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

TL;DR

This study observes and models coherent vibrational dynamics in methyl radicals following bond breaking, using femtosecond x-ray spectroscopy to reveal quantum beating and anharmonic effects in the umbrella mode.

Contribution

It provides the first detailed quantum-mechanical characterization and real-space reconstruction of vibrational dynamics in methyl radicals post-dissociation.

Findings

Observation of coherent vibrational motion via x-ray energy shifts

Identification of quantum beating patterns in vibrational dynamics

Reconstruction of real-space trajectories showing anharmonic effects

Abstract

We report on the observation of coherent molecular vibrations launched by the breaking of a molecular bond. The methyl radical, which is produced by $267\,\mathrm{nm}$ photodissociation of methyl iodide, is excited to high levels in its $\nu_2$ ``umbrella" vibrational mode by the dissociation. The ensuing coherent vibrational dynamics are observed by measuring ultrafast time-dependent changes in the x-ray transition energy from the C$1s$ 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 $\nu_2$ progression, although the fundamental frequencies of the $\nu_2$ mode are also observed. By constructing a fully quantum-mechanical model of the dynamics the coherent superposition is rigorously 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 $\nu_2$ mode.