Femtosecond Pump-Probe Spectroscopy of Gas-Phase Atomic and Molecular Systems Using the Free-Electron Laser FLASH

Abstract

This thesis explores ultrafast dynamics in atomic and molecular systems using pump–probe techniques at the free-electron laser (FLASH), with the goal of achieving the high temporal resolution required to resolve processes on the order of 100 fs and below. To address this challenge, an active feedback scheme based on a laser arrival time monitor is implemented to improve synchronization between optical laser and FEL pulses. This approach significantly enhances timing stability and is realized at the FL26 beamline of FLASH2, enabling an overall temporal resolution below 50 fs at the REMI endstation, as validated by photoionization experiments on xenon.

With this improved time resolution, the ionization and relaxation dynamics of xenon were investigated using extreme-ultraviolet (XUV) FEL pulses at photon energies near the 4d giant resonance (90 eV) and the Cooper minimum (160 eV). Ion time-of-flight spectroscopy reveals energy-dependent Auger–Meitner decay pathways and signatures of two-photon sequential ionization. Time-resolved XUV-pump–near-infrared-probe measurements identify transient intermediate states, yielding a characteristic lifetime of (49 ± 3) fs associated with the 4d double-core-hole decay at 160 eV.

The methodology is further extended to molecular systems through time-resolved Coulomb-explosion imaging experiments on CO2 and CS2 at the REMI endstation, triggered by 90 eV FEL pulses and probed with a near-infrared pulse. For both molecules, two-body and three-body fragmentation channels are observed, with contributions from both concerted and sequential breakup mechanisms. The intermediate dissociative cation states (CO2+* and CS2+*) show similar lifetimes of approximately 60 fs. Comparative analysis shows that the heavier mass and distinct electronic structure of sulfur lead to enhanced bending dynamics and greater stability of highly charged states in CS2.

Finally, UV-induced dynamics in 1-butanethiol were studied at the CAMP endstation using time-resolved ion and electron-spectroscopy. This measurement shows rapid fragmentation of the molecule, as well as hydrogen-transfer processes and charge redistribution. Photoelectron and Auger-Meitner spectra indicate excited-state lifetimes on the order of hundreds of femtoseconds, with C–S bond cleavage occurring on a 200 fs timescale. Despite spectral overlap arising from mixing of unresolved states due to finite experimental resolution, the combined results provide a picture of ultrafast relaxation followed by dissociation.

Overall, this work demonstrates how improved temporal resolution and complementary experimental techniques, i.e., multi-ion coincidence and simultaneous ion and electron spectroscopy, can enable detailed insight into the coupling between electronic and nuclear dynamics in atomic and molecular systems on femtosecond timescales.