STOPPING POWER OF A POLYATOMIC GAS 153 



azomethane which is 4 per cent "too low"; Forster (16) found H2O vapor 

 to have 3 per cent smaller stopping power than an equivalent mixture 

 of H2 and O2. Such results are usually discounted [for instance, cf. 

 Gray (10)]. 



Turning now to theory, we may recognize several more or less distinct 

 factors which should cause the stopping power of a diatomic or poly- 

 atomic molecule to differ to some extent from the simple sum of stopping 

 powers of the isolated atoms of its constituents. They are: 



1. Binding of atoms in a molecule changes the values of the possible 

 excitation energies (£„) of the system, and also of the associated oscil- 

 lator strengths (/„). 



2. The incident particle may transfer small quantities of energy to 

 vibrational modes of the molecule. 



3. Energy may be transferred to rotational modes of the molecule. 



For the last two effects the existence of a permanent dipole moment of 

 the molecule plays a decisive role ; for the first also it has, in a sense, an 

 indirect influence. We shall consider each of these factors, briefly, in 

 turn. 



The first factor is undoubtedly the most, and except at low particle 

 energies is probably the only, important one. It affects the stopping 

 power by altering the value for the molecule of the mean excitation en- 

 ergy 7, as may be seen from Eq. 3.* Insight into the working of this in- 

 fluence is gained by considering the Williams-Weizsacker method of 

 treating the collision problem (which was discussed above in terms of 

 impact parameters). For collisions in which the energy transfer to an 

 electron is great compared to the ionization potential of the latter the 



* Mention must again be made that the Bethe stopping-power formula 4 has not 

 been demonstrated to apply exactly in the case of a molecular medium. Conceiv- 

 ably, two sorts of complication may exist. One, that the spatial distribution of atoms 

 is not random, as inherently assumed in the derivation of formula 4, is almost al- 

 ways without influence. (It might have an effect for radiations of extremely great 

 specific ionization.) The other, that the valence electrons are necessarily far from 

 hydrogen-like, is in a general way covered by the alteration of 8„, /„ values discussed 

 above, provided that formula 3, as properly generalized for a many-electron system, 

 is valid. This effect has not been studied. However, Williams (17, cf. p. 24) has 

 mentioned the possibility that deviations from hydrogen-like binding of the electrons 

 in helium might be responsible for a 10 per cent departure in stopping power from 

 the prediction of the hydrogen-like model, the latter yielding too great a stopping 

 power. Such an effect in He would be a close analog to the effect in a valence bond 

 of a molecule. It would be of great interest to compare accurate stopping-power 

 data for He (which are not available at present) with the results of a refined theo- 

 retical calculation based on an accurate dispersion model, such as is provided by 

 the work of Vinti (18) and of Huang (19). (The former has shown, for example, 

 that 2 per cent of the total oscillator strength of the He electrons arises from double 

 excitations, which are automatically ignored in the hydrogen-like approximation.) 



