Vibrational energy transfer of excited CO molecules on NaCl(100): a non-equilibrium ab initio molecular dynamics...

CO/NaCl(100) at monolayer coverage is a weakly bound adsorbate system for which unusual phenomena after infrared laser excitation of adsorbate vibrations have been observed. Among them are very long vibrational lifetimes in the CO stretch mode (∼4 ms), efficient vibrational energy pooling, and orientational isomerization from a "C-bound" to an "O-bound" configuration. Here we use non-equilibrium ab initio molecular dynamics (neq-AIMD), coupled with time correlation function (TCF) techniques, to study vibrational relaxation dynamics and related processes after selectively exciting specific normal modes (CO internal stretch, frustrated rotation) across submonolayer and monolayer coverages of this system. We are mostly concerned with picosecond dynamics, and in one case we use recently proposed machine-learned, AIMD-based potentials for CO/NaCl, to explore significantly longer timescales (∼ns). To monitor the dynamics, we present atom- and mode-resolved transient vibrational spectra, kinetic energies and qualitative time-dependent local mode couplings. Time-dependent vibrational sum frequency (VSF) spectra are reported for a flipping CO in the CO/NaCl(100) monolayer, following frustrated bending mode excitation. Important highlights of this work are as follows: (i) localization of vibrational energy in the CO stretch modes on ps timescales and signatures of vibrational energy pooling on ns timescales; (ii) sub- to few-picosecond lifetimes of frustrated bending modes of both "C-bound" and "O-bound" CO adsorbates resulting from two dissipative channels, namely, the surface phonons and neighbouring adsorbates causing successive flipping, desorption and diffusion events; and (iii) identification of vibrational non-adiabatic channels in metastable "O-bound" geometries following CO stretch excitation.