Non-equilibrium x-ray spectroscopy using direct quantum dynamics

Abstract

Advances in experimental methodology aligned with technological developments, such as 3rd generation light sources, X-ray Free Electron Lasers, and High Harmonic Generation, have led to a paradigm shift in the capability of X-ray spectroscopy to deliver high temporal and spectral resolution on an extremely broad range of samples in a wide array of different environments. Importantly, the complex nature and high information content of this class of techniques mean that detailed theoretical studies are often essential to provide a firm link between the spectroscopic observables and the underlying molecular structure and dynamics. In this paper, we present approaches for simulating dynamical processes in X-ray spectroscopy based upon on-the-fly quantum dynamics with a Gaussian basis set. We show that it is possible to provide a fully quantum description of X-ray spectra without the need of precomputing highly multidimensional potential energy surfaces. It is applied to study two different dynamical situations, namely, the core-hole lifetime dynamics of the water monomer and the dissociation of CF + 4 recently studied using pump-probe X-ray spectroscopy. Our results compare favourably to previous experiments, while reducing the computational effort, providing the scope to apply them to larger systems. https://doi.

Time-resolved near-edge X-ray absorption fine structure of pyrazine from electronic structure and nuclear wave packet dynamics simulations

“…Alternatively, calculations could be performed using a reference state that has the appropriate electronic configuration (the one of the valence-excited state). This is commonly achieved 27,28,42,43,77 by using an open-shell reference determinant, which can be computed using the maximum overlap method (MOM) algorithm. 78 For example, in Ref.…”

Simulation of ultrafast excited-state dynamics and elastic x-ray scattering by quantum wavepacket dynamics

“…In the case of spectroscopy, this task requires the combination of electronic structure calculations and nonadiabatic dynamics simulations. This technique has been recently employed for the simulation of TR X-ray absorption [12][13][14][15][16][17] and emission spectra. 12 While X-ray spectroscopies are local, element-specific probes by their nature, X-ray scattering is global, i.e., arising from all electrons in the sample.…”

Excited state dynamics initiated by an electromagnetic field within the Variational Multi-Configurational Gaussian (vMCG) method

“…This approach assumes that the excitation is impulsive in that no nuclear dynamics can occur during excitation and that the pump pulse prepares a well-defined excited state. In the present contribution, we seek to facilitate an accurate framework for Gaussian based quantum dynamics to simulate time-resolved spectra [46,47,48,49]. We have used two model vibronic coupling Hamiltonians for pyrazine to explore the influence of the coupling between the external field and the GBFs within the framework of vMCG dynamics.…”