HIGH-RESOLUTION VISIBLE SPECTRUM FOR THE A(3)PI(1)-X(1)SIGMA(+) SYSTEM OF IBR

New high-resolution absorption and laser excitation spectra of the A(3 )II(1)-X(1) Sigma(+) system of IBr, consisting of approximately 4000 l ines of over 40 bands spanning the vibrational range v' = 6-29 and v'' = 1-4 for each of the two isotopomers, have been recorded and analyze d. Combined with previous microwave and infrared data and fluorescence progressions into the X state, and with new data obtained from laser excitation of the lowest levels of the A state, they formed the core o f a global fit to mass-reduced Dunham expansions that yields new molec ular constants for the A and X states of (IBr)-Br-79 and (IBr)-Br-81. Inability to fully account for all data within the experimental uncert ainties points to the existence of substantial perturbations or ''non- mechanical'' behaviour for A-state levels v' > 24.