The microwave Q branch of the distortion moment spectrum of 13CH4

Abstract

The distortion moment microwave spectrum of ¹³CH₄ has been studied with a conventional Stark modulated instrument having a sensitivity of ∼1×10⁻¹¹ cm⁻¹. Six pure rotational Q branch transitions have been observed between 8 and 20 GHz. With the assumption that the three octic tensor distortion constants L_4T, L_6T, and L_8T have the same values as in ¹²CH₄, values have been obtained for the one quartic and two sextic tensor distortion constants. These are (in Hz) D_T= (132 983.32±0.48); H_4T=−(17.0026±0.0018); H_6T= (11.1869±0.0011). The errors given here are the standard deviations obtained from the least squares analysis. The justification for the assumption is presented, and estimates are given for the absolute errors in D_T, H_4T, and H_6T, as well as for the isotopic shifts in L_4T, L_6T, and L_8T. The measured differences between the ¹³CH₄ and ¹²CH₄ values of D_T, H_4T, and H_6T are discussed with reference to recent theoretical calculations. A method is given of using these differences to calculate the tensor energy splittings of ¹³CH₄ from those previously published for ¹²CH₄.