Photoionization yield curves have been obtained for the major ions from CH 3 SH. Auxiliary studies with selectively deuterated species reveal that CD 3 S+ has a slightly lower threshold than CDzSH+ from CD 3 SH; but CHzSD+ has a lower threshold than CH 3 S+ from CH 3 SD. An analysis of alternative interpretations is presented. The isotopic studies also imply that CHzS+ is more stable than HCSH+, and HCS+ is more stable than CSH+. The value..:l H'f,O(HCS+) ~259.2±0.9 kcal/mol obtained in this study is in good agreement with a bracketed proton affinity for CS, but differs significantly from a recent ab initio calculation. This result confirms the view that HCS+ can be formed in interstellar clouds by exothermic reactions. The reaction CH 3 S+ /CHzSH+ ~HCS+ + Hz proceeds weakly at threshold, and much more strongly after overcoming an activation barrier of -0.9 eV. This ion yield curve is shown in Fig. 3, again with an enlarged threshold region. A very weak onset is observed at -1170 'A (-10.6 eV), slowly increasing to -1070 'A (-11.6 eV), where it abruptly grows and develops a cusp. To shorter wavelengths it then remains relative-Here aH, aD refer to the statistical degeneraCies of the two reactions, the G*'s are the sum of states in the respective activated complexes (or transition states), and E is the internal energy relative to the activation energy
An apparatus is described for performing molecular ion photofragment spectroscopy with an ultraviolet laser. Preliminary results are given for the photodissociation of H+2 , HD+, D+2 , D2O+, OD+, ND+3 , and ND+2 , using a laser energy of 6.42 eV. Where energetics permit alternative modes of dissociation, selectivity is observed and attributed to selection rules and correlation rules. With polyatomic ions, the distribution of recoil energies provides insight into the partition of excess energy between internal and kinetic energies of products. In the decomposition of ND+3 , results are obtained which call into question some recently reported thermochemical quantities.
Energy-dependent mass spectra are presented for the most prominent ions produced upon photoionization of hydroxylamine. Two distinct thresholds are oberved for mass 31, which we attribute to onsets for formation of HNO+ and NOH+. The corresponding heats of formation are ΔH°f,0 (HNO+) = 256.8±1.4 kcal/mol and ΔH°f,0 (NOH+) = 274.8±0.7 kcal/mol. The thresholds for mass 32, which we tentatively ascribe to H2NO+, corresponds to 224.6±0.2 kcal/mol for its heat of formation. Each of these entities can therefore be produced in an exothermic reaction of H+3 with NO, and represent mechanisms for depleting NO in the interstellar clouds. Experimental breakdown diagrams are generated and compared with predictions of QET. It is found necessary to invoke a three-body decomposition to explain some observations at higher energy. A theory is presented to accommodate this process within the framework of QET.
Infrared laser pumping of a supersonic beam followed by infrared fluorescence was employed to measure the energy dependence of the R↔T energy transfer. (AIP)
State-to-state cross sections for rotationally inelastic collisions of HF (v,J) with Ne, Ar, and Kr have been measured. Laser pumping of the molecular beam to the initial states v = 1, J = 1–6, and v = 2, J = 2, followed by infrared fluorescence, permitted measurements of relative cross sections with ‖ ΔJ ‖⩽8. The collision energy was varied between 4 and 16 kcal/mol. These cross sections could be fitted well using an inverse-power dependence on the rotational energy gap [due to Pritchard and co-workers; J. Chem. Phys. 70, 4155 (1979)] for rotational energy transfers of up to 55% of the initial translational energy. The energy-corrected sudden approximation was used to determine an ’’effective’’ collision length for rotationally inelastic scattering. The scattering is thought to occur predominantly on the repulsive wall of the intermolecular potential, except for the J = 1→J′ = 0 transition, which is shown to be sensitive to the depth of the van der Waals attractive well.
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