The term “halogen molecule” is intended to include the interhalide compounds as well as the elementary molecules. The existence of three of the ten possible molecules of this type, namely, IF, BrF, and CIF, has not yet been established, and we have not attempted any spectroscopic search for them. For F
2
the data are meagre, only an emission spectrum being known, and we have therefore confined our attention to the other six molecules, namely, CI
2
, Br
2
, I
2
, BrCl, ICI, and IBr. The spectra of these, with the exception of BrCl, have been extensively investigated, and partially interpreted. There are certain features, however, notably the continua and diffuse bands observed in emission, which have not yet been satisfactorily explained, and we have set out to survey the existing data systematically and, where necessary, to obtain new data with the object of accounting for these features. The problem of obtaining satisfactory data of continua and diffuse bands is definitely different from that involved in the case of line and ordinary band spectra. Not only is it more difficult to make accurate visual settings for the purpose of wave-length determinations, but it becomes of much more importance to record the intensity distribution, which is often very characteristic and undoubtedly significant. For these reasons we have thought it essential to obtain and reproduce microphotometer records in all experiments, and to determine wave-lengths solely from these records. Quite low dispersion has been used, and appears very suitable for a general survey, although a more detailed investigation using higher dispersion may well prove profitable later. It seemed important to us to get strictly comparable data in all observations, and it has therefore been necessary to redetermine some of the existing data, which although numerous are seldom complete or homogeneous. A number of entirely new data are also presented. Some consideration must first be given to the known band systems of these molecules, since they provide evidence relating to the location and characteristics of molecular levels, some at least of which may be expected to be concerned in the emission of the continua and diffuse bands.
Intensity distribution in the line emission spectrum of cesium