## Abstract

The high-frequency ultra-violet emission spectrum of HgF has been photographed and analysed into two systems which are shown to be due to a $^{2}\Pi _{r}{}^{-2}\Sigma $ transition between Hg atom-like levels. The constants are as follows: $ \matrix\format\l\kern.8em&\l\kern.8em&\l\kern.8em&\c \\ & \quad \nu _{e} & \quad \omega _{e} & x_{e}\omega _{e} \\ ^{2}\Pi _{{\textstyle\frac{3}{2}}} & 42999\cdot 6 & 469\cdot 4 & 10\cdot 05 \\ ^{2}\Pi _{{\textstyle\frac{1}{2}}} & 39060 & 506 & - \\ ^{2}\Sigma & \quad \quad 0 & 490\cdot 8 & 4\cdot 05 \endmatrix $ The doublet separation is related to the $^{3}$P width of the Hg atom, and the same separation is found in the spectra of the other Hg halides. It is shown that corresponding systems of the Zn and Cd halides should exist with doublet separations similarly related to the atomic $^{3}$P level. Most of these have been identified amongst existing data, and as a result analyses of the bands of CdF, CdCl and ZnBr have been given although there is some evidence that the data attributed to ZnBr really belong to ZnI. In addition a system of CdI different in type from those above is analysed. A probable $^{2}\Sigma $-$^{2}\Sigma $ system overlapping the main $^{2}\Pi $-$^{2}\Sigma $ system is found for most of the molecules. All these systems together with those arising from the resonance line $^{1}$P-$^{1}$S of the atom are discussed in terms of electron configurations of the states involved and an energy level diagram is presented.