Dissociation Energy of the HD Molecule

Fluorescence-excitation spectra of HD have been obtained in the region of ~84 nm showing two dissociation continua corresponding to the photofragments H (1s) +D (2s) and H (2s) +D (1s). Pulsed and tunable coherent radiation was used for excitation with detection by fluorescence emission of Ly-α radiation. By application of a delayed electric field for quenching of the photofragments, only the 2s atoms were selected for detection. At the high resolution achieved, the profiles of the two dissociation limits were clearly observed, and values of the thresholds obtained. The two limits are separated by 21.61±0.10 cm-1, almost 1 cm1 less than the expected difference in excitation energies of the D(2s) and H(2s) atoms. The lower limit, with subtraction of the H(2s-1s) energy, gives a value of D0(HD) = 36 405.83±0.10 cm-1 for the dissociation energy of the ground electronic state of HD, which agrees with the recent ab initio calculation1 of 36 405.784+0.10 cm-1. The apparent discrepancy (almost 10 times the quoted error of measurement) of the value derived from the higher limit remains to be explained. When the present value of D0(HD) is combined with known values of the ionization potentials of HD and H, the value Do(HD+) = 21 516±0.10 cm-1 is obtained for the dissociation energy of the ion, again in good agreement with theory.