STATIC CRYSTAL EFFECTS ON VIBRONIC STRUCTURE OF PHOSPHORESCENCE FLUORESCENCE AND ABSORPTION SPECTRA OF BENZENE ISOTOPIC MIXED CRYSTALS

The phosphorescence, fluorescence, and absorption spectra of the isotopic benzenes C6H6, C6H5D, p-C 6H4D2, and sym-C6H3D 3, present as dilute guests in a C6D6 host crystal at 4.2 °K, are obtained with sufficient spectral resolution to ascertain the magnitude of the crystalline site effects. The relative vibronic intensities in fluorescence and phosphorescence are compared. Two site effects are emphasized: site splitting of degenerate fundamentals and orientational effects. The former can occur for the isotopes C6H6 and sym-C6H3D3, while the latter is possible only for isotopes with less than a molecular threefold rotation axis. The observations show that both site-splitting and orientational effects do occur as a general rule on vibronic and vibrational states in benzene isotopic mixed crystals. An empirical correlation between the magnitudes of the site splitting, orientation effect and site (gas-to-crystal) shifts for in-plane and out-of-plane modes is noted. The phosphorescence of C6H6 and sym-C6H3D3 has been completely analyzed out to 0, 0- (v8+v1), while for that of C6D 5H the analysis of only the more intense bands near the electronic origin has been carried out. Some ground-state vibrations of p-C 6H4D2 are also presented but the phosphorescence spectrum, complicated greatly by both ground- and excited-state orientational effects, is not fully analyzed in the present work. Absorption spectra of these mixed crystals have yielded information concerning the orientational effect on the first excited singlet state of C6H 5D and p-C6H4D2 as well as site splitting of the v6′ vibrational levels of C6H 6. On heavily exposed photographic plates it has been possible to assign a number of transitions in the 13CC5H6 emission spectra. The 13CC5H6-nDn 0, 0 absorption spectra have also been identified. New absorptions, in the region of the 0, 0 transition of C6H6 and C6H 5D have been tentatively assigned on the basis of their intensity behavior as a function of guest concentration to resonance-pair lines and to lines from 13CC5H6-nDn and 13C 2C4H6-nDn.