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+(12A‘‘) + 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.