Abstract
In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4d5/2–σ* transition of a gas-phase di-iodomethane (CH2I2) target. This site-specific excitation generates a 4d core 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 same I: 4d resonance. 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.Received 9 June 2020Revised 27 February 2021Accepted 10 May 2021DOI:https://doi.org/10.1103/PhysRevX.11.031001Published 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. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhotodissociationPhysical SystemsMoleculesTechniquesTransient absorption spectroscopyAtomic, Molecular & Optical
Highlights
Spectroscopy of core-electron excitations in atoms and molecules has improved our understanding of their structure [1,2,3,4]
Many experiments on femtosecond molecular dynamics were made possible by the advances in ultrashort laser-pulse technology: for example, the spectroscopic investigation of femtosecond photodissociation dynamics initiated by bond breaking with strong-field infrared [10,11,12] and ultraviolet [8] laser pulses, which has more recently been augmented with site-specific spectroscopy through extreme-ultraviolet (XUV) and soft x-ray light from high-order harmonic generation [13,14,15,16,17]
The cation is initially prepared in the ground-state geometry of the neutral molecule since the nuclei did not have time to move significantly within the XUV excitation and successive Auger decay
Summary
Spectroscopy of core-electron excitations in atoms and molecules has improved our understanding of their structure [1,2,3,4]. The nonlinear interaction with two high-energy photons through a stimulated resonant x-ray Raman process [28,29,30] can be used to trigger a chemically relevant valence excitation of the molecule, with added site specificity through the localized interaction with a core-shell electron. The dissociation and wave-packet dynamics of di-iodomethane initiated by strong-field ionization has been time resolved and measured with site-specific XUV transient absorption spectroscopy [33]. Such structural dynamics typically evolve on the femtosecond timescale. We combine transient-absorption spectroscopy with a sequence of two femtosecond XUV-FEL pulses to time resolve the ultrafast structural dynamics of di-iodomethane after resonant sitespecific I∶ 4d5=2–σà excitation. Transient absorption spectroscopy hereby allows one to detect the charged final states of the molecular fragments, which may decay further on their way to an ion detector, and to resolve the real-time change of the molecular absorption spectra on the femtosecond timescale
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