Optimized pulses for Raman excitation through the continuum: verification using multi-configurational time-dependent Hartree-Fock

TL;DR

This paper verifies a Raman excitation mechanism in atoms using advanced quantum control methods and compares two computational approaches, demonstrating the process's robustness at lower intensities and its potential for multidimensional x-ray spectroscopy.

Contribution

It introduces a verification of a Raman excitation mechanism through continuum levels using MCTDHF, validated by TDCIS calculations, and explores optimal pulse sequences for atomic excitation.

Findings

Optimal Raman excitation involves sequential resonance-valence excitation.

At lower intensities, the TDCIS mechanism remains qualitatively valid.

MCTDHF predicts reduced populations compared to TDCIS by a factor of 4.

Abstract

We have verified a mechanism for Raman excitation of atoms through continuum levels previously obtained by quantum optimal control using the multi-configurational time-dependent Hartree-Fock (MCTDHF) method. For the optimal control, which requires running multiple propagations to determine the optimal pulse sequence, we used the computationally inexpensive time-dependent configuration interaction singles (TDCIS) method. TDCIS captures all of the necessary correlation of the desired processes but assumes that ionization pathways reached via double excitations are not present. MCTDHF includes these pathways and all multiparticle correlations in a set of time-dependent orbitals. The mechanism that was determined to be optimal in the Raman excitation of the Ne $1s^22s^22p^53p^1$ valence state via the metastable $1s^22s^12p^63p^1$ resonance state involves a sequential resonance-valence excitation. First, a long pump pulse excites the core-hole state, and then a shorter Stokes pulse transfers the population to the valence state. This process represents the first step in a multidimensional x-ray spectroscopy scheme that will provide a local probe of valence electronic correlations. Although at the optimal pulse intensities at the TDCIS level of theory the MCTDHF method predicts multiple ionization of the atom, at slightly lower intensities (reduced by a factor of about 4) the TDCIS mechanism is shown to hold qualitatively. Quantitatively, the MCTDHF populations are reduced from the TDCIS calculations by a factor of 4.