Rotational dependence of the dipole moment of CH3D

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Abstract
Radio frequency modulated side bands of the C18O2 P (18) laser line were used to perform infrared–infrared double resonance between the first order Stark components of the ν6 rP (10,1) transition of CH3D. The dipole moment in the J=10, K=1 level in the ground state has been measured to be (1.03±0.1) ×10−3 D, which is very different from the previously reported values of around 5.65×10−3 D. This difference has been explained as due to rotational dependence of the dipole moment. Combining the present result with the previous molecular beam measurements by Wofsy, Muenter, and Klemperer, we have obtained the rotational dependence of the dipole moment derived from Stark measurements in the form μStark(J,K) =∓{ (5.657±0.004)−(0.0427±0.0009) J (J+1) +(0.0696±0.0026) K2}×10−3 D. The upper sign would be consistent with ab initio calculations of the signs of the dipole derivatives. A theory was developed to calculate the above coefficients of J (J+1) and K2 for CH3D. It has been demonstrated that they can be completely predicted from the ϑzxy parameter of CH4. The prediction agreed with the observed values within the quoted uncertainties. This theoretical information was indispensable in determining which of the two observed double resonance signals corresponded to the ground state. The apparent contradiction of the present result with that of Ozier, Ho, and Birnbaum, who reported that the dipole moments of CH3D determined from the intensities of R branch spectral lines are independent of J, is explained as due to the fact that the rotational dependence of the dipole moment is different depending on the method of determination. Their measurements are shown to be consistent with the above parameters.