Knowledge Bank

Two-photon pure rotational transitions in the symmetric top $CF3CCH$ have been observed with a pulsed beam Fourier transform crossed-cavity spectrometer modified to allow the application of a static Stark field. Transient one-photon emission signals at $\nu o=17267$ MHz for transitions between the levels $(J=3,K,M)$ and (2,K.M) are generated for $KM\ne 0$ by the application of intense pulses at $\nu 0/2$. It has been demonstrated that the two-photon transitions occur within an effectively isolated two-level system as a result of the first order ac Stark effect. Quantitative studies of the intensities as a function of pulse length and power show that the two-photon transition probability in the microwave region is well represented by the theoretical model used $earlier1$ to treat multiphoton transitions in a two-level system whose one-photon frequency is $\lesssim 1MHz$. A description is given of the spectrometer with emphasis on the modifications made for two-photon studies.