Caged spin-orbit excited I*2P1/2+I*(2P1/2) atom pairs in liquids and in cryogenic matrices: Spectroscopy and dipolar quenching

Two-photon excitation of I2 in room temperature liquid solutions and in cryogenic rare gas matrices, in the 500–600 nm range, leads to fluorescence in the near infrared. The emission spectra are assigned to the I*I*(2P1/2+2P1/2)→I*I(2P1/2+2P3/2) transitions, namely, to contact pairs of spin excited atoms which are strictly bound by the solvent cage. The spectra are analyzed to characterize cage potentials and contact distances. In rare gas solids, the caged pair relaxes radiatively, τrad=3.9, 3.1, and 2.8 μs, in Ar, Kr, and Xe. In room temperature liquids, emission from both geminate and nongeminate pairs occurs, resulting in bimodal kinetics. The fluorescence yield strongly depends on the nature of the solvent. Through a set of measurements in hydrogenated and deuterated solvents, the quenching mechanism is ascribed to dipolar energy transfer to solvent molecules: the quenching efficiency is determined by the spectral overlap between vibrational overtones of the solvent and the solute emission.