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  • Letter
  • Open Access

Coherent vibrational dynamics in molecular bond breaking: Methyl radical umbrella mode probed by femtosecond x-ray spectroscopy

Christian A. Schröder1,2,*,†, John H. Hack1,2,*, Joshua L. Edwards1,2, Zhiyu Zhang1,2, J. Tyler Kenyon1,2, Qiyue Wang3, Han Wang3, Daniel M. Neumark1,2, and Stephen R. Leone1,2,4

  • *These authors contributed equally to this work.
  • †Contact author: caschroeder@lbl.gov
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Phys. Rev. Research 8, L022033 – Published 22 May, 2026

DOI: https://doi.org/10.1103/32v3-tgfd

Abstract

We report on the observation of coherent molecular vibrations in the methyl radical, which is produced by 267nm photodissociation of methyl iodide. The radical is excited to high levels in its ν2 “umbrella” vibrational mode by the dissociation, and the ensuing coherent vibrational dynamics are observed by measuring ultrafast time-dependent changes in the x-ray transition energy from the C1s to the singly occupied valence orbital. Due to symmetry, the real-space vibrational motion appears predominantly in the x-ray energy shift at the difference frequencies of the ν2 progression, although the fundamental frequencies of the ν2 mode are also observed. By constructing a quantum-mechanical model of the dynamics, the coherent superposition is characterized and the real-space motion of the radicals is reconstructed. The retrieved trajectories are dominated by pronounced quantum beating governed by the high degree of coherent excitation and the strong negative anharmonicity of the ν2 mode.

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Phys. Rev. Research 8, L022033– Published 22 May, 2026
  • Received 14 January 2026
  • Accepted 25 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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