Nonlinear Spectroscopy of Core and Valence Excitations Using Short X-Ray Pulses: Simulation Challenges

Abstract

Measuring the nonlinear response of electrons and nuclei to attosecond broadband X-ray radiation has become possible by newly developed free electron lasers and high harmonic generation light sources. The design and interpretation of these novel experiments poses considerable computational challenges. In this chapter we survey the basic description of nonlinear X-ray spectroscopy signals and the electronic structure protocols which may be used for their simulation.

Study of double core hole excitations in molecules by X-ray double-quantum-coherence signals: a multi-configuration simulation

“… 18 , 26 , 43 Other approaches include the density functional theory (DFT), 5 , 26 , 32 , 34 time-dependent DFT (TDDFT), 15 second-order Møller–Plesset (MP2), 25 MP4SDQ, 44 Configuration Interaction with singles and doubles (CISD), 33 and multi-reference CI (MRCI) 26 methods. Methods for core hole states and X-ray spectra such as DCH states are reviewed in ref. 8 .…”

“…The challenge of XDQC signal simulations lies in computing valence, SCH and DCH excited states with both accuracy and efficiency, and the corresponding transition dipole moments (TDMs) for valence–SCH and SCH–DCH state pairs. 8 Previous signal calculations 5 , 15 only considered the 2-site DCH states at the DFT level. The earlier work 5 treated the two core holes by using the equivalent core hole (ECH) approximation (also known as the Z + 1 approximation).…”

Density matrix-based time-dependent configuration interaction approach to ultrafast spin-flip dynamics

“…Thus, the relative SRIXS intensities in the respective energy ranges (0-1.5 eV for quintets and 1.5-8.2 eV for triplets in the case of [Fe(H 2 O) 6 ] 2+ system) would provide information on the time-evolution of the contribution of pure spin states to a mixed one. Given the rapid progress in high harmonic generation [4][5][6][7] and freeelectron lasers [8] and the expected establishment of time-resolved techniques such as SRIXS, [54][55][56] the experimental proof of the effect discussed here and its use for manipulating spin dynamics appears to be within reach.…”

“…The notable dependence of spin state yields on the pulse parameters calls for an experimental verification. A possible direct way to address such spin dynamics would be the upcoming time-resolved non-linear X-ray spectroscopy, e.g., stimulated resonant inelastic X-ray scattering (SRIXS) [54][55][56]. In this technique the mixed-spin core states can be projected onto the manifold of pure-spin valence states, which are usually energetically well separated in transition metal complexes, see, e.g., discussion in Refs.…”

Multidimensional resonant nonlinear spectroscopy with coherent broadband x-ray pulses

“…Calculations were performed at the stateaveraged restricted active space self-consistent field (SA-RASSCF) [34] level, which adequately considers the static correlation and orbital relaxation caused by core holes. Nonlinear x-ray spectroscopy poses new computational challenges for core excitations (see [35] for a review on different methods). RASSCF can treat the valence, SCH and DCH states at the same level, which gets a balanced accuracy for their couplings and the signals.…”