(v , 0) Werner bands for v = 0-4, using a narrow-band tunable extreme UV laser source at wavelengths λ = 92-105 nm in conjunction with the technique of 1 + 1 two-photon ionization. The measurements can be divided into three categories for which varying absolute accuracies were obtained. Special focus was on the B, v = 2-5 bands, where an accuracy of 0.004 cm −1 or δν/ν = 4 × 10 −8 is achieved. For transitions to B, v ≤ 13 and C, v ≤ 3 states the accuracy is 0.005 cm −1 or δν/ν = 5 × 10 −8 . Due to a different frequency mixing scheme uncertainties for B, v ≥ 13 and C, v = 4 are at the level of 0.011 cm −1 or δν/ν = 1.1 × 10 −7 . Inspection of combination differences between R(J ) and P(J + 2) lines shows that the accuracies are even better than estimated in the error budget. Based on the measurements of 138 spectral lines and the known combination differences, transition frequencies of 60 P-lines could be calculated as well, so that a data base of 198 accurately calibrated lines results for the Lyman and Werner bands of H 2 .Key words: vacuum UV, molecular spectroscopy, hydrogen, precision metrology. . Un examen des différences des combinaisons entre les raies R(J ) et P(J + 2) montre que les précisions sont meilleures que celles évaluées dans le budget des erreurs. Sur la base de mesures de 138 raies spectrales et de différences de combinaison connues, on a pu aussi calculer les fréquences de transition de 60 raies P et il en résulte qu'une base de données de 198 raies bien calibrées est disponible pour les bandes de Lyman et de Werner du H 2 .

Level energies of g symmetry states lying just below the n ¼ 2 dissociation threshold have been determined in a XUV + IR multi-step laser excitation experiment in HD, with an absolute accuracy of the excitation energy of 0.015 cm À1 . Intermediate B 1 R þ u energy levels have been calibrated by measuring transition frequencies in theLyman band with an accuracy of 0.007 cm À1 . It is shown that v ¼ 3 is the highest bound vibrational level in the shallow I 01 P g outer well potential in HD, and that only two rotational levels exist.