  We analyze the alignment of molecules generated by a pair of crossed
ultra-short pump pulses of different polarizations by a technique based on the
induced time-dependent gratings. Parallel polarizations yield an intensity
grating, while perpendicular polarizations induce a polarization grating. We
show that both configurations can be interpreted at moderate intensity as an
alignment induced by a single polarized pump pulse. The advantage of the
perpendicular polarizations is to give a signal of alignment that is free from
the plasma contribution. Experiments on femtosecond transient gratings with
aligned molecules were performed in CO2 at room temperature in a static cell
and at 30 K in a molecular expansion jet.
