Bastian C. Krüger

Vibrational inelasticity of highly vibrationally excited NO on Ag(111).

Multiquantum relaxation of highly vibrationally excited nitric oxide on noble metals has become one of the best studied examples of the Born-Oppenheimer approximations failure to describe molecular interactions at metal surfaces. When first reported, relaxation of highly vibrationally excited NO occurring in collisions with Au(111) surfaces exhibited the largest vibrational inelasticity seen in molecule-surface collisions, and no system has been found to date exhibiting a greater vibrational inelasticity. In this work, we compare the relaxation of NO(v = 11) in scattering events on Ag(111) to that on Au(111). The relaxation probability and the average vibrational energy loss are much higher when scattering from Ag(111). We discuss possible reasons for this remarkable phenomenon, which may be related to the dissociation of NO, possible on Ag(111) at lower energy compared with Au(111).

Controlling an electron-transfer reaction at a metal surface by manipulating reactant motion and orientation.

The loss or gain of vibrational energy in collisions of an NO molecule with the surface of a gold single crystal proceeds by electron transfer. With the advent of new optical pumping and orientation methods, we can now control all molecular degrees of freedom important to this electron-transfer-mediated process, providing the most detailed look yet into the inner workings of an electron-transfer reaction and showing how to control its outcome. We find the probability of electron transfer increases with increasing translational and vibrational energy as well as with proper orientation of the reactant. However, as the vibrational energy increases, translational excitation becomes unimportant and proper orientation becomes less critical. One can understand the interplay of all three control parameters from simple model potentials.

Intermediate state dependence of the photoelectron circular dichroism of fenchone observed via femtosecond resonance-enhanced multi-photon ionization

The intermediate state dependence of photoelectron circular dichroism (PECD) in resonance-enhanced multi-photon ionization of fenchone in the gas phase is experimentally studied. By scanning the excitation wavelength from 359 to 431 nm, we simultaneously excite up to three electronically distinct resonances. In the PECD experiment performed with a broadband femtosecond laser, their respective contributions to the photoelectron spectrum can be resolved. High-resolution spectroscopy allows us to identify two of the resonances as belonging to the B- and C-bands, which involve excitation to states with 3s and 3p Rydberg character, respectively. We observe a sign change in the PECD signal, depending on which electronic state is used as an intermediate, and are able to identify two differently behaving contributions within the C-band. Scanning the laser wavelength reveals a decrease of PECD magnitude with increasing photoelectron energy for the 3s state. Combining the results of high-resolution spectroscopy and femtosecond experiment, the adiabatic ionization potential of fenchone is determined to be IPaFen=(8.49±0.06) eV.

Vibrational Relaxation of Highly Vibrationally Excited CO Scattered from Au(111): Evidence for CO– Formation

Electronically nonadiabatic dynamics can be important in collisions of molecules at surfaces; for example, when vibrational degrees of freedom of molecules are coupled to electron-hole-pair (EHP) excitation of a metal. Such dynamics have been inferred from a host of observations involving multiquantum relaxation of NO molecules scattered from metal surfaces. Electron transfer forming transient NO- is thought to be essential to the nonadiabatic coupling. The question remains: is this behavior usual? Here, we present final vibrational state distributions resulting from the scattering of CO(vi = 17) from Au(111), which exhibits significantly less vibrational relaxation than NO(vi = 16). We explain this observation in terms of the lower electron affinity of CO compared to NO, a result that is consistent with the formation of a transient CO- ion being important to CO vibrational relaxation.