Knowledge Bank

The window region $(1100-1300\backslash AA)$ of the photoabsorption spectrum of $O2$ has been the subject of a number of $studies1$. However, the large interactions between Rydberg and valence states of the same symmetry, predicted by Buenker and $Peyerimhoff2$ on the basis of ab initio calculations, lead to many unusual spectral complexities. We have been studying the spectrum in conjunction with sem-empirical modelling using coupled Schrodinger equations (CSE) calculations which enable us to take account of these interactions. By fitting to our measurements of the spectrum, we have determined diabatic potential energy curves, coupling strengths and electronic transition moments. The calculations have enabled us to explain many features of the spectrum which were previously unexplained or incorrectly assigned. Our measurements have been of the absolute photoabsorption cross section of $O2$ at 79 K and 297 K using both a conventional monochromator system and a system using vacuum-ultraviolet radiation generated from pulsed dye lasers. These measurements give accurate information on predissociation broadening and linestregth which is vital for the numerical modelling using CSE calculations. A number of unusual features of the spectrum will be presented to illustrate the techniques.