Probing Proton Dynamics in Molecules on an Attosecond Time Scale

TLDR

Laser ionization in strong fields creates a chirped electron wavepacket that imparts a chirp to the photons emitted during recombination. The study demonstrates a high‑order harmonic generation technique to probe nuclear dynamics and structural rearrangement on a sub‑femtosecond timescale. The method uses high‑order harmonic generation and the chirp of emitted light to achieve 100‑attosecond resolution with an 8‑fs pulse in a single shot, and applies this to CH₄/CD₄ to observe proton rearrangement within a few femtoseconds. The technique accurately reproduces ultrafast nuclear dynamics in H₂/H₂⁺ and reveals proton rearrangement in methane within a few femtoseconds.

Abstract

We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization.