Momentum-resolved X-ray photoelectron spectroscopy of CS₂ photofragmentation

Time-resolved X-ray photoelectron spectroscopy (TRXPS) provides a unique view into the evolving electron charge densities and binding energies of specific atoms in a molecule undergoing a photochemical change. While TRXPS measurements provide rich information on the chemical environment of a particular atomic species, contributions from the same species present in distinct molecular fragments can overlap and obscure one another. Here, we present results of a TRXPS study of the photodissociation of gas-phase CS₂ probed above the S 2p edge that utilizes electron-ion covariance techniques to separate the overlapping contributions of the S and CS fragments. Both the X-ray photoelectrons and the Coulomb-exploded ions are collected in a double-sided velocity map imaging (VMI) instrument on each laser shot, and the correlations between the momentum-resolved ion yields and the measured photoelectron spectrum are used to assign chemical shift values to each fragment. We find that the S 2p binding energy of bare atomic sulfur produced by the CS₂ photodissociation differs by as much as 10 eV from the value obtained from photoemission studies of solid-state elemental sulfur, likely due to the chemical shift of the solid phase. Our results demonstrate the utility of correlation methods for disentangling congested photoelectron spectra and provide a path forward for obtaining high-quality inner-shell spectra of molecular transition states and fragment channels at new high repetition rate X-ray free electron lasers.