Physical Chemistry Seminar: Dr. Barratt Park, Max Planck Institute for Biophysical Chemistry

Physical Chemistry Seminar: Dr. Barratt Park, Max Planck Institute for Biophysical Chemistry (Host: Steve Boxer)

"Mode-specific and axis-specific mechanisms for energy exchange between polyatomic molecules and surfaces"

About the Seminar

The exchange of energy between molecules and surfaces plays a crucial role in heterogeneous catalysis. Molecular beam surface scattering experiments have substantially enhanced our understanding of energy exchange mechanisms. However, to date, almost all quantum state-resolved scattering experiments have focused on atoms or diatomic molecules and almost nothing is known about how the added degrees of freedom in polyatomic molecules influence dynamics at surfaces. I will discuss two results from ongoing experiments on formaldehyde scattering.

1. The conversion of translation to rotational motion often plays a major role in the trapping of small molecules at surfaces, a crucial first step for surface chemistry. We employ a new rotationally-resolved 1+1′ resonance-enhanced multiphoton ionization (REMPI) scheme to measure the rotational distribution of formaldehyde molecules directly scattered from the Au(111) surface. The results indicate a pronounced propensity to excite a-axis rotation (twirling) rather than b- or c-axis rotation (tumbling or cartwheeling), and are consistent with a sterically induced rotational rainbow scattering model. The results suggest that a-axis rotation profoundly affects trapping probability at high incidence energies (> 0.6 eV)
2. In recent decades, molecular beam scattering experiments from metal surfaces have demonstrated the importance of non-adiabatic coupling between electron hole pair excitation and molecular vibration (eHP-V coupling). Research on diatomic molecules suggests that the eHP-V mechanism involves transient electron transfer from the surface into the molecular LUMO. To examine this effect in polyatomic molecules, we have scattered formaldehyde, prepared in different vibrational levels of the metastable ã 3A2 electronic state from a low-work function Cs-covered surface and made quantitative measurement of the exo-electrons emitted from the surface. Preliminary results indicate that the efficiency of exo-electron generation from ν4 (out-of-plane wag) is significantly greater than that of ν2 (CO stretch), which is consistent with expectations for Franck-Condon overlap between the neutral ã 3A2 state and the ground electronic state of the formaldehyde anion. 

About the Speaker

Barratt Park was raised in South Carolina and The Netherlands before attending Davidson College, where he studied chemistry and minored in math. During his PhD at MIT under Bob Field, he focused on high-resolution spectroscopy of small polyatomic molecules. He built the first instrument capable of broadband (>10 GHz) Fourier-transform millimeter-wave spectroscopy, and helped develop Chirped-Pulse Millimeter-Wave Spectroscopy into a powerful tool for chemical kinetics and dynamics. He also used multiple-resonance techniques to investigate excited molecules "in the act" of isomerizing, and to characterize conical intersections and avoided crossings relevant to the atmospheric photodissociation of SO2.

During his Humboldt Fellowship with Alec Wodtke in Göttingen, Germany, Barratt changed his focus to molecular dynamics and kinetics at surfaces. He elucidated the trapping mechanism of formaldehyde on Au(111), and he investigated mode-specific dynamics of nonadiabatic electron transfer between polyatomic molecules and surfaces. Currently, he is working on an advanced new ion imaging method, which is being used to unravel the kinetics and site-specific mechanisms for heterogeneously catalyzed reactions. Outside of the lab, Barratt enjoys classical music and is the lead singer for Germany's favorite band, "Alec Wodtke and the Heartbreakers."