Ludger Inhester

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 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.

Efficient electronic structure calculation for molecular ionization dynamics at high x-ray intensity

We present the implementation of an electronic-structure approach dedicated to ionization dynamics of molecules interacting with x-ray free-electron laser (XFEL) pulses. In our scheme, molecular orbitals for molecular core-hole states are represented by linear combination of numerical atomic orbitals that are solutions of corresponding atomic core-hole states. We demonstrate that our scheme efficiently calculates all possible multiple-hole configurations of molecules formed during XFEL pulses. The present method is suitable to investigate x-ray multiphoton multiple ionization dynamics and accompanying nuclear dynamics, providing essential information on the chemical dynamics relevant for high-intensity x-ray imaging.

The Low Barrier Hydrogen Bond in the Photoactive Yellow Protein: A Vacuum Artifact Absent in the Crystal and Solution

There has been considerable debate on the existence of a low-barrier hydrogen bond (LBHB) in the photoactive yellow protein (PYP). The debate was initially triggered by the neutron diffraction study of Yamaguchi et al. ( Proc. Natl. Acad. Sci., U. S. A. , 2009 , 106 , 440 - 444 ) who suggested a model in which a neutral Arg52 residue triggers the formation of the LBHB in PYP. Here, we present an alternative model that is consistent within the error margins of the Yamaguchi structure factors. The model explains an increased hydrogen bond length without nuclear quantum effects and for a protonated Arg52. We tested both models by calculations under crystal, solution, and vacuum conditions. Contrary to the common assumption in the field, we found that a single PYP in vacuum does not provide an accurate description of the crystal conditions but instead introduces strong artifacts, which favor a LBHB and a large 1H NMR chemical shift. Our model of the crystal environment was found to stabilize the two Arg52 hydrogen bonds and crystal water positions for the protonated Arg52 residue in free MD simulations and predicted an Arg52 pKa upshift with respect to PYP in solution. The crystal and solution environments resulted in almost identical 1H chemical shifts that agree with NMR solution data. We also calculated the effect of the Arg52 protonation state on the LBHB in 3D nuclear equilibrium density calculations. Only the charged crystal structure in vacuum supports a LBHB if Arg52 is neutral in PYP at the previously reported level of theory ( J. Am. Chem. Soc. , 2014 , 136 , 3542 - 3552 ). We attribute the anomalies in the interpretation of the neutron data to a shift of the potential minimum, which does not involve nuclear quantum effects and is transferable beyond the Yamaguchi structure.

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

Because of the high intensity, X-ray free electron lasers allow one to create and probe double core ionized states in molecules. The decay of these multiple core ionized states crucially determines the evolution of radiation damage in single molecule diffractive imaging experiments. Here we have studied the Auger decay in hydrides of first row elements after single and double core ionization by quantum mechanical ab initio calculations. In our approach the continuum wave function of the emitted Auger electron is expanded into spherical harmonics on a radial grid. The obtained decay rates of double K-shell vacancies were found to be systematically larger than those for the respective single K-shell vacancies, markedly exceeding the expected factor of two. This enhancement is attributed to the screening effects induced by the core hole. We propose a simple model, which is able to predict core hole decay rates in molecules with low Z elements based on the electron density in the vicinity of the core hole.

Cationic and Anionic Impact on the Electronic Structure of Liquid Water

Hydration shells around ions are crucial for many fundamental biological and chemical processes. Their local physicochemical properties are quite different from those of bulk water and hard to probe experimentally. We address this problem by combining soft X-ray spectroscopy using a liquid jet and molecular dynamics (MD) simulations together with ab initio electronic structure calculations to elucidate the water-ion interaction in a MgCl2 solution at the molecular level. Our results reveal that salt ions mainly affect the electronic properties of water molecules in close vicinity and that the oxygen K-edge X-ray emission spectrum of water molecules in the first solvation shell differs significantly from that of bulk water. Ion-specific effects are identified by fingerprint features in the water X-ray emission spectra. While Mg2+ ions cause a bathochromic shift of the water lone pair orbital, the 3p orbital of the Cl- ions causes an additional peak in the water emission spectrum at around 528 eV.

Chemical Understanding of the Limited Site-Specificity in Molecular Inner-Shell Photofragmentation

In many cases fragmentation of molecules upon inner-shell ionization is very unspecific with respect to the initially localized ionization site. Often this finding is interpreted in terms of an equilibration of internal energy into vibrational degrees of freedom after Auger decay. We investigate the X-ray photofragmentation of ethyl trifluoroacetate upon core electron ionization at environmentally distinct carbon sites using photoelectron-photoion-photoion coincidence measurements and ab initio electronic structure calculations. For all four carbon ionization sites, the Auger decay weakens the same bonds and transfers the two charges to opposite ends of the molecule, which leads to a rapid dissociation into three fragments, followed by further fragmentation steps. The lack of site specificity is attributed to the character of the dicationic electronic states after Auger decay instead of a fast equilibration of internal energy.

Erratum: “Core hole screening and decay rates of double core ionized first row hydrides” [J. Chem. Phys.138, 164304 (2013)]

We discovered a sign error in our implementation of the (correct) expression for the exchange interaction between the Auger electron and the bound electrons (Eq. (2)). As a consequence, the Auger rates reported in the aforementioned article 1 are not correct. We have corrected the error in our implementation and recomputed the single and double core Auger rates for all molecules. After correcting the sign of the exchange potential the computed Auger decay rates are slightly smaller. The deviation to previously published values is slightly larger, but still comparable to the deviations between the two theoretical works 2, 3 and experiments. 4–6 Since both single and double core Auger rates were overestimated, the conclusions we draw, which were concerned with the ratio between the double and single-core Auger rates, remain unaltered. Therefore, with this correction Tables I–III and Figure 1 of the original article need to be replaced by the tables and the figure in this erratum. The tables and figures can be found in the supplemental material. 7

Auger Spectrum of a Water Molecule after Single and Double Core-Ionization by Intense X-Ray Radiation

The high intensity of new x-ray sources such as Free Electron Lasers (FEL) offers the possibility to do single-shot molecule diffraction experiments. Even for small molecules, the dynamics induced by the radiation damage in such experiments are not yet fully understood. In particular, double core-ionized molecules are expected to be created in considerable quantity.We have therefore studied the electronic and nuclear dynamics of water molecules in single and double core ionized states by means of electronic transition rates and ab initio molecular dynamics (MD) simulations. From MD trajectories Auger transition rates were computed based on continuum electronic wavefunctions obtained by explicit integration of the coupled radial Schrodinger equations. The calculated spectra for different molecular geometries were accumulated to account for the effects of nuclear dynamics during the core-hole lifetime.In contrast to the single core ionized water molecule, we found that dissociation dynamics of double core-ionized water have strong effect on the resulting electron emission spectra. In addition, we found that the single core hole lifetime is slightly smaller than the value obtained in earlier theoretical works. Finally, we predict that the lifetime of double core ionized states is significantly lower than half of the lifetime of a single core hole.