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Chapter: 4 Atomic and nuclear physics
    Section: 4.4 Free electrons and ions in gases
        SubSection: 4.4.5 Electron diffusion

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4.4.5 Electron diffusion

The coefficient of electron diffusion De may be calculated from the equation De/μ = ηp/Ne, where μ is the electron mobility, N is the number of gas molecules per m3 at s.t.p., e is the electronic charge, p is the pressure in N m−2 and η is the ratio of the mean agitation energy of an electron to that of a gas molecule. η is also known as the Townsend energy factor. Experimental values of η are given below as a function of E/p. For very low values of E/p or E/N, De/μ = kT/e, i.e. the electrons are in equilibrium with the gas.


Field strength per unit pressure (E/p)/(V m N−1)

Gas

0.5

1

2

3

4

5

10

20

30

40

                     

H2  .    .    .    .    .    .    .

     6

   10

   18

  23

  28

  32

  55

  100  

  130  

  150  

He .    .    .    .    .    .    .

   37

   74

 126

  152  

  176  

   

   

   

   

   

N2 .    .    .    .    .    .    .

   16

   25

   34

  39

  42

  44

  52

  68

  89

  114  

O2 .    .    .    .    .    .    .

   11

   18

   32

  41

  47

  52

  68

  96

  133  

   

Air .    .    .    .    .    .    .

     7

   15

   26

  34

  39

  42

  50

  68

  90

  106  

CO     .    .    .    .    .    .

     5

     8

   14

  19

  23

  27

  38

  48

  68

   

NO     .    .    .    .    .    .

     7

     8

   10

  11

  12

  14

  38

   

   

   

Ar  .    .    .    .    .    .    .

 220

 320

 316

  312  

  310  

  314  

  324  

   

   

   

CO   .    .    .    .    .    .

       1.3

       1.6

     2

    4

    9

  17

  58

  89

  117  

   

Cl2      .    .    .    .    .    .

   

   

   

   

   

   

  53

  71

  77

  82

Br2      .    .    .    .    .    .

   

   

   

  40

  52

  59

  74

  82

  89

  91

I2    .    .    .    .    .    .    .

   

   

   

   

   

   

  27

  76

  91

  99

 

 

 

 

 

 

 

 

 

 

 

Most of the values in the above table were obtained in the 1920s and are from Healey and Reed (1941). Later determinations have shown only small changes for H2, He, N2 and air but the values for O2 over a limited range are higher than those in the above table (see Huxley et al., 1959). D/μ values are quoted in modern literature.

J.W. Leake

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