Predissociation of the A 2Π3/2 state of IO studied by cavity ring-down spectroscopy

Abstract

The IO A²Π_3/2–X²Π_3/2 (v′,0) bands with v′ = 0–5 and the (1,1), (2,1) and (3,1) hot bands have been investigated in absorption using cavity ring-down spectroscopy. Analysis of the spectra gives refined band origins and rotational constants for the v′ levels and reveals strongly v′-dependent predissociation rates for the A²Π_3/2 state. Fitting of spectral lineshapes for the rotationally resolved (2,0) band shows that v′ = 2 undergoes a rotation-induced predissociation, most probably via coupling to a ²Σ⁻ 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 apparently J′-independent predissociation rates. The (1,0), (4,0) and (5,0) bands are sufficiently lifetime-broadened that no rotational structure is evident. Fits to the band contours give average homogeneous (FWHM) linewidths for the various vibrational bands of 0.30 ± 0.03 cm⁻¹ for v′ = 0, 6 ± 1 cm⁻¹ for v′ = 1, 0.80 ± 0.05 cm⁻¹ for v′ = 3, 9 ± 2 cm⁻¹ for v′ = 4 and 60 ± 10 cm⁻¹ for v′ = 5. The dominant predissociation mechanism for v′ = 0,1,3,4, and 5 is attributed to spin–orbit coupling between the A²Π_3/2 state and one or more Ω = 3/2 repulsive states.