Single vibronic level fluorescence spectra from the S1(1B2u) state of p-difluorobenzene vapor

${}^1\text{B}{}_{\mathrm{2u}}\text{→}{}^1\text{A}{}_{\mathrm{1g}}$ fluorescence spectra have been obtained after pumping each of 19 bands in the S₁ ← S₀ absorption of p-difluorobenzene under collision-free conditions. The fluorescence analyses provide new assignments of some absorption bands and confirmations of others so that the assignment of the principal structure in absorption is essentially secure. Only the identity of some sequence forming modes remains unconfirmed. The assignments confirm a number of excited-state fundamentals and provide new values for others (Table I). Analysis of the weaker structure in zero-point S₁ fluorescence gives a full picture of vibrational activity in the S₁ ← S₀ transition. In so far as can be determined, the vibrational activities in fluorescence and absorption are in complete correspondence. The vibrational activity in the allowed p-difluorobenzene transition replicates to a good degree that found in the forbidden transition of benzene. Evidence suggesting a nonplanar S₁ state is found. A Franck-Condon analysis is given for progressions in three modes, and several strong Fermi resonances are identified in the S₁ state.