Infrared diode laser spectroscopy of the PCl radical

The vibration‐rotation spectrum of the PCl radical has been observed by infrared diode laser spectroscopy. The radical was generated by an ac discharge in a PCl3 and H2 mixture. The observed lines have been assigned to the v=1–0 and v=2–1 bands of P35Cl and to the v=1–0 band of P37Cl. All the observed lines were included in a least‐squares analysis, where ground‐state parameters were fixed to microwave values and isotopic relations were assumed to hold for molecular constants of the two isotopic species. The equilibrium values for vibrational changes of the rotational, the spin–spin interaction, and the spin‐rotation interaction constants have thus been determined very accurately. It was found that the sign of the αλ constant of PCl is the same as that of NCl, but is opposite to those of O2, SO, S2, and SeO.