Ultrafast Imaging of Molecular Dynamics Using Ultrafast Low-Frequency Lasers, X-ray Free Electron Lasers, and Electron Pulses

Abstract

The requirement of high space-time resolution and brightness is a great challenge for imaging atomic motion and making molecular movies. Important breakthroughs in ultrabright tabletop laser, X-ray, and electron sources have enabled the direct imaging of evolving molecular structures in chemical processes, and recent experimental advances in preparing ultrafast laser and electron pulses resulted in molecular imaging with femtosecond time resolution. This Perspective presents an overview of the versatile imaging methods of molecular dynamics. High-order harmonic generation imaging and photoelectron diffraction imaging are based on laser-induced ionization and rescattering processes. Coulomb explosion imaging retrieves molecular structural information by detecting the momentum vectors of fragmented ions. Diffraction imaging encodes molecular structural and electronic information in reciprocal space. We also present various applications of these ultrafast imaging methods in resolving laser-induced nuclear and electronic dynamics.

Monitoring the Evolution of Relative Product Populations at Early Times during a Photochemical Reaction

“…The field includes studies of the transformations a molecule can undergo following electronic excitation by absorbing an ultraviolet (UV) or visible photon. The desire to understand the formation mechanisms of products in photochemical processesthe photoproductshas helped stimulate the development of a plethora of time-resolved spectroscopic and theoretical tools for investigating the dynamics of excited-state molecules. − …”

Ultrafast Imaging of Jahn–Teller Distortion and the Correlated Proton Migration in Photoionized Cyclopropane

“…Advances in laser technology, especially ultrashort intense lasers, provide a powerful tool to image and steer this ultrafast reaction process. In recent decades, ultrafast proton migration and isomerization, occurring in various organic molecules interacting with the strong laser field, have attracted much attention. − Furthermore, the control of proton migration can be realized by adjusting parameters of the laser pulses. − Utilizing the femtosecond laser pulses, the pump–probe experiments have been carried out to obtain dynamic information on the isomerization and proton migration process in real time. − …”

Molecular photodissociation dynamics revealed by Coulomb explosion imaging

“…28 Returning to the gas phase, ultrafast electron diffraction probe methods attracted early attention, 29 but achieving sufficiently intense, short duration electron pulses to enable measurable signals from dilute gas phase samples is a major challenge -that is now being overcome at leading international facilities. [30][31][32][33] Coulomb explosion imaging (CEI) is another emerging technique, that is increasingly finding use both as a route to exploring the fragmentation of multiply charged cations and as a means of probing molecules in the act of dissociating. Removing several electrons from an isolated molecule yields a multiply charged cation that dissociates by Coulomb explosion (CE).…”