CO_2-laser-microwave double-resonance spectroscopy of D_2CO: precise measurement of the dipole moment in the ground state

Abstract

The method of CO2-laser-microwave double resonance (LMDR) with an intense electric field was used to measure Stark shifts of ground-state microwave transitions of D2CO. Thirty LMDR signals originating from 15 K-doublet transitions were observed, associated with the infrared transitions of the ν4 and ν6 bands. Least-squares analysis of the observed LMDR signals yields precise values of the coefficients in the dipole-moment expansion, μ0 + μJJ(J + 1) + μKK2: μ0, 2.347 134(8) D; μJ, −4.76(10) × 10−6 D; μK, −28.7(18) × 10−6 D; where one-standard-deviation uncertainties are given in parentheses. The infrared–infrared double-resonance signals of PH3, which were calibrated against the OCS dipole moment, were used for the electric-field calibration, allowing us to determine the dipole moment with a precision of 10 parts in 106 (ppm). However, the absolute accuracy of the dipole moment obtained is 50 ppm, as limited by the uncertainty of the OCS dipole moment. The effective dipole moment for the 11,0 ← 11,1 transition measured in the present study agrees well with the effective dipole moment for the 11,0 rotational level from a molecular-beam electric resonance experiment. The μJ and μK coefficients calculated from Watson’s θγαβ constants agree well with the experimental values.