2-PHOTON MICROWAVE TRANSITIONS WITHIN A 2-LEVEL SYSTEM

Two-photon pure rotational transitions in the symmetric top CF3 CCH 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 v0 = 17 267 MHz for transitions between the levels (J = 3,K,M) and (2,K,M) are generated for KM ≠ 0 by the application of intense pulses at v0/2. 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 by Meerts, Ozier, and Hougen [J. Chem. Phys. 90, 4681 (1989)] to treat multiphoton transitions in a two-level system whose one-photon frequency is ≲ 1 MHz. A description is given of the spectrometer with emphasis on the modifications made for two-photon studies. © 1990 American Institute of Physics.