SPECTROSCOPY AND RELAXATION DYNAMICS OF METASTABLE ELECTRONICALLY EXCITED-STATES OF IODINE IN RARE-GAS MATRICES

The spectroscopy and relaxation dynamics of I2 isolated in solid rare gas (Rg) matrices have been studied using laser excitation and wavelength resolved fluorescence techniques. 193 nm excitation produced the D' 3-PI(2g)-A' 3-PI(2u) spectrum from I2:Ar and a broad emission feature centered at 423 nm from I2:Kr. Both visible and UV excitation of dilute I2:Rg matrices produced the A 3-PI(1u)-X 1-SIGMA(0g+) and A' -X emission systems. A state lifetimes of 143 +/- 10, 102 +/- 10, and 71 +/- 10-mu-s were observed in Ar, Kr, and Xe matrices, respectively. The fluorescence decay characteristics of I2 (A') were dependent on the excitation wavelength used. I2:Ar (1:2500) matrices yielded A' state lifetimes of 6 +/- 1 and 53 +/- 3 ms for 193 and 532 nm excitation, respectively. The difference in lifetimes reflected the relaxation dynamics of different initial state distributions. 193 nm excitation populated I2 (A') on a fast time scale (< 10 ns). Consequently, the short lifetime corresponded to radiative decay. 532 nm excitation populated low-energy metastable states through B 3-PI (Ou+) state predissociation. The long I2 (A') decay time resulted from slow excitation by energy transfer from metastable electronically excited states. Sequential two-photon excitation measurements supported this interpretation by demonstrating the presence of I2 molecules in metastable states other than A and A'. Concentration dependences of the A' state fluorescence decays were observed. These were attributed to perturbation facilitated energy transfer processes.