Probing spin–vibronic dynamics using femtosecond X-ray spectroscopy

Abstract

Ultrafast pump-probe spectroscopy within the X-ray regime is now possible owing to the development of X-ray Free Electrons Lasers (X-FELs) and is opening new opportunities for the direct probing of femtosecond evolution of the nuclei, the electronic and spin degrees of freedom. In this contribution we use wavepacket dynamics of the photoexcited decay of a new Fe(ii) complex, [Fe(bmip)2]2+ (bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine), to simulate the experimental observables associated with femtosecond Fe K-edge X-ray Absorption Near-Edge Structure (XANES) and X-ray emission (XES) spectroscopy. We show how the evolution of the nuclear wavepacket is translated into the spectroscopic signal and the sensitivity of these approaches for following excited state dynamics.

Non-equilibrium x-ray spectroscopy using direct quantum dynamics

“…Previous work 29,30 has highlighted the challenge associated with the exponential scaling of the grid-based approaches for simulating time-resolved X-ray spectra. Importantly, the GBF approach presented here avoids this problem making it possible to incorporate the effect of more than 3-4 nuclear degrees of freedom.…”

“…Previous methods for calculating non-equilibrium X-ray spectra have post-processed quantum dynamics simulations. 29,30 In these cases, the X-ray spectrum of the nonstationary wavepacket is calculated as the weighted sum of spectra calculated at each grid point used to describe the nuclear wavepacket. The weighting corresponds to the magnitude of the nuclear wavepacket at that grid point.…”

“…For nuclear dynamics approaches based upon a fully (electrons and nuclei) quantum description will clearly provide the best approach, but usually suffer from a scaling problem associated with the direct product grid representation. Consequently, they are usually limited to reduced coordinated space models 29,30 or small molecules for which the full multidimensional potential energy surface can be calculated. 31 Consequently, a simpler method is to adopt a semiclassical approach and represent the nuclear motion using an ensemble of classical trajectories.…”

Probing Competing Relaxation Pathways in Malonaldehyde with Transient X-Ray Absorption Spectroscopy

“…42 Increasing effort is being devoted towards enabling theoretical support for transient X-ray spectroscopies both in terms of accurate calculations of valence-to-core-excited state spectroscopic signals and inclusion of coupled electron-nuclear dynamics. Currently available methods for simulating TRXAS include density functional theory (DFT), 10,[43][44][45][46] restricted active space self-consistent field without (RASSCF) 47,48 and with second-order perturbation correction (RASPT2), 46,49 algebraicdiagrammatic construction (ADC), 50-52 coupled cluster 13,[53][54][55] and combined DFT/multireference configuration interaction. 56 Effects of nuclear dynamics, and in particular non-adiabatic transitions, have been treated at varying degrees of complexity and accuracy using fulldimensionality trajectory- 10,48 and Gaussian wavepacket-based 46,51,57 dynamics as well as reduced-dimensionality quantum dynamics.…”

“…56 Effects of nuclear dynamics, and in particular non-adiabatic transitions, have been treated at varying degrees of complexity and accuracy using fulldimensionality trajectory- 10,48 and Gaussian wavepacket-based 46,51,57 dynamics as well as reduced-dimensionality quantum dynamics. 44,45,54 We employ both RASPT2 58,59 and ADC 60,61 to simulate the oxygen K-edge TRXAS signals of the proposed photoexcited decay pathways in malonaldehyde, including internal conversion mediated by ESIHT and C=C torsional motion as well as singlet-triplet intersystem crossing. We provide a link between spectroscopic signatures and underlying electron and nuclear dynamics by analyzing initial valence and final core-excited states along interpolated reaction coordinates.…”