Single-triplet excitation spectrum of the CO-He complex. I. Potential surfaces and bound-bound CO(a~3PI - X~1epsilon~+) transitions

摘要

The interaction of He with metastable CO(a~3PI) gives rise to two adiabatic potential surfaces of reflection symmetry A' and A" which were calculated with the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples, RCCSD(T). Two diabatic potentials were constructed and fitted analytically; the appropriate form of the angular expansion functions was derived from general invariance properties. From variational calculations on these diabatic potential surfaces we obtained the quasibound vibration-rotation-spin levels of the CO-He complex in its lowest triplet state. Only the lower spin-orbit levels of this complex with approximate quantum number OMEGA = 0 of the CO(a~3PI) monomer were found to be stable with respect to dissociation into He and triplet CO. The potential and the bound van der Waals levels of the ground state CO (X ~1EPSILON~+)-He complex were recalculated and used in combination with the triplet excited state wave functions to compute the line strengths and the bound-bound part of the single-triplet excitation spectrum of the CO-He complex. The spin-forbidden singlet-triplet transitions access mainly the higher spin-orbit levels with |OMEGA| = 1, but these were found to undergo rapid predissociation. The companion Paper II explicitly studies this process, predicts the excited state lifetimes, and generates the bound-continuum part of the CO-He single-triplet spectrum.