[Pboc. Roy. Soc. Victoria, 35 (N.S.), Pt. II., 1923]. 



Art. XVII. — The High Frequency K Series Absorption 

 Spectrum of Erbium. 



By L. H. martin, B.Sc. 

 (Communicated by Professor T. H. Laby). 



(With one Text Fig.) 



[Read 9th November, 1922.] 



A critical absorption " edge " in the X-ray spectrum of an element 

 lias a wave length such that the element absorbs X-rays of shorter 

 wave length than that of the edge, more than X-rays of longer wave 

 length. 



For the heavier elements we find one such edge in the K series, 

 three in the L series, and five in the M series^^. 



It is found experimentally^ that if the element previously used as 

 an absorber is now used as a target in an X-ray tube, the K series 

 of this element are emitted only when the voltage applied to the 

 electrodes of the tube equals, or is superior to Vk given by the quan- 

 tum relation, 



hv ^= vk^ 

 where h = 6.556 X 10-27 erg. sec. e = 1.59 i x 10 -20 e.m. C.G.S. 

 units and i^k is the frequency of the critical absorption edge, which is 

 also greater than the frequency of the shortest K emission line i.e., 



^K > ^Ky > ^K^ > J'k" > 'k** (^) where v^y v,./? i^k^ ^'k^ 



are the frequencies of the K emission lines. 



In virtue of these facts, critical absorption phenomena have as- 

 sumed an important significance in the physical model of the atom 

 as developed by Kossel, Bohr, Sommerfeld, and Wentzel. They are 

 expressions of " levels " of energy within the atom. 



It is found that we can form a table of the frequencies of these 

 absorption edges [k; l l^ l^; m m^ m,, m ^^r^, etc.] the frequency of 

 every X-ray emission line being expressed as the difference of the 

 frequencies of two edges properly chosen^, every emission line cor- 

 xesponding to a definite pair of energy levels. We have for example: — 



Ka = Li _^ K La - Mi -^ L^ 

 Kyg = M, -> K L/^ = M2 -^ L, 



by which we mean, that the frequency of the K,^ line equals the dif- 

 ference between the frequencies of the L and K critical absorption 

 edges, or in terms of our atomic structure, the K^ line is emitted by 

 an atom when an electron falls from the L electron shell to the K 

 electron shell. 



