Millimeter/submillimeter Spectroscopy of PH2CN ({\tilde{X}} 1A') and CH3PH2 ({\tilde{X}} 1A'): Probing the Complexity of Interstellar Phosphorus Chemistry

Millimeter/submillimeter spectra of PH₂CN ({\tilde{X}} ¹A') and CH₃PH₂ ({\tilde{X}} ¹A') have been recorded for the first time using direct absorption techniques. This work extends previous measurements of both molecules beyond the 10-50 GHz range. Both species were created in the presence of an AC discharge by the reaction of phosphorus vapor and either cyanogen and hydrogen (PH₂CN) or methane (CH₃PH₂). Twelve rotational transitions of PH₂CN were recorded over the region 305-422 GHz for asymmetry components K_a = 0 through 8. For CH₃PH₂, eight rotational transitions were measured from 210-470 GHz with K_a = 0 through 16; these spectra exhibited greater complexity due to the presence of internal rotation, which splits the K_a = 1, 2, and 3 asymmetry components into A and E states. Combined analyses of the millimeter/submillimeter and previous microwave data were performed for both molecules. For PH₂CN, the spectra were fit with a Watson S-reduced asymmetric top Hamiltonian, resulting in more accurate rotational and centrifugal distortion constants. In the case of CH₃PH₂, an asymmetric top internal-rotation Hamiltonian was employed in the analysis, significantly improving the rotational and torsional parameters over previous microwave estimates. Searches for both molecules were subsequently conducted toward Sgr B2(N), using the 12 m telescope of the Arizona Radio Observatory (ARO). Neither species was identified, with abundance upper limits, relative to H₂, of f (PH₂CN/H₂) < 7.0 × 10^-12 and f (CH₃PH₂/H₂) < 8.4 × 10^-12. The nitrogen analogs NH₂CN and CH₃NH₂ are therefore more abundant in Sgr B2(N) by factors of >2 and >200, respectively.