Insights from Ultrafast Optical Pump - X-Ray Probe Studies of Adsorbate Excitation

Understanding reaction pathways and details of adsorbate-substrate bond breaking is crucial for accurate modeling of catalytic reactions and analysis of prospective catalyst materials. While theoretical calculations are an invaluable tool, there is a large gap between DFT calculations on ideal, periodic slab models, and actual catalytic reactions on active surfaces. Ultrahigh vacuum (UHV) surface science experiments can bridge part of this gap, giving information on adsorbate excitation, bond breaking, and reaction rates, in a controlled environment. Femtosecond high-intensity optical laser pulses are known to populate excited adsorbate states and initiate surface reactions [1], which together with an ultrashort x-ray probe pulse with tunable delay can give time-resolved and element-specific information on the system dynamics [2]. Using this setup we aim to elucidate key reaction mechanisms underlying important catalytic processes, and study them on a sub-picosecond timescale. Recent results on a few model adsorbate systems, such as CO/Ru(0001) [3] and C/Ni(100) [4], show that we can observe and distinguish the time scales of electron excitation, phonon thermalization, and excitation of various adsorbate modes. Detailed theoretical analysis is necessary in order to separate the different effects. About to start operations soon, the upgraded XFEL LCLS-II at SLAC will offer unprecedented time-resolution, enabling even more detailed studies of the time development of surface reactions, for a wide range of catalytic processes.

[1] Bonn et al., Science 285, 1042 (1999)

[2] Öström et al., Science 347, 978 (2015)

[3] Wang et al., Phys. Chem. Chem. Phys. 22, 2677 (2020)

[4] Schreck et al., in preparation