Quantum State-Resolved Studies of Methane/Surface Scattering by Vibrational Spectroscopy

Methane dissociation is the rate limiting step in the steam reforming process used by the chemical industry to convert natural gas into a mixture of H₂ and CO known as synthesis gas. To better understand the microscopic mechanism and reaction dynamics of methane chemisorption, we use vibrational spectroscopies and infrared lasers for quantum state-resolved studies of methane dissociation and state-to-state scattering on Ni and Pt surfaces [1]. Our experiments prepare surface incident methane molecules in specific ro-vibrational quantum states by state-selective infrared laser excitation in a molecular beam. The state prepared molecules then collide with a clean single crystal transition metal surface in ultrahigh vacuum and both reactive and non-reactive processes are monitored by infrared spectroscopic techniques.
Surface bound methyl species as products of the dissociative chemisorption of methane are detected by Reflection Absorption Infrared Spectroscopy (RAIRS). RAIRS allows for real-time and in-situ monitoring of the uptake of chemisorbed methyl species enabling quantum state-resolved measurements of reactive sticking coefficients. RAIRS is also used to study the vibrationally bond selective dissociation of partially deuterated methane demonstrating that a single quantum of C-H stretch excitation is sufficient to achieve bond-selective chemisorption. Furthermore, RAIRS allows for site specific detection of reaction products used to measure separately the dissociation probability of methane on steps and terraces sites on Pt(211) [2-3].
Non-reactive, inelastic energy transfer is probed by combining infrared laser tagging of scattered molecules with bolometric detection. These first methane state-to-state scattering experiments yield state-resolved information about rotational and vibrational energy transfer between the incident molecule and the solid surface [4]. Furthermore, for scattering from a reactive surface, we observe evidence for surface-induced vibrational energy redistribution (SIVR) in the scattered molecules, the extend of which is related to the catalytical activity of the target surface. [5-7]. For methane scattering from the inert Au(111) surface, we are able to detect, for the first time to our knowledge, conservation of wavefunction reflection parity in molecule-surface scattering [8] which is a uniquely quantum mechanical effect.

References
[1] H.J. Chadwick and R.D. Beck, Chem. Soc. Rev. 2016, 45, 3576-3594.
[2] H.J. Chadwick, H. Guo, A. Gutiérrez Gonzáles; J.P. Menzel, B. Jackson, R.D. Beck, J. Chem. Phys. 2018, 148, 1470.
[3] A. Gutiérrez Gonzáles. F.F. Crim, R.D. Beck, J. Chem. Phys. 2018, 149, 74701.
[4] J. Werdecker, M.E. van Reijzen, B.J. Chen, R.D. Beck, Phys. Rev. Lett. 2018, 120, 53402.
[5] J. Werdecker, J. Werdecker, B.-J. Chen, M.E. van Reijzen, A. Farjamina, B. Jackson, R.D. Beck, Phys. Rev. Res. 2020, 2, 43251.
[6] P. Floß, C.S. Reilly, D.J. Auerbach, R.D. Beck, Frontiers in Chemistry 2023, 11, 88.
[7] C.S. Reilly, P. Floß, B-J. Chen, D.J. Auerbach, R.D. Beck, J. Chem. Phys. 2023, 158 21.
[8] C. Reilly et al., Science 387, 962–967 (2025).