Hyperfine structure of the BaI X ²Σ + and C²Π states

Optical-microwave double resonance measurements were carried out to find the hyperfine structure constants of the v=0 level of the BaI X 2∑+ state. These were combined with sub-Doppler optical measurements of the BaI C 2Π-X 2∑ + (0,0) band in order to derive the hyperfine structure constants of the excited state. We have determined the following molecular constants (in MHz) where the numbers in parentheses represent one standard deviation in a least squares fit: for the BaI X 2∑+ state, γ″=75.8501(33), b″=93.117(19), c″=52.170(54), and eQq″=-33.62(12), and for the BaI C 2Π state, a′=263(53), b′+c′=-430(212), d′=-66.7(1.4), and eQq′=-214(11). The Fermi contact interaction and the electric quadrupole coupling constants for both the BaI X and C states appear to arise from the distortion of closed-shell I- orbitals by the field of the Ba + ion. In the BaI X state, the charge distribution on the Ba + center is directed away from I- while in the C state toward I-.