SPECTROSCOPY OF METASTABLE SPECIES IN A FREE-JET EXPANSION - THE BETA[-A TRANSITION IN IBR

The A 1((II)-I-3) State of IBr is observed in a free-jet expansion of IBr in Ar, following photolysis with ArF laser irradiation. Laser exci tation spectra are obtained for the beta 1(P-3(2)) <-- A transition in medium (0.5 cm(-1)) and high (0.08 cm(-1)) resolution. Sixteen v'-v'' bands recorded in high resolution are rotationally analyzed, providin g the first detailed characterization of the lowest three vlevels in t he A state. An additional similar to 50 bandheads in the v'' = 0 and 1 progressions are recorded for both (IBr)-Br-79 and (IBr)-Br-81. The n ew data, which sample v' levels in the range 31-71, are combined with existing information for low v in the beta state to provide improved c onstants for the beta state. These results are merged with least-squar es parameters from a detailed reanalysis of existing data for the A an d X((1) Sigma(+)) states to yield spectroscopic parameters valid for v = 0-20 in the X state and v = 0-26 in the A state. Among these result s are the first precise estimates of the equilibrium parameters in the A state: T-c = 12 369.68 (14) cm(-1)omega(e) = 134.22(13) cm(-1), B-e = 0.042 40 (5) cm(-1), R(e) = 2.8583 (16) Angstrom.