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

Quasi-bound resonances of H-2 are produced via two-photon photolysis of H2S molecules as reactive intermediates or transition states and detected before decay of 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 resonances with lifetimes of multiple mu s, lying energetically above the dissociation limit of the electronic ground state X-1 Sigma(+)(g) of H-2, were observed as X(v, J) = (7, 21)*, (8, 19)*, (9, 17)*, and (10, 15)*, while also the short-lived (similar to 1.5 ns) quasi-bound resonance X(11, 13)* was probed. The present paper gives a detailed account on the identification of the quasi-bound or shape resonances, based on laser detection via F-1 Sigma(+)(g) -X-1 Sigma(+)(g) two-photon transitions, and their strongly enhanced Franck-Condon factors due to the shifting of the wave function density to large internuclear separation. In addition, the assignment of the rotational quantum number is verified by subsequent multi-step laser excitation into autoionisation continuum resonances. Existing frameworks of full-fledged ab initio computations for the bound region in H-2, including Born-Oppenheimer, adiabatic, non-adiabatic, relativistic and quantum-electrodynamic contributions, are extended into the energetic range above the dissociation energy. These comprehensive calculations are compared to the accurate measurements of energies of quasi-bound resonances, finding excellent agreement. They show that the quasi-bound states are in particular sensitive to non-adiabatic contributions to the potential energy. From the potential energy curve and the correction terms, now tested at high accuracy over a wide range of energies and internuclear separations, the s-wave scattering length for singlet H+H scattering is determined at a = 0.2735(31)(39)a(0). It is for the first time that such an accurate value for a scattering length is determined based on fully ab initio methods including effects of adiabatic, non-adiabatic, relativistic and QED with contributions up to ma(6). [GRAPHICS] .