Abstract
The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or hole) dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump–x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first tentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics.Received 26 September 2020Revised 22 March 2021Accepted 21 June 2021DOI:https://doi.org/10.1103/PhysRevX.11.031048Published 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 SocietyPhysics Subject Headings (PhySH)Research AreasElectron correlation calculations for atoms & ionsSingle- and few-photon ionization & excitationUltrafast phenomenaPhysical SystemsMoleculesX-ray lasersTechniquesX-ray absorption spectroscopyAtomic, Molecular & Optical
Highlights
One of the most fundamental problems in photophysics is the electronic response of a quantum system to an impulse
We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump–x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment
We have been able to demonstrate the capability to distinguish between the dynamics arising from the removal of an electron from the 6a and 7a orbitals using the variation of the probe photon energy
Summary
One of the most fundamental problems in photophysics is the electronic response of a quantum system to an impulse. Impulsive excitation creates time-dependent oscillations in the charge density, i.e., an electronic wave packet or superposition of electronic states This electronic superposition resulting from ionization produces rapid evolution in the electron hole (here referred to as a “hole”) density that may result in movement of charge density within the molecular frame, a process termed “charge migration” [1]. While this initial charge dynamics is purely electronic in nature, eventually this charge disturbance will couple to other degrees of freedom in the system such as nuclear motion, leading to a localization of the charge. A deeper knowledge of these fundamental processes, including concepts of charge directed reactivity [4], may be used to steer photochemical reactions
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