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BELL SYSTEM TECHNICAL JOURNAL 



atoms. That their shifts do not agree very well with the valuec of 

 extraction-energies suggested by certain spectroscopic data is not in the 

 least surprising. None of the spectroscopic data was obtained with 

 solid carbon ; and the superficial electrons of the atom are most sensi- 

 tive of all to such changes of environment as occur when it is in- 

 corporated into a lattice. In all probability, the frequency-shifts of 

 these lines, when divided by h, are the best values yet available for 

 the amounts of energy required to extract superficial electrons from 

 carbon atoms in the graphite lattice. ^^ 



500 



300 200 100 500 400 



SECONDS OF ARC, CRYSTAL B (2dei) 



100 



Fig. 10 — X-rays scattered by graphite, the peak on the left being the unshifted line. 

 (B. Davis, D. P. Mitchell; Physical Review.) 



Davis and Mitchell point out a feature of these scattered rays which 

 should be stressed : the shifted lines are sharp, implying that the quanta 

 simply extract electrons, without endowing them with divers quantities 

 of kinetic energy to boot. They later observed such lines in the rays 

 scattered by aluminium. 



The General Principle 



The general principle of which all these scattered phenomena of 



scattering are special illustrations now stands forth very clearly, and 



^* It seems paradoxical that the quanta which collide with free electrons should 

 confer on them several hundred of equivalent volts of energy, while those which 

 strike these loosely-bound superficial electrons should lose but half-a-dozen. The 

 contrast is due to the requirement of conservation of momentum; where only two 

 particles are involved the relatively large energy -transfer is entailed, but where there 

 are three among which the momentum may be divided, the limitation ceases. 



