Applied Physics | Watching molecules in action: Using ultrafast X-ray spectroscopies to probe atomic and electronic dynamics.

The fate of a photoactive molecule is determined by the electronic and structural rearrangements that follow excitation. Femtosecond X-ray free electron lasers (XFELs) have made it possible to use X-ray spectroscopy to probe changes in electronic configuration and atomic structure as a function of time, beginning from the initial excited state. Both ‘movies’ of coherent or ballistic motion and ‘snaphots’ of local minima or kinetic intermediates are possible. Polarization anisotropy, long exploited in ultrafast optical measurements, permits decomposition of the X-ray transient difference signal into contributions along the direction parallel to the transition dipole initially pumped, and perpendicular to this transition dipole. This decomposition allows the analysis of asymmetric sequential structural changes of photoexcited molecules in isotropic solution. In many cases, it is possible to combine methods and probe complementary spectroscopies to obtain more complete insight into the dynamics.