## Abstract

The absorption spectrum of gaseous BiO has been photographed at a resolution of about 300 000. The ground state X$_1$, is the $\Omega = \frac{1}{2}$ component of a $^2\Pi$ state in which the spin-orbit coupling must be very large, $\sim$ 1 eV. The doubling in X$_1$ is large, and for $\nu = 0, \Delta\nu_{cd} \sim 0.187 (J + \frac{1}{2}).$ Structure of 16 bands has been analysed: the bands arise from transitions from X$_1$ to four states. $A ^2\Pi_{\frac{1}{2}}, B(^4\Sigma^-) \Omega = \frac{1}{2}, C(^2\Delta_{\frac{3}{2}}) \Omega = \frac{3}{2}$ and $D(^2\Pi_{\frac{1}{2}}) \Omega = \frac{1}{2}$. They show a number of unusual features, not least of which is the fact that, with the exception of D-X$_1$, the lines in the regions analysed, $25\frac{1}{2} \leqslant J \leqslant 90\frac{1}{2}$, have a half-width, $\Delta\nu_{\frac{1}{2}} \sim 0.25 cm^{-1}$, apparently independent of J, and far larger than the Doppler width. It is shown that this effect arises from unresolved nuclear magnetic hyperfine doubling of the levels in the ground state. The rotational levels in the single level of state D to be observed are predissociated both at high and at low J, but for intermediate values comparatively sharp lines are observed, by a partial cancellation of the h.f.s. between states D and X$_1$. The case b designation of state B is $^4\Sigma^-$, but the structure of the bands show that the transition is better described as $\Omega = \frac{1}{2} - \Omega = \frac{1}{2}$, in which the perpendicular transition moment is somewhat larger than the parallel one.