[en] A new technique, vacuum-ultraviolet laser-absorption spectroscopy, has been developed to quantitatively determine the absolute density of H and H₂ within a plasma. The technique is particularly well suited to measurement in a plasma, where high charged particle and photon background complicate other methods of detection. The high selectivity and sensitivity of the technique allows for the measurement of the rotational-vibrational state distribution of H₂ as well as the translational temperature of the atoms and molecules. The technique has been used to study both pulsed and continuous H/sup /minus// ion-source plasma discharges. H₂ state distributions in a multicusp ''volume'' H/sup /minus// ion- source plasma show a high degree of internal excitation, with levels up to v = 5 and J = 8 being observed. The method is applicable for a very wide range of plasma conditions. Emission measurements from excited states of H are also reported. 17 refs., 9 figs