Microwave harmonics from a low-pressure discharge in different gases

Observations of the 2nd harmonic of 35GHz from low-pressure gas discharges have proved compatible with the theories of Swan and of Kino and Krenz. For different gases, the results show that the discharge is characterised largely by the size of the collision cross-section for electrons with neutral molecules. High-order-harmonic radiation was weak for reasons which are indicated.