Low energy electron collisions in H2S and H2Se: Structure in dissociative attachment cross-sections

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Abstract

Dissociative electron attachment between 0 and 4 eV has been investigated in hydrogen sulfide and hydrogen selenide with an improved electron resolution (0.040 eV). HS− and HSe− cross-sections versus electron energy present vertical onsets revealing that the potential surfaces of the resonances which are reached around 2 eV are bound. A well-developed and intriguing structure is observed in HS−, S−, HSe− and Se− cross-sections. It could reveal interferences due to an attractive resonance having a lifetime of the order of one vibrational period. The strong similarity between the anion behaviour in H2S and H2Se is in contrast with H2O where no dissociative attachment process occurs in this energy range.

Introduction

Low energy electron scattering with H2S have been the object of many studies during the last decades, both experimentally [1], [2], [3], [4], [5], [6], [7], [8] and theoretically [9], [10], [11], [12], [13], [14], [15] implying several resonant states. If calculations are in rather good agreement with scattering measurements, many questions are still open to understand the dissociative processes. Very recently calculations have renewed the interest in the problem. R matrix calculations have been focussed on the elastic scattering and electronic excitation [12]. Using the local complex potential model, detailed dynamics calculations could reproduce the angular behaviour of H− fragments in H2O and H2S [13]. More recent calculations by the same group, taking into account three resonant potential surfaces [14], [15] were successful in reproducing qualitatively the dissociation data in H2O concerning the major negative ions produced. These calculations could possibly be extended to similar processes at low energy in H2S and H2Se. It is well known that below about 10 eV, electron collisions with molecules are dominated by the temporary capture of the incident electron by the target, forming a short lived metastable anion (“resonance”), which can decay either by ejection of the incident electron, possibly leaving the molecule vibrationally excited, or by dissociation leading to a stable negative ion, and one or several neutral fragments via the dissociative electron attachment (DEA) process. Therefore, the knowledge of the character (energy, symmetry, width) of the involved resonances is important to understand electron collisions in a given molecule.

In H2S between 1 and 10 eV the DEA process is known [1], [2], [3], [4] to give rise to HS−, S− and H− anions. To explain the observation of HS− around 2 eV a shape resonance of 2A1 symmetry was originally proposed [3]. On the other hand, the vibrational excitation cross-section versus electron energy of the stretch modes 0 0 1 and 1 0 0 [5] was displaying, besides a peak at threshold, a structureless cross-section, peaking around 2.5 eV and extending up to 4 eV. It was also presenting an almost isotropic behaviour, in agreement with the proposed 2A1 resonance symmetry.

Angular measurements of H− anions around 5.5 and 7.5 eV [4], using the O’Malley–Taylor theory, demonstrated that these anions were produced by DEA through 2B1 and 2A1 resonant states. A few years later a theoretical study [10] found a 2B2 resonance to take into account the 2–3 eV energy process, the 6–8 eV region being dominated by a 2A1 resonance. Such a 2B2 resonance was also reported in calculations of elastic scattering in H2S and H2Se [11] and also recently by Gupta and Baluja [12]. Despite the convincing assignment of Azria et al. [4] (only one partial wave was involved in their analysis), these authors [12] did not find any evidence of a 2B1 resonance at 5.5 eV. Very recently, Haxton et al. [13] considering a 2B1 resonance, and using the complex local potential model, were able to take into account the overall angular behaviour for the H− + HS 2∏ (v = 0) process at 5.5 eV, and even have a qualitative agreement for the behaviour of the process H− + HS 2∏ (v = 1). However, they do not mention any other resonance in H2S, at lower or higher energy. The nature of the resonant states involved in H2S from 1 to 10 eV appears therefore somewhat controversial and certainly needs to be clarified.

Using an improved electron energy resolution, the present paper deals with detailed observations of HS− and S− anion yields versus electron energy produced via DEA processes in the range 0–4 eV. An energy loss spectrum showing high-excited vibrational levels of H2S is also presented. The well developed structure observed in DEA cross-sections may help to precise the nature of the involved resonant states, and its understanding could be a motivation for improved dynamics calculations on resonant potential surfaces. Similar results in H2Se are also presented for comparison

Section snippets

Experimental

The experimental set up used to study HS−, S− and electron scattering in H2S is an electrostatic electron spectrometer having two hemispherical energy analysers in tandem, both in the electron gun and the analyser section. The energy resolution ranges from 0.025 to 0.060 eV (FWHM) with electron currents ranging from 0.5 to 5 nA. Rotation of the analyser section allows angular behaviour of the scattered electrons. Mass analysis of anions and cations is performed by a time of flight system using a

H2S

The existence of three separated peaks for S− ions (Fig. 1) from 1 to 12 eV, as well as the observation of H− ions at 5.5 and 7.5 eV [4] suggest the occurrence of at least three resonant states. The present study is focussed on HS−, and S− ions produced in the energy range 1–4 eV.

Within the electron energy resolution (0.040 eV), HS− cross-section versus electron energy (Fig. 2a), presents a vertical onset at 1.60 ± 0.02 eV (most probable value of the electron energy), a value in excellent agreement

Discussion

Whereas DEA processes around 1–4 eV appear very similar in H2S and H2Se, the situation is totally different for H2O. In this molecule, if the DEA leading to H− anions resembles the H2S case, observations for dissociations giving rise to O− and OH− ions are strongly different. Indeed, even if the thermodynamical threshold for O− + H2 is 3.57 eV [29], O− ions are not observed below 7 eV. Furthermore, the dissociation limit for OH− + H is 3.29 eV [29], however OH− is not observed via the direct DEA

Conclusion

Using a better electron energy resolution we have observed a well-developed structure in the DEA cross-section versus electron energy (1–4 eV) for H2S and H2Se. The vertical onsets in HS− and HSe− reveal attractive resonant potential surfaces. The rather regular structure in S− and Se− cross-section from 1 to 4 eV could reveal a predissociation at large distance of these surfaces by dissociative states. At an energy as low as 2 eV, Feshbach resonances are not likely, shape resonances are more

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