Fourier transform spectroscopy of BaO: New ground-state constants from theA (1)Sigma(+)-X (1)Sigma(+) chemiluminescence

The A (1)Sigma(+)-X (1)Sigma(+) emission spectrum of BaO excited in a Broid a-type oven was revisited using a Fourier transform spectrometer. Chemilumi nescence was observed from excited BaO molecules generated from the Ba+N2O reaction, and 82 vibrational bands of (BaO)-Ba-138 were measured in the ran ge 8900-21 000 cm(-1) at a resolution of 0.04 cm(-1). Vibrational levels we re observed up to v' = 20 of the ground state and up to v' = 11 for the exc ited state. In addition, 72 bands from the minor isotopomers (BaO)-Ba-137, (BaO)-Ba-136, and (BaO)-Ba-135 were detected with v' up to 14 and v(') up t o 4. Over 15 500 lines with J' up to 119 were measured with a precision of about 0.005 cm(-1), which shows that the Broida oven is an excellent source for high-resolution emission spectroscopy. Our main goal here is to obtain an optimum set of molecular constants for the ground electronic state. To avoid complications due to the well-known perturbations for v(A)' greater t han or equal to 1 of the A (1)Sigma(+) state, our unified combined-isotopom er analysis used a novel combination of a Dunham expansion description of t he ground state with independent band-constants for data involving A-state level v(A)' = 0, while fitting to an independent term value for each observ ed A-state v(A)' greater than or equal to 1 vibration-rotation level of eac h isotopomer. Significantly improved ground-state constants and A-state v(A )' = 0 band constants are obtained, together with term values for some 1372 higher-v' A-state levels. (C) 2000 American Institute of Physics. [S0021-9 606(00)01732-3].