Nonresonant two-photon pulsed field ionization of CH3S formed in photodissociation of CH3SH and CH3SSCH3

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Abstract
Threshold photoelectron (PE) spectra for CH3S formed in the photodissociation of CH3SH and CH3SSCH3 in the photon energy range of 36 850–38 150 cm−1 have been measured using the nonresonant two-photon pulsed field ionization (N2P-PFI) technique. Both spin–orbit states CH3S(X̃ 2E3/2) and CH3S(2E1/2) are observed from CH3SH and CH3SSCH3 in this photodissociation energy range. However, negligible intensities of vibrationally excited CH3S radicals are produced from CH3SH. In the case of CH3S from CH3SSCH3, the population ratio CH3S(ν3=1)/CH3S(ν3=0) is estimated to be ≊0.18. The simulation of the N2P-PFI-PE spectra reveals that the rotational temperature for CH3S(X̃ 2E3/2,1/2) formed by photodissociation of CH3SH is ≊200–250 K and the branching ratio CH3S(2E1/2)/CH3S(X̃ 2E3/2) is 0.5±0.1. For CH3S(X̃ 2E3/2,1/2) produced from CH3SSCH3, the rotational temperature for CH3S(X̃ 2E3/2,1/2) is ≊800–900 K and the branching ratio CH3S(2E1/2)/CH3S(X̃ 2E3/2) is 1.1±0.2. This experiment demonstrates that the PFI-PE spectroscopic method can be a sensitive probe for nascent rovibronic state distributions of photoproducts. Furthermore, the simulation also shows that the photoionization dynamics of CH3S may involve rotational angular momentum changes up to ±4. The ionization energy and C–S stretching frequency for CH3S+(X̃ 3A2) are determined to be 74 726±8 cm−1 (9.2649±0.0010 eV) and 733±5 cm−1, respectively. The spin–orbit splitting for CH3S(X̃ 2E3/2,1/2) is 257±5 cm−1, in agreement with the literature values.