Knowledge Bank

A microwave cavity spectrometer of a Stark-dc-voltage sweep type has been developed. Two rectangular cavity resonators were made: One operates in the X-band $TE10n\prime \prime$ ($n\prime \cong 20$) mode and the other in the K-band $TE10n\prime \prime$ ($n\cong 120$) mode. A Stark electrode which is placed at the center of each cavity is supplied with a dc sweep voltage as well as with a 100 kHz sine-wave modulation voltage. The cavity is iris-coupled Co an arm of a microwave magic T in an EPR apparatus from which the electromagnet has been removed. The signal-to-noise ratios obtained with the samples of 20 ppm $H2CO$ and 4.6 ppm $NH3$ in air, for example, have led to the minimum detectable absorption coefficients of this spectrometer to be $0.6\times 10-12cm-1$ for the X-band and $1\times 10-12cm-1$ for the K-band with a time constant of 30 seconds. We also report an EPR apparatus reduced in size and weight. The apparatus which was equipped with a $TE011$ mode cylindrical cavity resonator in X-band was designed only to detect $NO2$ in exhaust gases. The $NO2$ signal is observed at 1 atm sample pressure without interference from and $O2$ and NO. The minimum detectable limit is $O2$ ppm of $NO2$.