Electronic and vibrational states of biphenyl crystals using two-photon excitation spectroscopy

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Abstract
The two‐photon excitation spectra are reported for biphenyl‐h10 and biphenyl‐d10 neat and mixed crystals at 77 and 1.8 K. The region studied involves the low energy electronic states in the 3000–2850 Å region. We have presented background in regard to the theory of the TPE anisotropy for crystals containing more than one molecule in the unit cell, and for k=0 exciton levels having g total symmetry. An important conclusion is that previous theoretical studies of two‐photon anisotropy in crystals are shown to have neglected the effects due to birefringence in using real representations for the incident fields. Expressions valid for anisotropic crystals are given here. The 0–0 transition is the strongest band seen in two‐photon excitation and is located at 33128 cm−1, coincident with the energy of a previously observed mixed magnetic–electric dipole transition. The anisotropy of the TPE was measured in all possible polarizations involving principal faces for the 0–0 transition of crystals at 77 K. The results showed that the state at 33128 cm−1 is of the 1B3g type in the idealized symmetry group D2h. Evidence for crystal field mixing was observed from studies on the ab face, where the oriented gas predicts zero intensity. A number of new vibrational states of 1B3g were identified and attributed to particular species on the basis of their two‐photon fluorescence excitation anisotropy. These were 578 (b3g), 1049, 1451, and 1483 (Ag total symmetry). Our results confirm the existence of two electronic states of biphenyl in the 3000 Å region.