Collisional energy dependence of Penning ionization and dissociative fluorescence of CS2 by metastable argon

Using the crossed molecular beam method with time-of-flight velocity selection the collisional energy dependence was determined for total Penning ionization of CS2 by Ar(3P2,0) over the energy range 0.015–1.72 eV, and for dissociative fluorescence producing CS(A1Π) over the energy range 0.026−0.428 eV. The cross sections for the two competing processes were found to have nearly the same functional velocity dependence. Both functions decreased as v−n up to 0.08 eV, with n values of 1.02 for ionization and 0.88 for formation of CS(A1Π). Above 0.08 eV the velocity dependence becomes more gradual with n=0.60 for both processes up to 0.40 eV, and at higher collision energies the ionization velocity dependence again becomes steeper, corresponding to a larger n value than at low energies. The similarity of the cross section energy dependence functions indicates that the major dependence at low energy is for formation of a common ArCS2* precursor from which either autoionization or predissociation to form CS(A1Π) occurs, and the low energy n values suggest the presence of a chemical well in the Ar*–CS2 potential. Initial experiments using an electronically pulsed metastable argon beam gave cross section energy dependence functions which were dependent on the argon source pressure and temperature, and these effects were attributed to Ar*–Ar electronic energy exchange collisions within the time-of-flight beam. This was confirmed by changing to mechanical pulsing, which removed the dc Ar beam and eliminated the source pressure and temperature effects.