Isomerization of CO on a NaCl(100) Surface: Evidence for Resonance Enhanced Quantum Tunneling in Condensed Phase

by Arnab Choudhury
Cumulative thesis
Date of Examination:2024-05-28
Date of issue:2024-06-20
Advisor:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Alec M. Wodtke
Referee:Prof. Dr. Martin Suhm
Persistent Address: http://dx.doi.org/10.53846/goediss-10544

 

 

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Abstract

English

Carbon monoxide (CO) physisorbed on a NaCl(100) surface can undergo an isomerization reaction at cryogenic temperatures. In equilibrium conditions, the CO molecule absorbs on the surface with the C-atom closer to the Na+ ion of the substrate. Upon vibrational excitation, these CO molecules in a monolayer on the surface can flip from the C-bound orientation to another isomer where the O-atom is closer to the Na+ ion. In this thesis work, I have explored the flipping of the CO molecules from the O-bound orientation back to the C-bound orientation using Fourier transform infrared spectroscopy. As this reaction is studied for different isotopologues of CO, we observe that the isomerization from the O-bound to the C-bound orientation on the surface occurs through quantum tunneling. Moreover, the tunneling rates for different isotopologues do not obey a monotonic mass dependence. We have observed that a heavier mass has a higher tunneling rate compared to a lighter mass contradicting our prior knowledge of quantum tunneling. The basic concept of quantum mechanics in the context of wavepackets being scattered from a potential barrier, suggests that a lighter mass should always have a higher tunneling rate owing to its longer De-Broglie wavelength. This thesis work shows that the standard notion of quantum mechanics cannot accurately describe reactions happening in a condensed phase. The experimental observations have guided the development of a theoretical framework to explain quantum tunneling in condensed phases, where the consideration of bound states and the possibility of resonances between them through the potential barrier is of utmost importance. This framework depicts that, in the studied system these resonances between the C-bound and the O-bound states open “Gateways” for the CO molecules to tunnel through the barrier.
Keywords: Surface science; Condensed phase chemistry; Infrared spectroscopy; CO on NaCl(100); Orientational isomerization; Gateway tunneling; Quantum tunneling
 
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