Polarization studies in multiphoton absorption spectroscopy

Using the principles of quantum electrodynamics, the theory of two‐, three‐, and four‐photon absorption in polyatomic gases and liquids is developed. Expressions are derived for the rates of single‐frequency absorption from plane polarized, circularly polarized, and unpolarized light. It is shown that for n‐photon absorption with n?3, the rate for unpolarized radiation is in each case expressible as a linear combination of the rates for plane polarized and circularly polarized light; no such relationship exists for four‐photon absorption. For each multiphoton process, it is demonstrated how the fullest information about the symmetry properties of excited states can be derived by a simple linear processing of the results from experiments with different polarizations. A detailed examination of the selection rules is made, based on a reduction of the molecular transition tensor into irreducible components, and a new classification scheme is introduced to assist with the interpretation of experimental results. Finally, it is shown that the theory may also be applied to resonance‐enhanced multiphoton ionization spectroscopy.