# Electron correlation in beryllium: Effects in ground state, short-pulse photoionization and time-delay studies

## TL;DR

This paper uses a 3D time-dependent RASSCF method to study electron correlation effects in beryllium's ground state, photoionization spectra, and time-delay, demonstrating improved convergence and insights into correlation impacts.

## Contribution

It introduces the application of TD-RASSCF with flexible active space selection to accurately model electron correlation effects in beryllium's ionization dynamics.

## Key findings

- TD-RASSCF achieves better convergence than MCTDHF for ground state.
- Photoelectron spectra depend on the chosen active space.
- Electron correlation significantly influences photoionization time-delay.

## Abstract

We apply a three-dimensional (3D) implementation of the time-dependent restricted-active-space self-consistent-field (TD-RASSCF) method to investigate effects of electron correlation in the ground state of Be as well as in its photoionization dynamics by short XUV pulses, including time-delay in photoionization. First, we obtain the ground state by propagation in imaginary time. We show that the flexibility of the TD-RASSCF on the choice of the active orbital space makes it possible to consider only relevant active space orbitals, facilitating the convergence to the ground state compared to the multiconfigurational time-dependent Hartree-Fock method, used as a benchmark to show the accuracy and efficiency of TD-RASSCF. Second, we solve the equations of motion to compute photoelectron spectra of Be after interacting with a short linearly polarized XUV laser pulse. We compare the spectra for different RAS schemes, and in this way we identify the orbital spaces that are relevant for an accurate description of the photoelectron spectra. Finally, we investigate the effects of electron correlation on the magnitude of the relative time-delay in the photoionization process into two different ionic channels. One channel, the ground state channel in the ion, is accessible without electron correlation. The other channel is only accessible when including electron correlation. The time-delay is highly sensitivity to the choice of the active space, and hence to the account of electron-electron correlation.