Two-color photodissociation of the lithium molecule: Anomalous angular distributions of fragments at high laser intensities

Two-step photodissociation angular distributions are calculated by a coupled-equation approach for the 1 ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$→1 ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$→1 ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ transition in ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$. The initial excitation is induced by a low-intensity visible laser in order to select the rovibrational state. The Π-Π transition is effected by an infrared laser that is varied from weak (perturbative) to intense (nonperturbative) regimes. It is shown that laser-induced orientation effects and laser-induced resonances (trapped states) considerably alter the angular distribution of the photodissociating fragments, creating anomalies in some instances at high intensities where a nonperturbative, numerical approach is essential.