Intense laser dissociation of $\mathrm{D}_2{}^{+}$: From experiment to theory

Accurate angularly resolved photodissociation spectra are recorded using electric discharge for preparing the molecular ions, under intense (about ${10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$), short (about 140 fs) laser pulse excitation at 748 nm wavelength. Such experiments are particularly convenient for the interpretation level of theoretical models that neglects ionization-dissociation competition. Abel transformation relates the center-of-mass dissociation probability to the laboratory frame observations. A spatial field average over the laser focus area, together with some statistics on molecular initial rovibrational states, yields to fragments kinetic and angular distributions that are directly comparable to experimental data. Due to small-angle Coulomb explosion, not explicitly taken into account by the model, the theory-versus-experiment confrontation can be quantitatively conducted only for angles exceeding $\pi ∕7$. Very good agreement is obtained for spectra including both single- and two-photon processes that are enhanced at such high laser intensities. The level of accuracy which is reached advocates both for the correct arrangement of the different steps of the theory and for the basic mechanisms (namely, bond softening and vibrational trapping) that are used as interpretative tools.

Figure 1

The $\mathrm{D}_2{}^{+}$ photodissociation experiment based on the ionization of ${\mathrm{D}}_2$ using a discharge source.

Figure 2

(Color online) Two-dimensional experimental photodissociation image of $\mathrm{D}_2{}^{+}$ using a 784.5-nm laser pulse of duration 140 fs and peak intensity of $1.5{10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$.

Figure 3

Single- and three-photon dressed BO potential energy curves of $\mathrm{D}_2{}^{+}$. The thin horizontal lines indicate the vibrational levels of $\mathrm{D}_2{}^{+}$, whereas the dashed horizontal lines are for the vibrational levels of $\mathrm{H}_2{}^{+}$.

Figure 4

Vibrational levels populations as experimentally estimated and fitted in the present work (rectangles).

Figure 5

Fragment kinetic energy distributions (cut at $\alpha =0$) resulting from one- and two-photon dissociation of $\mathrm{D}_2{}^{+}$ (solid line for the experimental and dashed line for the calculated spectra, respectively). The vertical lines correspond to fragments momenta originated from the one- (solid line) and two- (dotted line) photon dressed vibrational levels.

Figure 6

Same as for Fig. 5, the additional (dotted) curve being the single-photon contribution to the calculated spectrum

Figure 7

Coulomb explosion branching ratio ${\mathcal{R}}_c$ as a function of ${\alpha }_0$ (solid line). Are also displayed $\Delta$ (dotted line) and ${\mathcal{P}}_D^{{\alpha }_0>}$ (dashed line). See text for the notations.

Figure 8

Same as for Fig. 5, but for cuts $\alpha =\pi ∕7$ (a) and $\alpha =\pi ∕4$ (b).

Figure 9

Fragments angular distributions resulting from single-photon dissociation of $\mathrm{D}_2{}^{+}$ for cuts corresponding to $k_{\rho }=7$, 9, and 10 (solid line for the experimental and dashed line for the calculated spectra, respectively).

Figure 10

Angular distributions for fragments resulting from two-photon dissociation of $\mathrm{D}_2{}^{+}(v=4)$, for different intensities. The solid line for $I=3\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$, the dotted line for $I=5\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$, and the dashed line for $I=7\times {10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$.

Figure 11

Same as for Fig. 10, but for $\mathrm{H}_2{}^{+}(v=3)$.