Surface strain hampers dissociation and induces curious rotational-alignment effect for HCl on the Au/Cu(111) alloy surface

We present the first six-dimensional quantum dynamics study of HCl on a strained pseudomorphic monolayer of Au deposited on a Cu(111) substrate, utilizing a newly developed machine learning-based potential energy surface. The strain in the surface lattice, resulting from a 12.62% compression of the Au monolayer, induces a significant high barrier height (1.81 eV) and a tight saddle point. These effects lead to a marked suppression of the reactivity of ground-state HCl (v = 0) on Au/Cu(111), while the dissociation probability of vibrationally excited (v = 1) HCl increases substantially. As a result, the vibrational efficiency is notably higher compared to those for HCl on pure Au(111) and alloyed Au/Ag(111) surfaces. In addition, the surface strain induces a distinctive rotational alignment effect in the title reaction, where for HCl in the (v = 0, j) states, dissociation is most favorable when an HCl molecule collides with its rotation perpendicular to the Au/Cu(111) surface (the cartwheel alignment). In contrast, for HCl in the (v = 1, j) states, the opposite (helicopter) alignment is preferred. This leads to a pronounced difference in the effects of rotational excitation, depending on whether HCl is initially in the ground or excited vibrational state.