Improved potential-energy and dipole-moment functions of the ground electronic state of phosphorus nitride

Abstract

Ab initio calculations of the PN potential and electric dipole moment in the $X^1{\Sigma }^{+}$ ground electronic state were performed at short bond lengths, $r=0.2$–0.8 Å, and semi-empirical analytical potential-energy and dipole-moment functions were constructed based on all available experimental and theoretical information. The analytical forms for the potential-energy functions include the Extended Morse Oscillator and the Extended Hulburt-Hirshfelder potential. The dipole-moment function of PN was presented by our irregular and rational functions previously used for CO. The potential-energy and dipole-moment model functions were fitted simultaneously to the experimental line positions and permanent dipoles at $v=0$-2, as well as to the ab initio data from our present and previous studies. With these new functions, the improved line list for the ground electronic state of ³¹P¹⁴N was calculated. We show that the new analytic representations of the potential and dipole moment functions help significantly reduce the numerical noise in the intensities of high overtones as well as the associated saturation at high wavenumbers leading to the so-called “overtone plateaus” in spectra of diatomic molecules (see Medvedev et al., J. Mol. Spectrosc., 330, 36 (2016)) and thus provide reliable transition intensities at very high transition frequencies. The 3-0 band is identified as vibrational anomaly, and rotational anomalies inside this and some other bands are found.