Fragmentation due to centrifugal forces in the photodissociation of ${{\mathrm{H}}_2}^{+}$ in intense laser fields

By means of quantum-dynamical and classical trajectory calculations of ${{\mathrm{H}}_2}^{+}$ photodissociation in strong laser fields, it is shown that for certain combinations of pulse durations and intensities the rotational dynamics can lead to fragmentation. In that case, the photofragments exhibit characteristic angular distributions. The classical calculations provide a transparent physical picture of this mechanism which is also very well established in collisions between atomic nuclei or liquid droplets: nonrotating systems are stable, whereas rotating systems fragment due to the decrease of the fragmentation barrier with increasing angular momentum.

Figure 1

Fragment angular distributions $P_D\left(\theta \right)$ (left, solid lines), and corresponding angular momentum distributions $P_l\left(t\right)$ (right, linear density plots) of dissociating ${{\mathrm{H}}_2}^{+}$, initially in its rotational and vibrational ground state, in a cw laser with $\lambda =230\mathrm{nm}$, $I=70{\mathrm{TW}/\mathrm{cm}}^2$ (a,b) and $I=180{\mathrm{TW}/\mathrm{cm}}^2$ (c,d). The dotted lines in (a) and (c) correspond to the reduced one-Floquet-surface calculations (see text).

Figure 2

Same as in Fig. 1, but for a pulsed laser with $T=$ 30 fs (a,b), 85 fs (c,d), and 240 fs (e,f) and $\lambda =230\mathrm{nm}$, $I=180{\mathrm{TW}/\mathrm{cm}}^2$.

Figure 3

Typical shapes of the angular distributions $P_D\left(\theta \right)$ belonging to different dissociation mechanisms (BS, BH, and CF) emerging in the fragmentation of ${{\mathrm{H}}_2}^{+}$ depending on the initial state $\nu$, $l_i$ (solid lines). The dotted lines correspond to the reduced one-Floquet-surface calculations. The Frank-Condon-averaged distribution is shown as well (d). The laser parameters are $\lambda =266\mathrm{nm}$, $T=$ 130 fs, and $I=120{\mathrm{TW}/\mathrm{cm}}^2$.

Figure 4

Classical picture of centrifugal fragmentation. Upper: Floquet potential surface $V_F(R,\theta )$ of ${{\mathrm{H}}_2}^{+}$ with two sample trajectories for $\lambda =230$ nm, $I=70{\mathrm{TW}/\mathrm{cm}}^2$, and $T=$ 300 fs (cw). Lower: Effective potential $V_{\mathrm{eff}}\left(R\right)=V_F\left(R\right)+\frac{l^2}{2\mu R^2}$ at $\theta =0^{\circ }$ for angular momenta $l=0$ and $l=9$. The fragmentation barrier is clearly smaller for $l=9$ due to centrifugal forces. For comparison, the energy of the lowest vibrational state is plotted as a straight line.