Forbidden Lines

Abstract

One of the most fundamental laws of spectrum analysis is the Ritz combination principle, a law which was developed early in the history of the analysis of spectra. This principle states that the frequencies, or wave numbers, of all of the lines of the spectrum of a given atom or ion can be expressed as the differences between a relatively small number of terms. According to our present picture of atomic structure this principle is interpreted by stating that the electrons of an atom can exist in certain states or modes of vibration each of which has a definite energy. A line of the spectrum of the atom is emitted when the atom changes from one state to another, each particular pair of states corresponding to a different line. The energy of the line emitted comes from the difference in the energy of the two states or terms between which the transfer occurs and the frequency of the line is equal to this energy difference divided by Planck's constant ℏ. The term values whose differences give the frequencies of the observed lines are therefore proportional to the energies of the various possible states of the atom.