Vibrational energy pooling in CO on NaCl(100): Simulation and isotope effect

The phenomenon of vibrational energy pooling in a CO monolayer on a NaCl(100) surface is investigated using a kinetic simulation approach. The kinetic Monte Carlo (KMC) method requires as, input the rate constants for each available channel of vibrational energy flow in the system: laser excitation; vibrational relaxation to the substrate; vibrational energy transfer between nearest neighbor CO molecules on the surface; radiation. These rates were computed using perturbation theory and available experimental information. Our simulations predict a dramatic isotope effect in which energy is seen to preferentially pool on heavier isotopomers of CO if a natural abundance sample of CO is optically pumped. Simulations of an isotopically pure CO/NaCl(100) system continuously pumped by a laser source demonstrate that vibrational energy pools containing more and more quanta can be formed by using more intense lasers until a theoretical crossover is reached where vibrational relaxation rates become faster than vibrational energy pooling rates. This crossover occurs at n = 18 for (CO)-C-12-O-16. Finally, the effect of temperature on the vibrational dynamics in the CO/NaCl(100) system is explored with the result that finite temperature facilitates energy redistribution within the monolayer, hastening the overall vibrational relaxation dynamics.