ALBERT

Notes: The desorption of CH4 physisorbed on Ni(111) is observed to be induced by collision with Ar atoms incident with energies less than 2 eV. The absolute cross section for collision-induced desorption of CH4 in the low coverage limit of an isolated CH4 molecule and from a saturated CH4 monolayer is measured as a function of the kinetic energy and incident angle of the Ar beam. The dominant mechanism for collision-induced desorption is determined to involve the direct collision of the incident Ar with the physisorbed CH4. Indirect, surface mediated desorption processes and multiple desorptions are found to be unimportant. Three-dimensional, classical molecular dynamics simulations based upon a hard sphere/hard cube model of the direct collision mechanism show that the complicated dependence of the desorption cross section at low CH4 coverage on the Ar energy and incident angle is the result of two competing dynamical effects: the increase in the geometrical collision cross section and the decrease in the Ar kinetic energy that can be transferred to CH4 motion normal to the surface as the Ar incident angle increases. Multiple Ar–CH4 collisions and mirror collisions are found to make relatively minor contributions to the cross section for collision induced desorption. Normal energy accommodation during the CH4-surface collision plays a significant role in determining the threshold energy for desorption. At high CH4 coverage, the obstruction of small impact parameter, head-on Ar–CH4 collisions by neighboring CH4 molecules at large angles of incidence is the origin of the difference in the cross section observed for low and high CH4 coverage.

Notes: The dissociation of CH4 physisorbed on Ni(111) at 46 K is observed to be induced by the impact of incident inert gas atoms. The dynamics and mechanism of this new process, collision induced dissociative chemisorption, are studied by molecular beam techniques coupled with ultrahigh vacuum electron spectroscopies. The absolute cross section for collision induced dissociation is measured over a wide range of kinetic energies (28–109 kcal/mol) and incident angles of Ne, Ar, and Kr atom beams. The cross section displays a complex dependence on the energy of the impinging inert gas atom characteristic of neither total nor normal energy scaling. Quantitative reproduction of the complex dependence of the cross section on the Ar and Ne incident energy by a two-step, dynamical model establishes the mechanism for collision induced dissociation. Collision induced dissociation occurs by the impulsive transfer of kinetic energy upon collision of Ar or Ne with CH4, followed by the translationally activated dissociative chemisorption of the CH4 upon its subsequent collision with the Ni surface. The dependence of the probability of activated dissociation on the resultant CH4 normal energy derived from the fit of the model to the experimental cross section is in excellent agreement with the results of a previous study of the translationally activated dissociative chemisorption of CH4 on Ni(111). Collision induced activation and translational activation are shown to be consistent mechanisms for providing energy to CH4 to surmount the barrier to dissociative chemisorption.

Notes: The activation of the dissociative chemisorption of CO on Ni(111) by translational and vibrational energy is probed. Molecular beam techniques produce CO molecules with high kinetic energies and with some vibrational excitation. Thermal desorption and high resolution electron energy loss spectroscopy detect the product of the chemisorption event. The maximum translational and vibrational energies attainable in these experiments, 45 and 18 kcal/mol, respectively, are observed not to activate the dissociative chemisorption of CO. These experiments are sensitive to dissociation probabilities as small as 2×10−6 and 9×10−4 at the maximum values of translational and vibrational energy, respectively. It is concluded that translational energies greater than 45 kcal/mol do not contribute to the CO dissociation rate at high pressures. Rather, the potential energy surface of the CO–Ni(111) interaction likely requires vibrational excitation greater than the amount that can be achieved in this experiment for activation of the C 3/4 O bond.

Notes: The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≈2106 cm−1 ) of top-site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO-covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution 〈1 ps and monochromator resolution 〈1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD〈0 show features characteristic of a perturbed free induction decay, which are expected based on multiple-level density-matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the time scale for relaxation do not depend (within experimental error) on coverage for 0.1≤aitch-thetaCO≤0.5 ML or on temperature for 150≤Ts≤300 K. A model for coupled anharmonic oscillators qualitatively explains the tD〉0 spectra in terms of a population-dependent decrease in frequency of the one-phonon band, as opposed to a transition involving a true CO(v=2) two-phonon bound state. The rapid relaxation time and its insensitivity to Ts and aitch-thetaCO are consistent with electron–hole pair generation as the dominant decay mechanism.

Notes: The effect of translational energy on the molecular chemisorption of CO on a Ni(111) surface is used as a probe of the dynamics of the adsorption process. Initial adsorption probabilities, apparent saturation coverages, spatially resolved Auger coverage profiles, and high resolution electron energy loss spectra of CO deposited on the Ni surface from a supersonic molecular beam are measured as a function of translational energy. It is found that the initial adsorption process for CO molecules incident with energies less than 4 kcal/mol differs from that for molecules incident with higher energies. Molecules with kinetic energies below 4 kcal/mol adsorb with an initial adsorption probability of 0.85±0.04 and a high apparent saturation coverage. Molecules with translational energies between 7 to 30 kcal/mol have an initial adsorption probability of 0.46±0.03, and an apparent saturation coverage approximately half that of the low energy molecules. Since the CO packing density and the final chemisorption states are shown to be independent of incident energy, the two apparent saturation coverages are the result of a difference in the surface area over which the CO molecules are spread. This is verified by spatially resolved Auger coverage profile measurements. Molecules at low energies are initially adsorbed with higher mobility than those incident with larger translational energies. High resolution electron energy loss spectra and thermal desorption spectra show no translational energy-induced dissociation. The frequency shift of the bridge-bonded CO stretching mode measured at the periphery of the molecular beam image shows that the energy-induced difference in the CO mobility during the chemisorption process is qualitatively similar on both the clean and partially CO-covered surface. These results are interpreted as evidence for two adsorption pathways into the molecular chemisorption state. Molecules incident on the surface with low energies are identified as mobile precursor molecules to the molecularly chemisorbed molecules. The precursor molecules have access to the molecular chemisorption state via a low energy pathway. As the incident translational energy is increased beyond the effective 4–7 kcal/mol energy barrier, a new adsorption pathway directly into the lessmobile chemisorption state becomes accessible. The natures of the precursor molecule, the effective energy barrier and the low energy pathway are discussed.

Notes: A rapid site exchange process is observed in the equilibrated chemisorbed layer of CO on Ni(111). Following adsorption at 298 K, the relative populations of CO adsorbed on atop sites and twofold bridge sites are monitored by the high resolution electron energy loss intensities of the respective CO vibrational modes as a function of surface temperature. Since equilibrium is established, the binding energy difference between the terminal and bridge adsorption sites is determined. The bridge site is more stable than the atop site by 0.94±0.15 kcal/mol at a coverage of 0.13. As the coverage is increased to 0.42, the difference in binding energies decreases to 0.44±0.07 kcal/mol. At saturation coverage, 0.5, the binding energy difference effectively becomes very large, resulting in CO occupation of the twofold bridge sites exclusively.