``Forbidden'' rotational spectra of phosphine and arsine

``Forbidden'' rotational transitions with the selection rules ΔJ =0, Δk = ± 3 (k = ± 1 ← ± 2) caused by centrifugal distortion of molecules have been observed in PH3, PD3, and AsH3 by using a conventional Stark modulation spectrometer. The observation confirms the existence of Δk = ± 3 transitions that were first considered for rotational equilibration of interstellar NH3. The distinction between the Δk = ± 3 transitions and the Δk = ± 1 (ortho-para) transitions was made by observing Stark shifts of the spectral lines. The observed spectra provide accurate information on rotational parameters that are complementary to those previously obtained. The C constants for PH3, PD3, and AsH3 were determined to be 117 488.09±2.2 MHz, 58 972.20±3.0 MHz, and 104 882.584±0.03 MHz, respectively. From these constants the molecular structures were determined.