Differential multiphoton absorption by chiral molecules and the effect of permanent moments

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Abstract
The effects of permanent moments for molecules with no centre of inversion or plane of symmetry are considered, including the generalisation of circular dichroism to multiphoton processes, in several circumstances. The absorption rate for n left-handed circularly polarised photons is compared with that for n right-hand circularly polarised photons by introducing generalised Einstein B coefficients. Semiclassical theory and quantum electrodynamics are shown, within perturbation theory, to make equal predictions. The differential rates are considered both for chiral absorbers in locked-in positions relative to the geometry of the beam and for freely rotating molecules. The presence of permanent electric dipole moments, in addition to transition moments, allows multiphoton absorption to take place in situations where absorption is not possible with transition moments only. The two- and three-photon cases are developed in some detail for a single light beam. The effects of both magnetic transition moments and electric quadrupole permanent and transition moments are considered.