X-Ray absorption spectra of microsolvated metal cations.

Core excited states in clusters or bulk medium are known to undergo a process of internal ionisation, whereby the excited electron delocalises throughout the medium. This delocalisation is visible in the shifting and broadening of lines in X-ray absorption spectra, and it impacts the electronic decay initiated by photoabsorption. In this paper we study the delocalisation of electrons excited from the 1s core orbital of Na(+) and Mg(2+) ions in microsolvated Na(+)(H2O)m and Mg(2+)(H2O)m clusters (m = 1-6) by computing the X-ray absorption spectra and electron distributions in different core excited states. We show that addition of water ligands to the ion leads to more and more pronounced delocalisation of the core-to-valence 1s → 3p and core-to-Rydberg 1s → 4p excitations. Even for the compact 1s → 3p excitation the excited electron is mostly located on the water molecules when the solvation shell is complete. We also found that the degree of delocalisation strongly depends on the cluster geometry and the ionic charge. These results indicate that even in small microsolvated clusters delocalisation of core excited electrons is substantial and will affect the following electronic decay. The accuracy and transferability of our results are corroborated by the good agreement between our XAS spectra of microsolvated Na(+) and experimental X-ray absorption spectra of dilute NaCl solutions.

Ab initio calculation of ICD widths in photoexcited HeNe.

Excitation of HeNe by synchrotron light just below the frequency of the 1s → 3p transition of isolated He has been recently shown to be followed by resonant interatomic Coulombic decay (ICD). The vibrationally resolved widths of the ICD states were extracted with high precision from the photoion spectra. In this paper, we report the results of ab initio calculations of these widths. We show that interaction between electronic states at about the equilibrium distance of HeNe makes dark states of He accessible for the photoexcitation and subsequent electronic decay. Moreover, the values of the calculated widths are shown to be strongly sensitive to the presence of the non-adiabatic coupling between the electronic states participating in the decay. Therefore, only by considering the complete manifold of interacting decaying electronic states a good agreement between the measured and computed ICD widths can be achieved.