Vibrational energy pooling in CO on NaCl(100): Methods

Vibrational states as high as n=15 have been experimentally observed in CO molecules adsorbed in a monolayer on the NaCl(100) surface after pumping the n=0→1 vibrational transition with a short (5 μs) infrared laser pulse. These high states become populated from successive single vibrational quantum exchanges between CO molecules on the surface, CO(m)+CO(n)→CO(m−1)+CO(n+1), mediated by dipole–dipole interactions and driven by the anharmonicity of the CO bond vibration. The rates for all of the possible channels of vibrational energy flow in the CO/NaCl(100) system, exchange, relaxation, and fluorescence, were calculated using perturbation theory for a model in which the CO bond vibration is treated as a Morse oscillator and is coupled to a bath of harmonic oscillators with a Debye density of states representing the underlying NaCl substrate. These rates form a Master equation that governs the overall vibrational population dynamics of CO molecules in the monolayer, and was solved using kinetic Monte Carlo (KMC) techniques. Time-dependent vibrational population distributions, Pn(t), representing the probability of finding a CO molecule in the monolayer in vibrational state n at time t, were obtained from the KMC simulations. The results are in good accord with experiment. The maximum achievable excitation is found to be limited by a crossover in the vibrational relaxation and excitation transfer rates with increasing quantum number.