The lowest triplet state (3)A ' of H-3(+): Global potential energy surfaceand vibrational calculations

The adiabatic global potential energy surface of the H-3(+) system for the lowest triplet excited state of A' symmetry was computed for an extensive g rid of conformations around the minimum region at full configuration intera ction ab initio level, using a much more extended basis set than in a previ ous paper from the same authors. An accurate global fit (rms error lower th an 27 cm(-1) for energies lower than dissociation into separated atoms and lower than 5 cm(-1) for energies lower than the dissociation channel) to th ese ab initio points and also to part of the previous calculated points (fo r a total of 7689 energies in the data set) of the lowest triplet excited s tate of A' symmetry is obtained using a diatomics-in-molecules approach cor rected by one symmetrized three-body term with a total of 109 linear parame ters and 1 nonlinear parameter. This produces an accurate global potential which represents all aspects of the bound triplet excited state of H-3(+) i ncluding the minima and dissociation limits, satisfying the correct symmetr y properties of the system. The vibrational eigenstates have been calculate d using hyperspherical coordinates with symmetry adapted basis functions wi th the proper regular behavior at the Eckart singularities. The accuracy of the vibrational levels thus obtained is expected to be better than 2 cm(-1 ) with respect to unknown experimental values. Due to the presence of three equivalent minima at collinear geometries (D-infinityh) the lower vibratio nal levels are close to triple degenerate. Since the interconversion barrie r between the three minima is about 2640 cm(-1), these states split for the upper excited vibrational levels. Such splitting can provide a key feature to identifying the unassigned transitions amongst the many H-3(+) lines th at have been observed in hydrogen plasmas. (C) 2001 American Institute of P hysics.