A STUDY OF THE ABSOLUTE PHOTOABSORPTION CROSS-SECTION AND THE PHOTOIONIZATION QUANTUM EFFICIENCY OF SULFUR-HEXAFLUORIDE FROM THE IONIZATION THRESHOLD TO 420-ANGSTROM

The absolute photoabsorption cross section and the photoionization quantum efficiency of sulphur hexafluoride have been measured using a double ion chamber and monochromated synchrotron radiation. The absorption spectrum is unusual because all the prominent features can be associated with transitions into valence states. This behaviour is caused by a barrier in the molecular potential which leads to a suppression of Rydberg structure and an enhancement of intervalence features. The photoionization quantum efficiency reaches a value close to unity for wavelengths shorter than 600 angstrom, thus resolving an earlier anomaly. Although the Rydberg structure is weak, and superimposed upon a strong background, excitation into these states produces a noticeable effect on the photoionization quantum efficiency. A sum rule analysis has been carried out by combining the present absolute photoabsorption cross section with similar data covering the remaining wavelength regions.