Ultrafast Electronuclear Dynamics of ${\mathrm{H}}_2$ Double Ionization

The ultrafast electronic and nuclear dynamics of ${\mathrm{H}}_2$ laser-induced double ionization is studied using a time-dependent wave packet approach that goes beyond the fixed nuclei approximation. The double ionization pathways are analyzed by following the evolution of the total wave function during and after the pulse. The rescattering of the first ionized electron produces a coherent superposition of excited molecular states which presents a pronounced transient ${\mathrm{H}}^{+}{\mathrm{H}}^{-}$ character. This attosecond excitation is followed by field-induced double ionization and by the formation of short-lived autoionizing states which decay via double ionization. These two double ionization mechanisms may be identified by their signatures imprinted in the kinetic-energy distribution of the ejected protons.

Figure 1

(a) Theoretical and experimental proton kinetic energy spectra at $4.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The measured pulse duration (FWHM) is $10\pm 2\mathrm{fs}$ and the calculation is for 11.7 fs. (b) Theoretical spectrum at $5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and for $\tau =2.67\mathrm{fs}$. The dotted lines and dashed lines correspond to recollision-induced field-assisted double ionization (RFDI) and to recollision-induced autodouble ionization (RADI).

Figure 2

(a) Single (SI) and double (DI) ionization probabilities as a function of time at $5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and for $\tau =2.67\mathrm{fs}$. The electric field $E(t)$ is superimposed on this graph. The vertical line marks the end of the pulse. (b)–(d) Nuclear probability distributions $P_k(R,t)={\iint }_{{\Gamma }_k}|\Psi (R,z_1,z_2,t){|}^{2}d{z}_{1}d{z}_2$ as a function of internuclear distance $R$ and time $t$ in the three regions ${\Gamma }_k$ corresponding to ${\mathrm{H}}_2$ (b), ${\mathrm{H}}_2^{+}$ (c) and ${\mathrm{H}}_2^{2+}$ (d).

Figure 3

Electronic probability distribution $|\Psi (R,z_1,z_2,t){|}^2$ (log scale) at different times and internuclear distances. The white arrows represent the photoelectron flux. Panels (a)–(d) correspond to points (1)–(4) in Fig. 2d.