Infrared Detection of Molecular Internal Energy after Interactions with Platinum Surfaces.

After molecules have desorbed from or been scattered by a surface, they carry detailed information about their interactions in their internal degrees of freedom. The results discussed in this dissertation come from three interactions: CO(,2) scattered by platinum, CO desorbed from platinum, and CO(,2) formed by CO oxidation on a platinum surface. Infrared emission was detected and analyzed with a Fourier transform infrared spectrometer to determine the population distributions of rotational and vibrational states of the molecules in the gas phase after interaction with the surface. In order to minimize gas phase collisions that perturb the state distributions and yet have densities high enough for good signal-to-noise, a nozzle free jet was used. The directionality and velocity of the molecules in the jet improved the signal-to-noise by about an order of magnitude for a given density at the surface. The scattering of CO(,2) was studied by the detection of infrared emission from its asymmetric stretch (AS) mode. Even though the energy of interaction between CO(,2) and platinum is small and therefore the residence time on the surface is short (particularly in the surface temperature range used here, 700 K to 1500 K), the accommodation of the asymmetric stretch mode was surprisingly high and dropped as surface temperature increased. The high value is in agreement with vibrational deactivation results of other research groups. Perhaps more surprising is that very few of the molecules excited in the asymmetric stretch mode were also excited in other vibrational modes. The rotational populations did not depend on initial translational energy and were Boltzmann distributed (except for the first few rotational states) with temperatures considerably below the surface temperature. For molecules that did not change their vibrational state, translational effects were observed. For CO, because of the long surface residence time, the desorption process was studied. This was confirmed by the equilibrium distributions of vibrationally excited states with temperature equal to that of the surface. Despite the length of time the molecules must have remained on the surface to come to vibrational equilibrium, the rotational mode was not fully equilibrated. The rotational distributions were markedly non-Boltzmann and varied considerably with the surface temperature. Because CO(,2) did not chemisorb on platinum after formation by CO oxidation, it was found to escape the surface with a considerable amount of internal energy. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.