Paper SS-ThP8
State-Resolved Measurements of CH₄ Activation on Ni(111) Without Laser Excitation

Thursday, October 23, 2008, 6:00 pm, Room Hall D

We describe experimental work that yields vibrational-state-resolved reaction probabilities for a polyatomic molecule without the need for selective laser excitation of the target state. The method relies on a detailed knowledge of the vibrational structure of the molecule under study, its vibrational cooling dynamics in a supersonic expansion, and an understanding of how individual vibrational states contribute to the state-averaged reactivity measured in the experiment. The reagents in beam-surface scattering measurements of surface reactivity typically have a well-defined translational energy and a narrow distribution of rotational states, but the vibrational state distribution of the reagents remains nearly thermalized at the nozzle source temperature. As nozzle temperatures are raised to access higher incident kinetic energies, the thermal population of excited vibrational states grows. The high vibrational state density of polyatomic molecules can result in hundreds, or even thousands of vibrational states contributing to the measured reactivity. In addition to yielding reaction probabilities averaged over many internal states, the state or group of states that dominate reactivity may vary as a function of incident kinetic energy, even at a fixed nozzle source temperature. Recent results from state-resolved measurements of methane activation on Ni(111) allow us to model vibrational-state-averaged beam-surface data to gain insight into how reactivity scales with increasing vibrational excitation. We use this approach to extract a state-resolved reaction probability for the v=0 vibrational ground state of methane dissociating on a Ni(111) surface. At intermediate nozzle temperatures, only the v=0, ν₂, and ν₄ vibrations have sufficient population to impact reactivity. Non-equilibrium vibrational cooling in the expansion relaxes ν₂ to ν₄. Knowledge of S₀ for the v=0 state allows us to extract S₀ for the ν₄ vibrational fundamental of the "umbrella" bending vibration.