Precision measurement of quasi-bound resonances in H₂ and the H + H scattering length

Quasi-bound resonances of H$_2$ are produced via two-photon photolysis H$_2$S molecules as reactive intermediates or transition states, and detected before decay the parent molecule into three separate atoms. As was previously reported [K.F. Lai et al., Phys. Rev. Lett. 127, 183001 (2021)] four centrifugally bound quantum with lifetimes multiple $\mu$s, lying energetically above dissociation limit electronic ground state X$^1\Sigma_g^+$ H$_2$, were observed X($v,J$) = (7,21)$^*$, (8,19)$^*$, (9,17)$^*$, (10,15)$^*$, while also short-lived ($\sim 1.5$ ns) quasi-bound resonance X(11,13)$^*$ probed. The present paper gives a detailed account on identification shape resonances, based laser detection F-X transitions, their strongly enhanced Franck-Condon factors due to shifting wave function density large internuclear separation. In addition, assignment rotational number is verified by subsequent multi-step excitation autoionization continuum resonances. Existing frameworks full-fledged ab initio computations for region in including Born-Oppenheimer, adiabatic, non-adiabatic, relativistic quantum-electrodynamic contributions, extended energetic range energy. These comprehensive calculations compared accurate measurements energies finding excellent agreement. Etc.