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Chapter: 4 Atomic and nuclear physics
    Section: 4.4 Free electrons and ions in gases
        SubSection: 4.4.3 Ionic recombination

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4.4.3  Ionic recombination

The neutralization of electrical carriers of opposite sign in a gas is termed recombination. The positive carrier is a positive ion while the negative carrier may be a negative ion or an electron depending on the nature of the gas. This table deals only with ionic recombination, and only with the situation in which the ions are randomly distributed throughout the gas (see Loeb, 1955). Then dn+/dt = dn/dt = −αn .n+, where n+ and n are the ion densities and α is the recombination coefficient. α is commonly taken as a constant. Experimentally, however, measurements show that α changes with ion age, presumably due to a change in the nature of the ions owing to the presence of small quantities of impurity. Further there is a small dependence on ion density, α decreasing with increasing ion density. α is dependent on pressure and temperature and from 105 N m−2 to a few hundred N m−2 is adequately described by the theory of Thomson. For pressures above 106 N m−2 the Langevin theory is employed. Three-body and mutual neutralization processes are by far the most important for ionic recombination. For a general theoretical discussion, see Loeb (1955).

All values given below are at 1.01 × 105 N m−2 pressure unless otherwise stated.


Gas

H2

N2

O2

Air

Ar (impure)

α/(10−12 m3 s−1)   .    .    .    

0.28 (22°C)

1.06 (22°C)

1.32 (2°C)

1.65 (18°C)

1.06 (22°C)

     

2.08 (25°C)

   
           

Dependence on ion density, pressure and temperature
n = No. of ions/m3; p = pressure; T = temperature
 

Air at 15°C    .    .    .    .    .    .    .

n/(1012 m−3)

   1.5

   2.7

   3.1

   4.1

   4.4

 

α/(10−12 m3 s−1)

   2.65

   2.5

   2.5

   2.3

   2.3

 
  

 p/103 N m− 2)   .    .    .    .    .

 5

 10

 20

 30

40

 50

 60

 70

 80

 90

 100

110

 Air 18°C α /(10−12 m3 s−1)  .

0.12

0.21

0.37

0.55

0.7   

 0.88 

 1.05 

 1.22 

1.4  

 1.55 

 1.72 

 

 O2 20°C α /(10−12 m3 s− 1) . 

 

0.7   

1.3   

1.6   

1.75

 1.88 

 1.96 

 2.01 

 2.05 

 2.08 

 2.08 

 2.08 

 
 

  p/(105 N m−2) .    . 

4.6

6.4   

7.9   

8.0  

9.7  

12.5    

15.1    

17.4    

17.8    

21.1    

22.0   

 23.6    

25.7    

 Air 18 °C 
   α /(10−13 m3 s−1)

 10.58

 8.44

  

 6.98

 5.94

 4.80

 4.07

  

 3.44

 2.90

  

 2.59

 2.37

 CO2 18 °C
   α /(10−13 m3 s−1)   

 5.76

  

 3.58

  

  

 2.24

  

 1.56

  

  

 1.24

  

  

  
 

 O2 T/K   .    .    .    .    .   

200

250

300

350

400

 α /(10−12 m3 s−1)    .    . 

3.7

2.7

1.9

1.5

1.2

           

J.W. Leake

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