Observation of Double Excitations in the Resonant Inelastic X-ray Scattering of Nitric Oxide

Abstract

The nitrogen K-edge resonant inelastic x-ray scattering (RIXS) map of nitric oxide (NO) has been measured and simulated to provide a detailed analysis of the observed features. High-resolution experimental RIXS maps were collected using an in situ gas flow cell and a high-transmission soft x-ray spectrometer. Accurate descriptions of the ground, excited, and core-excited states are based upon restricted active space self-consistent-field calculations using second order multiconfigurational perturbation theory. The nitrogen K-edge RIXS map of NO shows a range of features that can be assigned to intermediate states arising from 1s→π* and 1s→Rydberg excitations; additional bands are attributed to doubly excited intermediate states comprising 1s→π* and π→π* excitations. These results provide a detailed picture of RIXS for an open-shell molecule and an extensive description of the core-excited electronic structure of NO, an important molecule in many chemical and biological processes.

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

“…2 and 19 are also shown. The curves were computed with a (1,4,20) orbital space using either a restricted or unrestricted Kohn-Sham ground state reference, denoted as RKS and UKS, respectively. The behaviour of the potential energy curves is qualitatively correct for the UKS calculations, up to the Coulson-Fisher (CF) point, [72][73][74] beyond this point unphysical kinks are observed due to spin-contamination in the ground-state reference determinant.…”

“…Furthermore, since the spectral resolution is not limited by the large core-hole lifetime broadening, it can deliver information on the nuclear motion and associated potential energy surfaces through a vibrational substructure of electronic transitions. Recent studies ranging from isolated molecules, [2][3][4][5] molecules in solution, [6][7][8][9] fluctuating networks in liquid phase 10 and to correlated materials 11,12 demonstrate its versatility. RIXS also draws substantial interest for its applicability in pump-probe spectroscopies, [13][14][15][16][17] specially in the context of modern X-ray Free Electron Laser experiments.…”

Modeling of the spectroscopy of core electrons with density functional theory

“…One note of caution when applying TDDFT to study these processes is that it is possible that some of the intermediate states may have multiple unpaired electrons where static correlation becomes important. TDDFT is likely to perform poorly for these states and a genuine multireference treatment may be required 224 …”

“…TDDFT is likely to perform poorly for these states and a genuine multireference treatment may be required. 224…”

A time-dependent density functional theory protocol for resonant inelastic X-ray scattering calculations

“…Although RIXS has been a well established technique in the study of condensed matter systems, as core-excited states in these systems are relatively easy to access without the need of high-intensity light sources [4] (due to the high concentration of atomic centers), the continuous advance in light source technologies are allowing RIXS to also flourish in the context of gas/solution-phase molecular spectroscopy [5][6][7][8][9][10][11][12][13]. As RIXS enters the domain of larger and more complex molecular systems, the need for reliable and inexpensive electronic structure computational methods to aid in the prediction and interpretation of complicated spectral features becomes crucial.…”