Quantum-Controlled Collisions of H₂ Molecules

Abstract

The amount of information that can be obtained from a scattering experiment depends upon the precision with which the quantum states are defined in the incoming channel. By precisely defining the incoming states and measuring the outgoing states in a scattering experiment, we set up the boundary condition for experimentally solving the Schrodinger equation. In this Perspective we discuss cold inelastic scattering experiments using the most theoretically tractable H 2 and its isotopologues as the target. We prepare the target in a precisely defined rovibrational (v, j, m) quantum state using a special coherent optical technique called the Stark-induced adiabatic Raman passage (SARP). v and j represent the quantum numbers of the vibrational and rotational energy levels, and m refers to the projection of the rotational angular momentum vector j on a suitable quantization axis in the laboratory frame. Selection of the m quantum numbers defines the alignment of the molecular frame, which is necessary to probe the anisotropic interactions. For us to achieve the collision temperature in the range of a few degrees Kelvin, we co-expand the colliding partners in a mixed supersonic beam that is collimated to define a direction for the collision velocity. When the bond axis is aligned with respect to a well-defined collision velocity, SARP achieves stereodynamic control at the quantum scale. Through various examples of rotationally inelastic cold scattering experiments, we show how SARP coherently controls the dynamics of anisotropic interactions by preparing quantum superpositions of the orientational m states within a single rovibrational (v, j) energy state. A partial wave analysis, which has been developed for the cold scattering experiments, shows dominance of a resonant orbital that leaves its mark in the scattering angular distribution. These highly controlled cold collision experiments at the single partial wave limit allow the most direct comparison with the results of theoretical computations, necessary for accurate modeling of the molecular interaction potential.

Theoretical Insights into the Dynamics of Gas-Phase Bimolecular Reactions with Submerged Barriers

“…Another exciting new frontier is cold and ultracold chemistry, ,− in which the reactions necessarily require the absence of an intrinsic reaction barrier. , So far, many cold collision studies have focused on nonreactive scattering, , but progress has already been made for cold reactive scattering. , For instance, the reactivity of the slightly exoergic KRb + KRb → K 2 + Rb 2 reaction has recently been elucidated at the state-to-state level near 200 nK. , This reaction proceeds without a barrier via an extremely long-lived intermediate in which energy randomization is likely to be close to completion . While most of the measured state-to-state reaction probabilities are in reasonably good agreement with statistical theory predictions, there are some outliers that are not fully understood.…”

Quantum stereodynamics of cold molecular collisions

“…For j = 2, the experiments considered three specific preparations: H-SARP (pure m = 0), a bi-axial state X-SARP (a linear combination of m = ±1), and V-SARP (a linear combination m = 0, ±2). 37 In particular, for j = 2 the V-SARP preparation corresponds towhere we have assumed molecule 1 with angular momentum j 1 is polarized. When integrated over the azimuthal angle ζ , the DCS for a given SARP preparation is given by 39 Note that the interference term between different initial m -states in the SARP preparation drops off up on integrated over ζ .…”

“…Details of the SARP method and its applications to rotational quenching in atom-diatom and diatom-diatom collisions have recently been reviewed by Mukherjee. 37 Here, we focus on the theoretical work motivated by experiments in which the alignment of the molecular axis can be selected and varied. [38][39][40][41][42][43][44][45][46][47][48][49] The article is organized as follows.…”

“…Recently Perreault et al 27–31 and Zhou et al 32,33,36,37 demonstrated that one can study cold collisions in intrabeam scattering without actually cooling or trapping the collision partners. The technique first demonstrated by Suits and coworkers 81,82 for atom-atom collisions involves co-expanding the collision partners in the same molecular beam.…”

“…[74][75][76][77][78] Rotational quenching and excitation in CO and O 2 by collisions with He and H 2 have been the topic of several experiments in recent years using cryogenic techniques at temperatures as low as 5 K. 79,80 Quantum close-coupling calculations were highly successful in reproducing the experimental results that also revealed narrow resonances in the energy dependence of the rotational excitation cross sections. 74,76,77,79,80 Recently Perreault et al [27][28][29][30][31] and Zhou et al 32,33,36,37 demonstrated that one can study cold collisions in intrabeam scattering without actually cooling or trapping the collision partners. The technique first demonstrated by Suits and coworkers 81,82 for atom-atom collisions involves co-expanding the collision partners in the same molecular beam.…”