Stereodynamical control of a four-atom reaction: HD + OH → D + H 2 O

To date, studies of differential steric effect in reactive collisions have been limited to atom + molecule systems. How stereodynamics is affected when the other reactant is also a molecule remains an open question. Here, we present a combined high-resolution crossed-beam scattering and full-dimensional quantum dynamical study of the HD ( v = 1, j = 2) + OH → D + H 2 O reaction, in which the HD molecular axis was initially prepared either parallel or perpendicular to the relative velocity using the stimulated Raman pumping scheme. The OH reactant was unpolarized and in its ground vibrational state, with a rotational temperature of ~85 K in the experiment, and was restricted to the ground rovibrational state in the theory. Pronounced stereodynamic effects are observed in both the backward scattering intensity and the product angular distributions. Detailed analysis of scattering wavefunctions reveals that the initial m = ±2 states access a noncoplanar dynamical transition state, distinct from that accessed by m = 0 ( m denotes the projection of j along the direction of relative velocity). The nonzero dihedral angle, a geometric degree of freedom absent in triatomic reactions, leads to reduced reactivity and enhances sideward scattering for the perpendicular configuration, while leaving the product rovibrational state distributions essentially unchanged. In addition, the alignment of the HD rotational angular momentum is preserved in the rotational direction of the product H 2 O.