Radiative lifetime of the a 3Σ+ state of HeH+ from ab initio calculations

The first metastable triplet state of HeH(+) was found to be present in ion beam experiments, with its lifetime estimated to be between hundreds of milliseconds and thousand of seconds. In this work, we use ab initio methods to evaluate the radiative lifetimes of the six vibrational levels of the a (3)Σ(+) of HeH(+). The transition a (3)Σ(+)→X (1)Σ(+) is spin-forbidden, but acquires intensity through spin-orbit interaction with the singlet and triplet Π states. Large scale CASSCF/MRCI calculations using an adapted basis set were performed to determine the potential energy curves of the relevant states of HeH(+) as well as the matrix elements of the dipole and spin-orbit operators. The wave functions and energies of the vibrational levels of the a (3)Σ(+) and X (1)Σ(+) states are obtained using a B-spline method and compared to previous works. We find that the radiative lifetime of the vibrational levels increases strongly with v, the lifetime of the v=0 state being 150 s. We also analyze the contributions from discrete and continuum parts of the spectrum. With such a long lifetime, the a (3)Σ(+) state could have astrophysical implications.