Simulation and visualization of attosecond stimulated x-ray Raman spectroscopy signals in trans-N-methylacetamide at the nitrogen and oxygen K-edges

Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy

Attosecond X-ray pulses are short enough to capture snapshots of molecules undergoing nonadiabatic electron and nuclear dynamics at conical intersections (CoIns). We show that a stimulated Raman probe induced by a combination of an attosecond and a femtosecond pulse has a unique temporal and spectral resolution for probing the nonadiabatic dynamics and detecting the ultrafast (∼4.5 fs) passage through a CoIn. This is demonstrated by a multiconfigurational self-consistent-field study of the dynamics and spectroscopy of the furan ring-opening reaction. Trajectories generated by surface hopping simulations were used to predict Attosecond Stimulated X-ray Raman Spectroscopy signals at reactant and product structures as well as representative snapshots along the conical intersection seam. The signals are highly sensitive to the changes in nonadiabatically coupled electronic structure and geometry.

Experimental and theoretical XPS and NEXAFS studies of N-methylacetamide and N-methyltrifluoroacetamide

Experimental and theoretical spectra of N-methylacetamide and N-methyltrifluoroacetamide at the K-edges are reported.

Stimulated X-ray Raman scattering – a critical assessment of the building block of nonlinear X-ray spectroscopy

With the invention of femtosecond X-ray free-electron lasers (XFELs), studies of light-induced chemical reaction dynamics and structural dynamics reach a new era, allowing for time-resolved X-ray diffraction and spectroscopy. To ultimately probe coherent electron and nuclear dynamics on their natural time and length scales, coherent nonlinear X-ray spectroscopy schemes have been proposed. In this contribution, we want to critically assess the experimental realisation of nonlinear X-ray spectroscopy at current-day XFEL sources, by presenting first experimental attempts to demonstrate stimulated resonant X-ray Raman scattering in molecular gas targets.

Characterizing the Intermediates Compound I and II in the Cytochrome P450 Catalytic Cycle with Nonlinear X-ray Spectroscopy: A Simulation Study

Cytochrome P450 enzymes are an important family of biocatalysts that oxidize chemically inert CH bonds. There are many unresolved questions regarding the catalytic reaction intermediates, in particular P450 Compound I (Cpd-I) and II (Cpd-II). By using simple molecular models, we simulate various X-ray spectroscopy signals, including X-ray absorption near-edge structure (XANES), resonant inelastic X-ray scattering (RIXS), and stimulated X-ray Raman spectroscopy (SXRS) of the low- and high-spin states of Cpd-I and II. Characteristic peak patterns are presented and connected to the corresponding electronic structures. These X-ray spectroscopy techniques are complementary to more conventional infrared and optical spectroscopy and they help to elucidate the evolving electronic structures of transient species along the reaction path.

Three-dimensional attosecond resonant stimulated X-ray Raman spectroscopy of electronic excitations in core-ionized glycine

We investigate computationally the valence electronic excitations of the amino acid glycine prepared by a sudden nitrogen core ionization induced by an attosecond X-ray pump pulse. The created superposition of cationic excited states is probed by two-dimensional transient X-ray absorption and by three dimensional attosecond stimulated X-ray Raman signals. The latter, generated by applying a second broadband X-ray pulse combined with a narrowband pulse tuned to the carbon K-edge, reveal the complex coupling between valence and core-excited manifolds of the cation.

Multidimensional x-ray spectroscopy of valence and core excitations in cysteine

Several nonlinear spectroscopy experiments which employ broadband x-ray pulses to probe the coupling between localized core and delocalized valence excitation are simulated for the amino acid cysteine at the K-edges of oxygen and nitrogen and the K- and L-edges of sulfur. We focus on two-dimensional (2D) and 3D signals generated by two- and three-pulse stimulated x-ray Raman spectroscopy (SXRS) with frequency-dispersed probe. We show how the four-pulse x-ray signals [Formula: see text] and [Formula: see text] can give new 3D insight into the SXRS signals. The coupling between valence- and core-excited states can be visualized in three-dimensional plots, revealing the origin of the polarizability that controls the simpler pump-probe SXRS signals.

