chapter Thirteen 



METASTABLE ATOMS AND ELECTRONS 



PRODUCED BY 

 RESONANCE RADIATION IN NEON 



In studies of arcs in pure neon at a few millimeters pressure, we have 

 observed some phenomena which indicate that the resonance radiation from 

 the arc can travel through the un-ionized neon beyond the end of the arc for 

 distances of 20 to 30 centimeters, or more, and the absorption of this radia- 

 tion produces, by excitation of the gas and by subsequent collisions of the 

 second kind, metastable atoms at the rate of at least lo-^^ cm"^ sec"^. In these 

 experiments the neon arc carrying currents of approximately one ampere 

 was passed through a long tube between a hot cathode at one end of the 

 tube and an anode placed near the center. One or more collecting electrodes 

 were placed in the tube beyond the end of the arc, and, in some experiments, 

 movable collectors were used whose distance from the end of the arc could 

 be varied. 



The metastable atoms thus produced beyond the end of the arc diffuse 

 into contact with the walls or with metallic electrodes and liberate electrons 

 from the glass or metal surfaces which have velocities of tbe order of 5 to 

 10 volts. The walls thus become positively charged and within the space 

 there is a definite concentration of electrons. If a collector is maintained at, 

 say, 70 volts negative with respect to the anode, the electrons emitted by it 

 serve as a measure of the concentration of the metastable atoms. The con- 

 centration of metastable atoms diffusing out of the arc should fall to i/e^^ 

 of its value every time the distance is increased by an amount equal to 0.4 

 of the radius of the tube. Within a few centimeters of the end of the arc this 

 theoretical decrement is actually observed, but at greater distances the con- 

 centration of metastable atoms decreases far more slowly. 



The width of the line corresponding to the resonance radiation in the arc 

 is far greater than that of the absorption line in the unexcited neon, which is 

 determined mainly by the Doppler effect. Assuming this cause of broaden- 

 ing of the absorption line, it can be calculated that between parallel planes 

 the energy absorbed per unit volume in the gas is inversely proportional to 

 the distance from the source and thus, as the end of the arc is approximately 



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