The O(D-1) yield from O-2 photodissociation near H Lyman-alpha (121.6 nm)

The solar H Lyman-alpha line is, through O-2 photodissociation, an importan t source of O(D-1) production throughout the mesosphere and lower thermosph ere. To ascertain the energy balance in this altitude region, it is necessa ry to know the O(D-1) yield across the solar H Lyman-alpha feature, since H Lyman-alpha absorption by O-2 at similar to 80 km accounts for a substanti al fraction of the solar radiation absorbed in the mesosphere. An earlier l aboratory study had provided a value of 0.44+/-0.05 for the O(D-1) yield at the center of the solar H Lyman-alpha line, where the profile shows a mini mum in intensity due to strong self-reversal of the line. Using tunable las er radiation, we have determined the O(D-1) yield from O-2 photodissociatio n across the entire H Lyman-alpha profile from 121.2 to 121.9 nm, at a spec tral resolution of 0.0015 nm (1 cm(-1)). The results reveal a strongly wave length-dependent window in the O(D-1) yield, the origins of which are expla ined using calculations based on a coupled-channel Schrodinger-equations mo del of the O-2 photodissociation. The calculations, which show significant isotopic dependence near H Lyman-alpha, predict that the depth of the quant um-yield window will increase significantly as the temperature is lowered. (C) 1999 American Institute of Physics. [S0021-9606(99)00604-2].