THE SEARCH FOR DIRECT VIBRATIONAL-EXCITATION IN GAS-SURFACE COLLISIONS OF CO WITH AU(111)

I have examined the dynamics of energy transfer in the CO/Au(111) system to determine the probability of direct vibrational excitation. In contrast to earlier studies of NO/Ag (111), NH3/Au(111), and H-2/Cu(111), no direct vibrational excitation is observed. Measurements have been made using molecular beam techniques to control the collision energy E(i) and angle theta(i) and using laser ionization detection to probe the internal state distributions of the scattered molecules. The probability of direct vibrational excitation is found to remain below 10(-3) for E(i) up to 1.4 eV at surface temperatures T(s) of 300 and 800 K and theta(i)=10-degrees. I have also determined the probability of deexcitation for CO(v=1) colliding with Au(III). Within the uncertainties (approximately 20%), no deexcitation is apparent for E(i) in the range 0.2-1.1 eV with T(s)=300 K. With T(s)=800 K and E(i)=1.1 eV, the survival probability was also indistinguishable from unity. As part of this study, I have determined the variation of the trapping probability of CO on Au (111) as a function of kinetic energy for different surface temperatures, and I report rotational distributions for scattered CO for E(i) of 0.5, 0.9, and 1.4 eV with theta(i)=10-degrees. In contrast to the weak coupling to molecular vibration, a high degree of rotational excitation is observed, yielding pronounced rotational rainbows. Using beams with approximately 1% v=1 populations, I find that the rotational distributions of scattered v=1 molecules are the same as those for scattered v=0 within the uncertainties of the measurements.