Polarized Singlet–Triplet Absorption Spectra of Normal and Deuterated Anthracene Crystals

The singlet–triplet absorption spectra of anthracene‐h₁₀ and anthracene‐d₁₀ crystals have been measured along the $a$ and $b$ crystal axes for temperatures between 350 and 4°K. The delayed fluorescence intensity was used to monitor the absorption. The Franck–Condon overlap factor was found to change with temperature, and the change is discussed in terms of variations in the C–C bond order. A vibrational analysis of the low temperature spectra is given.