The Phosphidogen Radical, PH2: Terahertz Spectrum and Detectability in Space

Abstract

Abstract Pure rotational transitions of phosphidogen, PH2, in its ground electronic ( X 2B1) and ground vibrational states have been observed with the Cologne terahertz spectrometer using backward wave oscillators as radiation sources in the frequency region between 458 and 951 GHz. The PH2 radical was generated by immersing solid red phosphorus in a dc glow discharge of molecular hydrogen buffered with helium, with a ratio He/H2∼3. A total of 107 new lines have been measured, comprising 21 distinct sets of rotational transitions through N and Ka of 7 and 6, respectively. These include four important sets of R-branch transitions with N=2←1 and N=3←2. In addition to the fine structure splitting, all measured rotational transitions exhibit smaller splittings due to hyperfine interactions involving the three nuclei. The achieved frequency accuracy of the measurements ranges between 30 and 500 kHz, but is typically 50–100 kHz. The newly measured PH2 transitions were analyzed together with existing field-free data to derive highly accurate molecular parameters, which are considerably more extensive than those reported in the existing literature. Among the determined parameters are the rotational constants A=273 783.512 (38) MHz, B=242 347.640 (42) MHz, C=126 343.827 (34) MHz, centrifugal distortion constants through eighth order, and fine and hyperfine constants with distortion corrections up to and including sextic and quartic terms, respectively. This new set of molecular parameters allows highly accurate and reliable frequency predictions of the rotational spectrum up to about 2 THz. The strongest transitions, both measured and predicted, should be the basis of future astronomical searches. Abundance estimates indicate PH2 to be detectable in young hot-molecular-core regions of interstellar clouds or in circumstellar envelopes of oxygen-rich or carbon-rich stars.