MOTION OF GASEOUS IONS IN STRONG ELECTRIC FIELDS 



24J} 



obtained for the polarization force have a potentially wide field of ap- 

 plication when measurements of ion drift are extended to low tempera- 

 ture. In the meantime, the results apply occasionally at room 

 temperature, whenever we deal with a small ion and are not bothered 

 by special scattering mechanisms having large cross section. An example 

 of this are the molecular noble gas ions in the parent gas whose drift 

 velocities were measured by Hornbeck^^' ^^ and Varney.^^ Table IV shows 

 the measured mobility at standard gas density measured for these ions, 

 in comparison with a value obtained from equation (131). The field 

 range from which the observed mobility was obtained is intermediate. 

 There is not only good numerical agreement, but the experiments follow 

 the theory also in that there is little variation of the observed value 



Table IV 



Mobilities at standard density of the noble gas molecular ions. Comparison of 



the experiment with a formula based on the polarization force only. 



with the field. The discrepancy between the two columns can be used to 

 determine a hard collision cross section which is to be superimposed on 

 the polarization force, as is suggested in the so-called Langevin model. 



inc. VELOCITY DISTRIBUTION FUNCTION FOU ELECTRONS 



We have almost exhausted the results achieved for intermediate 

 field conditions. For the sake of completeness I shall mention shortly 

 the intermediate field distribution function for electrons whose derivation 

 we owe to the ingenuity of Davydov. 



The derivation does not differ in principle from the one presented in 

 Section IIC for the electrons in the high field case. The distribution 

 function is first expanded in spherical harmonics. For group theoretical 

 reasons the scattering term in the Boltzmann equation is diagonal in 



32 Davydov, B., Phys. Zeits. Sowjetunion., 8, p. 59, 1935. See also Reference 4, 

 pp. 349-350. 



