Fourier transform infrared emission spectroscopy of the b(1)Pi-a(1)Sigma(+) system of BN

The emission spectrum of BN has been investigated in the 1800-9000 cm(-1) region using a Fourier transform spectrometer. BN was formed in a microwave discharge of He with a trace of BCl3 and N-2. The bands observed in the 3000-7800 cm(-1) interval have been assigned as the b(1) Pi-a(1) Sigma(+) transition, with the 0-0 band at 3513.99040(43) cm(-1). This transition is analogous to the A(1) Pi(u)-X(1) Sigma(g) (Phillips) system of the isoelectronic C-2 molecule. The rotational analysis of the 0-0, 1-1, 1-0, 2-1, 3-2, 2-0, 3-1, 4-2, and 4-1 bands has been obtained and the molecular constants for the b(1) Pi and a(1) Sigma(+) states have been determined. A local perturbation has been observed in the v = 1 vibrational level of the b(1) Pi state near J = 18 caused by the interaction with the v = 3 vibrational level of the a(1) Sigma(+) state. The principal equilibrium constants for the a(1) Sigma(+) state are: omega(e) = 1705.4032(11) cm(-1), omega(e)x(e) = 10.55338(52) cm(-1), B-e = 1.683771(10), alpha(e) = 0.013857(16) cm(-1), and r(e) = 1.2745081(37) Angstrom. Although the b(1) Pi-a(1) Sigma(+) transition has recently been seen in emission from boron nitride trapped in solid neon matrices [J. Chem. Phys. 104, 3143-3146 (1996)], our work represents the first observation of this transition of BN in the gas phase. (C) 1996 Academic Press, Inc.