Photodissociation of ${\mathrm{Br}}_2$ molecules in an intense femtosecond laser field

We experimentally demonstrate the photodissociation process of ${\mathrm{Br}}_2$ molecules in the intense femtosecond laser field by a dc-sliced ion velocity map imaging technique. We show that four fragment ions $\mathrm{B}{\mathrm{r}}^{n+}\left(n=1–4\right)$ are observed, and their kinetic energy increases while their angular distribution decreases with the increase of the charge number. We prove that the low (or high) charged fragment ions result from the photodissociation of the low (or high) charged parent ions. We explain the changes of the kinetic energy and angular distribution in these fragment ions by considering the potential energy curves of these parent ions that involve both the interaction of the Coulomb repulsive energy and chemical bonding energy. We also explain the experimental observation that the measured kinetic energy release in the experiment is much smaller than the theoretical calculation by enhanced ionization at a critical distance.

Figure 1

The mass spectrum of ${\mathrm{Br}}_2$ molecules excited by the femtosecond laser pulse with the laser intensity of $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

The sliced images of the fragmental ions ${\mathrm{Br}}^{4+}$ (a), ${\mathrm{Br}}^{3+}$ (b), ${\mathrm{Br}}^{2+}$ (c), and ${\mathrm{Br}}^{+}$ (d).

Figure 3

The kinetic energy diagrams of the fragment ions ${\mathrm{Br}}^{4+}$ (a), ${\mathrm{Br}}^{3+}$ (b), ${\mathrm{Br}}^{2+}$ (c), and ${\mathrm{Br}}^{+}$ (d) calculated from Fig. 2.

Figure 4

The angular distributions of these $P$₁ peaks in ions ${\mathrm{Br}}^{4+}$ (a), ${\mathrm{Br}}^{3+}$ (b), and ${\mathrm{Br}}^{2+}$ (c) (blue open circles), together with these $P$₃ peaks in ions ${\mathrm{Br}}^{3+}$ (a), ${\mathrm{Br}}^{2+}$ (b), and ${\mathrm{Br}}^{+}$ (c) (red open squares).

Figure 5

Ab initio potential energy curves of the parent ions $\mathrm{B}{{\mathrm{r}}_2}^{m+}\left(m=1–8\right)$ calculated by MP2/6-311G++($d,p$) in gaussian 09 package.