SPECTROSCOPY OF METASTABLE SPECIES IN A FREE-JET EXPANSION - THE BETA[-A TRANSITION IN BR-2

The A 1(u)((3) Pi) state of Br-2 is observed in free-jet expansions of UV-photolyzed BBr3/Ar mixtures and electrically discharged Br-2/Ar mi xtures. Laser excitation spectra are recorded for the beta 1(g)(P-3(2) ) <-- A transition, providing the first high-resolution data for upsil on = 0 in the A state. Thirteen upsilon'-0 '' bands spanning upsilon' = 23-33 are rotationally analyzed, yielding a total of 475 assignments . The new results are combined with existing spectroscopic data for up silon' = 0-10 to yield an improved description of the beta state and t he following constants (cm(-1)) for upsilon = 0 of the A state: T-0 (B r-79(2)) = 13 984.22(5), T-0 ((BrBr)-Br-79-Br-81) = 13 983.80(5), T-0 (Br-81(2)) = 13 683.3265), B-0 '' (Br-79(2)) = 0.05815(2). The new inf ormation for the A state is analyzed together with existing data for l evels upsilon = 7-24 and 30-34 to achieve compact expressions for the vibronic energy and rotational constant in terms of the standard polyn omials in (upsilon + 1/2) at low upsilon, merging smoothly into near-d issociation expansions (NDEs) at high upsilon. The resulting ''mixed r epresentations'' have the usual properties of the polynomials at low u psilon and yield the estimates (cm(-1), for Br-79(2)): T-e = 13 910, D -e = 2147, omega(e) = 150, B-e = 0.05844, and R(e) = 2.704 Angstrom. T he NDEs have superior extrapolating ability above the highest observed level to dissociation, which occurs at upsilon(D) = 41.7. Spectroscop ic constants and RKR potentials are tabulated for the first similar to 40 upsilon levels of both states, and Franck-Condon factors are given for transitions from the low upsilon '' levels. (C) 1997 Academic Pre ss.