Stimulated electronic x-ray Raman scattering

We demonstrate strong stimulated inelastic x-ray scattering by resonantly exciting a dense gas target of neon with femtosecond, high-intensity x-ray pulses from an x-ray free-electron laser (XFEL). A small number of lower energy XFEL seed photons drive an avalanche of stimulated resonant inelastic x-ray scattering processes that amplify the Raman scattering signal by several orders of magnitude until it reaches saturation. Despite the large overall spectral width, the internal spiky structure of the XFEL spectrum determines the energy resolution of the scattering process in a statistical sense. This is demonstrated by observing a stochastic line shift of the inelastically scattered x-ray radiation. In conjunction with statistical methods, XFELs can be used for stimulated resonant inelastic x-ray scattering, with spectral resolution smaller than the natural width of the core-excited, intermediate state.

Multicolor operation and spectral control in a gain-modulated x-ray free-electron laser

We show that the spectral properties of a self-amplified spontaneous emission x-ray free-electron laser can be controlled by modulating the gain in magnetic undulators, thus producing one or several spectral lines within a single few femtosecond pulse. By varying the magnetic field along the undulator and the electron beam transport line, the system we demonstrate can tailor the x-ray spectrum to optimally meet numerous experimental requirements for multicolor operation.

Double-core excitations in formamide can be probed by X-ray double-quantum-coherence spectroscopy

The attosecond, time-resolved X-ray double-quantum-coherence four-wave mixing signals of formamide at the nitrogen and oxygen K-edges are simulated using restricted excitation window time-dependent density functional theory and the excited core hole approximation. These signals, induced by core exciton coupling, are particularly sensitive to the level of treatment of electron correlation, thus providing direct experimental signatures of electron and core-hole many-body effects and a test of electronic structure theories.

Core and valence excitations in resonant X-ray spectroscopy using restricted excitation window time-dependent density functional theory

We report simulations of X-ray absorption near edge structure (XANES), resonant inelastic X-ray scattering (RIXS) and 1D stimulated X-ray Raman spectroscopy (SXRS) signals of cysteine at the oxygen, nitrogen, and sulfur K and L(2,3) edges. Comparison of the simulated XANES signals with experiment shows that the restricted window time-dependent density functional theory is more accurate and computationally less expensive than the static exchange method. Simulated RIXS and 1D SXRS signals give some insights into the correlation of different excitations in the molecule.

Amplified x-ray emission from core-ionized diatomic molecules

We predict high-gain x-ray lasing in molecular nitrogen by ultrafast core ionization with an x-ray free-electron laser source. To estimate the spectral and temporal output of this molecular x-ray laser, we solve generalized Maxwell-Bloch equations, keeping track of the electronic and nuclear degrees of freedom. The spectrum of the amplified x-ray emission shows a strong dependence on the gain-length product. Whereas the emission at small gain length is similar to the relatively broad x-ray fluorescence band, the spectrum is determined by a single frequency in the linear gain region. The vibrational wave packet dynamics during the x-ray emission process is examined. By preparation of the initial vibrational quantum state, the x-ray emission frequency can be tuned within the fluorescence band. The present scheme is applicable to other homo- and heteronuclear diatomic systems, thereby extending the spectral range of coherent x-ray radiation sources based on amplification on bound transitions.

Two-dimensional stimulated resonance Raman spectroscopy of molecules with broadband x-ray pulses

Expressions for the two-dimensional stimulated x-ray Raman spectroscopy (2D-SXRS) signal obtained using attosecond x-ray pulses are derived. The 1D- and 2D-SXRS signals are calculated for trans-N-methyl acetamide (NMA) with broad bandwidth (181 as, 14.2 eV FWHM) pulses tuned to the oxygen and nitrogen K-edges. Crosspeaks in 2D signals reveal electronic Franck-Condon overlaps between valence orbitals and relaxed orbitals in the presence of the core-hole.