NONRESONANT 2-PHOTON PULSED-FIELD IONIZATION PHOTOELECTRON SPECTROSCOPIC STUDY OF CH3SH AND CH3CH2SH

The threshold photoelectron spectra for CH3SH and CH3CH2SH in the phot on energy regions of 75600-77400 and 74600-76400 cm(-1), respectively, have been measured using the nonresonant two-photon pulsed field ioni zation photoelectron (N2P-PFI-PE) spectroscopic technique. The respect ive ionization energies (Ifs) for CH3SH and CH3CH2SH are determined to be 76262+/-5 cm(-1) (9.4553+/-0.0006 eV) and 74951+/-5 cm(-1) (9.2927 +/-0.0006 eV). The theoretical IEs of 9.46 eV for CH3SH and 9.30 eV fo r CH3CH2SH obtained by the Gaussian-2 (G2) procedure are in excellent agreement with the experimental values. Ab initio calculations for CH3 CH2SH and CH3CH2SH+ at the G2 level indicate that both CH3CH2SH and CH 3CH2SH+ exist as a gauche- or a trans-conformer with the gauche-confor mer slightly more stable. Comparison of the experimental and theoretic al results show that the main vibrational features resolved in the PFI -PE spectra can be assigned to excitation of vibrational modes involvi ng the H-S torsional, C-S stretching, and/or H-S bending vibrations of CH3SH+ and gauche-CH3CH2SH+. A doublet observed for the H-S torsional mode is tentatively attributed to mixing of the harmonic vibrational levels due to the low potential barriers between the potential wells o f the gauche- and trans-conformers of CH3CH2SH and CH3CH2SH+. The vibr ational structures observed in the PFI-PE spectra for CH3SH and CH3CH2 SH are consistent with the expectation that ionization involves the ej ection of a mostly nonbonding electron, associated with the S.