EPFL doctorate Award 2011 - Yoder Bruce

“For his pioneering Ph.D. thesis work on gas/surface reaction dynamics using laser-aligned and quantum-state-selected molecules to uncover the stereochemistry of methane chemisorption.”

Steric Effects in the Chemisorption of Vibrationally Excited Methane on Nickel.

Steric effects in a chemical reaction reveal information about the reactive potential energy surface, making studies of stereochemistry a powerful probe of microscopic chemical dynamics.
In this thesis, steric effects in the dissociative chemisorption of quantum state-prepared methane on the (100) and (110) orientations of a nickel surface are detected and quantified for the first time. Exploiting a newly available, continuous-wave, high-power, infrared optical parametric oscillator, we produced an intense, quantum state-prepared molecular beam of methane by rapid adiabatic passage. We used the linearly polarized infrared excitation of C-H stretch modes of two methane isotopologues [CH4(v3) and CD3H(v1)] to align methane’s angular momentum and vibrational transition dipole moment in the laboratory frame. Variation of the rotational branch (P, Q, or R) used for excitation indicates that alignment of u is responsible for the observed alignment dependent reactivity. The highest reactivity is detected with u aligned parallel to the surface plane and lowest for u aligned along the surface normal. Potential explanations for the observed steric effect are discussed in terms of electronically non-adiabatic effects and an alignment-dependent reaction barrier height.
The results in this thesis demonstrate and quantify specific steric requirements for this benchmark gas-surface reaction and will serve as a stringent test of multi-dimensional dynamics calculations.