Tensor patterns and polarization ratios for three-photon transitions in fluid media

Excitation of polyatomic molecules by the simultaneous absorption of three photons provides a method for observing and unambiguously assigning molecular electronic and vibronic transitions which may be inaccessible to one- or two-photon spectroscopy. Such three-photon allowed states may be forbidden to one-photon processes by reason of symmetry, even though the states may be parity (g→u) allowed. Also, the states may be one-photon allowed but lie at inaccessibly high energies, such as in the vacuum ultraviolet. Calculation of the polarization dependence of the three-photon absorption intensity requires a knowledge of the third rank tensor pattern for the three-photon transition. These patterns are presented here for the symmetry species of several molecular point groups, and their applications are discussed. Tensor patterns for the special case of three-photon excitation by a single laser are also presented. In several symmetry species the polarization ratio (circular/linear) for excitation of fluid media becomes 5/2. The intensities vanish for a few species possessing antisymmetric tensors. This phenomenon of ’’identity forbiddenness’’ is discussed.