CAGED SPIN-ORBIT EXCITED I-ASTERISK(P-2(1 2))+I-ASTERISK(P-2(1/2)) ATOM PAIRS IN LIQUIDS AND IN CRYOGENIC MATRICES - SPECTROSCOPY AND DIPOLAR QUENCHING/

Two-photon excitation of I-2 in room temperature liquid solutions and in cryogenic rare gas matrices, in the 500-600 nm range, leads to fluo rescence in the near infrared. The emission spectra are assigned to th e II*(P-2(1/2)+P-2(1/2))-->I*I(P-2(1/2)+P-2(3/2)) transitions, namely , to contact pairs of spin excited atoms which are strictly bound by t he solvent cage. The spectra are analyzed to characterize cage potenti als and contact distances. In rare gas solids, the caged pair relaxes radiatively, tau(rad) = 3.9, 3.1, and 2.8 mu s, in Ar, Kr, and Xe. In room temperature liquids, emission from both geminate and nongeminate pairs occurs, resulting in bimodal kinetics. The fluorescence yield st rongly depends on the nature of the solvent. Through a set of measurem ents in hydrogenated and deuterated solvents, the quenching mechanism is ascribed to dipolar energy transfer to solvent molecules: the quenc hing efficiency is determined by the spectral overlap between vibratio nal overtones of the solvent and the solute emission. (C) 1997 America n Institute of Physics. [S0021-9606(97)00444-3].