Dynamics of linear and T-shaped Ar-I-2 dissociation upon B <- X optical excitation: A dispersed fluorescence study of the linear isomer

We report the dispersed fluorescence spectra of the linear and the previous ly well-studied T-shaped isomers of Ar-I-2 following B <-- X optical excita tion for v(pump)=16-26, below the I-2 dissociation limit. The linear isomer has a continuum excitation spectrum. For excitation at the highest pumping energy (v(pump)=26), the product vibrational state distribution is nearly identical to that observed for excitation above the I-2(B) dissociation lim it; it shows a broad, nearly Gaussian distribution of I-2(B) vibrational st ates, with about 22% of the available excess energy deposited in translatio n of the Ar+I-2. This gives direct evidence that the "one-atom cage" effect seen above the I-2(B) dissociation limit is attributable to the linear Ar- I-2 isomer. The product vibrational state distribution becomes increasingly Poisson for decreasing excitation energies, and only about 7% of the exces s energy is deposited in translation for v(pump)=16. The bond energy in the linear isomer is determined from the spectra, 170(+/- 1.5)less than or equ al to D-0"(linear Ar-I-2(X))less than or equal to 174(+/- 1.5) cm(-1). A bo nd energy of D-0"(T-shaped Ar-I-2(X))=142 +/- 15 cm(-1) is estimated based on the linear to T-shaped population ratio observed in the beam, which is a bout 90 cm(-1) smaller than that determined from fluorescence spectra. We s uggest that electronic quenching in the T-shaped isomer is nearly 100% for the highest vibrational level produced by vibrational predissociation. (C) 1999 American Institute of Physics. [S0021-9606(99)02041-3].