Static Crystal Effects on the Vibronic Structure of the Phosphorescence, Fluorescence, and Absorption Spectra of Benzene Isotopic Mixed Crystals

The phosphorescence, fluorescence, and absorption spectra of the isotopic benzenes C₆H₆, C₆H₅D, p‐C₆H₄D₂, and sym‐C₆H₃D₃, 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 ${\text{0, 0\hspace{0.17em}−\hspace{0.17em}(ν}}_8{\mathrm{\hspace{0.17em}+\hspace{0.17em}\nu }}_1)$ , while for that of C₆D₅H the analysis of only the more intense bands near the electronic origin has been carried out. Some ground‐state vibrations of p‐C₆H₄D₂ 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 C₆H₅D and p‐C₆H₄D₂ as well as site splitting of the ${\nu }_6\prime$ vibrational levels of C₆H₆. On heavily exposed photographic plates it has been possible to assign a number of transitions in the ¹³CC₅H₆ emission spectra. The ${}^{13}{\mathrm{CC}}_5{\mathrm{H}}_{\text{6−n}}{\mathrm{D}}_n$ 0, 0 absorption spectra have also been identified. New absorptions, in the region of the 0, 0 transition of C₆H₆ and C₆H₅D 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 ${}^{13}{\mathrm{CC}}_5{\mathrm{H}}_{\text{6−n}}{\mathrm{D}}_n$ and ${}^{13}{\mathrm{C}}_2{\mathrm{C}}_4{\mathrm{H}}_{\text{6−n}}{\mathrm{D}}_n$ .