Time-resolved X-ray fragmentation probing of molecular isomerization

Short intense X-ray pulses, now available at FELs, are already having a strong impact on the AMO science. In addition to scattering or resonant absorption, it is possible to use X-ray radiation to probe processes on molecular timescales through non-resonant absorption by initiating a molecular fragmentation in a time-resolved manner. Core-ionized molecules composed of light elements predominantly relax through Auger decay into multiply charged molecular ions, which subsequently fragment through Coulomb repulsion. The fragmentation patterns (ion time-of-flight spectra, ion-kinetic energy release, Auger spectra, etc.) encode information about instantaneous nuclear geometry and momenta. Unlike intense IR laser field fragmentation, X-ray fragmentation occurs in the weak-field regime. In order to test the potential of time-resolved X-ray fragmentation for probing isomerization, we selected the example of ring opening of 1,3-cyclohexadiene. In a time-resolved UV-pump - X-ray fragmentation-probe experiment performed at Linac Coherent Light Source at SLAC we observed an increase in the average ion-KER and an increase in the number of lighter fragments upon photoexcitation. We discuss how the evolving fragmentation patterns reflect the structural change that the molecule is undergoing.