The O„D... yield from O2 photodissociation near H Lyman-a „121.6 nm..

The solar H Lyman-a line is, through O2 photodissociation, an important source of O( D) production throughout the mesosphere and lower thermosphere. To ascertain the energy balance in this altitude region, it is necessary to know the O(D) yield across the solar H Lyman-a feature, since H Lyman-a 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 0.4460.05 for the O(D) yield at the center of the solar H Lyman-a 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(D) yield from O2 photodissociation across the entire H Lyman-a profile from 121.2 to 121.9 nm, at a spectral resolution of 0.0015 nm ~1 cm). The results reveal a strongly wavelength-dependent window in the O(D) 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-a, predict that the depth of the quantum-yield window will increase significantly as the temperature is lowered. © 1999 American Institute of Physics. @S0021-9606~99!00604-2#