Journal of Physical Chemistry A, Vol.101, No.36, 6513-6522, 1997
Dissociation of CH3SH+ by collisional activation: Evidence of nonstatistical behavior
We have measured the absolute total cross sections for CH2SH+(CH3S+), CH2S+, HCS+, HS+, CH3+, and CH2+ produced by the collision-induced dissociation (CID) reaction of CH3SH+(1(2)A '') + Ar in the center-of-mass collision energy range of 1-36 eV. While the onset for CH3+ is consistent with the thermochemical threshold for the formation of CH3+ + SH, the onsets for other product ions are higher than their corresponding thermochemical thresholds. Using a charge transfer probing technique, we conclude that the m/e = 47 amu ions observed in the CID reaction have mostly the CH2SH+ structure. The relative yields for CH2SH+, CH2S+, HCS+, HS+, CH3+, and CH2+ formed in the CID reaction, which strongly favor the C-S bond scission process leading to the formation of CH3+ + SH, are significantly different from those measured in previous photoionization and charge exchange studies. Since the CH3+ + SH channel is not among the most stable product channels, this observation suggests that the collision-activated dissociation of CH3SH+ is nonstatistical. The high yield for CH3+ + SH observed in CID is attributed to the more efficient translational to vibrational energy transfer for the C-S stretch than for the C-H stretches of CH3SH+, and to weak couplings between the low-frequency C-S and the high-frequency C-H stretching vibrational modes of CH3SH+. The differences in excitation mechanisms for CH3SH+ via collision activation, photoionization, and charge exchange are responsible for the different fragment ion distributions from CH3SH+ observed in these experiments.