Complete Polarization Study of the Two-Photon Absorption of Liquid 1-Chloronaphthalene

An apparatus is described which permits the measurement of all three of the parameters governing the polarized two‐photon absorption of a fluid. The measurement is performed by using suitable combinations of linear and circular polarized photons. The apparatus permits an experimental check on the basic theory of two‐photon absorptions. We find that predictions based on second‐order p·A interactions are quantitatively obeyed. Knowledge of all three absorption parameters allows the use of exact symmetry rules to assign the symmetry of the excited state. In 1‐chloronaphthalene, we find that the region of greatest absorption (38 000–42 000 cm⁻¹) is governed by a nearly symmetric absorption tensor with nonzero trace, indicating a naphthalenelike ${}^1{A}_g^{-}{\leftarrow }^1{A}_g^{-}$ transition of the molecule. In the weaker region (32 000–36 000 cm⁻¹) the tensor is assymetric with nonzero trace. This means that the weaker band cannot be due to any two‐photon allowed naphthalenelike transition. Structure is seen in the spectrum of the parameter ${\mathrm{(\delta }}_G{\mathrm{-\; \delta }}_H{\mathrm{)/\delta }}_F,$ indicating that it is probably the vibronically induced ${}^1{B}_{\text{2u}}^{+}{\leftarrow }^1{A}_g^{-},$ which is well known, lie close to this energy from one‐photon studies. We find that δ_F and δ_H are very similar throughout the spectrum, leaving open the possibility that in the theoretical expressions for the two‐photon absorption, the sum over states is dominated by a single most important virtual state.