Highly enhanced reactivity of HCl on the Ag/Au(111) alloy surface via rotational quantum state excitation.

Rotational excitations of reactants are often considered to have little impact on chemical reactivity compared to the excitations of vibrational modes and translational motion. Here, we reveal a significant influence of the rotational excitation of HCl on its dissociation on an Ag/Au(111) alloy surface. This finding is based on six-dimensional time-dependent wave packet calculations performed on an accurately fitted machine learning potential energy surface. Rotational energy is found to be 90 times more effective in driving the reaction than an equivalent amount of translational energy. The reactivity of HCl on Ag/Au(111) is diminished due to a charge transfer effect, which induces a strongly non-monotonic dependence of the minimum energy path on molecular orientation, resulting in the HCl molecule deviating from the minimum barrier. We identify and detail two underlying mechanisms by which rotationally excited HCl can overcome the charge transfer effect between Au and Ag atoms, leading to enhanced reactivity and an intriguing rotational-alignment phenomenon. This finding underscores the profound impact that rotational excitations can exert on molecular reactivity, emphasizing a delicate interplay between various forms of molecular energy and their respective contributions to the reaction mechanism.