Time-Resolved Core-Level Photoelectron Spectroscopy of Glycine Fragmentation: A Case Study on the Capabilities and Limitations of Site-Selective Probes at XFELs

Abstract

This paper presents the time-resolved X-ray photoelectron spectroscopy (TR-XPS) setup at the Swiss Free-Electron Laser (SwissFEL) Maloja endstation for investigating the ultrafast dynamics of gas-phase molecules with site-specificity. As a case study, we investigate the dissociative photoionization of glycine. By tracking the C1s, N1s, and O1s core-level edges, we resolve the site-specific dynamics following multiphoton ionization. Guided by ab initio theory, the measurements are modeled using a global kinetic fit. Using this model, we report a glycine parent cation lifetime of 385 fs, consistent with the theoretically predicted barrier to C–C bond rupture. We track fragment separation in real time via carboxyl red-shifts, which signal the relaxation of Coulombic repulsion between the COOH and the iminium ion (CH₂NH₂⁺). Finally, we address fundamental technical challenges, including the temporal limitations imposed by laser-induced optical sidebands in the presence of high intensity fields, understanding the modulated GSB signal, and the complexities of the theoretical framework that couples ab initio molecular dynamics with high-level quantum-chemical calculations of core level binding energies.