PREDISSOCIATION OF THE A(2)PI(3 2) STATE OF IO STUDIED BY CAVITY RING-DOWN SPECTROSCOPY/

The IO A (2)Pi(3/2)-X (2)Pi(3/2) (upsilon',0) bands with upsilon' = 0- 5 and the (1,1), (2,1) and (3,1) hot bands have been investigated in a bsorption using cavity ring-down spectroscopy. Analysis of the spectra gives refined band origins and rotational constants for the upsilon' levels and reveals strongly upsilon'-dependent predissociation rates f or the A (2)Pi(3/2) state. Fitting of spectral lineshapes for the rota tionally resolved (2,0) band shows that upsilon' = 2 undergoes a rotat ion-induced predissociation, most probably via coupling to a (2)Sigma( -) state, with lifetimes for the rotational levels that range from ca. 1 ns at J' = 1.5 to 15 ps at J' = 50.5. In contrast,however, the (0,0 ) and (3,0) bands, which are also rotationally structured, exhibit app arently J'-independent predissociation rates. The (1,0), (4,0) and (5, 0) bands are sufficiently lifetime-broadened that no rotational struct ure is evident. Fits to the band contours give average homogeneous (FW HM) linewidths for the various vibrational bands of 0.30 +/- 0.03 cm(- 1) for upsilon' = 0, 6 +/- 1 cm(-1) for upsilon' = 1, 0.80 +/- 0.05 cm (-1) for upsilon' = 3, 9 +/- 2 cm(-1) for upsilon' = 4 and 60 +/- 10 c m(-1) for upsilon' = 5. The dominant predissociation mechanism for ups ilon' = 0,1,3,4, and 5 is attributed to spin-orbit coupling between th e A (2)Pi(3/2) state and one or more Omega = 3/2 repulsive states.