Cross sections forv=0rarr;1, 2, 3 excitation in I2+He collisions have been measured over the kinetic energy range from threshold to 0.4 eV by means of a new experimental technique in which pulsed molecular beams are crossed at a variable intersection angle and scattered products are state selectively detected by laser induced fluorescence. The values of the measured cross sections range from about 0.2 Aring;2forv=1 at 0.4 eV to about 10minus;5Aring;2forv=3 at the lowhyphen;energy detection limit. The kinetic energy dependences of the cross sections for excitingv=1, 2, and 3 are approximately linear, quadratic, and cubic, respectively. The experimental cross sections are compared with those predicted by a simple classical model and by the detailed quantum calculations reported by Schwenke and Truhlar in the accompanying article. It appears that vibrational excitation in this system comes primarily from nearly impulsive collisions with the steeply repulsive part of the interaction potential in approximately collinear geometries. The most probable rotational excitation is small, but even very highJstates are excited with small probability.
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