Nuclear dynamics in resonant inelastic X-ray scattering and X-ray absorption of methanol

Abstract

We report on a combined theoretical and experimental study of core-excitation spectra of gas and liquid phase methanol as obtained with the use of X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). The electronic transitions are studied with computational methods that include strict and extended second-order algebraic diagrammatic construction [ADC(2) and ADC(2)-x], restricted active space second-order perturbation theory, and time-dependent density functional theory—providing a complete assignment of the near oxygen K-edge XAS. We show that multimode nuclear dynamics is of crucial importance for explaining the available experimental XAS and RIXS spectra. The multimode nuclear motion was considered in a recently developed “mixed representation” where dissociative states and highly excited vibrational modes are accurately treated with a time-dependent wave packet technique, while the remaining active vibrational modes are described using Franck–Condon amplitudes. Particular attention is paid to the polarization dependence of RIXS and the effects of the isotopic substitution on the RIXS profile in the case of dissociative core-excited states. Our approach predicts the splitting of the 2a″ RIXS peak to be due to an interplay between molecular and pseudo-atomic features arising in the course of transitions between dissociative core- and valence-excited states. The dynamical nature of the splitting of the 2a″ peak in RIXS of liquid methanol near pre-edge core excitation is shown. The theoretical results are in good agreement with our liquid phase measurements and gas phase experimental data available from the literature.

TD-DFT simulations of K-edge resonant inelastic X-ray scattering within the restricted subspace approximation

“…The first line refers to the case where no restrictions are imposed # of roots ---10 452 1 30 5 150 1 30 20 600 1 30 35 1050 1 10 20 200 1 15 20 300 1 20 20 400 1 30 decay processes in the dissociated fragments, conserving the kinetic energy upon dissociation. 59 Based on a fully quantumdynamical treatment of the dissociation processes the spectral contribution of the so called molecular band on the low energy loss side of the peak, corresponding to scattering in the intact molecule, could be separated from the quasi-atomic peak at the high energy loss side, which results from scattering to the electronic ground state of the dissociated fragments. Similar agreement is obtained for gas-phase water in the comparison between the experimental spectra upon scattering through the |1s O À1 4a 1 1 i state by Weinhardt et al, 56 who provided the experimental spectrum, and the theoretical simulations.…”

“…Nuclear dynamics also often lead to violation of the electronic quadrupole selection rules in polyatomic systems. 1,63,64 Furthermore, a growing effort is being devoted to the development of RIXS as a technique to study vibrational excitations and dynamics in molecules, 2,59,65,66 liquids and solutions, 10,67,68 as well as in solids. 11,69 Therefore, it is important to discuss how nuclear dynamics effects can be incorporated into the framework discussed in the previous sections.…”

“…will be considered as a reference point, as it has been shown to be a robust and straightforward way of estimating core-hole induced relaxation. 70,71 All vibrationally resolved spectra were computed using the time-dependent wave packet implementation of the Kramers-Heisenberg amplitudes as described by Vaz da Cruz et al 59 Nuclear dynamics in gas-phase H 2 O…”

Vibrational Resonant Inelastic X-Ray Scattering Inliquid Acetic Acid: A Ruler for Molecular Chain Lengths

“…The RIXS profile was computed using the wave packet software as in Refs. 6,8,20,26 using the potential energy curves (PECs) along the O-H and C-O stretching modes. The ground and core-excited PECs were determined at the RASSCF/ANO-RCC-VTZP level (with 10 electrons in 13 active molecular orbitals) followed by RASPT2 calculations to account for dynamic electron correlation effects.…”

“…1 (a)). Also in corresponding measurements on methanol 8 and acetone 7 , it was found that the long vibrational progression of the RIXS spectra of the free molecules was largely preserved in the liquid. These observations suggest that the intermolecular interactions, albeit significant, only play a minor role for the RIXS dynamics.…”