Knowledge Bank

The H$2$As=O radical has been identified for the first time by laser-induced fluorescence (LIF) and single vibronic level (SVL) emission techniques. The radical was generated by a pulsed electric discharge in a mixture of AsH$3$ and CO$2$ and high-pressure argon and detected by observation of the $B~2A\prime -X~2A\prime$ electronic transition in the 510-410 nm region. Moderate resolution LIF and SVL emission spectra of H$2$AsO, D$2$AsO, and HDAsO have been recorded and analysis shows unequivocally that the spectrum is due to the arsenyl radical. High-resolution spectra of the $000$ bands of H$2$AsO and D$2$AsO, which consist of strong $a$-type and weaker $c$-type transitions, revealed spin-splittings and small, but significant arsenic hyperfine splittings due to a Fermi contact interaction in the ground state. The effective molecular structures of H$2$AsO in the ground and excited states have been determined from the rotational constants and will be discussed in the context of the analogous nitroxyl (X$2$N=O) and phosphonyl (X$2$P=O) radicals.