Mapping and controlling ultrafast dynamics of highly excited H2 molecules by VUV-IR pump-probe schemes

We used ultrashort femtosecond vacuum ultraviolet (VUV) and infrared (IR) pulses in a pump-probe scheme to map the dynamics and nonequilibrium dissociation channels of excited neutral ${\text{H}}_{2}$ molecules. A nuclear wave packet is created in the $B\phantom{\rule{0.16em}{0ex}}^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ state of the neutral ${\text{H}}_{2}$ molecule by absorption of the ninth harmonic of the driving infrared laser field. Due to the large stretching amplitude of the molecule excited in the $B\phantom{\rule{0.16em}{0ex}}^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ electronic state, the effective ${\text{H}}_{2}{}^{+}$ ionization potential changes significantly as the nuclear wave packet vibrates in the bound, highly electronically and vibrationally excited $B$ potential-energy curve. We probed such dynamics by ionizing the excited neutral molecule using time-delayed VUV-or-IR radiation. We identified the nonequilibrium dissociation channels by utilizing three-dimensional momentum imaging of the ion fragments. We found that different dissociation channels can be controlled, to some extent, by changing the IR laser intensity and by choosing the wavelength of the probe laser light. Furthermore, we concluded that even in a benchmark molecular system such as ${\text{H}}_{2}$*, the interpretation of the nonequilibrium multiphoton and multicolor ionization processes is still a challenging task, requiring intricate theoretical analysis.