The O(¹D) yield from O₂ photodissociation near H Lyman-α (121.6 nm)

The solar H Lyman-α line is, through O₂ photodissociation, an important source of O(¹D) production throughout the mesosphere and lower thermosphere. To ascertain the energy balance in this altitude region, it is necessary to know the O(¹D) yield across the solar H Lyman-α feature, since H Lyman-α absorption by O₂ at ∼80 km accounts for a substantial fraction of the solar radiation absorbed in the mesosphere. An earlier laboratory study had provided a value of 0.44±0.05 for the O(¹D) yield at the center of the solar H Lyman-α line, where the profile shows a minimum in intensity due to strong self-reversal of the line. Using tunable laser radiation, we have determined the O(¹D) yield from O₂ photodissociation across the entire H Lyman-α profile from 121.2 to 121.9 nm, at a spectral resolution of 0.0015 nm (1 cm^-1). The results reveal a strongly wavelength-dependent window in the O(¹D) yield, the origins of which are explained using calculations based on a coupled-channel Schrödinger-equations model of the O₂ photodissociation. The calculations, which show significant isotopic dependence near H Lyman-α, predict that the depth of the quantum-yield window will increase significantly as the temperature is lowered.