When carbon monoxide goes “upside down”: vibrational signatures of CO at NaCl(100) from <i>ab initio</i> molecular dynamics

CO adsorbed on NaCl(100) is a model system for surface science showing a rich variety of interesting phenomena. It features several adsorption phases like tilted/antiparallel or perpendicular/upright, very long vibrational lifetimes of the CO internal stretch (IS) mode, anharmonicity-driven vibrational energy pooling, "C-bound" <i>vs.</i> "O-bound" adsorption, and heavy-atom gateway tunneling during CO inversion at low temperatures. Typically, these features and phenomena are experimentally probed by stationary and time-resolved vibrational spectra, exhibiting characteristic differences between the various adsorption modes and phases. To gain atom- and time-resolved insight into vibrational response of CO molecules on NaCl(100), vibrational density of states (VDOS), infrared (IR) and vibrational sum frequency (VSF) spectra are computed from velocity velocity correlation functions (VVCFs) by <i>ab initio</i> molecular dynamics (AIMD) for various coverages, temperatures and phases. In agreement with experiments, we find that increasing CO ("C-bound") coverages as well as CO inversion lead to redshifts of the CO IS mode. We predict more diffuse spectra at <i>T</i> = 300 K compared to 30 K, reflecting the disorder of adsorbates and monolayer instability at room temperature. Analyzing molecularly decomposed and internal VDOS curves as well as computed non-linear correlation matrices give further insight into the complex molecular dynamics underlying the vibrational spectra, notably for the low-frequency regime where frustrated rotations, translations and intermolecular motions come into play. On a methodological side, we also test and discuss some intricate details of how to compute IR and VSF response using a modified formulation of the VVCF methods [Ohto <i>et al.</i>, <i>J. Chem. Phys.</i>, 2015, <b>143</b>, 124702], by including time and angle-dependent dipole and polarizability derivatives as well as intermolecular couplings by cross correlations. Their effect on computed vibrational spectra is studied. These findings provide a detailed, microscopic insight into the picosecond vibrational spectra and dynamics of CO on NaCl(100), highlighting the effects of temperature, coverage, and changes in adsorbate orientation.