Reflection spectra of the first singlet transition of crystalline phenanthrene

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Abstract
Reflection spectra in the range of the first singlet transition are reported for the (001) face of a phenanthrene crystal at 1.5 K and at several higher temperatures. The spectra were taken at near normal incidence with light polarized parallel and perpendicular to the monoclinic b axis. The (001) face was obtained by cleaving a crystal grown from the melt. For the a-polarized transition the reflectivity of the 0–0 component is 30% and the stopband is approximately 5 cm−1 wide. The 0–0b transition has a much lower reflectivity, close to 16%, and a stopband almost 40 cm−1 wide. The dielectric function was calculated for the 0–0 region by fitting the spectrum to that of a series of Lorentzian oscillators and also by using the Kramers-Kronig transformation. The factor group splitting in reflection is measured to be 41 cm−1, while the same splitting from the derived absorption spectra is 46 cm−1. This difference is due to a red shift of the a-polarized absorption peak and a blue shift of the b-polarized absorption peak from the corresponding peaks in the reflection spectrum. The absorption linewidths are narrower than previously reported. The temperature dependence of the linewidths is discussed.