Impact of intense laser pulses on the autoionization dynamics of the $2s2p$ doubly excited state of He

The photoionization of a helium atom by short intense laser pulses is studied theoretically in the vicinity of the $2s2p{}^{1}P$ doubly excited state with the intention to investigate the impact of the intensity and duration of the exciting pulse on the dynamics of the autoionization process. For that purpose, we solve numerically the corresponding time-dependent Schrödinger equation by applying the time-dependent restricted-active-space configuration-interaction method (TD-RASCI). The present numerical results clearly demonstrate that the Fano interferences can be controlled by a single high-frequency pulse. As long as the pulse duration is comparable to the autoionization lifetime, varying the peak intensity of the pulse enables manipulation of the underlying Fano interference. In particular, the asymmetric profile observed for the $2s2p{}^{1}P$ doubly excited state of He in the weak-field ionization can be smoothly transformed to a window-type interference profile.

Figure 1

Total ionization yield of helium as a function of the photon energy $\omega$, computed across the $2s2p{}^{1}P$ doubly excited state for different pulse durations $T$ (indicated for each panel) and peak intensities $I_0$ (indicated in the legends in W/${\mathrm{cm}}^2$). To enable comparison, all curves are shown on the relative vertical scale by normalization to unity at the photon energy of $\omega =58.5$ eV. The vertical dashed lines in the middle panel indicate the photon energies at which the spectra depicted in Fig. 3 have been computed.

Figure 2

Fano parameter $q$ as a function of the product of pulse duration and peak intensity, $T{I}_0$, for pulses with different duration $T$ (see legend). The values of $q$ were extracted from the profiles depicted in Fig. 1 using parametrization [8, 43]. The standard deviations of the fitted $q$ parameters are indicated by the vertical error bars. Note also the break in scale on the horizontal axis.

Figure 3

Photoelectron spectra of helium computed for $T=20$ fs laser pulses at different peak intensities (indicated in the legend in W/${\mathrm{cm}}^2$) at the photon energies of $\omega =59.6$ eV (left panel) and $\omega =60.0$ eV (right panel). Each spectrum is normalized to the corresponding peak intensity of the pulse.