First and Second Triplets of Solid Benzene

Abstract

Absorption spectra have been taken of the O2-perturbed first (3B1u) and second (3E1u) 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.