Angular resolved kinetic-energy distributions of fragments resulting from dissociation induced by intense, short, linearly polarized laser pulses are calculated using an accurate three-dimensional Fourier transform method in spherical coordinates. The rotational excitation of the molecule leads, in general, to an alignment of the photofragments with respect to the field-polarization vector. But, unexpectedly, increasing the field strength may also produce less aligned fragments at the higher kinetic energies of the multiphoton above-threshold-dissociation spectrum. ${\mathrm{H}}_2^{+}$ photodissociating by interaction with an Nd:YAG laser at $\lambda =532\mathrm{}$ nm and for intensities of $10-50\hspace{0.5em}\mathrm{TW}/{\mathrm{cm}}^2$, is taken as an illustrative example, for which some angular resolved experimental spectra are available. A comprehensive interpretation is provided within the field-dressed Floquet picture by referring to two strong field mechanisms; namely, the potential barrier lowering (also responsible for bond softening), and the nonadiabatic transitions (also responsible for the vibrational trapping). Calculations are presented for three specific initial vibrational states leading to strongly anisotropic angular distributions that are discussed.

• Received 16 November 1998

DOI:https://doi.org/10.1103/PhysRevA.60.406