Knowledge Bank

Fluorescence-excitation spectroscopy has been used to measure the threshold for the second dissociation limit of $H\_\{2\}(B^\{\backslash prime\}^\{1\}\backslash Sigma\{\_\{u\}\}^\{\backslash prime\})\; =\; H\; (1s)\; +\; H\; (2s)$ at $118377.06\pm 0.04cm-1$, and to determine a precise value for the dissociation energy of the ground electronic state. Coherent radiation near 84.5nm was used to excite a beam of hydrogen gas, and tuned over this threshold region. Metastable 2s atoms formed in the dissociation process were detected by quenching with an electric field, delayed -- 200 ns after the excitation pulse. This delay time was sufficient to allow molecules excited in the B and B^{\prime} states to decay by radiation, long before the 2s atoms were quenched to produce Lyman-$\alpha$ radiation. This it was possible to observe the dissociation limit clearly, without overlapping molecular features, and to derive a value for the dissociation energy of $H2$, namely, $Do(H2)=36118.11\pm 0.08cm-1$. This is in good agreement with the latest theoretical $value1$ of $36118.09\pm 0.10cm-1$, which includes nonadiabatic, relativistic, and radiative corrections. When the present value is combined with recent experimental values of the ionization potentials of H and $H2$ it yields $D0$ ($H2+)=21379.36\pm 0.08cm-1$, also in good agreement with theory.