The influence of grain mantles on the formation of hydrogen molecules on grain surfaces

The physical adsorption energy,E, of hydrogen molecules on various substrates at temperatures between 5 and 30 K and at the lowest practicable gas densities has been measured. Values ofE/k are for condensed CO 340 K, CO₂ 800 K, H₂O 850 K and for ‘dirty’ graphite 980 K and ‘dirty’ copper 800 K. From these measurements temperature ranges in which H atoms might combine on the surface to form H₂ molecules are estimated. Duley has discussed the formation and composition of condensed gas mantles on interstellar grains. The effects of such mantles in promoting and poisoning hydrogen molecule formation are discussed.