Quantum Control of Molecular Collisions Near 1 K

The purpose of a scattering experiment is to probe molecular forces that drive chemistry at the most fundamental quantum level. However, the amount of information that can be extracted is often limited by the presence of a large number of initial states that blurs details of the interaction dynamics. To clearly understand the quantum dynamics of molecular processes it’s absolutely essential to eliminate averaging over the initial states. To control collisions at the quantum level we prepare a scattering sensitive population in a single orientational m quantum sublevel of a rovibrationally excited (v, j) molecular eigenstate using a coherent optical method called Stark-induced adiabatic Raman passage (SARP). To accomplish cold collisions, we developed a molecular beam technique that reduces the relative speed of the colliding partners by co-expanding them in a single supersonic beam. Using SARP prepared (v, j, m) quantum states of HD, and D₂ we studied cold inelastic collisions in a supersonic beam co-expanded with its collision partners He, H₂, and D₂ molecules. The co-expansion brings the collision temperature down to a few Kelvin thereby reducing the number of input orbital states to l = 0, 1, 2. The small number of orbital states and a well-defined internal state prepared by SARP in the input channel has allowed us to develop a partial wave analysis for the scattering angular distribution. The partial wave analysis of the rotationally inelastic scattering has revealed strong stereodynamic preference and existence of a collisional resonance that momentarily binds the reactant molecules in a quasi-bound state within the centrifugal barrier of an orbital state. In my talk I will give an overview of the ongoing quantum state controlled cold collision research in our laboratory.