On the infrared fluorescence of monolayer 13CO:NaCl(100)

Abstract

Computations are presented to describe and analyze the high levels of infrared laser induced vibrational excitation of a monolayer of absorbed 13 CO on a NaCl(100) surface. Extending the vibrational site-to-site surface hopping technique of Corcelli and Tully, kinetic Monte Carlo computations are used to incorporate single-quantum vibrational pooling and depooling of the 13 CO by phonon excitation to allow up to the n = 45 vibrational state under different lasing conditions. Previously unpredicted pooling peaks at n > 16 are calculated and, under the highest fluence conditions, pooling up to the n = 32 state is found in the calculation. These results lead to the prediction of a secondary local maximum in the dispersed fluorescence of monolayer CO:NaCl(100) under sufficiently high fluence excitation conditions. At times on the order of ms, we recover similar behavior for both high and low fluence results. The calculations confirm that, for situations where the Debye frequency limited n domain restriction approximately holds, the vibrational state population deviates from a Boltzmann population linearly in n, a result that we have derived earlier theoretically for a domain of n restricted to one-phonon transfers. This theoretically understood term, linear in n, dominates the Boltzmann term and is responsible for the inversion of the population of vibrational states, P n .

Theory of vibrational equilibria and pooling at solid-diatom interfaces

“…3). Nonetheless, when plotted on the observable, total overtone fluorescence scale, I(t) vs. t, as in the computational comparison with experiment, 3 the difference from single exponential for the monolayer result is barely noticeable, as in Fig. 4.…”

“…The dynamics in the Monte Carlo simulations are quite complex, [1][2][3] in containing hundreds of first-order and secondorder reactions, but can be treated as having to an effective single exponential decay when there is a rapid equilibration process among the states n as follows. Consider the average number of quanta in any one site:…”

“…(7) by comparing with kinetic Monte Carlo results on the ms experimental timescale (Ref. 3 gives further details of the calculation). The result is seen in Fig.…”

“…Nevertheless, the results demonstrate that it is a useful description of the inverted distribution with its cutoff at n = 10. 3 as the slope of ln(P n ) + E n /kT, vs. n, the vibrational state. For the P n calculated near the end of a pulse of the conditions of the previous monolayer experiment, 10 γ = 131.…”

“…1,2 In this paper, we describe such a statistical theory to explain two key effects: [1][2][3][4][5] (1) an inversion of the population of CO vibrational states and (2) the origin of the single exponential overtone fluorescence decay, the many contributing second-order and first-order steps in the mechanism notwithstanding.…”