We analyze the alignment of molecules generated by a pair of crossed ultrashort 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 ${\mathrm{CO}}_2$ at room temperature in a static cell and at $30\mathrm{K}$ in a molecular expansion jet.

• Received 22 March 2006

DOI:https://doi.org/10.1103/PhysRevA.75.013419