Katzenstein Distinguished Lecture Series

This talk contrasts molecular collisions, particularly reactive collisions, in the familiar “warm” domain with the “ultracold” regime (< 1 mK), where the relative deBroglie wavelengths become long compared with the range of interaction of the collision partners.  Ultracold collisions have much greater sensitivity to entrance channel interactions, so offer the prospect of tuning by external fields to control onset of reaction.  Unfortunately, other aspects introduce handicaps.  In ultracold collisions, kinematic constraints impose severe limitations on the observable dynamical properties.  Also, the very slow approach of the reactants allows rapid rovibrational motions of a target molecule to average the interactions.  The resonance and tunneling effects that occur thus pertain to a “dressed” potential energy surface, which can differ in important ways from the PES derived as usual from electronic structure calculations and the Born-Oppenheimer approximation.  In the exit channel for appreciably exoergic reactions, the deBroglie wavelengths become short, so the exit dynamics are much like those for warm collisions.   There is much of special interest to explore in the “cold” collision regime (between 1 mK and a few K), as illustrated in analogous nuclear physics.   The current “alkali age” of cold molecule research can be expanded in chemical scope by drawing on work done decades ago, in the early alkali era of warm molecular beam experiments.