Absorption and dispersion of thallium vapour

Abstract

In recent years the study of absorption of light by vapours of metals of Group III of the periodic table has attracted the attention of many physicists. For the chief difference between the spectra of metals of this group and those of the elements of the first two groups is that the convergence frequency of the principal series is smaller than that of the subordinate series, a fact not easily reconciled with the quantum theory, unless the observed principal series is of the second type, as Foote and Mohler have pointed out in “The Origin of Spectra.” Recent experiments of Grotrian and Carroll indicate that, in the case of metals of this group, the ground orbit of the valence electron is 1, π orbit, and not 1, σ orbit, as in the case of other metals. According to Bohr’s theory, for the vapour of an element to absorb lines corresponding to a given series in its spectrum, it is necessary that in the vapour there should be a fairly large number of atoms with orbits corresponding to the first term of the pulse of radiation to be absorbed. It is therefore possible to obtain information as to the quantum state of the atoms in the normal state by observation of the absorption spectrum of the non-luininous vapour.