Ionization Branching Ratio Control with a Resonance Attosecond Clock

We investigate the possibility to monitor the dynamics of autoionizing states in real-time and control the yields of different ionization channels in helium by simulating extreme ultraviolet (XUV) pump IR-probe experiments focused on the $N=2$ threshold. The XUV pulse creates a coherent superposition of doubly excited states which is found to decay by ejecting electrons in bursts. Prominent interference fringes in the photoelectron angular distribution of the $2s$ and $2p$ ionization channels are observed, along with significant out-of-phase quantum beats in the yields of the corresponding parent ions.

Figure 1

Charge density after the XUV-pump pulse, at small (top row) and large (bottom row) radii. At each breathing cycle, the metastable wave packet, formed by a coherent superposition of doubly excited states, ejects a burst of electrons. The peak of the free electron density originating close to the nucleus results in a wave front which propagates outward at almost constant speed, up to very large distances.

Figure 2

Partial differential photoelectron spectra in the $2s$ (top row) and $2p$ (bottom row) ionization channels after the XUV-pump pulse (a),(d) and after the IR pulse with two different time delays $\Delta t$ between pump and probe pulses separated by half the IR period: 15.53 fs (b),(e), and 16.87 fs (cf.). $x$ axis: cosine of the photoelectron ejection angle with respect to the laser polarization; $y$ axis: photoelectron energy in atomic units. The interplay between the direct ionization by the XUV and the multiphoton ionization of the DES due to the IR-probe results in prominent interference fringes, with a characteristic energy spacing $\Delta ϵ=2\pi \hslash /\Delta t$.

Figure 3

Increase in the yields of the $2s$ and $2p$ ${\mathrm{He}}^{+}$ excited parent ions, due to the IR probe stage, as functions of the time delay between the pump and probe pulses. Both yields are modulated by large quantum beats, due to the interplay between $s{p}_2^{+}$ and $s{p}_3^{+}$ ${}^{1}P^o$ DES, which are out of phase by as much as 60°. The continuous curves are obtained by fitting the computed points with a sine function plus a quadratic background.