Spectroscopy and relaxation dynamics of metastable electronically excited states of iodine in rare gas matrices

Abstract

The spectroscopy and relaxation dynamics of I 2 isolated in solid rare gas (Rg) matrices have been studied using laser excitation and wavelength resolved fluorescence techniques. 193 nm excitation produced the D′ 3Π(2 g)-A′ 3Π(2 u) spectrum from I 2:Ar and a broad emission feature centered at 423 nm from I 2:Kr. Both visible and UV excitation of dilute I 2:Rg matrices produced the A 3Π(1 u)-X 1Σ(0 g +) and A′-X emission systems. A state lifetimes of 143±10, 102±10, and 71±10 μs were observed in Ar, Kr, and Xe matrices, respectively. The fluorescence decay characteristics of I 2(A′) were dependent on the excitation wavelength used. I 2: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 I 2(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 3Π(0 u +) state predissociation. The long I 2(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 I 2 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.