Abstract
THE OAO 3 Copernicus ultraviolet satellite has provided important new data on the abundances of elements in the interstellar gas in the direction of many O and B-stars along lines of sight of low optical depth1–4. This information may make it possible to choose between the following theories of molecular hydrogen formation in interstellar clouds: (1) physical adsorption of H-atoms on to cold dielectric grains and their subsequent recombination and desorption5; (2) H2 recombination on graphite grains6; and (3) hydrogen recombination by non-activated chemisorption on transition metal grains7. Indirect methods of testing the above theories are essential because there is a glaring lack of experimental data, especially for processes (1) and (2). Also, it does not seem possible that in the near future radio astronomers will be able to measure the abundances of the various hydrogen ions (such as H2+ and H3+) that the above theories predict8.
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