Formation of a long-lived collision complex in alkali diatomic-rare gas collisions and rotational excitation via a mode-specific dissociation

We report an unusual pressure dependence of the gas phase dynamics of alkali diatomics in 1Π excited states in their interactions with certain collision partners. This behavior consists of a change in the relative rates of the ΔJ=+1 and ΔJ=−1 collision-induced transitions, which results in a dominance of rotational excitation at high pressures. This effect occurs only for the e Λ-doublet component of the excited 1Π state. Also observed is a change in the polarization of the parent line such that the polarization ratio increases with increasing pressure. The dependence of this behavior on intermolecular potential, reduced duration of collision, rotational state, and temperature lead us to propose that a long-lived complex or orbiting resonance is responsible for the effects observed. This species appears to be formed during the lifetime of the excited state of the diatomic. Knowledge of the molecule-fixed geometry of the Λ doublets allows us to propose a structure for the long-lived species and to speculate that the observed characteristic rotational excitation results from specific vibrational and rotational predissociation processes within the complex.