Molecular Dissociative Ionization and Wave-Packet Dynamics Studied Using Two-Color XUV and IR Pump-Probe Spectroscopy

We present a combined theoretical and experimental study of ultrafast wave-packet dynamics in the dissociative ionization of ${\mathrm{H}}_2$ molecules as a result of irradiation with an extreme-ultraviolet (XUV) pulse followed by an infrared (IR) pulse. In experiments where the duration of both the XUV and IR pulses are shorter than the vibrational period of $\mathrm{H}_2{}^{+}$, dephasing and rephasing of the vibrational wave packet that is formed in $\mathrm{H}_2{}^{+}$ upon ionization of the neutral molecule by the XUV pulse is observed. In experiments where the duration of the IR pulse exceeds the vibrational period of $\mathrm{H}_2{}^{+}$ (15 fs), a pronounced dependence of the ${\mathrm{H}}^{+}$ kinetic energy distribution on XUV-IR delay is observed that can be explained in terms of the adiabatic propagation of the $\mathrm{H}_2{}^{+}$ wave packet on field-dressed potential energy curves.

Figure 1

Measured time-dependent ${\mathrm{H}}^{+}$ kinetic energy distributions for two-color $\mathrm{XUV}+\mathrm{IR}$ dissociative ionization of ${\mathrm{H}}_2$, with 7 fs IR laser pulses (a) and 35 fs IR laser pulses (b). In the latter case the XUV pulse consisted of a train of attosecond laser pulses.

Figure 2

Calculated time-dependent kinetic energy distributions resulting from model calculations for two-color $\mathrm{XUV}+\mathrm{IR}$ dissociative ionization of ${\mathrm{H}}_2$, making use of a 7 fs FWHM IR pulse (a) and a 35 fs FWHM IR pulse (b). In the latter case the XUV pulse consisted of a train of attosecond laser pulses.

Figure 3

Fourier transform of the ${\mathrm{H}}^{+}$ fragment yield integrated over the energy range between 0 and 1.2 eV for the experimental (a) and theoretical (b) results, in the case of a 7 fs FWHM IR laser pulse. The spectra reveal the two-level beats that are responsible for the observed time dependence. The insets show an energy-resolved Fourier transformation and reveal a correlation between the fragment KE release and the vibrational level occupied prior to dissociation, observed both in the experiment and the simulations.

Figure 4

Potential energy curves of $\mathrm{H}_2{}^{+}$ in a 750 nm laser-dressed diabatic representation (black solid lines). Also indicated are the lower adiabatic curves resulting from the diagonalization of the radiative interaction for two intensities reached at the center of the pulse: $I=3·{10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ (dashed red line) and $I={10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ (dotted red line). $E_7$, $E_8$ and $E_9$ represent the kinetic energies issued from $v^{+}=7$, 8 and 9 for XUV-IR delays of 100 fs (a) and 0 fs (b).