Determination of atomic hydrogen densities in gases by a deuterium based Lyman-alpha absorption method

We discuss the variant of a method for the determination of absolute hydrogen atom densities based on the absorption of the hydrogen Lyman-alpha line. Previously, we demonstrated that by using a simple vacuum ultraviolet spectrometer with low resolution an accuracy of better than 50% for the determined hydrogen atom densities can be obtained for transmissions ranging from 10% to 90%. For transmissions outside of this range excessive errors occur, thus, limiting the usefulness of the method to a certain range of hydrogen atom densities, depending on absorber length and temperature. This range of atomic hydrogen densities accessible to the measurement can be extended by the new method that consists of using well-defined mixtures of hydrogen and deuterium in the absorber. Using a source of either hydrogen or deuterium Lyman-alpha radiation, only one sort of atoms contributes to the absorption. Thus, by selecting an appropriate mixture of hydrogen and deuterium, the range of atom densities accessible to the measurement can be extended to higher densities. Using well-defined mixtures ranging from 1% hydrogen in deuterium to 2% deuterium in hydrogen as absorbers, we were able to determine hydrogen atom densities up to a factor 100 higher than those measured previously in pure hydrogen. Using mixtures down to the natural abundance of deuterium in hydrogen, the measurement of even higher atom densities seems possible. The effect of the difference of the energies of dissociation of hydrogen and deuterium has been investigated using a computer model of relevant chemical processes. In most situations, this effect is smaller than the experimental error. The main limitation of the method is its sensitivity to absorption by impurities, as many molecular gases absorb Lyman-alpha radiation.