectrum of SiH (in black) compared with our calculated spectrum (in red) in the vicinity of the X<sup>+</sup> ¹Σ<sup>+</sup>(v<sup>+</sup> = 0) ← X<sup>2</sup>Π<sub>Ω</sub>(v = 0) photoionizing transition. The calculated spectrum has been convolved with a Gaussian line shape (FWHM = 1.2 cm<sup>-1</sup>). The black and blue combs provide the rotational assignments for Ω = 1/2 and Ω = 3/2, respectively. The band marked by an asterisk is assigned to the X<sup>+</sup> ⁴Σ<sup>-</sup><sub>g</sub> ← X<sup>3</sup>Σ<sup>-</sup><sub>g</sub> photoionizing transition of Si<sub>2</sub>.</figcaption></figure>

lations of the rotational lines, only the ionization energy was adjusted to match the experimental positions. Our final simulation depicted in Figure 4 was obtained with the X<sup>+</sup> ¹Σ<sup>+</sup> (v<sup>+</sup> = 0, J<sup>+</sup> = 0) ← X<sup>2</sup>Π<sub>1/2</sub> (v = 0, J = 1/2) transition located at 64050.52 cm<sup>-1</sup>. After applying the field-induced shift correction, this yields an adiabatic ionization energy of 64051.8(10) cm<sup>-1</sup>.

This value cannot be directly compared to that reported by Chen *et al.*, 7.934(5) eV (63992(40) cm⁻¹) [21]. As noted in Ref. [21], the spin-orbit coupling in the ground electronic state of the neutral species (*A*<sub>SO</sub> = 143.82 cm⁻¹ [29]) was neglected, as the two spin-orbit components could not be resolved. Therefore, their value should be corrected by adding half the spin-orbit constant, yielding a revised value of 7.943(5) eV (or 64063(40) cm⁻¹), which is in good agreement with our result.

An additional spectral signature is observed at 63825.6 cm⁻¹ in Figure 4, which we assign to the X<sup>+</sup> ⁴Σ<sup>-</sup><sub>g</sub> ← X<sup>3</sup>Σ<sup>-</sup><sub>g</sub> photoionizing transition of Si<sub>2</sub>. This band is not rotationally resolved because of the much smaller rotational constants of the ground electronic states of Si<sub>2</sub> (*B*<sub>e</sub> = 0.2390 cm<sup>-1</sup> [31]) and Si<sub>2</sub><sup>+</sup> (*B*<sub>e</sub> = 0.2322 cm<sup>-1</sup> [32]). After applying the field-induced shift correction, the position of the band (without calculating the rotational contour) yields an ionization energy of 63826.9(20) cm<sup>-1</sup>, which further refines the value obtained by Chen *et al.* of 7.919(5) eV (= 63871(40) cm<sup>-1</sup>) [33]. The slight discrepancy between the two measurements is most likely due to an underestimation of the error bars in Ref. [33].
## 4. Conclusion

Silylidyne radicals have been studied near their first ionization threshold using PFI-ZEKE photoelectron spectroscopy. The radicals were produced in a cold molecular beam *via* an electric discharge coupled with a pulsed valve. Thanks to the versatility of our VULCAIM setup in terms of spectral resolution, both ion yield and photo-electron spectra were recorded at low resolution over a broad energy range, and at high resolution around the first adiabatic ionization threshold. The rotationally resolved spectrum was successfully assigned using the well-established molecular constants of SiH [29] and SiH$^+$ [30], enabling a precise determination of the adiabatic ionization energy ($E_1(\text{SiH})/hc = 64051.8(10) \text{ cm}^{-1}$). The adiabatic ionization energy of Si$_2$ molecule was also refined ($E_1(\text{Si}_2)/hc = 63826.9(20) \text{ cm}^{-1}$).

With this work, we have demonstrated that our setup is not only efficient for high-resolution measurements but also capable of covering a very broad energy range for free radical spectroscopy. This performance is comparable to that typically achieved with synchrotron-based setups, but with improved spectral resolution and faster acquisition times. The optimization of mass-analyzed threshold-ionization spectroscopy [28], as previously implemented in an earlier version of our spectrometer [24], will further enhance our system, making it ideally suited for photoionization spectroscopy of free radicals.

## Acknowledgement(s)

UGJA and BEGA are delighted to dedicate this article to Professor Frédéric Merkt and would like to thank him for shaping the scientists they have become, and for being, and continuing to be, both a source of inspiration and a role model.

## Funding

This work has received the financial support from the French "Agence Nationale de la Recherche" (ANR) under Grant Nos. ANR-17-CE30-0031-01 (Project PRIMA) and ANR-21-CE29-0017 (Project ZEPHIRS). This work has also received the financial support from the "Institut de Physique" (INP) of CNRS, the Paris Ile-de-France Region (DIM ACAV and DIM ACAV+), and the University of Paris-Saclay (Labex PALM). This work was supported by the Programme National "Physique et Chimie du Milieu Interstellaire" (PCMI), the "Programme National de Planétologie" (PNP), and the "Commission Spécialisée Astronomie-Astrophysique" (CSAA) of CNRS/INSU with INC/INP cofunded by CEA and CNES. I. A. benefited from a State grant under the France 2030 plan, as part of the QuanTEdu-France project, under the reference ANR-22-CMAS-0001

## Conflict of Interest

The authors declare no conflict of interest.
## Data Availability

Data supporting this article are available from the corresponding author upon reasonable request.

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