CO2 laser–microwave double resonance spectroscopy of NH3: Precise measurement of dipole moment in the ground state

Abstract

CO2 laser–microwave double resonance (LMDR) spectroscopy with an intense electric field was applied to the NH3 molecule. Using infrared transitions in the ν2 band for pumping, 76 and 37 LMDR signals originating from 25 and 16 inversion transitions in the ground vibrational state were observed for 14NH3 and 15NH3, respectively. The applied electric field was calibrated with a precision of 10 ppm by a new method using the infrared–infrared double resonance signals of PH3, which have been calibrated against the OCS LMDR signals. The observed signals were analyzed to yield the precise dipole moment with rotational dependence, which was expanded as μ0+μJJ(J+1) +μKK2+⋅⋅⋅. The coefficients determined are, with one-standard-deviation uncertainties in the parentheses, μ0=1.471 932(7) D, μJ=1.8736(34)×10−4 D, and μK =−3.5550(47)×10−4 D for 14NH3, and μ0=1.471 964(11) D, μJ =1.9244(77)×10−4 D, and μK =−3.5903(98)×10−4 D for 15NH3. The absolute accuracy of the dipole moment is, however, limited to 50 ppm as determined by the uncertainty of the OCS dipole moment. The effective dipole moments for the J,K=1, 1 and 1,0 levels of 14NH3 calculated from the present coefficients are consistent with the results of a molecular beam electric resonance experiment. The μJ and μK coefficients are interpreted in terms of Watson’s θαβγ constants, resulting in a good agreement between the observed and theoretical values. The consistency between the two isotopic species is also confirmed.