Anomalously strong two-electron one-photon X-ray decay transitions in CO caused by avoided crossing.

Franz Hennies,Conny Såthe,Victor Kimberg,Loïc Journel,Alessandro Nicolaou,Gheorghe S Chiuzbăian,Freddy F Guimarães,Rafael C Couto,Marc Simon,Hans Agren,Nicolas Jaouen,Faris Gel’Mukhanov,Marco Guarise,Victor Ekholm,Jan-Erik Rubensson,Jan Lüning

doi:10.1038/srep20947

Abstract

The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentials offered by the resonant inelastic X-ray scattering (RIXS) technique is utilized to unravel an anomalously strong two-electron one-photon transition from core-excited to Rydberg final states in the CO molecule. High-resolution RIXS measurements of CO in the energy region of 12–14 eV are presented and analyzed by means of quantum simulations using the wave packet propagation formalism and ab initio calculations of potential energy curves and transition dipole moments. The very good overall agreement between the experimental results and the theoretical predictions allows an in-depth interpretation of the salient spectral features in terms of Coulomb mixing of “dark” with “bright” final states leading to an effective two-electron one-photon transition. The present work illustrates that the improved spectral resolution of RIXS spectra achievable today may call for more advanced theories than what has been used in the past.

Highlights

Decay transitions in RIXS of CO near the 1σ → 2π core excited state, leading to the E′, E and G final states, respectively
Experimental high-resolution RIXS spectra excited near the O 1s → 2π resonance of CO (Fig. 2) using circularly polarized
The potential energy curves of the states involved in the RIXS process (Fig. 2), along with transition dipole moments between them are based on state-of-the-art ab initio theory

Summary

Introduction

Decay transitions in RIXS of CO near the 1σ → 2π core excited state, leading to the E′, E and G final states, respectively. Resulting in coupled non-adiabatic electronic-nuclear dynamics involving the potential surfaces of the diabatic “dark” and “bright” states. Fast insight into the problem can be reached using adiabatic approximation[7,8,10,11], by neglecting the kinetic energy operator in nuclear Hamiltonians In this case the solution of the two-states eigenvalue problem (Eq 3) is straightforward and explains the mixing of the “bright” and “dark” states. This results in adiabatic potential energy curves where the level crossing is avoided with increase of the strength of the coupling (Eq 2). In spite of that the two representations provide the same final results[8,10,11], we use the diabatic representation (3), which is better from the computational point of view[10,11]

Methods

Results

Conclusion