CO (a3Π) quenching at a metal surface: Evidence of an electron transfer mediated mechanism

Vibrationally Mode-Specific Molecular Energy Transfer to Surface Electrons in Metastable Formaldehyde Scattering from Cesium-Covered Au(111)

Nonadiabatic interaction of adsorbate nuclear motion with the continuum of electronic states is known to affect the dynamics of chemical reactions at metal surfaces. A large body of work has probed the fundamental mechanisms of such interactions for atomic and diatomic molecules at surfaces. In polyatomic molecules, the possibility of mode-specific damping of vibrational motion due to the effects of electronic friction raises the question of whether such interactions could profoundly affect the outcome of chemistry at surfaces by selectively removing energy from a particular intramolecular adsorbate mode. However, to date, there have not been any fundamental experiments demonstrating nonadiabatic electron-vibration coupling in a polyatomic molecule at a surface. In this work, we scatter excited metastable formaldehyde and formaldehyde-d2 from a low work function surface and detect ejected exoelectrons that accompany molecular relaxation. The exoelectron ejection efficiency exhibits a strong dependence on the vibrational mode that is excited: out-of-plane bending excitation (ν4) leads to significantly more exoelectrons than does CO stretching excitation (ν2). The results provide clear evidence for mode-specific energy transfer from vibration to surface electrons.

Theoretical study of laser cooling of the TlF+ molecular ion

The feasibility of the thallium monofluoride TlF+ molecular ion towards laser cooling is brought into focus through an electronic structure study. Ab initio calculations are carried out to investigate the four lowest-lying electronic states, X2Σ+, (1)2Π, (2)2Σ+ and (2)2Π, including the spin-orbit coupling effect by employing the Complete Active Space Self Consistent Field (CASSCF) method at the Multireference Configuration Interaction (MRCI) level of theory while invoking Davidson correction (+Q). Potential energy and permanent dipole moment curves are used to determine the corresponding spectroscopic constants and some other equilibrium parameters. Vibrational parameters of vibrational states and transition dipole moments between possible transitions are computed. The calculated parameters are then used to conduct a theoretical study focusing on the potential possibility of TlF+ ionic molecule to be laser cooled on the (2)2Π1/2(ν')-X2Σ+1/2(ν'') transition based on Di Rosa's criteria. With the results obtained being promising, a laser cooling optical cycling scheme is proposed to illustrate the number of pump lasers needed with the corresponding wavelengths that were found to lie within the ranges covered by a specific scientific laser.

Fundamental mechanisms for molecular energy conversion and chemical reactions at surfaces

The dream of theoretical surface chemistry is to predict the outcome of reactions in order to find the ideal catalyst for a certain application. Having a working ab initio theory in hand would not only enable these predictions but also provide insights into the mechanisms of surface reactions. The development of theoretical models can be assisted by experimental studies providing benchmark data. Though for some reactions a quantitative agreement between experimental observations and theoretical calculations has been achieved, theoretical surface chemistry is in general still far away from gaining predictive power. Here we review recent experimental developments towards the understanding of surface reactions. It is demonstrated how quantum-state resolved scattering experiments on reactive and nonreactive systems can be used to test front-running theoretical approaches. Two challenges for describing dynamics at surfaces are addressed: nonadiabaticity in diatomic molecule surface scattering and the increasing system size when observing and describing the dynamics of polyatomic molecules at surfaces. Finally recent experimental studies on reactive systems are presented. It is shown how elementary steps in a complex surface reaction can be revealed experimentally.

A new Stark decelerator based surface scattering instrument for studying energy transfer at the gas-surface interface

We report on the design and characterization of a new apparatus for performing quantum-state resolved surface scattering experiments. The apparatus combines optical state-specific molecule preparation with a compact hexapole and a Stark decelerator to prepare carrier gas-free pulses of quantum-state pure CO molecules with velocities controllable between 33 and 1000 m/s with extremely narrow velocity distributions. The ultrahigh vacuum surface scattering chamber includes homebuilt ion and electron detectors, a closed-cycle helium cooled single crystal sample mount capable of tuning surface temperature between 19 and 1337 K, a Kelvin probe for non-destructive work function measurements, a precision leak valve manifold for targeted adsorbate deposition, an inexpensive quadrupole mass spectrometer modified to perform high resolution temperature programmed desorption experiments and facilities to clean and characterize the surface.

Adsorbate enhancement of electron emission during the quenching of metastable CO at metal surfaces

A monolayer of adsorbed rare gas dramatically enhances electron emission when quenching CO(a³Π) at metal surfaces.