Plate 9] I n t r o d u c t io n The phosphorus hydride band at A 3400 was first reported by Geuter (1907) in an account of an investigation of the spectra associated with phosphorus. It was obtained with a discharge tube containing phosphorus in addition to hydrogen. Pearse (1930) photographed the band with high dispersion, showed that the electronic transition involved was of the type sIIi -> and attributed the band to the molecule PH. Since this analysis further theoretical work on this type

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... f transition has been published by Hebb (1936), and calculations of the rotational constants for the 377 state of PH have been made by Gilbert (1936) and Budo (1936) . A study of the distribution of intensity in the rotational structure has been made by Noland and Jenkins (1936). The present paper is devoted to a study o f the changes produced in the band structure by replacing the hydrogen in the molecule by deuterium. E x p e r im e n t a l In the previous work of Pearse the source used was a small H-type discharge tube containing hydrogen and phosphorus, requiring about 4 hr. to photograph the A 3400 band in the 2nd order of a 10 ft. concave grating. It was soon apparent that higher dispersion would be necessary to resolve the structure of the PD band sufficiently for a satisfactory analysis. The source was therefore improved until photographs could be taken con veniently in the 4th order of the grating. A diagram of the tube used is shown in figure 1 . The electrodes were of aluminium; the constriction was about 10 cm. in length and 3 mm. in diameter; and the whole tube could be immersed in water to prevent overheating. The discharge was maintained by means of a D.C. generator, the current being about 0*5 amp. with a potential difference of 1500 V. With this source, good photographs could be obtained in the 4th order with exposures of 2 hr. Deuterium was obtained from heavy water supplied by Messrs Imperial Chemical Industries, Ltd., and was guaranteed by them to be 99*9 % pure. The A 3400 hands of P H and P D 267 reduction to wave numbers was carried out with the aid of Kayser's Tabette der Schwingungszahlen. T h e s t r u c t u r e o p t h e b a n d s The appearance of the phosphorus hydride bands with high dispersion and the resolution of their structure into the constituent branches can be seen from plate 9. The relations between the various branches can be % J -K+l ± . 8 Z t h J=K