The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

The photoinduced relaxation dynamics of nucleobases and their thionated analogs have been investigated extensively over the past decades motivated by their crucial role in organisms and their application in medical and biochemical research and treatment. Most of these studies focused on the spectroscopy of valence electrons and fragmentation. The advent of ultrashort x‐ray laser sources such as free‐electron lasers, however, opens new opportunities for studying the ultrafast molecular relaxation dynamics utilizing the site‐ and element‐selectivity of x‐rays. In this review, we want to summarize ultrafast experiments on thymine and 2‐thiouracil performed at free‐electron lasers. We performed time‐resolved x‐ray absorption spectroscopy at the oxygen K‐edge after UV excitation of thymine. In addition, we investigated the excited state dynamics of 2‐tUra via x‐ray photoelectron spectroscopy at sulfur. For these methods, we show a strong sensitivity to the electronic state or charge distribution, respectively. We also performed time‐resolved Auger–Meitner spectroscopy, which shows spectral shifts associated with internuclear distances close to the probed site. We discuss the complementary aspects of time‐resolved x‐ray spectroscopy techniques compared to optical and UV spectroscopy for the investigation of ultrafast relaxation processes.

We present time-resolved ultraviolet-pump x-ray probe Auger spectra of 2-thiouracil. An ultraviolet induced shift towards higher kinetic energies is observed in the sulfur 2p Auger decay. The difference Auger spectra of pumped and unpumped molecules exhibit ultrafast dynamics in the shift amplitude, in which three phases can be recognized. In the first 100 fs, a shift towards higher kinetic energies is observed, followed by a 400 fs shift back to lower kinetic energies and a 1 ps shift again to higher kinetic energies. We use a simple Coulomb-model, aided by quantum chemical calculations of potential energy states, to deduce a C–S bond expansion within the first 100 fs. The bond elongation triggers internal conversion from the photoexcited S ₂ to the S ₁ state. Based on timescales, the subsequent dynamics can be interpreted in terms of S ₁ nuclear relaxation and S ₁ -triplet internal conversion.

Experimental Auger electron spectra of C, N, and O 1s ionized and excited HCNO are compared with theoretical counterparts. A simple minimal-basis LCAO approach provides reliable estimates for the significantly different Auger transition rates.

In this paper, we report X-ray absorption and core-level electron spectra of the nucleobase derivative 2-thiouracil at the sulfur L₁- and L_2,3-edges. We used soft X-rays from the free-electron laser FLASH2 for the excitation of isolated molecules and dispersed the outgoing electrons with a magnetic bottle spectrometer. We identified photoelectrons from the 2p core orbital, accompanied by an electron correlation satellite, as well as resonant and non-resonant Coster–Kronig and Auger–Meitner emission at the L₁- and L_2,3-edges, respectively. We used the electron yield to construct X-ray absorption spectra at the two edges. The experimental data obtained are put in the context of the literature currently available on sulfur core-level and 2-thiouracil spectroscopy.

We report a study on the fragmentation of core-ionized and core-excited isocyanic acid, HNCO, using Auger-electron/photoion coincidence spectroscopy. Site-selectivity is observed both for normal and resonant Auger electron decay. Oxygen 1s ionization leads to the CO⁺ + NH⁺ ion pairs, while nitrogen 1s ionization results in three-body dissociation and an efficient fragmentation of the H–N bond in the dication. Upon 1s → 10a′ resonant excitation, clear differences between O and N sites are discernible as well. In both cases, the correlation between the dissociation channel and the binding energy of the normal Auger electrons indicates that the fragmentation pattern is governed by the excess energy available in the final ionic state. High-level multireference calculations suggest pathways to the formation of the fragment ions NO⁺ and HCO⁺, which are observed although the parent compound contains neither N–O nor H–C bonds. This work contributes to the goal to achieve and understand site-selective fragmentation upon ionization and excitation of molecules with soft x-ray radiation.