The vibrational spectrum of deuterated phosphaethyne: A quantum mechanical, classical, and semiclassical analysis

The vibrational spectrum of deuterated phosphaethyne (DCP) is analyzed in t erms of quantum-mechanical variational calculations, classical mechanics (p eriodic orbits), and an effective Hamiltonian model. The quantum mechanical and classical calculations are performed with a new, spectroscopically acc urate potential energy surface. The spectrum is governed by a 2 : 1 DC stre tch : CP stretch anharmonic resonance, which already exists for the fundame ntals. The bending degree of freedom is to a large extent decoupled. It is shown that several bifurcations in the classical phase space profoundly inf luence the quantum spectrum. For example, a new progression, which does not exist at very low excitation energies, comes into existence at intermediat e energies. In contrast to HCP, the pure bending states gradually evolve al ong the isomerization path with increasing bending quantum number. (C) 2000 American Institute of Physics. [S0021-9606(00)00120-3].