Theory of sub-Doppler Autler–Townes splitting in molecules: Alignment and orientation of the angular momentum in nonpolar molecules

Abstract

Within a density matrix framework, an analytical expression for the optical–optical double resonance (OODR) line shape for a molecule in the presence of a strong coupling laser is derived. The coupling laser is tuned to a transition between the upper level in the OODR sequence and another excited level which is not part of the OODR sequence. The pump and probe lasers are treated perturbatively, while the coupling laser is treated exactly. The line shape reveals sub-Doppler Autler–Townes (AT) splitting of the states |J,m〉 within the highest excited level. The splitting depends on |m|(m) for linearly (circularly) polarized coupling fields. m-state selectivity and therefore alignment and orientation of the angular momentum can be achieved by tuning the probe laser in the presence of a coupling field which is sufficiently strong to resolve the m-dependent AT splittings. m selectivity is analyzed for e and f parity 1Πg levels in diatomic molecules. A method to orient nonpolar diatomics using circularly polarized lasers is proposed.