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Electron Impact Fragmentation Dynamics of Carbonyl Sulfide: A Combined Experimental and Theoretical Study

Soumya Ghosh, Narayan Kundu, Aryya Ghosh, Dhananjay Nandi

TL;DR

The paper addresses how low- to intermediate-energy electrons induce fragmentation of carbonyl sulfide (OCS) through DEA and IPD channels. It combines absolute S$^{-}$ cross sections measured via relative flow calibration with MCTDH-based quantum-dynamical calculations using EOM-CCSD PECs for linear and bent geometries to reproduce and interpret observed resonances. Key findings include identification of S$^{-}$ DEA resonances at $1.2$, $5.0$, $6.8$, and $10.2$ eV and the dominant role of bent configurations at higher energies, along with Wannier-threshold–based IPD thresholds that agree with thermochemical data. The work demonstrates how electronic structure and nuclear dynamics intertwine in electron-induced OCS dissociation and provides a framework for studying similar polyatomic systems relevant to plasmas, atmospheres, and astrochemistry.

Abstract

In this study, we examine the interactions of low- to intermediate-energy electrons (0$-$45 eV) with carbonyl sulfide (OCS). These collisions lead to the formation of several anionic fragments, including C$^-$, O$^-$, S$^-$, and SO$^-$. When the incident electron energy is below the first ionization potential of the molecule, dissociative electron attachment (DEA) process dominates, primarily yielding O$^-$ and S$^-$ fragments. At higher energies, beyond the ionization potential, ion-pair dissociation (IPD) becomes the dominant process, resulting in the emergence of additional fragments such as C$^-$ and SO$^-$. This leads to an increasingly intricate mechanism, necessitating a detailed analysis to elucidate the ion-pair dissociation pathways. The absolute cross section for S$^-$ ions has been determined using the well-established relative flow technique. Theoretical cross sections are calculated using the multi-configurational time-dependent hartree (MCTDH) method, with each potential energy curve obtained from equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations. The computed values are in excellent agreement with the experimental data. The analysis reveals contributions from both linear and bent anionic resonant states. Due to low count rates, only relative cross section curves have been obtained for the O$^-$ and SO$^-$ ions. At higher energies, the ion pair thresholds are evaluated using the Wannier threshold law, yielding values consistent with those derived from thermochemical data.

Electron Impact Fragmentation Dynamics of Carbonyl Sulfide: A Combined Experimental and Theoretical Study

TL;DR

The paper addresses how low- to intermediate-energy electrons induce fragmentation of carbonyl sulfide (OCS) through DEA and IPD channels. It combines absolute S cross sections measured via relative flow calibration with MCTDH-based quantum-dynamical calculations using EOM-CCSD PECs for linear and bent geometries to reproduce and interpret observed resonances. Key findings include identification of S DEA resonances at , , , and eV and the dominant role of bent configurations at higher energies, along with Wannier-threshold–based IPD thresholds that agree with thermochemical data. The work demonstrates how electronic structure and nuclear dynamics intertwine in electron-induced OCS dissociation and provides a framework for studying similar polyatomic systems relevant to plasmas, atmospheres, and astrochemistry.

Abstract

In this study, we examine the interactions of low- to intermediate-energy electrons (045 eV) with carbonyl sulfide (OCS). These collisions lead to the formation of several anionic fragments, including C, O, S, and SO. When the incident electron energy is below the first ionization potential of the molecule, dissociative electron attachment (DEA) process dominates, primarily yielding O and S fragments. At higher energies, beyond the ionization potential, ion-pair dissociation (IPD) becomes the dominant process, resulting in the emergence of additional fragments such as C and SO. This leads to an increasingly intricate mechanism, necessitating a detailed analysis to elucidate the ion-pair dissociation pathways. The absolute cross section for S ions has been determined using the well-established relative flow technique. Theoretical cross sections are calculated using the multi-configurational time-dependent hartree (MCTDH) method, with each potential energy curve obtained from equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations. The computed values are in excellent agreement with the experimental data. The analysis reveals contributions from both linear and bent anionic resonant states. Due to low count rates, only relative cross section curves have been obtained for the O and SO ions. At higher energies, the ion pair thresholds are evaluated using the Wannier threshold law, yielding values consistent with those derived from thermochemical data.

Paper Structure

This paper contains 11 sections, 8 equations, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Measurement procedure
  3. Experimental
  4. Computational
  5. Result and discussion
  6. Mass spectrum
  7. S$^{-}$ production
  8. The DEA process
  9. The IPD process
  10. O$^{-}$, SO$^{-}$ and C$^{-}$ production
  11. Conclusion

Figures (5)

  • Figure 1: (a) The mass spectra of anionic fragments generated at an incident electron energy of $11.2$ eV reveal the presence of O$^{-}$ and S$^{-}$ ions. At this energy, the O$^{-}$ ion signal is notably weak, resulting in a poor signal-to-noise ratio. (b) The mass spectra of anionic fragments generated at an incident electron energy of $40.8$ eV reveal the presence of C$^{-}$, O$^{-}$, S$^{-}$ and SO$^{-}$ ions.
  • Figure 2: The absolute cross section of the S$^{-}$ ions, corrected for electron beam intensity, is shown. Below $4$ eV (indicated in red), the electron flux is too low (comparable to the dark noise level) rendering the calculated cross sections in this range unreliable. Although a linear correction for electron beam intensity was applied, the anion production in this low-energy region does not follow a linear trend. The inset displays the absolute cross section of S$^{-}$ ions for incident electron energies between $4$ and $45$ eV, where the electron flux is sufficiently high to ensure reliable measurements.
  • Figure 3: The calculated cross sections for the formation of S$^{-}$ ions using the MCTDH method are shown for both C$_{\infty\text{V}}$ (linear geometry, blue curve) and C$_{\text{S}}$ ($135.4^{\circ}$ bent geometry, red curve). (a) Cross section calculation near the 1.2 eV DEA resonance for both linear and bent geometries. (b) Cross section calculation near the 5 eV DEA resonance for both geometries. (c) Cross sections for the 6.8 eV resonance in linear and bent configurations. (d) Another 6.8 eV resonance in the bent geometry is displayed separately for clarity, as it shares the same electron energy range as (c). (e) Cross section near 10.2 eV shown only for the bent geometry, as no linear resonance is present at this energy.
  • Figure 4: Ion yield as a function of electron energy in the threshold region for (a) S$^{-}$, (b) O$^{-}$ and (c) SO$^{-}$ ions. The vertical markers indicate the appearance energies obtained from the Wannier law fit.
  • Figure 5: (a) The ion yield curve for O$^{-}$ ions over the $0-45$ eV incident electron energy range exhibits weak peaks near $3.5$ eV and $11$ eV, indicating possible low-intensity resonances. (b) The ion yield curve for SO$^{-}$ ions over the $0-45$ eV incident electron energy range shows no distinct DEA peaks. Instead, a gradual increase in signal is observed in the ion-pair dissociation region, suggesting that SO$^{-}$ formation occurs primarily through ion pair production.