128 ILLINOIS STATE ACADEMY OF SCIENCE 



THE MOLECULAR SPECTRUM OF AMMONIA 



B. J. Spence, Northwestern University 

 Preliminary Report 



A number of years ago it was found that hydrogen 

 chloride showed a double branched absorption band at 

 a wavelength of approximately 45,000 A. U. This wave- 

 length is found in the near infra-red spectrum. The 

 bands were accounted for by Bjerrum (Nernst Fest- 

 schrift, 1911) assuming that the diatomic molecule ro- 

 tated about a line at right angles to the line joining the 

 atomic centers and that the atomic centers vibrated along 

 the line joining them. Such a system of molecules will 

 absorb energy from a beam of radiation passing through 

 them corresponding to the frequency of rotation and 

 also the frequency of the combined frequencies of rota- 

 tion and vibration. The double branched band will have 

 frequencies corresponding to fv ± f r , where fv is the fre- 

 quency of vibration of the molecule and fr is the fre- 

 quency of rotation. 



Later experiments revealed the fact that the double 

 branched band was not simple but made up of a number 

 of fine bands. Bjerrum modified his theory involving the 

 quantum theory. His theory was not altogether satis- 

 factory, and later Lenz (Verb. d. D. Phys. Ges. 31, 632, 

 1919) following the idea of the stationary state of Bohi 

 in his atomic theory arrived at a more satisfactory ex- 

 pression for the wavelength of these bands. The Lenz 

 theory assumes that the molecules exist in a series of 

 stationary states in regard to rotation and vibration, 

 and that energy is emitted or absorbed during a transi- 

 tion between stationary states. Lenz's expression for the 

 frequency of the emitted or absorbed radiation is 



h mh 



f =• nf „ + =t 



8n 2 I 4rrl 

 where f and f are the frequencies of the absorbed radia- 

 tion and the atomic vibration frequency respectively, h 

 is the Planck constant, I the moment of inertia of the 

 molecule and m and n are small integers characterizing 



