The (1)(3)Pi electronic state of LaF

Rotational studies of bands of the infrared systems (1)(3)Pi --> (1)(3)Delt a and (1)(3)Pi (1) --> X(1)Sigma (+) of lanthanum monofluoride were carried out with the aim of characterizing the (1)(3)Pi state, assuming the previo us representations of X(1)Sigma (+) and (1)(3)Delta (1-3). The vibrational levels of (1)(3)Pi involved in the analyzed transitions were v = 0, 1, 2 of the Omega = 0 component, nu = 0, 1 of the Omega = 1 component, and nu = 0 of the Omega = 2 component. Perturbations were observed in both systems whi ch were ascribed to spin-uncoupling interactions between (1)(3)Pi (2)(nu = 0) and (1)(3)Pi (1)(nu = 1) levels, A 9 x 9 matrix representation at equili brium of the complex of interacting levels (nu = 0, 1,2) was constructed, e ach diagonal nu -block corresponding to a tripler model of the rovibrationa l (3)Pi Hamiltonian. The wavenumbers of some 1910 lines in the two systems were simultaneously fitted (rms error of the fit of about 0.0053 cm(-1)), t hus yielding a consistent set of accurate spectroscopic constants for the ( 1)(3)Pi state: the spin-uncoupling interaction parameter was determined as B-0.1(Pi) = 0.010917(13) cm(-1). (C) 2001 Academic Press.