Intrinsic channel closing in strong-field single ionization of ${\mathrm{H}}_2$

The ionization of ${\mathrm{H}}_2$ in intense laser pulses is studied by numerical integration of the time-dependent Schrödinger equation for a single-active-electron model including the vibrational motion. The electron kinetic-energy spectra in high-order above-threshold ionization are strongly dependent on the vibrational quantum number of the created $\mathrm{H}_2{}^{+}$ ion. For certain vibrational states, the electron yield in the midplateau region is strongly enhanced. The effect is attributed to channel closings, which were previously observed in atoms by varying the laser intensity.

Figure 1

Envelopes of kinetic-energy spectra of (right-going) ATI electrons leaving the $\mathrm{H}_2{}^{+}$ ion in the $v=4$ vibrationally excited state. The various laser intensities correspond to ponderomotive potentials between $U_{\mathrm{p}}=3.13\omega$ and $U_{\mathrm{p}}=3.93\omega$.

Figure 2

Envelopes of kinetic-energy spectra of (right-going) ATI electrons produced by a laser pulse of the intensity $I=9.696\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$ $(U_{\mathrm{p}}=3.73\omega )$, plotted for vibrational states of the $\mathrm{H}_2{}^{+}$ ion from $v=0,\dots ,7$. In (a), the spectra are divided by the total yield of the corresponding vibrational state (cf. Fig. 3). In (b) the spectra show the correct weighting with respect to their yield.

Figure 3

Occupation of vibrational states after a laser pulse with an intensity of $I=9.696\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$ $(U_{\mathrm{p}}=3.73\omega )$, split up into contributions from the left and right parts of the grid. The Franck-Condon overlap is plotted for comparison. The distribution has been normalized to total probability one.

Figure 4

Electronic yield within the energy window $40–50\mathrm{eV}$ (cf. Fig. 1), plotted versus vibrational state of the $\mathrm{H}_2{}^{+}$ ion for different laser intensities. In (a), the underlying electron spectra were divided by the total yield of the corresponding vibrational state (cf. Fig. 3) as in Fig. 2a. In (b), spectra weighted correctly with respect to their yield were taken as in Fig. 2b.