Laser-induced molecular alignment in dissociation dynamics

Fragment angular distributions resulting from photodissociation induced by intense, short, linearly polarized laser pulses are investigated using a two-dimensional time-dependent split operator wave-packet propagation technique. The rotational excitation of the molecule leads, in general, to an alignment of the photofragments with respect to the field polarization vector but, unexpectedly, the reverse situation may also happen for some specific wavelengths for which fragmentation mainly occurs in a plane nearly orthogonal to the polarization axis. A detailed and comprehensive interpretation is provided referring to two typical wavelength regimes corresponding either to the blue or to the red wing of the single photon absorption line shape. The relevant strong field mechanisms upon which this interpretation rests are, respectively, the bond softening and the vibrational trapping of the molecule by the radiation which induces substantial modifications on the internal force field, viewed in the radiatively dressed adiabatic frame. An illustrative example, concerning the photodissociation of an isotropic distribution of ortho-${\mathrm{H}}_2^{+}$ in the ${}_{}{}^2{}_{}{}^{}$ ${\mathcal{J}}_{\mathit{g}}^{+}$(J=1/2, N=1,v=0) initial state, pertaining to Hund’s (b) case, reveals that, at λ=1600 Å, an efficient alignment is obtained with femtosecond pulses delivering an intensity in the range ${10}^{13}$–${10}^{14}$ W/${\mathrm{cm}}^2$, as a consequence of the bond softening mechanism. More surprising is the possibility of observing fragments in a near orthogonal direction to the laser polarization, predicted by the trapping mechanism and confirmed by quantum calculations at shorter wavelengths, such as λ=800 Å.