PURE ROTATIONAL SPECTRUM AND ELECTRIC DIPOLE MOMENT OF CH3D

The pure rotational spectrum of CH3D in the ground vibrational and electronic state has been observed using a far-infrared grating spectrometer. This spectrum is of particular interest because the electric dipole moment n arises entirely from an isotopic substitution. Ten distinct lines have been observed in the fre-quency range from 40 to 120 cm-1, and have been identified with the (J→J+1) transitions for the rota-tional levels J = 5-14. For the (6→7) line, the absolute intensity and the linewidth were determined by a curve of growth method. It was found that, for this line, the dipole moment μ = (5.68±0.30) X 10-3D and the broadening parameter γ0= (0.075±0.012) cm -1/amagat. This measurement constitutes the first correct experimental determination of the dipole moment of CH3D. By making further intensity measure-ments on other lines, it was shown that, to within 6%, μ is independent of J in the range 5≤J≤12. A detailed account of the experimental methods used to make the intensity measurements is given. It was also determined that the rotational constant B0= (3.882±0.002) cm-1 and the distortion constant Dj= (7.7±0.3) X10-5cm-1.