Normal State of the Helium Atom. 129 



-argument for hypothesis (c) cannot ba stated with equal 

 brevity. 



The experiments of Franck and Knipping (I. c.) on the 

 excitation potentials of helium indicate the voltages at which 

 various new radiation frequencies begin to appear, without 

 determining what these frequencies actually are. They find 

 that the first critical potential for slightly impure helium 

 occurs at 20*45 volts, which should be sufficient to knock 

 an atom into the state (2, s) ; but that if the helium is 

 pure, the first critical potential is that required to knock an 

 atom from the normal state to (2, S), viz. 21*25 volts. This 

 shows that when atoms are thrown out to the energy level 

 (2, s) in impure helium, they emit some kind of radiation, 

 and therefore drop to some lower energy level. They do not 

 fall directly back to the normal state, for the line (N) — (2, s) 

 is not observed spectroscopically. Hence they must drop to 

 some other stationary state (i) whose existence is conditioned 

 by the presence of an impurity. Since the atoms eventually 

 find their way back to the normal state, we may assume that 

 the energy of the state (i) is intermediate between the energies 

 of (2, s) and (N). The frequencies emitted at 20*45 volts 

 would be 



n=(0 -(2, S ) 



and 



p,=(N)-(0. 



These frequencies should satisfy the relation 



Y=h(v 1 + v 2 ). 



One way in which such an intermediate state might occur 

 would be through the formation of a metastable compound 

 of an atom of helium in the state (2, ,9) and an atom of the 

 impurity X. The emission of the frequency v x would then 

 accompany the formation of the compound molecule, and the 

 emission of v 2 its disintegration into an atom of normal helium 

 and an atom of X. 



Another possible explanation of the existence of such an 

 intermediate energy level would be to suppose that a positive 

 ion of the element X colliding with an atom of helium in the 

 state (2, s) is able to rob the latter of its outer electron, 

 emitting at the same time a quantum of radiation of 

 frequency v r . The helium ion might then in turn collide 

 with an atom of X in a somewhat different manner, and by 

 robbing the X atom of the outermost electron revert to the 

 normal state while emitting radiation of frequency v % * 



Let us turn our attention now to the excitation of the line 

 Phil. Mag. S. 6. Vol. 42. No. 247. July 1921. K 



