First and Second Triplets of Solid Benzene

Abstract

Absorption spectra have been taken of the O₂‐perturbed first (³B_1u) and second (³E_1u) triplets of solid benzene at 4.2°K. Spectra of both C₆H₆ and C₆D₆ were obtained. The (0–0) bands of the first triplet occur at 29 674±25 cm⁻¹ for C₆H₆ and 29 851±25 cm⁻¹ for C₆D₆. For the second triplet they lie at 36 560 cm⁻¹±50 for C₆H₆ and 36 784±50 cm⁻¹ for C₆D₆. The result for the first triplet of C₆H₆ compares very favorably with Evans' gas‐phase O₂‐perturbed spectrum. It is also in satisfactory agreement with Nieman's accurate phosphorescence measurements on isotopic mixed crystals of benzene which place the C₆H₆ (0, 0) band position in the crystal at 29 657.1 cm⁻¹. Many precautions were taken to eliminate the possibility of mis identification of the second triplet. The observation that the O₂‐enhanced first triplet and the O₂‐enhanced bands in the 36 600‐cm⁻¹ region always appear together and with approximately the same relative intensities is considered to be the best evidence for the assignment. However, the rather broad structure obtained by the O₂‐perturbation technique does not allow all the uncertainties in the identification to be completely removed, nor does it allow a detailed study of this interesting state. A detailed evaluation of the purity of the benzene is made, and a method is described for the preparation of material having ultrahigh spectroscopic purity. Crystals, up to 5 cm in length, of this very highly purified C₆H₆ and C₆D₆ were studied at 4.2°K to ascertain if the singlet—triplet absorptions could be seen in the absence of a perturbation. The long crystals showed some sharp and some broad (Δν≈150 cm⁻¹) absorptions starting at 36 947±50 cm⁻¹ in C₆H₆ and at 37 147±50 cm⁻¹ in C₆D₆. The broad absorptions correlate reasonably well with the features assigned to the second triplet in the O₂‐perturbation experiments. The first triplet is too weak to be observed in the long‐crystal experiments. The position of the second triplet lies about 3000 cm⁻¹ above that given by the Pariser—Parr calculation. This places the second triplet about nine‐tenths rather than half of the distance from the lowest triplet to the lowest excited singlet.