Adiabatic excitation of rotational ladder by chirped laser pulses

We discuss rotational excitation of molecules by a pair of left and right circularly polarized laser pulses with opposite chirps. The pulses are supposed to be short enough (picosecond or femtosecond) to prevent relaxation, sufficiently intense to induce adiabatic evolution, and far-off-resonant, e.g., infrared. This technique has been demonstrated recently by Villeneuve et al. [ Phys. Rev. Lett. 85 542 (2000)] in rotational dissociation of molecules. We analyze the properties of this technique by using the concepts of level crossing and adiabatic evolution, which allow us to derive analytically the conditions for efficient excitation. We analyze both the cases of intuitive (divergent frequencies) and counterintuitive (convergent frequencies) chirps and examine various initial conditions, including a single J state, coherent and incoherent superpositions of J states. We propose a technique, which can create superrotors by applying a pair of appropriately timed narrow pulses.