HIGHLY EXCITED VIBRATIONAL-STATES OF HCP AND THEIR ANALYSIS IN TERMS OF PERIODIC-ORBITS - THE GENESIS OF SADDLE-NODE STATES AND THEIR SPECTROSCOPIC SIGNATURE

We present quantum mechanical bound-state calculations for HCP((X) ove r tilde) using an nb initio potential energy surface, The wave functio ns of the first 700 states, corresponding to energies roughly 23 000 c m(-1) above the ground vibrational state, an visually inspected and it is found that the majority can be uniquely assigned by three quantum numbers, The energy spectrum is governed, from the lowest excited stat es up to very high states, by a pronounced Fermi resonance between the CP stretching and the HCP bending mode leading to a clear polyad stru cture. At an energy of about 15 000 cm(-1) above the origin,the states at the lower end of the polyads rather suddenly change their bending character. While all states below this critical energy avoid the isome rization pathway, the states with the new behaviour develop nodes alon g the minimum energy path and show large-amplitude motion with H swing ing from the C-to the P-end of the diatomic entity. How this structura l change can be understood in terms of periodic classical orbits and s addle-node bifurcations and how this transition evolves with increasin g energy is the focal point of this article. The two different types o f bending motion are clearly reflected by the rotational constants. Th e relationship of our results with recent spectroscopic experiments is discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)015 46-8].