a view from modern soft X-ray spectroscopies

Soft X-ray spectroscopies are powerful tools for probing the local electronic and molecular orbital structures of materials in different phases and various environments. While modern spectroscopic tools using soft X-ray synchrotron photons perspicuously reveal the molecular orbital (MO) structure in detail, structures remain widely unknown in the liquid phase since many of these techniques could only be applied to solutions very recently. Furthermore, the interactions and dynamics of molecules in the liquid phase are especially complicated compared to those in gas and solid phases and thereby impede the understanding of functional materials in solution. This review presents recent developments using soft X-ray radiation for probing the electronic structure of ions and molecules in solution. The presented X-ray absorption, emission, and photo-electron spectroscopy studies exhibit the powerful contributions of soft X-ray liquid spectroscopies in the last few years

recent techniques and applications using soft X-ray spectroscopy

The aim of a more precise knowledge about molecular structures and the nature of chemical bonds is the driving force behind the development of numerous experimental methods and theories. Recent soft X-ray based techniques provide novel opportunities for tackling the structure and the dynamics of chemical and biochemical systems in solution. In our research group we are developing experimental methods for mapping the electronic structure and dynamics of molecular systems in solution during bond-building and breaking using soft X-ray absorption and emission spectroscopy. The combination of such recent developments with conventional spectroscopy as well as theoretical modeling allows us to address open questions about hydrogen bonds, thermodynamics and active centers of biological systems. Based on the core-hole clock and pump–probe spectroscopy dynamics on the time scale from sub-femtoseconds up to picoseconds can be revealed

Multi-reference approach to the calculation of photoelectron spectra including spin-orbit coupling

X-ray photoelectron spectra provide a wealth of information on the electronic structure. The extraction of molecular details requires adequate theoretical methods, which in case of transition metal complexes has to account for effects due to the multi-configurational and spin-mixed nature of the many-electron wave function. Here, the Restricted Active Space Self-Consistent Field method including spin-orbit coupling is used to cope with this challenge and to calculate valence and core photoelectron spectra. The intensities are estimated within the frameworks of the Dyson orbital formalism and the sudden approximation. Thereby, we utilize an efficient computational algorithm that is based on a biorthonormal basis transformation. The approach is applied to the valence photoionization of the gas phase water molecule and to the core ionization spectrum of the $\text{[Fe(H}_2\text{O)}_6\text{]}^{2+}$ complex. The results show good agreement with the experimental data obtained in this work, whereas the sudden approximation demonstrates distinct deviations from experiments

an iron L-edge X-ray absorption study of the active centre

Iron L-edge X-ray absorption spectra of the active centre of myoglobin in the met-form, in the reduced form and upon ligation to O2, CO, NO and CN are presented. The strength of ligation with the iron centre is finger-printed through the variation of the L3 : L2 intensity ratio. Charge Transfer Multiplet calculations are performed and give qualitative information about oxidation states as well as charge transfer

Multiplateau structure in photoemission spectra of strong-field ionization of dense media

Strong-field ionization of dense molecular gases in a short infrared laser pulse is studied by means of photoelectron spectroscopy combined with a liquid microjet technique. By increasing the gas density, we observe how the laser- assisted electron scattering on neighboring particles becomes a dominant mechanism of hot electron emission. The angle-resolved energy distributions of rescattered electrons are obtained by analyzing the density dependency of emission spectra. A semiclassical consideration of electron trajectories is shown to provide a good description of experimental spectra. The model predicts the existence of four energy plateaus. Two cutoffs at higher energies are evident in the spectra

Soft-X-Ray Absorption Spectroscopy and Ab Initio Calculations

Nonradiative decay channels in the L-edge fluorescence yield spectra from transition-metal–aqueous solutions give rise to spectral distortions with respect to x-ray transmission spectra. Their origin is unraveled here using partial and inverse partial fluorescence yields on the microjet combined with multireference ab initio electronic structure calculations. Comparing Fe2+, Fe3+, and Co2+ systems we demonstrate and quantify unequivocally the state- dependent electron delocalization within the manifold of d orbitals as one origin of this observation

spin-state and metal coordination revealed from resonant inelastic X-ray scattering and electronic structure calculations

The local electronic structure of the cobalt centre-ion of Co(III) protoporphyrin IX chloride dissolved in dimethyl sulfoxide (DMSO) liquid solution is studied by resonant inelastic X-ray scattering (RIXS) spectroscopy at the cobalt L-edge. The resulting cobalt 2p partial-fluorescence-yield (PFY) X-ray absorption (XA) spectrum, integrated from RIXS spectra, is simulated for various possible spin-states and coordination of the cobalt centre by using the newly developed density functional theory/restricted open shell single excitation configuration interaction (DFT/ROCIS) method. Comparison between experiment and calculation shows that the cobalt ion (3d6 electronic configuration) adopts a low-spin state with all six 3d electrons paired, and the cobalt centre is either 5-coordinated by its natural ligands (one chloride ion and four nitrogen atoms), or 6-coordinated, when binding to an oxygen atom of a DMSO solvent molecule. Analysis of the measured RIXS spectra reveals weak 3d–3d electron correlation, and in addition a value of the local HOMO–LUMO gap at the Co sites is obtained

Charge transfer to solvent dynamics in iodide aqueous solution studied at ionization threshold

We explore the early-time electronic relaxation in NaI aqueous solution exposed to a short UV laser pulse. Rather than initiating the charge transfer reaction by resonant photoexcitation of iodide, in the present time-resolved photoelectron spectroscopy study the charge-transfer-to-solvent (CTTS) states are populated via electronic excitation above the vacuum level. By analyzing the temporal evolution of electron yields from ionization of two transient species, assigned to CTTS and its first excited state, we determine both their ultrafast population and relaxation dynamics. Comparison with resonant- excitation studies shows that the highly excited initial states exhibit similar relaxation characteristics as found for resonant excitation. Implications for structure and dynamical response of the hydration cage are discussed