Microwave Determination of the Probability of Collision of Electrons in Helium

A previously reported microwave method for determining the collision probability for momentum transfer of slow electrons has been modified so that a variation in average electron energy from 0.012 ev to 3 ev may be obtained. Measurements of the ratio of the real part to the imaginary part of the electron conductivity are performed in the afterglow of a pulsed helium discharge in a microwave resonant cavity. The average electron energy is varied by applying a microwave electric field in the afterglow and, under appropriate assumptions, the average electron energy is determined theoretically from this field. Measurements are also obtained by varying the gas temperature from 77°K to 700°K. The value of the collision probability for momentum transfer in helium is 18.3±2 percent ${\mathrm{cm}}^2$/${\mathrm{cm}}^3$ per mm Hg from 0 to 0.75 electron volts and increases slowly to a peak value of 19.2±2 percent at 2.2 ev.