Speaker
Description
We present a new approach to model sequential ionization processes, employing a series of R-matrix with time-dependence (RMT) calculations to model the behaviour of the residual ion stages, and a density matrix to describe the coherences between the residual ion states. This combination allows us to describe correlated dynamics through multiple ionization stages.
RMT is a technique that can study the dynamics present in laser-driven, multielectron system on ultrafast timescales [1]. RMT partitions the system into two distinct regions. In the ‘inner’ region (close to the nucleus), the correlation between electrons is described fully. In the ‘outer’ region, electron exchange is neglected, and an ionized electron moves under the influence of the laser and the long-range potential of the ion.
Density matrix theory has been applied to understand the attosecond-scale dynamics (see [2] for an example studying correlations between photoelectrons in sequential photoionization). The density matrix can summarise the ionic quantum state left after ionization. The population of the residual ion states is contained in the diagonal elements, while the off-diagonal elements contain the correlation coefficients between these states.
As a demonstration, we model pump-probe ionisation of Ne$^+$ to Ne$^{3+}$ using an XFEL with photon energies of 95 eV [3]. We start with Ne$^+$ in its $^2$P$^o$ ground state with either $m=0$ or $m=1$. An initial XUV photon ionizes Ne$^+$ to Ne$^{2+}$. After a time delay (up to 1.2 fs), a second XUV pulse photoionises Ne$^{2+}$ to Ne$^{3+}$. The main observable of interest is the momentum-resolved angular distribution of the photoelectrons. Separate angular distributions are obtained for each residual ion state, including orbital and spin magnetic quantum numbers.
The angular distribution of the outgoing electron is affected by changes in delay time. For a delay time of 0.39 fs, the distribution is more aligned along the polarization axis compared to a delay time of 0.94 fs. This change is caused by the coherent superposition of the Ne$^{2+}$ ion after ionisation by the pump pulse. This superposition is seen for the $m=0$ levels of the $^1$S and $^1$D states, as these two states are built from the same independent electron basis states.
[1] A. Brown et al., Comput. Phys. Commun., 250 107062 (2020)
[2] L.A.A Nikolopoulos, Phys. Rev. Lett. 111 093001 (2013)
[3] W. Decking et al., Nat. Photonics, 14 391-397 (2020)
