The search for direct vibrational excitation in gas–surface collisions of CO with Au(111)

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Abstract
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 H2/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 θ 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 θ i =10°. I have also determined the probability of deexcitation for CO(v=1) colliding with Au(111). Within the uncertainties (∼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 θ i =10°. In contrast to the weak coupling to molecular vibration, a high degree of rotational excitation is observed, yielding pronounced rotational rainbows. Using beams with ∼1% v=1 populations, I find that the rotational distributions of scatteredv=1 molecules are the same as those for scatteredv=0 within the uncertainties of the measurements.