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

Abstract

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 absorption 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 considerable 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 calculations and ab initio molecular dynamics (MD) simulations. From the MD trajectories, photoionization and Auger transition rates were computed based on electronic continuum wavefunctions obtained 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 different 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 resulting electron emission spectra. The lifetime of the double core ionized water was found to be significantly shorter than half of the single core hole lifetime.

X-ray multiphoton ionization dynamics of a water molecule irradiated by an x-ray free-electron laser pulse

“…The motion of the nuclei may alter the electronic structure in the molecule, which could have impact on the resulting electronic transition rates. We note, however, that in the water molecule Auger decay rates are quite constant with respect to the molecular geometries that appear during the typical lifetime of single or double core vacancies [49,50]. Since core ionization on oxygen is dominant here, total photoionization cross section may not be affected by the nuclear motion.…”

Using covariance mapping to investigate the dynamics of multi-photon ionization processes of Ne atoms exposed to X-FEL pulses

“…figure 1). The presence of this island is expected since core-ionized Ne atoms have an Auger decay lifetime of ∼2.4 fs [15], while the LCLS pulse duration was ∼8 fs, which implies that there is a significant probability for the core-ionized system to decay by Auger electron emission before the second X-ray photon is absorbed. The competing twophoton formation of DCHs requires removal of two core electrons before the Auger decay sets in, and, within the present experimental conditions, it is expected to be less probable than the PAP sequence.…”

“…The latter is approximated as half the crosssection for ionizing the first 1s electron based on simple statistical arguments (one 1s electron compared with two). The Auger decay rate for a double core hole was set to 3 × Γ 1s 1 [15].…”

Core hole screening and decay rates of double core ionized first row hydrides

“…As described in detail before 14 Auger decay rates were calculated using a single center expansion of the electronic wave functions. 17 The bound part of the molecular states was calculated from Configuration Interaction (CI) calculations, while the continuum part was determined by solving the Schrödinger equation in the potential generated by the left-behind molecular cation.…”

“…For the water molecule, we have recently calculated an even larger rate increase by a factor of three. 14 A possible explanation for this pronounced rate increase over single core hole Auger decay are electronic relaxation effects. In particular, a single or double core hole in an atom or molecule induces relaxation of the surrounding electron density, such that the remaining electrons tend to screen the core hole.…”