The Potential Energy Surface for the Electronic Ground State of H 2Se Derived from Experiment

Abstract

The present paper reports a determination of the potential energy surface for the electronic ground state of the hydrogen selenide molecule through a direct least-squares fitting to experimental data using the MORBID (Morse oscillator rigid bender internal dynamics) approach developed by P. Jensen [ J. Mol. Spectrosc. 128, 478-501 (1988); J. Chem. Soc. Faraday Trans. 2 84, 1315-1340 (1988)]. We have fitted a selection of 303 rotation-vibration energy spacings of H 2 80Se, D 2 80Se, and HD 80Se involving J ≤ 5 with a root-mean-square deviation of 0.0975 cm −1 for the rotational energy spacings and 0.268 cm −1 for the vibrational spacings. In the fitting, 14 parameters were varied. On the basis of the fitted potential surface we have studied the cluster effect in the vibrational ground state of H 2Se, i.e., the formation of nearly degenerate, four-member groups of rotational energy levels [see I. N. Kozin, S. Klee, P. Jensen, O. L. Polyansky, and I. M. Pavlichenkov. J. Mol. Spectrosc., 158, 409-422 (1993), and references therein]. The cluster formation becomes more pronounced with increasing J. For example, four-fold clusters formed in the vibrational ground state of H 2 80Se at J = 40 are degenerate to within a few MHz. Our predictions of the D 2 80Se energy spectrum show that for this molecule, the cluster formation is displaced towards higher J values than arc found for H 2 80Se. In the vibrational ground state, the qualitative deviation from the usual rigid rotor picture starts at J = 12 for H 2 80Se and at J = 18 for D 2 80Se, in full agreement with predictions from semiclassical theory. An interpretation of the cluster eigenstates is discussed.