Rotational rainbows in the vibrational predissociation of ICl–He complexes

Strongly peaked ICl fragment rotational distributions are observed following vibrational predissociation of ICl–He B state complexes containing two or three quanta of ICl vibrational excitation. The nascent rotational distributions of the ICl product exhibit two distinct maxima, occurring at j=7 and j=16. A theoretical analysis demonstrates that the two maxima are due to rotational rainbows, arising from the He atom scattering off of the I and Cl ends of the ICl molecule. The vibrational predissociation of ICl–He B(vB=2) is simulated with a semiclassical scattering theory which is analogous to that developed by Schinke [J. Chem. Phys. 85, 5049 (1986)] for direct photodissociation. Vibrational predissociation is modeled as a rotationally inelastic ‘‘half-collision,’’ following deactivation of the ICl vibration. The final rotational angular momentum of the ICl fragments is determined from exact classical trajectories and in a sudden limit on a model potential energy surface. The calculated ICl product rotational distribution, like the experimentally observed distributions, is bimodal.