Microwave Spectrum of Phosphine, Arsine, and Stibine

Abstract

The microwave spectrum of the mono-deuterated phosphine (P${\mathrm{H}}_2$D), arsine (As${\mathrm{H}}_2$D), and stibine (Sb${\mathrm{H}}_2$D) has been measured. Although a supposedly pure sample of stibine was prepared, phosphine and arsine proved to be major impurities. They were separated from the sample; the microwave spectrometer was used to monitor the distillation process. The quadrupole fine structure associated with the absorption transitions for As${\mathrm{H}}_2$D is used to illustrate the special assumptions which may be made with these molecules having an axis of charge symmetry. These assumptions allow one to rotate the coordinate system from the chargesymmetry axes to the inertial axes without thereby adding unknown cross derivatives to the descriptions of the dyadic $\nabla E_{\mathrm{zz}}$. Interpretation of the spectrum of Sb${\mathrm{H}}_2$D is more complicated. It is not found possible to fit the theory to the observed lines without making use of the second-order perturbations caused by a nearby level of different symmetry but of the same $J$. This perturbation involves the cross derivatives which had previously been assumed to be zero. With the assumption of cross derivatives approximately one percent of the diagonal terms, the theory fitted experiment within the experimental error. The electric quadrupole interaction in arsine and stibine is thus determined.