Laser-Intensity Dependent Vibrational Excitation and Alignment of Molecular Ions in the Ultrafast Multiphoton Regime

${\mathrm{H}}_2$ molecules were ionized in the ultrafast ($\sim 150\mathrm{fs}$) multiphoton regime (263 nm, $\sim {10}^{13}\mathrm{W}{\mathrm{cm}}^{-2}$). Earlier experiments investigated the kinetic energies of electrons or ions only. Using a unique experiment, we show that the vibrational excitation of molecular ions contains essential information about the dynamics of the process. In addition, we prove some earlier interpretations wrong. A realistic model based on vibronically excited intermediates, Stark shifting into resonance, reproduces the measurements, demonstrating that resonances continue to be important in the femtosecond regime. This eventually enables ultrafast control of the vibrational excitation of molecular ions, which is relevant to the whole field of molecular physics and physical chemistry.

Figure 1

Experimental KER spectra at (a) $I=1.5\times {10}^{13}\mathrm{W}{\mathrm{cm}}^{-2}$ and (b) $I=5.4\times {10}^{13}\mathrm{W}{\mathrm{cm}}^{-2}$. (c),(d) Modeled spectra at intensities corresponding to (a) and (b).

Figure 2

(a) Experimental relative populations of $v^{+}=0–10$ of ${\mathrm{H}}_2^{+}$ produced by MPI of ${\mathrm{H}}_2$ at 263 nm. (b) Results of a model based on REMPI with ponderomotive shifts of resonances. (c) Including photodissociation of ${\mathrm{H}}_2^{+}$. Dashed lines refer to Fig. 1 (see text).

Figure 3

Potential-energy diagram showing the four relevant excited states of ${\mathrm{H}}_2$ [19] and the ground state of ${\mathrm{H}}_2^{+}$. The states are $B$ ${}^1\Sigma _u^{+}$, $C$ ${}^1\Pi _u$, $B^{\prime }$ ${}^1\Sigma _u^{+}$, and $D$ ${}^1\Pi _u$.