Non-empirical CI potential curves for the ground and excited states of PH and its positive ion

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Abstract
A series of calculated potential curves is reported for the low lying valence and (selected) Rydberg states of the PH molecule by employing the multi-reference (single- and) double-excitation CI method. A large AO basis including f functions on phosphorous is chosen for this study and up to twenty main configurations are used to generate a given MRD-CI space in the accompanying theoretical treatment. Very good agreement is obtained between calculation and experiment for a variety of spectroscopic constants and a number of predictions for electronically excited states of PH are made for cases in which no comparable empirical data are yet available. The repulsive 5∑- state is shown to be responsible for the predissociation of several states of the neutral system and is also thought to explain the failure to detect a number of low lying Rydberg states experimentally. Analogous calculations for the PH+ molecular ion have also been carried out and spectroscopic constants for this system are reported. In this case the 4Π state plays much the same role as the neutral 5∑- species in bringing about predissociation in its electronic spectrum. Finally a comparison is made throughout between the results for PH and PH+ and a number of isovalent systems, especially NH and NH+ and also SiH.