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LXX. Impact Ionization by Low-Speed Positive }l-Ions in 

 Hydrogen. By A. J. Saxton, M.Sc, Assistant Lecturer 

 in Physics, The University of She/field*. 



Introduction. 



^^HERE now exists a considerable amount ol evidence on 

 J- the conditions necessary to ionize a " normal " atom 

 of a gns. In every case we are concerned with the energy 

 exchanges between the atom to be ionized and the source of 

 the ionizing energy. 



We may formulate the conditions governing this inter- 

 change of energy as follows : — 



(a) In order just to ionize a "normal" atomf of a par- 

 ticular gas or vapour, i. e. to detach completely one electron 

 from it so that this electron possesses no kinetic energy as 

 a result of the process (single ionization), always requires 

 the same total absorption of energy whatever the nature of 

 the ionizing source. Thus whether the atom is ionized by 

 electron impact or by the absorption of radiation, the total 

 ionization energy necessary to change it from the "normal" 

 unexcited state to the ionized state is always the same. 

 The long wave-length limit (threshold frequency) for the 

 photo-electric effect in the vapour thus corresponds to the 

 ionization potential for electron impact. 



(b) " Single" ionization of an atom may be produced by 

 absorption of the ionization energy from one or more of the 

 following sources : — 



1. Impact by an electron. 



2. Impact by a positive ion. 



3. Absorption of radiation (photo-electric ionization). 



4. Impact by " normal" atoms (thermal ionization). 



(c) If only one of the above sources is concerned, e. g. 

 electron impact (about which we possess the most data), the 

 absorption of the energy may take place in a single process 

 or by stages ; in this case by successive electronic collisions. 

 If a partially ionized atom receives the extra amount of 

 energy necessary to ionize before radiating the portion it 

 has already received, ionization will result. 



* Communicated by Prof. S. R. Milner, F.R.S. 



t With polyatomic molecules energy may be required first to dis- 

 sociate the molecule. 



Phil. Mag. S. 6. Vol. 44. No. 263. Xov, 1922. 3 G 



