Advanced photoelectron- and ion-imaging for chemical-dynamics studies

Abstract

Ultrafast molecular dynamics unfold on femtosecond timescales and often produce multiple charged fragments whose correlated momenta encode transient molecular structure. Accessing this information at modern high-repetition-rate light sources poses a fundamental experimental challenge: conventional velocity-map imaging (VMI) spectrometers rely on frame-based CCD detection, precluding simultaneous multi-mass acquisition, suppressing coincidence measurements at high count rates, and limiting quantitative access to correlated fragmentation dynamics.

This thesis demonstrates how event-resolved momentum imaging enables direct access to correlated ultrafast dynamics in hydrogen-bonded systems. Applied to the water dimer ($\mathrm{(H_2O)_2}$), a representative example of such systems and fundamental atmospheric species, simultaneous multi-mass detection identified thirteen ion–radical fragmentation pathways—six previously unreported—demonstrating a substantially richer dissociation landscape than earlier studies suggested. Recent extensions of this approach to kinetic-energy-resolved covariance measurements further uncover energy-dependent proton-transfer and ion–radical formation dynamics, providing direct insight into ultrafast energy redistribution in ionized hydrogen-bonded networks.

The methodology was further validated under high-flux conditions at the Free Electron Laser Hamburg (FLASH), where shot-resolved three-dimensional ion momentum distributions were recorded at 250\,kHz without reliance on symmetry-based reconstruction. It was subsequently applied to Coulomb explosion imaging of $\mathrm{CS_2}$ following site-selective soft X-ray ionization, resolving transient bent and stretched geometries during dissociation.

In parallel, the development of a photonic-crystal-fiber-based vacuum ultraviolet source enabled angle-resolved photoemission measurements of the topological insulator $\mathrm{Bi_2Se_3}$, resolving its Dirac-cone surface-state dispersion with signal-to-noise ratios comparable to established methods. Although distinct from the molecular studies, this demonstration establishes a compact VUV platform compatible with time-resolved photoemission and single-photon ionization experiments.

These advances are enabled by integrating hybrid pixel, event-driven Timepix detectors into VMI instrumentation. Nanosecond-scale timestamping of individual particles permits simultaneous multi-mass detection, high-rate multi-hit capability, and continuous acquisition beyond the intrinsic limitations of CCD-based systems. Systematic characterization of detector response under high-flux conditions yields a refined model of multi-hit energy deposition, improving quantitative accuracy by up to 70\,\% and ensuring robust performance in this regime.

Together, these developments transform VMI from a single-mass, frame-limited technique into a scalable, multi-parameter imaging platform capable of quantitative, high-throughput momentum measurements. By uniting detector innovation with concrete physical applications, this work advances VMI toward reaction-microscope functionality and substantially broadens the range of ultrafast molecular and electronic dynamics accessible at next-generation light sources.