Molecular alignment using coherent resonant excitation: A new proposal for stereodynamic control of chemical reactions.

For the mode-selective control of chemical reaction, we present a new approach of molecular alignment using coherent resonant interaction with low intensity midinfrared optical pulses. Under coherent excitation, the alignment of vibrationally excited molecules becomes a function of the optical pulse area. Depending on the type of transition, with certain values of the pulse areas, a narrow group of magnetic substates are selectively excited, which results in aligning the rotational axis of the molecular ensemble. It is shown that for a P-type transition, significant alignment in the excited vibrational state can be realized using a resonant midinfrared pulse of area approximately 2pi. Under the steady state excitation (pulse duration longer than the vibrational relaxation time), the molecular alignment is destroyed due to saturation. We design a polarization spectroscopy experiment to coherently excite and probe the molecular alignment in real time.