Spectra of Other Elements

A further complication must now be described. The terms of many spectra may be divided into two S, P, D, . . . groups, with in each of which the combination rules just enunciated are obeyed. Combinations, however, may take place between terms of one group and those of the other. In such cases the rules are the same as before, with the exception of the one relating to the azimuthal quantum number, k, which becomes:—the difference of the k values must be o or To distinguish the groups from one another, the term symbols for one of them are often followed by a dash at the top right hand corner; thus—S', P', etc. We may, therefore, have such combinations as PP', DD', SD'. . . but not SP', PD'. . . . It is immaterial which of the groups is denoted by the dash, for there is, in fact, no difference between their properties except that indicated by the rules of combination.

Identification of Multiplets.

Well-developed series are usually found only in systems of low multiplicity, although the rule is not invariable. Singlet, doublet and triplet series have long been known, but it was not until 1922, through the work of Miguel A. Catalan, that the existence of systems of higher multi plicity was discovered. In spectra containing such systems it ap pears that the energy radiated is mainly distributed among the numerous lines of the earlier members of the series (i.e., those corresponding to the smaller values of n), the later members being absent or relatively inconspicuous.

Although for descriptive purposes it has been thought desirable to deduce the structure of a complex spectrum from the multi plicity of its terms, the practical problem is, of course, the reverse; namely, to deduce and classify the terms from the regu larities observed among the lines. In systems of low multiplicity, with well-developed series, this can often be done by careful inspection after some experience has been obtained, the numerical values of the terms being determined by the calculation of a Hicks or Ritz formula. With higher multiplicities, however, one

looks for groups of lines belonging to particular combinations, such as the quartet PD combination referred to above. These are detected by the recurrence of certain wave-number differ ences between the lines in a group. and to a lesser extent by the relative intensities of the lines.

The true numerical values of the terms in these systems of high multiplicity cannot be determined unless sufficient members of a series are found to permit the calculation of a formula. Rela tively accurate values, however, can always be found from the observations. Thus, if the value of one term be known or assumed, all the other terms participating in a group of combi nations can be at once evaluated.

A good example of one of the many multiplets occurring in the arc spectrum of iron is illustrated in Plate II., No. 7, this representing the combination of a septet D and a septet F term, giving r lines.

Zeeman

Effects.—The analysis of a spectrum is often facili tated by a study of the Zeeman effect, i.e., the splitting of the lines into components when the source of light is placed in a strong magnetic field. (See ZEEMAN EFFECT.) The group, or pattern, into which each line is dissected depends, in general, on the quantum numbers, r, k, j, of its component terms, and, largely through the work of A. Lando, an algebraical expression involv ing r, k, j, has been found which in many spectra represents with extreme accuracy the number and relative separations of the components of a term. The pattern characteristic of each pos sible combination—e.g., a combination—can thus be con structed, and by comparison with observation the lines can be classified. The pattern is not unique in every case—for example, all lines of a singlet system show the same pattern—but it is nevertheless a very valuable aid in analysis, and may be a crucial factor in deciding between alternative possibilities.