Dissociation and Ionization of $\mathrm{H}_2{}^{+}$ by Ultrashort Intense Laser Pulses Probed by Coincidence 3D Momentum Imaging

Laser-induced dissociation and ionization of $\mathrm{H}_2{}^{+}$ were simultaneously measured using coincidence 3D momentum imaging, allowing direct separation of the two processes, even where the fragment kinetic energy is the same for both processes. The results for 45 and 135 fs 790 nm pulses with an intensity of approximately $2.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ differ from each other much more than one would expect from previous measurements with longer pulses. Ionization was negligible for the longer pulse and was strongly aligned along the laser polarization for the shorter pulse, but showed no structure in its kinetic energy distribution. In addition, the ionization to dissociation ratio was found to be much smaller than theoretically predicted for $\mathrm{H}_2{}^{+}$.

Figure 1

Schematic of the experimental setup (see Ref. 20).

Figure 2

(a) Angle vs KER distribution for dissociation in a 135 fs, $I_0=2.4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ laser pulse, (b) including only the intensities between 64% and 100% of $I_0$. The vertical bars mark the expected KER of the unperturbed vibrational levels (their length represents the population of the state, assuming a Franck-Condon distribution). (c) Field-dressed (Floquet) Born-Oppenheimer curves for intensities relevant to the measurement. The minimum and maximum KER values are associated with the barrier height and well minimum for each $I_0$.

Figure 3

Angle vs KER distributions: (a) and (b) are similar to those shown in Fig. 2, but for a 45 fs pulse; (c) and (d) show ionization by a 45 fs pulse of $2.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and $4.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ peak intensities, respectively.