[Paper Review] Correlation-driven transient hole dynamics resolved in space and time in the isopropanol molecule

J. P. Marangos, Taran Driver|arXiv (Cornell University)|Jul 30, 2021

Laser-Matter Interactions and Applications69 references42 citations

TL;DR

This study resolves ultrafast, correlation-driven transient hole dynamics in isopropanol using x-ray pump/x-ray probe spectroscopy with site- and state-specific sensitivity, achieving femtosecond temporal and spatial resolution. It reports the first direct observation of hole dynamics driven by frustrated Auger-Meitner transitions, confirming theoretical predictions of quasi-exponential hole decay near the oxygen atom, advancing real-time observation of charge migration in molecules.

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.

Motivation & Objective

• To directly observe ultrafast electron hole dynamics in molecules following impulsive ionization.
• To resolve the spatial and temporal evolution of transient hole states in isopropanol with site- and state-specific sensitivity.
• To probe the role of electron correlation in driving hole dynamics, particularly in inner valence cationic states.
• To verify theoretical predictions of hole decay dynamics in systems where Auger decay is energetically frustrated.
• To establish a method for real-time, site-specific observation of charge migration and transfer in polyatomic molecules.

Proposed method

• Performed x-ray pump/x-ray probe experiments at the Linac Coherent Light Source (LCLS) using 100-200 eV x-rays to ionize isopropanol molecules.
• Employed resonant x-ray probing to selectively interrogate electron density near the oxygen atom, enabling site-specific detection of transient hole states.
• Combined time-resolved photoelectron spectroscopy with ab initio theoretical calculations using ADC(2)x and EOM-IP-CCSD methods to model cationic states and hole dynamics.
• Used a combination of x-ray absorption and emission spectroscopy to map the evolution of inner valence hole states (7a−1 and 6a−1) over time.
• Applied few-femtosecond temporal resolution to resolve dynamics on the attosecond to few-femtosecond timescale.
• Correlated experimental data with theoretical simulations to distinguish between different types of charge migration, particularly the Bixon-Jortner-type decay.

Experimental results

Research questions

• RQ1 How do transient electron hole states evolve in space and time following inner-valence ionization in isopropanol?
• RQ2 What is the role of electron correlation in driving hole dynamics when Auger decay is energetically forbidden?
• RQ3 Can frustrated Auger-Meitner transitions be directly observed and characterized in real time?
• RQ4 To what extent does the hole density exhibit quasi-exponential decay due to coupling to a quasi-continuum of cationic states?
• RQ5 How does the coherence of the electronic superposition influence observable oscillations in hole density?

Key findings

• The study reports the first direct observation of transient hole dynamics driven by frustrated Auger-Meitner transitions in isopropanol, with hole decay occurring on a timescale consistent with theoretical predictions.
• The hole state localized near the oxygen atom exhibits quasi-exponential decay, confirming the Bixon-Jortner model of coupling to a quasi-continuum of cationic states.
• The measured hole lifetime of approximately 1.2 fs for the 7a−1 state is in excellent agreement with ab initio calculations using the ADC(2)x method.
• Site-specific probing via resonant x-ray spectroscopy confirmed that the hole dynamics are localized near the oxygen atom, with minimal contribution from remote molecular regions.
• The experimental data show clear evidence of electronic coherence and partial revivals in the hole density, indicating that dephasing is not immediate.
• The combination of x-ray pump/probe and ab initio theory enables unambiguous assignment of the observed dynamics to specific inner-valence ionization channels (7a−1 and 6a−1).