Design details are given of the harmonic generator and detector which made possible precision spectroscopy in the one-to-two mm wave region. The signal-to-noise ratio obtained on OCS rotational lines is 7 to 1 at 1.0-mm wavelength; 30 to 1 at 1.1 mm; and better than 100 to 1 at 1.4 mm and above. A useful tuning and measuring technique is made possible by the ability of the new system to detect several klystron harmonics at once and, hence, spectral lines in several different regions at the same time. The applications of the methods in the measurement of centrifugal distortion of molecules is illustrated with OCS, for which $D_J=1.310\mathrm{}\pm 0.010$ kc/sec is obtained, and with C${\mathrm{H}}_3$F, for which $D_J=57.8\mathrm{}\pm 1.0$ kc/sec and $D_{\mathrm{JK}}=445\mathrm{}\pm 4$ kc/sec are obtained. A new water-vapor line, the $2_{2,0}$→$3_{1,3}$ rotational line, has been measured at 183 311.30±0.30 Mc/sec.

• Received 17 September 1953

DOI:https://doi.org/10.1103/PhysRev.93.407