Fluorescence-excitation spectroscopy near 84.5 nm has been used to measure the threshold for the dissociation limit of H 2 (^' , X W + )=H(1J-) + H(2J) at 118377.06±0.04 cm" 1 . This yields a value of Do(\\i) =36 118.11 ±0.08 cm" 1 for the dissociation energy of the ground electronic state X( Sy Lg) of Fh, in agreement with the latest theoretical calculation which includes nonadiabatic, relativistic, and radiative corrections. When combined with recent determinations of the ionization energy of H2, the present £>o(H2) leads to the value Do(H2 + ) =21 379.36 ±0.08 cm" 1 , also in good agreement with theoretical values.

Fluorescence-excitation spectroscopy near -84 nm has been used to measure the threshold for the dissociation of H~{B' 'X"+ ) =H{1s)+H(2s) at 118377.06+0.04 cm ' and the corresponding limit for D2 at 119029.72+0.04 cm '. The onset of dissociation was observed clearly, free from overlapping molecular lines, by the use of delayed quenching of 2s atoms produced by the photoexcitation pulse. These measurements have yielded the values Do(H2)=36118. 11+0. 08 cm ' and Do(D2)=36748. 38+0.07 cm ' for the dissociation energies of the ground electronic states X 'Xg+, in agreement with the latest ab initio calculations, which include nonadiabatic, relativistic, and radiative corrections. From the present values and recently determined ionization energies of H& and D&, the dissociation energies of the respective ions were found to be Do(H2+)=21379. 37+0.08 cm ' and Do(D2+)=21711.64+0.07 cm ', also in agreement with recent ab initio calculations.PACS number(s): 35.20.6s, 33.20.Ni, 32.50.+d

The measurement of the dissociation energy of hydrogen remains an important problem in spectroscopy for comparison with theoretical calculations. We report on a preliminary determination of the dissociation energy of H from a fluorescence-excitation spectrum obtained using laser-generated tunable radiation near 85 nm. The radiation was tuned to excite the uppermost vibrational levels of the B state of H and the higher-lying dissociation continuum from the ground state X(v" = 0), while the fluorescence intensity was monitored. Features which were blended in previous work1 are resolved, and they correspond well to predicted2 R(1) and P(1) branches of the X B (v' = 38, 39) transitions, thus confirming Stwalley's identification of lines in the spectrum of H near the dissociation limit of the B state. This limit is then found by extrapolating the P(1) branch positions using Stwalley's technique,2 and from the onset of the dissociation continuum1. The B state dissociates into the products H(1s) + H(2p); therefore, the binding energy of the ground state of H is found by subtracting the Lyman-energy from the B state dissociation limit. From the extrapolation, a value of 36118.6 0.2 cm1 is obtained which agrees with Stwalley's estimate,2 and is 0.5 cm1 higher than a recent theoretical calculation by Kolos et al., and a measurement by Eyler et al. using multiphoton spectroscopy. A less precise value, found from the onset of the continuum, is limited by the presently available resolution.