Fourier Transform Infrared Emission Spectroscopy of the b1Pi-a1Sigma+ 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 N2. The bands observed in the 3000-7800 cm-1 interval have been assigned as the b1Pi-a1Sigma+ transition, with the 0-0 band at 3513.99040(43) cm-1. This transition is analogous to the A1Piu-X1Sigma+g (Phillips) system of the isoelectronic C2 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 b1Pi and a1Sigma+ states have been determined. A local perturbation has been observed in the v = 1 vibrational level of the b1Pi state near J = 18 caused by the interaction with the v = 3 vibrational level of the a1Sigma+ state. The principal equilibrium constants for the a1Sigma+ state are: omegae = 1705.4032(11) cm-1, omegaexe = 10.55338(52) cm-1, Be = 1.683771(10), alphae = 0.013857(16) cm-1, and re = 1.2745081(37) A. Although the b1Pi-a1Sigma+ 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.