The lowest triplet state A′3 of H3+: Global potential energy surface and vibrational calculations

Abstract

The adiabatic global potential energy surface of the H3+ system for the lowest triplet excited state of A′ symmetry was computed for an extensive grid of conformations around the minimum region at full configuration interaction ab initio level, using a much more extended basis set than in a previous paper from the same authors. An accurate global fit (rms error lower than 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 these ab initio points and also to part of the previous calculated points (for a total of 7689 energies in the data set) of the lowest triplet excited state of A′ symmetry is obtained using a diatomics-in-molecules approach corrected by one symmetrized three-body term with a total of 109 linear parameters and 1 nonlinear parameter. This produces an accurate global potential which represents all aspects of the bound triplet excited state of H3+ including the minima and dissociation limits, satisfying the correct symmetry properties of the system. The vibrational eigenstates have been calculated using hyperspherical coordinates with symmetry adapted basis functions with 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∞h) the lower vibrational levels are close to triple degenerate. Since the interconversion barrier 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 H3+ lines that have been observed in hydrogen plasmas.