The band spectrum of lead fluoride (PbF). II

The investigation was undertaken to explain the apparently conflicting band-spectral evidence for the nature of the ground states of the diatomic germanium, tin and lead halides. Investigations of the band spectra of the germanium and tin monohalides by Jevons (1926, 1937), Jenkins and Rochester (1937) and others show that the ground states of these molecules are doublets of the electronic type 2Π, with 2Π3/2 ~ 2Π½ separations of approximately 1000 cm.-1 for the germanium compounds and 2400 cm.-1 for the tin compounds. Ground states of the type 2Π are predicated by theory if it is assumed that the molecules are formed from unexcited metal and halide atoms. The spectra of the lead monohalides, of which that of PbF is typical, apparently do not show doublet multiplicity, only one system (A1) being found (Part I, Rochester, 1936). Morgan (1936) observed this system in absorption, and suggested that the apparent absence of a second component system (A2) was due to the magnitude of the doublet separation in the ground state. He pointed out that if the doublet separation was as large as 7000 cm.-1 system A2 would not occur in absorption but could occur in emission and would lie in the near infra-red region. In the ultra-violet region of the spectrum the germanium and tin monohalides have intense systems of the type 2Σ⇆2Π and it is to be expected that the spectrum of PbF will have similar spectra in the same region. The following paper gives an account of the results of a search for these spectra.