Knowledge Bank

Using Ab Initio State Averaged MCSCF Plus CI Methods Chemical Research, Development, and Engineering Center Aberdeen Proving Ground, MD 21010-5423 The radiative lifetime for the $a3\Sigma u+$ state in neutral $He2\ast$ was calculated including spin-orbit (SO) interactions between the $a3\Sigma 1+$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi u$ manifold of states, and the repulsive ground state $X1\Sigma g$ and the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi g$ manifold. The SO interactions are treated through first-order perturbation theory wherein the first-order corrections to the wavefunctions are calculated directly by solving a set of linear equations (representing the first-order perturbation expression) involving SO matrix elements between the zeroth-order $X1\Sigma g+$ or $a3\Sigma U+$ wavefunctions, and the list of Configuration State Functions generated by a second-order CI for the states of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi g$ or $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi u$ symmetries, respectively (1). This partially alleviates the problem of solving explicitly for many eigenstates of the $3\Pi g$ or $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi u$ symmetries. The results show a rapidly changing electric transition dipole moment over the bound region of the $a3\Sigma u+$, i.e. $r(He-He)=1.5$ to 4.0 bohr. A vibrational analysis gives the lifetime of the v-O level of $a3\Sigma u+$ to be around 18 secs, which is consistent with a recent experimental estimate of 10 secs (v-?) as a lower bound.