Potential Energy Surfaces for the Na(3p)N2, Na(3p)HCN and Na(3p)C2H2 Complexes:Ab Initio Calculations and Model Considerations

Abstract

Dedicated to Prof. Edgar Heilbronner on the occasion of his 80th birthday Motto: Hier kocht der Chef, kommen Sie dennoch! Highly correlated ab initio calculations for the potential-energy surfaces of the systems NaÀN 2 , NaÀHCN, and NaÀC 2 H 2 have been performed, where the Na atom is in the 3s ground state or in the 3p excited state, and the molecules are kept fixed in their equilibrium configurations. The purpose of these calculations was to enable computer simulation of optical-scattering experiments. In this paper, model considerations are presented that allow easy qualitative understanding of the shapes of the calculated surfaces.Introduction. ± Modern quantum chemistry is a field of both physics and chemistry. No need to discuss its chemical merits in this journal. But surprisingly often also for the interpretation of physical experiments on systems more complex than an atom and less complex than a crystal, quantum chemistry can also make significant contributions. Various kinds of results based on precise numerical calculation of molecular wave functions are required. A few recent examples from my own work, where transition probabilities [1], quantum-defect surfaces [2], or spectroscopic assignments [3] were needed, may illustrate the situation. However, with increasing precision of quantumchemical results their transparency is lost, and while the results must be accepted, physical understanding can often not account for every numerical detail. The present study was carried out to support the interpretation of optical-collision experiments with molecules [4], providing high-precision interaction potentials for intermolecular forces in electronically excited states, and a physical interpretation of the results is attempted.One great advantage of quantum chemistry is the use of atomic units (a.u.) throughout, quantities are given in a.u. Conversion factors: length: 1 a o 5.29181 0 À11 m; energies: 1 a.u. 4.3598´10 À18 J; dipole moments: 1 a.u. 8.4783´10 À30 Cm; quadrupole moments: 1 a.u. 4.4865´10 À40 Cm 2 .