Vibrational autoionization of state-selective jet-cooled methanethiol (CH₃SH) investigated with infrared + vacuum-ultraviolet photoionization

Journal Article · Tue Oct 24 00:00:00 EDT 2017 · Physical Chemistry Chemical Physics. PCCP (Print)

DOI:https://doi.org/10.1039/C7CP06433K· OSTI ID:1420066

Xie, Min ^[1]; Shen, Zhitao ^[1]; Pratt, S. T. ^[2]; Lee, Yuan-Pern  ^[3]

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1. National Chiao Tung University, Hsinchu (Taiwan). Department of Applied Chemistry and Institute of Molecular Science
2. Argonne National Lab. (ANL), Argonne, IL (United States)
3. National Chiao Tung University, Hsinchu (Taiwan). Department of Applied Chemistry and Institute of Molecular Science; Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei (Taiwan)

Vibrational autoionization of Rydberg states provides key information about nonadiabatic processes above an ionization threshold. In this work, we employed time-of-flight mass detection of CH₃SH⁺ to record vibrational-state selective photo-ionization efficiency (PIE) spectra of jet-cooled methanethiol (CH₃SH) on exciting CH₃SH to a specific vibrationally excited state with an infrared (IR) laser, followed by excitation with a tunable laser in the vacuum-ultraviolet (VUV) region for ionization. Autoionizing Rydberg states assigned to the ns, np, nd and nf series are identified. When IR light at 2601 (ν₃, SH stretching mode) and 2948 cm^-1 (ν₂, CH₃ symmetric stretching mode) was employed, the Rydberg series converged to the respective vibrationally excited (ν₃ and ν₂) states of CH₃SH⁺. When IR light at 3014 cm^-1 (overlapped ν₁/ν₉, CH₃ antisymmetric stretching and CH₂ antisymmetric stretching modes) was employed, Rydberg series converging to two vibrationally excited states (ν₁ and ν₉) of CH₃SH⁺ were observed. When IR light at 2867 cm^-1 (2ν₁₀, overtone of CH₃ deformation mode) and 2892 cm^-1 (2ν₄, overtone of CH₂ scissoring mode) was employed, both Δν = -1 and Δν = -2 ionization transitions were observed; there is evidence for direct ionization from the initial state into the CH₃SH⁺ (ν₄⁺ = 1) continuum. In all observed IR-VUV-PIE spectra, the ns and nd series show intensity greater than the other Rydberg series, which is consistent with the fact that the highest-occupied molecular orbital of CH₃SH is a p-like lone pair orbital on the S atom. Finally, the quantum yields for autoionization of various vibrational excited states are discussed. Values of ν₁ = 3035, ν₂ = 2884, ν₃ = 2514, and ν₉ = 2936 cm^-1 for CH₃SH⁺ derived from the converged limits agree satisfactorily with values observed for Ar-tagged CH₃SH⁺ at 3026, 2879, 2502, and 2933 cm^-1.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1420066
• Journal Information: Physical Chemistry Chemical Physics. PCCP (Print), Vol. 19, Issue 43; ISSN 1463-9076
• Publisher: Royal Society of Chemistry
• Country of Publication: United States
• Language: English

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