• Open Access

Characterizing Multiphoton Excitation Using Time-Resolved X-ray Scattering

Philip H. Bucksbaum, Matthew R. Ware, Adi Natan, James P. Cryan, and James M. Glownia
Phys. Rev. X 10, 011065 – Published 17 March 2020

Abstract

Molecular iodine is photoexcited by a strong 800-nm laser driving several channels of multiphoton excitation. The motion following photoexcitation is probed using time-resolved x-ray scattering, which produces a scattering map S(Q,τ). Temporal Fourier-transform methods are employed to obtain a frequency-resolved x-ray-scattering signal S˜(Q,ω). Taken together, S(Q,τ) and S˜(Q,ω) separate different modes of motion so that mode-specific nuclear oscillatory positions, oscillation amplitudes, directions of motion, and times may be measured accurately. Molecular dissociations likewise have a distinct signature, which may be used to identify both velocities and dissociation time shifts and also can reveal laser-induced couplings among the molecular potentials.

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  • Received 7 November 2019
  • Revised 15 January 2020
  • Accepted 7 February 2020

DOI:https://doi.org/10.1103/PhysRevX.10.011065

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.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic, Molecular & Optical

Authors & Affiliations

Philip H. Bucksbaum1,2,3, Matthew R. Ware3, Adi Natan3, James P. Cryan3,4, and James M. Glownia3,4

  • 1Department of Physics, Stanford University, Stanford, California 94305, USA
  • 2Department of Applied Physics, Stanford University, Stanford, California 94305, USA
  • 3Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
  • 4Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

Popular Summary

Molecules exposed to intense laser radiation absorb or scatter the photons in ways that do not occur for weaker light. While these nonlinear phenomena have been studied using conventional tools of optical spectroscopy, none of these optics-based methods can measure directly the subangstrom displacement and femtosecond motion of the atoms in response to the intense light. Here, we employ femtosecond x-ray laser scattering, a new method that fills this gap in our measurement capabilities.

Femtosecond x rays can answer several questions about the dynamics of motion in strong laser fields: How long does it take the atoms to move? Where is the motion within the molecule? What is the amplitude of this motion? When initially excited, do the molecular bonds first contract or expand?

We use femtosecond x rays to study molecular iodine irradiated by ultrafast pulses of 800-nm laser light. The scattered x rays reveal the molecule’s motion in several channels of vibration and dissociation following multiphoton excitation, with behavior largely in line with expectations.

Taken together, these measurements are movies that can validate model predictions of molecular motion that previously had to be compared to indirect spectroscopic measurements. Viewing the motion directly also provides new insights about the importance of different kinds of motion that control the outcomes in laser-matter interactions.

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Vol. 10, Iss. 1 — January - March 2020

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