Diode laser spectroscopy of the ν2 fundamental band of cis-HOPO

The absorption spectrum of the ν2 fundamental band of the cis-conformer of the transient molecule HOPO, namely the terminal PO stretching mode, has been detected and measured using diode laser spectroscopy. The molecule was generated in a discharge flow system containing hydrogen and white phosphorus vapour (P4) and a trace of oxygen. The spectrum has the appearance of an a-type band of a near prolate asymmetric top. Above Ka = 5 the spectrum is perturbed and transitions terminating on these higher Ka levels were excluded from the fit. The vibrational frequency and rotational constants derived from the unperturbed parts of the spectrum are compatible with new high precision ab initio calculations reported here. A combined fit of the ν2 band and the ν4 band data, measured earlier, was carried out. The ν2 band origin was determined to be 1258.539525(32) cm−1, approximately 5.5 cm−1 higher than the matrix value.