Mapping and controlling ultrafast dynamics of highly excited ${\mathrm{H}}_2$ 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 $B{}^1\mathrm{\Sigma }_u^{+}$ 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 $B{}^1\mathrm{\Sigma }_u^{+}$ 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.

Figure 1

(a) Schematic of the NWP dynamics in the $B{}^1\mathrm{\Sigma }_u^{+}$ state potential-energy curve of ${\text{H}}_2$. (b) Simulation of the wave-packet propagation (see text). (c) ${\text{H}}_2{}^{+}$ yield and (d) ${\text{H}}^{+}$ yield with corresponding Fourier transform power spectra (insets).

Figure 2

Potential-energy curves of ${\text{H}}_2/$ ${\text{H}}_2{}^{+}$ accessible by a single-photon (VUV) excitation and ionization. Vertical arrows in purple (ninth harmonic) and blue (11th harmonic) indicate the energy of the pump VUV pulses. Red (dark gray) (IR) and yellow (light gray) (third harmonic) arrows indicate the observed nonequilibrium transitions resulting in dissociative ionization into ${\text{H}}^{+}+\text{H}$.

Figure 3

(a) Sketch of the 3D ion momenta caused by VUV pump–IR probe and (b) VUV pump–VUV probe transitions. (c) Measured ${\text{H}}^{+}$ ion energies corresponding to projected slices through the momentum sphere in all three spatial directions shown below [(d) black solid, (e) blue dotted, and (f) red dashed line]. Intensities corrected for the momentum-dependent detection phase space. The identified nonequilibrium channels are labeled with roman numerals.

Figure 4

(a),(c) ${\text{H}}^{+}$ momenta and (b),(d) energies for two measurements at low IR intensity (top row) and high IR intensity (bottom row), demonstrating the IR-intensity-dependent branching ratio of the dissociation channels. The energies for projections in all three spatial directions are shown as black solid, blue dotted, and red dashed lines.