Victor Kimberg

Vibrational scattering anisotropy in O2 -- dynamics beyond the Born–Oppenheimer approximation

Born–Oppenheimer and Franck–Condon approximations are two major concepts in the interpretation of electronic excitations and modeling of spectroscopic data in the gas and condensed phases. We r ...

Selective gating to vibrational modes through resonant X-ray scattering

The dynamics of fragmentation and vibration of molecular systems with a large number of coupled degrees of freedom are key aspects for understanding chemical reactivity and properties. Here we present a resonant inelastic X-ray scattering (RIXS) study to show how it is possible to break down such a complex multidimensional problem into elementary components. Local multimode nuclear wave packets created by X-ray excitation to different core-excited potential energy surfaces (PESs) will act as spatial gates to selectively probe the particular ground-state vibrational modes and, hence, the PES along these modes. We demonstrate this principle by combining ultra-high resolution RIXS measurements for gas-phase water with state-of-the-art simulations.

Stochastic stimulated electronic x-ray Raman spectroscopy

Resonant inelastic x-ray scattering (RIXS) is a well-established tool for studying electronic, nuclear, and collective dynamics of excited atoms, molecules, and solids. An extension of this powerful method to a time-resolved probe technique at x-ray free electron lasers (XFELs) to ultimately unravel ultrafast chemical and structural changes on a femtosecond time scale is often challenging, due to the small signal rate in conventional implementations at XFELs that rely on the usage of a monochromator setup to select a small frequency band of the broadband, spectrally incoherent XFEL radiation. Here, we suggest an alternative approach, based on stochastic spectroscopy, which uses the full bandwidth of the incoming XFEL pulses. Our proposed method is relying on stimulated resonant inelastic x-ray scattering, where in addition to a pump pulse that resonantly excites the system a probe pulse on a specific electronic inelastic transition is provided, which serves as a seed in the stimulated scattering process. The limited spectral coherence of the XFEL radiation defines the energy resolution in this process and stimulated RIXS spectra of high resolution can be obtained by covariance analysis of the transmitted spectra. We present a detailed feasibility study and predict signal strengths for realistic XFEL parameters for the CO molecule resonantly pumped at the O1s→π* transition. Our theoretical model describes the evolution of the spectral and temporal characteristics of the transmitted x-ray radiation, by solving the equation of motion for the electronic and vibrational degrees of freedom of the system self consistently with the propagation by Maxwell equations.

Calculation of K-edge circular dichroism of amino acids: comparison of random phase approximation with other methods.

Soft x-ray natural circular dichroism of amino acids is studied by means of ab initio methods. Several approaches to evaluate the oscillator and rotary strengths of core-to-valence excitations are compared from the viewpoint of basis set dependence: ground-state Hartree-Fock (HF) orbital set employed in (i) random phase approximation (RPA), (ii) static exchange approach (STEX) (unrelaxed), (iii) core-ionized state HF orbital set applied in STEX (relaxed), and (iv) HF excited state orbital set for each core-to-valence excited state. Furthermore in (i) the PRA in the framework of the density functional method (DFT) is compared with the RPA where the ab initio HF orbital set is used. In (iv), the oscillator and rotary strengths evaluated by different orbital sets for the initial and final states, namely, nonorthogonal ground-state and core-excited HF orbitals, are compared with those evaluated by using the core-excited HF orbital set to describe the initial (ground) state. It was shown that, among considered methods, the RPA provides most consistent and less time-consuming results for circular dichroism core excitation spectra. Discussion of the low energy part of K edge circular dichroism spectra of five common amino acids obtained with the help of RPA is presented.

Gradual collapse of nuclear wave functions regulated by frequency tuned X-ray scattering

As is well established, the symmetry breaking by isotope substitution in the water molecule results in localisation of the vibrations along one of the two bonds in the ground state. In this study we find that this localisation may be broken in excited electronic states. Contrary to the ground state, the stretching vibrations of HDO are delocalised in the bound core-excited state in spite of the mass difference between hydrogen and deuterium. The reason for this effect can be traced to the narrow “canyon-like” shape of the potential of the state along the symmetric stretching mode, which dominates over the localisation mass-difference effect. In contrast, the localisation of nuclear motion to one of the HDO bonds is preserved in the dissociative core-excited state . The dynamics of the delocalisation of nuclear motion in these core-excited states is studied using resonant inelastic X-ray scattering of the vibrationally excited HDO molecule. The results shed light on the process of a wave function collapse. After core-excitation into the state of HDO the initial wave packet collapses gradually, rather than instantaneously, to a single vibrational eigenstate.

Theoretical studies of angle-resolved ion yield spectra of core-to-valence transitions of acetylene

Recent experimental results on angle-resolved photoion-yield spectroscopy (ARPIS) spectra near the core-to-valence excitation in acetylene show significant anisotropies in the spectral profile measured at 0 degrees and 90 degrees regarding to the polarization direction of x-ray photons. In the present work, a theoretical model is proposed to simulate the fine structure and anisotropy in ARPIS. This employs two-dimensional potential energy surfaces of the ground and core-excited states, as well as transition dipole moments, including symmetric and antisymmetric bending modes to account for Duschinsky effect. The ARPIS is simulated by evaluation of the ion flux, which is found as a projection of the excited state wave packet on a particular direction in the molecular frame. Numerical simulations explain qualitatively the angular dependence of the experimental spectra of the 1s-->1pi(g) ( *) and 1s-->3sigma(u) ( *) transitions. The effects of the lifetime of the core-excited state, the direction of the ion flux, and the transition dipole moment are discussed.

Angular anisotropy of the delay time of short pulses in impurity band based photonic crystals

We investigate the transmission of short pulses through one-dimensional impurity band based photonic crystals. We found a strong dependence of the delay time on the angle of incidence. The delay time is larger for larger incident angles for the transverse electrical mode, while the delay time of the transverse magnetic mode has a qualitatively different angular dependence, especially in the region below the Brewster angle. The strong anisotropy of the delay time is traced to the anisotropy of the group velocity which is directly related to the angular dependence of the impurity band structure.

X-ray lasing in the CO molecule

We theoretically demonstrate the feasibility of x-ray lasing in the CO molecule by the core ionization of the C K- and O K-shell by x-ray free-electron laser sources. Our numerical simulations are ...

Angular properties of band structures in one-dimensional holographic photonic crystals

One-dimensional photonic crystals with continuous distribution of the dielectric constant were fabricated by the use of photopolymer materials and laser holography. The angular dependence of light propagation through the system was studied experimentally and theoretically. It is shown that the Brewster angles for different bands are different, in contrast to the conventional two-layer Bragg reflector with a step-like distribution of the dielectric constant. Comparison of the theory with experimental data allowed us to define the parameters of the hologram—the dielectric contrast and the shrinkage of the structure.

Stimulated X-Ray Raman Scattering with Free-Electron Laser Sources

Stimulated electronic x-ray Raman scattering is the building block for several proposed x-ray pump probe techniques, that would allow the study of electron dynamics at unprecedented timescales. We present high spectral resolution data on stimulated electronic x-ray Raman scattering in a gas sample of neon using a self-amplified spontaneous emission x-ray free-electron laser. Despite the limited spectral coherence and broad bandwidth of these sources, high-resolution spectra can be obtained by statistical methods, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules and the results of a recent experiment in CO are discussed.