Vibrational relaxation of carbon dioxide at LiF(100)

Abstract

The vibrational relaxation of CO2 at LiF(100) has been investigated by monitoring infrared fluorescence from vibrationally excited molecules under conditions where they are relaxed primarily by collisions with the solid surface. The relaxation probabilities are found to be 0.65±0.10 at room temperature and 0.35±0.10 at 450 K. In order to understand better the vibrational relaxation results, angular distributions of CO2 scattered from LiF(100) were measured with a molecular beam scattering apparatus. At slow incident beam velocities, the trapping probability of CO2 at LiF(100) is essentially unity. Thus, in the vibrational relaxation measurements, where the incident velocity is even slower than in the scattering experiments, vibrational relaxation is preceded by trapping. Possible mechanisms for relaxation are discussed. Excitation of phonons in the solid and transfer of energy to other degrees of freedom of the molecule (i.e., translation, rotation, and other vibrational modes) are both plausible relaxation channels.

Scattering and trapping dynamics of gas-surface interactions: Vibrational excitation of CF3Br on graphite

“…It has been estimated that in the absence of fast de-excitation through coupling to electron-hole pairs, the vibrationally hot molecules can survive picoseconds and even nanoseconds on the surface. 20 The vibrational deexcitation is then governed by coupling to surface phonons and if the vibrational frequencies of the molecule are higher than the maximum phonon frequency the process must include a multiphonon excitation, which is occurring with low probability. In the present study the maximum phonon frequency in the z-direction (830 cm Ϫ1 ) is higher than the low vibrational frequencies of the molecule ͑see Table IV͒, and it is probably those vibrational modes that couples most strongly during the residence time on the surface.…”

“…Studies which have been performed under conditions favoring adsorption/desorption processes have revealed that relaxation of vibrationally hot molecules occur over a range of time scales, i.e., it may be accomplished in a few surface collisions or it may need as long as several nanoseconds. 20 Most of the systems mentioned above have been examined using laser spectroscopy. This is a powerful technique that gives state resolved information, but it is limited to mol-ecules for which sensitive spectroscopic detection schemes can be found.…”

TPD of nitric acid from BaNa–Y: evidence that a nanoscale environment can alter a reaction mechanism

“…54 If collisions with the walls of the zeolite pores can push the rate of the H + OH + M reaction into the high pressure limit (as justified in the ESIw), then in the 200-400 1C temperature range, the pseudo-second-order rate constant for H + OH + M is larger than the rate constant for H + NO 2 by a factor of B1.7. However, we expect the zeolite pore walls to act as a more efficient third body than an inert gas, [37][38][39] which would lead to a factor greater than 1.7. Further, the NO 2 molecules are dispersed across the entire volume of the reactor's chamber, which is 4410 3 times the volume of the zeolite cell, while the H and OH radicals may be produced and consumed within the zeolite pores.…”

“…35,36 As shown in the ESI,w using a generous value of 20 A ˚for the mean free path of a gas molecule in BaNa-Y, a collision cross-section of 100 A ˚2 for HNO 3 (g) in Ar, and assuming that collisions with the zeolite pore walls have the same efficiency as collisions with M (where M = Ar) the collision rate with the pore walls is equivalent to that in a gas mixture with a pressure of [Ar] 41 Â10 4 Torr. We expect collisions with the pore walls to be more efficient for vibrational energy transfer than collisions with Ar; [37][38][39] resulting in an effect analogous to that of an even greater pressure of Ar. The crossover from the low pressure to high pressure limit in the Lindemann mechanism for the thermal decomposition of HNO 3 occurs at B1 Â 10 4 Torr (see the ESIw).…”