Mechanical Vibrational Relaxation of NO Scattering from Metal and Insulator Surfaces: When and Why They Are Different

NO scattering from metallic and insulating surfaces represents contrasting benchmark systems for understanding energy transfer at gas-surface interface. Strikingly different behaviors of highly vibrationally excited NO scattered from Au(111) and LiF(001) were observed and attributed to disparate electronic structures between metals and insulators. Here, we reveal an alternative mechanical origin of this discrepancy by comparative molecular dynamics simulations with globally accurate adiabatic neural network potentials of both systems. We find that highly vibrating NO can reach for the high-dissociation barrier on Au(111), by which vibrational energy can largely transfer to translation or rotation and further dissipate into substrate phonons. This mechanical energy transfer channel is forbidden in the purely repulsive $\mathrm{NO}/\mathrm{LiF}(001)$ system or for low-vibrating NO on Au(111), where molecular vibration is barely coupled to other degrees of freedom. Our results emphasize that the initial state and potential energy landscape concurrently influence the mechanical energy transfer dynamics of gas-surface scattering.

Figure 1

Two-dimensional cuts of (a) the $\mathrm{NO}/\mathrm{LiF}(001)$ and (b) the $\mathrm{NO}/\mathrm{Au}(111)$ PESs as a function of the N-O distance ($r$) and the molecular height ($Z$) above the surface, with other coordinates (defined in Fig. S1 [44]) fixed at the adsorption state on LiF(001) and the dissociation transition state on Au(111). A representative trajectory (red dots) of each system is projected on top of the corresponding PES.

Figure 2

Comparison of vibrational state distributions of highly vibrationally excited NO scattered from (a) Au(111) and (b) LiF(001), including the adiabatic QCT results on the EANN PES (red), nonadiabatic molecular dynamics with electronic friction (MDEF) results with the scaled orbital-dependent friction tensor taken from Ref. [36] (green), and experimental data (black). Initial conditions are selected mimicking the corresponding experiments.

Figure 3

Kinetic energy and geometric evolutions as a function of time during a representative scattering trajectory for (a)–(c) $\mathrm{NO}(v_i=12,E_i=0.42\mathrm{eV})$ from LiF(001) and (d)–(f)$\mathrm{NO}(v_i=16,E_i=0.52\mathrm{eV})$ from Au(111), including the kinetic energy in NO translation ($E_{\mathrm{trans}}$, black), rotation ($E_{\mathrm{rot}}$, blue), and vibration ($E_{\mathrm{vib}}$, green), the average kinetic energy of surface atoms relative to the initial value ($E_{\mathrm{LiF}}$ and $E_{\mathrm{Au}}$, red), the N-O distance ($r$, magenta), and the molecular height ($Z$, gray).