Hetero-site Double Core Ionization Energies with Sub-electronvolt Accuracy from Delta-Coupled-Cluster Calculations

Abstract

Benchmark scalar-relativistic delta-coupled-cluster calculations of hetero-site double core ionization energies of small molecules containing second-row elements are reported. The present study has focused on the highspin triplet components of two-site double core-ionized states, which are single reference in character and consistent with the use of standard coupledcluster methods. Contributions to computed double core ionization energies from electron-correlation and basis-set effects as well as corrections to the core−valence separation approximation have been analyzed. On the basis of systematic convergence of computational results with respect to these effects, delta-coupled-cluster calculations have been shown to be capable of providing accurate double core ionization energies with remaining errors estimated to be below 0.3 eV, and thus they are recommended for use to facilitate experimental studies of two-site double core-ionized states that are involved in X-ray pump/X-ray probe studies of electronic and molecular dynamics following inner shell ionization or excitation.

Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis

“…Interpretation of ultrafast X-ray spectroscopic measurements is invariably linked with theory, which, for understanding structural dynamics of core-excited matter, is an ongoing challenge as described in some recent reviews [73,74]. There are already challenges at a very basic level, e.g., to predict the X-ray absorption spectrum of ground-state species with sub-eV accuracy [75] and to locate the positions of double-hole states [76]. Here, advances in equation-of-motion coupled-cluster (EOM-CC) approaches [77,78] have allowed a systematic convergence after application of the core-valence separation scheme [79] within the EOM-CC framework [80].…”

From synchrotrons for XFELs: the soft x-ray near-edge spectrum of the ESCA molecule

“…These methods were benchmarked on other simple molecules (NH 3 , H 2 CO, CO 2 ) and are thought to be generalizable. While most time-resolved NEXAFS spectra have been used to interpret dynamics due to optical/UV excitation of valence states [26,16], NEXAFS can also provide fingerprints to follow inner-shell-initiated molecular dynamics -a discipline which is now feasible and a natural extension to x-ray pump/probe methods at XFELs [17,89,90,91,92]. Ethyl trifluoroacetate has already proven itself a model for ESCA (electron spectroscopy for chemical analysis), and, now extends its reach to advance theoretical methods to describe complex innershell photoabsorption spectra.…”

Modeling of the spectroscopy of core electrons with density functional theory

“…This ionization can occur on a single atom or two different atoms. DFT based approaches to have been used to determine these ionization energies 201,202 and benchmark coupled cluster theory values are available 203 …”

“…DFT based approaches to have been used to determine these ionization energies 201,202 and benchmark coupled cluster theory values are available. 203 Resonant inelastic X-ray scattering (RIXS) is a fast-developing technique wherein absorption of a photon creates a core-excited intermediate state and emission from this state leads to the final state. 204 RIXS can provide information for a range of core-excited intermediate states with the capability of probing the evolution of the nuclear and electronic structure occurring in the intermediate state within the lifetime of core-excited state.…”