SUPERSONIC JET FLUORESCENCE AND GAS-PHASE ABSORPTION-SPECTRA OF INDAN - PUCKERING POTENTIAL IN S0 AND S1

Single vibronic level (dispersed) fluorescence spectra of indan in a supersonic jet have been obtained and interpreted. The observation of transitions in the puckering vibration ν27 of the five-membered ring with Δv27 even and odd confirm that the equilibrium conformation in S0 is puckered in a Cs configuration. The barrier to planarity of the carbon skeleton is 1979 cm-1. These spectra force a reinterpretation of some bands in the gas phase electronic absorption spectrum in which the 2701 band is now seen to be prominent. The observation of this and other 27vv+1 bands, which show splittings due to tunneling through the puckering barrier, indicate a lower barrier of about 1800 cm-1 in S1 but the v′27 = 0, 1, and 2 levels cannot be fitted very well to a quadratic-quartic potential. A probable reason for this is that the vibrational motion involved in ν27 in S1 is no longer a purely puckering motion. © 1991.