Two-photon microwave transitions within a two-level system

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Abstract
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 ν0=17 267 MHz for transitions between the levels (J=3,K,M) and (2,K,M) are generated for K M≠0 by the application of intense pulses at ν0/2. It has been demonstrated that the two‐photon transitions occur within an effectively isolated two‐level system as a result of the first order acStark 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. 9 0, 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.