Search for the First A1g−3 ← B2u+3 Transition in Naphthalene

The polarization of the triplet–triplet absorption spectrum of naphthalene-d8 in the 3250–6000-Å region was measured by the photoselection method. The spectrum 3250–4300 Å, which consisted of three strong and five weak lines, evidenced uniform polarization and therefore seemed to result solely from the transition B3g−3 ← B2u+3. The weak triplet–triplet absorption band in the 4300–6000-Å range showed four strong and several weak bands of mixed polarization, where some corresponded to that expected for the A1g−3 ← B2u+3 transition. The energy separation of this transition from the B2u+3 level and its oscillator strength were considerably smaller than were predicted from theoretical calculations.