Knowledge Bank

Some years $ago1$ we studied the structure of the low lying excited states of the rare gas excimers using a simple model hamiltonian based on parameterized quantities called the $\Sigma -\Pi$, spin-orbit, and singlet-triplet splittings. We were also able to estimate radiative lifetimes of the $1u$ sublevel of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u+$ state, which where used to select among the available experimental data, yielding values of 5, 3, 0.3, and $0.1\mu \sec$ for $Ne2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>},Ar2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>},Kr2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$, and $Xe2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ respectively. In subsequent kinetic studies the third component of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u+$ state, designated $Ou-$, has been supposed either to be entirely metastable, or has been otherwise ignored. In the present investigation we expand our pervious model hamiltonian to include rotational coupling among the $\mathrm{<mrow\; data-mjx-texclass="ORD">1,3</mrow>}\Sigma u+$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1,3</mrow>}\Pi u$ states. We find that for all four $Rg2\ast$ species the thermally averaged radiative lifetime of the $Ou-$ sublevel should be between 2 and $10\mu \sec$. In the case of $Ne2\ast$, which is well described by Hund's case (b) the lifetime is nearly independent to J (the $F1$ and $F3$ components have lifetimes about twice that of $F2$). While for $Xe2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$, which is essentially case (c), the lifetime varies inversely with $J(J+1)(\tau 1\gg \tau 3=\tau 2)$. In high-pressure gas experiments, collisional mixing of the three triplet components would probably prevent their separate observation. In a beam experiment, the low-J levels of $Xe2\ast Ou-$ would be quite long lived, and the total population would decay non-exponentially.