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.

High-resolution state-selected ion-molecule reaction studies using pulsed field ionization photoelectronsecondary ion coincidence method Rev. Sci. Instrum. 74, 4096 (2003); 10.1063/1.1599071Pulsed field ionization-photoelectron bands for CO 2 + (A 2 Π u and B 2 Σ u + ) in the energy range of 17.2-19.0 eV: An experimental and theoretical study Rotationally resolved pulsed field ionization photoelectron study of CO + (X 2 Σ + ,v + =0-42) in the energy range of 13.98-21.92 eV

The vacuum ultraviolet pulsed field ionization photoelectron (PFI-PE) band for OCS+(X 2Π) in the energy region of 11.09–11.87 eV has been measured using high resolution monochromatized synchrotron radiation. The ionization energies (IEs) for the formation of the (0,0,0) X 2Π3/2 and (0,0,0) 2Π1/2 states of OCS+ are determined to be 11.1831±0.0005 and 11.2286±0.0005 eV, respectively, yielding a value of 367±1.2 cm−1 for the spin–orbit splitting. Using the internally contracted multireference configuration interaction approach, three-dimensional potential energy functions (PEFs) for the OCS+(X 2Π) state have been generated and used in the variational Renner–Teller calculations of the vibronic states. The energies of all vibronic states (J=P) for J=1/2, 3/2, 5/2, and 7/2 have been computed in the energy range of ≈4000 cm−1 above the IE[OCS+(X 2Π3/2)] for the assignment of the experimental spectrum. By a minor modification of the ab initio PEFs, good correlations are found between the experimental and theoretical Renner–Teller structures. Similar to the PFI-PE bands for CO2+(X 2Πg) and CS2+(X 2Πg), weak transitions have been detected in the PFI-PE band for OCS+(X 2Π), which are forbidden in the Franck–Condon approximation. The nonvanishing single-photon ionization cross sections involving the excitation of the bending vibrational modes of OCS+, CO2+, and CS2+, in their ground electronic states are attributed to the symmetries of the geometry-dependent electronic transition dipole operator components.

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Strong preference is observed for the C–S bond scission process, leading to the formation of CH3++SH (CH3CH2++SH), in the collision induced dissociation (CID) reaction of CH3SH++Ar (CH3CH2SH++Ar). Since the dissociation energy of 81.4 kcal/mol (45.2 kcal/mol) for the CH3+–SH (CH3CH2+–SH) bond is significantly higher than that of 48 kcal/mol (33.9 kcal/mol) for the H–CH2SH+ [H–CH(CH3)SH+] bond, this observation indicates that the CID process is nonstatistical. The high yield for the C–S bond breakage process is attributed to the more efficient translational to vibrational energy transfer for the C–S stretching mode than for C–H and S–H stretching modes via collisional activation, and to weak couplings between the low frequency C–S and high frequency C–H and S–H stretching vibrational modes of CH3SH+ andCH3CH2SH+.

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of O2 in the energy range of 20.2–21.3 eV, covering the ionization transitions of O2+(B 2Σg−, v+=0–7, N+)←O2(X 3Σg−, v″=0, N″). Only the ΔN=−2, 0, and +2 (or O, Q, and S) rotational branches are observed in the PFI-PE bands for O2+(B 2Σg−, v+=0–7), indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the ionization transitions. This experiment allows the determination of accurate spectroscopic constants, such as ionization energy (20.29825±0.0005 eV) for the formation of O2+[B 2Σg−, v+=0, N+=1 (F2)] from O2(X 3Σg−, v″=0, N″=1), vibrational constants (ωe+=1152.91 cm−1, ωe+χe+=20.97 cm−1_, and rotational constants (Be+=1.255±0.0015 cm−1, αe+=0.0241±0.00037 cm−1_ for O2+(B 2Σg−, v+). The (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are measured to be ≈0.2–0.6 μs, which are significantly shorter than those of ≈1.9 μs observed for O2+(b 4Σg−, v+=0–5). The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are attributed to the higher kinetic energy releases (or velocities) of O++O fragments resulting from predissociation of the O2+(B 2Σg−, v+=0–6) ion cores. Rotationally resolved PFI-PE measurements also make possible the identification of the weak vibrational progression with the origin at 20.35 eV as associated with transitions to O2+(2Σu−, v+=0–7). The analysis of the rotationally resolved PFI-PE bands for O2+(2Σu−, v+=0 and 1) has yielded accurate rotational constants and IE values for these states. The rotational structures resolved in the O2+(2Σu−, v+=0 and 1) PFI-PE bands are contributed overwhelmingly by the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches, showing that the angular momenta for the outgoing photoelectron are restricted to l=0, 2, and 4. Based on simulation of the observed rotational structures, we also obtain the predissociative lifetimes for O2+(B 2Σg−, v+=0–7) and O2+(2Σu−, v+=0–1) to be in the range of 0.45–2 ps.

Pulsed field ionization-photoelectron bands for CO 2 + (A 2 Π u and B 2 Σ u + ) in the energy range of 17.2-19.0 eV: An experimental and theoretical study Rotationally resolved pulsed field ionization photoelectron study of CO + (X 2 Σ + ,v + =0-42) in the energy range of 13.98-21.92 eV J. Chem. Phys. 111, 8879 (1999); 10.1063/1.480259Rotationally resolved pulsed field ionization photoelectron bands of O 2 + (X 2 Π 1/2,3/2g ,v + =0-38) in the energy range of 12.05-18.15 eVThe vacuum ultraviolet pulsed field ionization-photoelectron ͑PFI-PE͒ spectra for CO 2 have been measured in the energy range of 13.6-14.7 eV, revealing complex vibronic structures for the ground CO 2 ϩ (X 2 ⌸ g ) state. Many vibronic bands for CO 2 ϩ (X 2 ⌸ g ), which were not resolved in previous photoelectron studies, are identified in the present measurement based on comparison with available optical data and theoretical predictions. As observed in the HeI photoelectron spectrum of CO 2 , the PFI-PE spectrum is dominated by the symmetry allowed 1 ϩ ͑symmetric stretch͒ vibrational progression for CO 2 ϩ (X 2 ⌸ g ). However, PFI-PE vibronic bands due to excitation of the symmetry disallowed 2 ϩ ͑bending͒ and 3 ϩ ͑asymmetric stretch͒ modes with both odd quanta, together with the symmetry allowed even quanta excitations, are clearly discernible. The simulation of rotational contours resolved in PFI-PE vibronic bands associated with excitation to the ͑ 1 ϩ ϭ0 -1, 2 ϩ ϭ0 -2, 3 ϩ ϭ0͒ vibrational levels has yielded accurate ionization energies for the formation of these vibronic states from CO 2 (X 1 ⌺ g ϩ ).