Auger spectrum of a water molecule after single and double core ionization.

The high intensity of free electron lasers opens up the possibility to perform single-shot molecule scattering experiments. However, even for small molecules, radiation damage induced by absorp- tion of high intense x-ray radiation is not yet fully understood. One of the striking effects which occurs under intense x-ray illumination is the creation of double core ionized molecules in con- siderable quantity. To provide insight into this process, we have studied the dynamics of water molecules in single and double core ionized states by means of electronic transition rate calcula- tions and ab initio molecular dynamics (MD) simulations. From the MD trajectories, photoioniza- tion and Auger transition rates were computed based on electronic continuum wavefunctions ob- tained by explicit integration of the coupled radial Schrödinger equations. These rates served to solve the master equations for the populations of the relevant electronic states. To account for the nuclear dynamics during the core hole lifetime, the calculated electron emission spectra for differ- ent molecular geometries were incoherently accumulated according to the obtained time-dependent populations, thus neglecting possible interference effects between different decay pathways. We find that, in contrast to the single core ionized water molecule, the nuclear dynamics for the double core ionized water molecule during the core hole lifetime leaves a clear fingerprint in the result- ing electron emission spectra. The lifetime of the double core ionized water was found to be sig- nificantly shorter than half of the single core hole lifetime