Large Δv′ transitions and isotope effects in the crossed beams inelastic scattering of I₂* (v′ = 35) from H₂ and D₂

Rotationally and vibrationally inelastic scattering from the level v′ = 35 with J′ = 8-11 in the B Ou+ electronic state of I2 have been followed in a crossed molecular beams experiment using H2 and D2 target gases. Center-of-mass collision energies are 89 and 103 meV, respectively. I2 B Ou+ 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 I2 scattering for Δv′'s extending to ±7. The cross sections are remarkably symmetric with respect to gain vs loss of vibrational quanta. They decrease exponentially with increasing |Δv′| for each target, but H2 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-1 for both H2 and D2, a small fraction of the 60-cm-1 vibrational quantum size. Rotationally inelastic scattering produces 2-3 times more rotational excitation with the heavier isotope D2, a consequence of angular momentum conservation. © 1988 American Chemical Society.