Large .DELTA.v transitions and isotope effects in the crossed beams inelastic scattering of excited iodine (v' = 35) from hydrogen and deuterium

Abstract

Rotationally and vibrationally inelastic scattering form the level v' = 35 with J' = 8-11 in the B O/sub u//sup +/ electronic state of I/sub 2/ have been followed in a crossed molecular beams experiment using H/sub 2/ and D/sub 2/ target gases. Center-of-mass collision energies are 89 and 103 meV, respectively. I/sub 2/ B O/sub u//sup +/ is excited by a laser pump and the inelastic scattering is followed by dispersed B ..-->.. X fluorescence. The signal-to-noise is good enough to obtain relative cross sections for I/sub 2/ scattering for ..delta..v's extending to +- 7. The cross sections are remarkably symmetric with respect to gain vs loss of vibrational quanta. They decrease exponentially with increasing absolute value ..delta..v' divides for each target, but H/sub 2/ has a steeper decline. Collision momentum rather than velocity differences appears to be responsible for this isotope effect. The mean vibrational energy change per state-changing collision is only about -9 cm/sup -1/ for both H/sub 2/ and D/sub 2/, a small fraction of the 60-cm/sup -1/ vibrational quantum size. Rotationally inelastic scattering produces 2-3 times more rotational excitation with the heavier isotope D/sub 2/, a consequence of angular momentum conservation