Spectroscopic application of a 'tunable source' in the 10 μM region: Laser-microwave two-photon spectroscopy

A tunable microwave frequency was added to, or subtracted from fixed frequency ¹³C ¹⁶O ₂ and ¹²C ¹⁸O ₂ laser lines using the nonlinearity of molecular absorption. In this way the frequency of 37 transitions of ¹⁴NH ₃ and 11 transitions of ¹⁵NH ₃ have been measured. Under favourable conditions Lamb-dip signals were observed and in these cases absolute frequency measurements with a precision of ± 3 MHz were made. In the case of the exact coincidence between the R(18) ¹³CO ₂ laser line and the ν _2asQ(5,4) transition of ¹⁵NH ₃ signals produced by multiple-photon processes were observed. Transitions in fundamental and hot-bands of the linear molecules HCCF and FCN have also been measured. New systems for the generation of submillimetre waves are proposed.