All-XUV Pump-Probe Transient Absorption Spectroscopy of the Structural Molecular Dynamics of Di-iodomethane

Abstract

In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite theI:4d5/2–σ*transition of a gas-phase di-iodomethane (CH2I2) target. This site-specific excitation generates a4dcore hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the sameI:4dresonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger.

Alternate abstract:

Plain Language Summary

A chemical bond between atoms is governed by the outermost, or valence, electrons of these atoms. Electrons located closer to the atomic core usually do not participate in these chemical bonds. These electrons are bound more strongly to the nucleus, so exciting such electrons requires a lot of energy. This energy has to be provided in a very defined quantity for every specific atomic species and even for the individual electrons within the respective atom. This allows one to select electrons simply by tuning in to the correct frequency for this very specific transition. In this work, we study the femtosecond intramolecular motion that is initiated by such a site-specific excitation within the di-iodomethane molecule,CH2I2.

To perform this experiment, we use free-electron laser pulses with femtosecond durations provided by the FLASH user facility in Hamburg, Germany. The high-energy photons are used to resonantly excite one iodine-specific transition in di-iodomethane at about 50 eV of photon energy. We then follow the dynamical evolution of the excited molecule via transient-absorption spectroscopy, which uses subsequent free-electron laser pulses to track changes in the molecule’s absorption of light over time. Our experimental findings, combined with calculated absorption spectra, reveal the timing of excited-state geometry changes of the singly charged molecular cation (CH2I2+) on its dissociating pathway.

These results light the way towards site-specific control of chemical reactions.