Strongly driven resonant Auger effect treated by an open-quantum-system approach

Nina Rohringer,Robin Santra

doi:10.1103/physreva.86.043434

Abstract

We present theoretical studies of a two-step resonant Auger process at high x-ray intensity. Tuning a short x-ray pulse to the initially closed resonant channel of the 1s-2p transition in singly ionized neon, the initially neutral neon target is valence ionized. Subsequently, the strong resonant x-ray field transfers an inner-shell electron to the created outer valence vacancy, thereby creating a core-excited state. The strong resonant coupling, giving rise to Rabi oscillations involving a core transition, results in a modification of the resonant Auger-electron spectral line profile. If the valence photoelectron remains unobserved, the system of the residual ion undergoing the resonant Auger decay can be treated by an open quantum system approach. The resonant Auger-electron spectral line shape is shown to be determined by an analog of the reduced density matrix that depends on two time arguments. The equations of motion of this reduced density matrix are derived and numerical results are presented, in support of the recent experimental verification [E. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011)] of this nonlinear x-ray optical effect.

Highlights

The recent start up of x-ray free electron laser (XFEL) sources [1,2,3] opened the opportunity to study fundamental interactions of femtosecond, high-intensity radiation with atoms in the x-ray regime [4,5,6,7]
In addition to stimulated emission, the core hole can undergo an Auger decay, resulting in damped Rabi oscillations. (Spontaneous emission is not important in the problem considered here.) The spectral line shape of the resonant Auger electron is modified in the strong-field limit [8,9,10,11], i.e., when the Rabi oscillation period is comparable to the Auger lifetime of the core hole
Equation (11) highlights the fact that the Auger-electron spectral line shape can be determined by eliminating both continua related to the photoelectron and the Auger electron, at the expense of introducing an object related to the reduced ionic density matrix which treats memory effects and depends on two time arguments

Summary

INTRODUCTION

The recent start up of x-ray free electron laser (XFEL) sources [1,2,3] opened the opportunity to study fundamental interactions of femtosecond, high-intensity radiation with atoms in the x-ray regime [4,5,6,7]. (Spontaneous emission is not important in the problem considered here.) The spectral line shape of the resonant Auger electron is modified in the strong-field limit [8,9,10,11], i.e., when the Rabi oscillation period is comparable to the Auger lifetime of the core hole. Another effect resulting in a change of the spectral line shape in the regular Auger process is the Autler-Townes Stark splitting of the final state, induced by strong resonant excitation of the final state by an x-ray field [12]. Assumptions for the driving x-ray field will be compared, i.e., transform limited femtosecond pulses and pulses from a self-amplified spontaneous emission (SASE) XFEL source

THEORETICAL APPROACH

Expansion of the wave function

Calculation of resonant Auger-electron spectral line shape

Equations of motion

SASE pulses of limited temporal coherence

CONCLUSIONS