Vibrational Excitation in Molecular Collisions

The collision energy dependence of the total cross sections for state resolved translation to vibration energy transfer was measured for several neutral systems. Measurements were made for vibrationally inelastic collisions of iodine with helium, neon, and hydrogen isotopes, as well as collisions of aniline and paradifluorobenzene with helium, all in the thermal energy range. Our new experimental technique uses pulsed supersonic molecular beams for initial state selection, crossed at a variable intersection angle for kinematic, continuously tunable collision energy selection. The scattered products are state-selectively detected in the intersection region by laser induced fluoroscence. The iodine cross section energy dependences are approximately linear, quadratic, and cubic for v = 0 to 1, 2, and 3 excitations respectively, as expected from a classical-quantal correspondence principle model. Extreme mode specificity was observed in the polyatoms as only 3 of about 20 energetically accessible vibrations were observed to be collisionally excited. In aniline, the 2 lowest frequency modes were excited. The cross section for single quantum excitations of the inversion mode of the amine group is a strongly decreasing function over the 20 to 250 meV collision energy range. The other observed mode, an out-of-plane bend of the amine group shows a linear onset at threshold for single quantum excitations. The only vibration excited in paradifluorobenzene was the lowest frequency, out-of-plane fluorine bend. Excitation of 1 and 2 quanta was observed. The cross sections both have approximately linear onset at threshold.