The absolute photoabsorption, photoionisation and photodissociation cross sections and the photoionisation quantum efficiency of carbon dioxide have been measured using a double ion chamber and monochromated synchrotron radiation. In addition to structure associated with well established Rydberg series, some new features have been observed which are interpreted in terms of transitions into Rydberg states converging onto the C-2 Sigma(g)(+) upsilon(1)(+) = 1, upsilon(2)(+) = 0, upsilon(3)(+)= 1 threshold. The photoionisation quantum efficiency measurements indicate that the decay of Rydberg states by dissociation into neutral products is a significant de-excitation route. Rydberg states belonging to the Henning sharp series are strongly predissociated, whereas autoionisation is the dominant decay mechanism for states associated with the Henning diffuse series. The photodissociation cross section is compared to the CO A(1)II --> X(1) Sigma(+) fluorescence spectrum produced through photodissociation of CO2, and many similarities are apparent. A sum rule analysis has been carried out by combining the present absolute photoabsorption measurements with similar data covering the remaining wavelength regions.

Vibrational branching ratios have been measured for the H2CO+ X B-2(2) state using photoelectron spectroscopy and monochromated synchrotron radiation. In the wavelength range encompassed by the present study the photoionization cross section is dominated by intense autoionizing resonances, most of which belong to Rydberg series converging onto the B 2A1 threshold. Autoionization from these Rydberg states causes considerable variations to be observed in the vibrational branching ratios, The present data have been recorded over several Rydberg members, and the results indicate that autoionization tends to produce a vibrational distribution in the ion that is characteristic of the vibrational level in the neutral excited states.

The absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of oxygen have been measured using a double ion chamber and monochromated synchrotron radiation. In addition to structure associated with well established Rydberg series, some new features have been observed which are interpreted in terms of transitions into Rydberg states converging onto the 3 2Πu ionization threshold. The photoionization quantum efficiency measurements indicate that the decay of Rydberg states by dissociation into neutral products is a significant de-excitation route, particularly for the low vibrational members of the H 3Πu state. A sum rule analysis has been carried out by combining the present absolute photoabsorption measurements with similar data covering the remaining wavelength regions.

The absolute photoabsorption cross section and the photoionization quantum efficiency of nitrogen have been measured using a double ion chamber and monochromated synchrotron radiation, In the wavelength range encompassed by the present study, several well-established Rydberg series give rise to prominent structure in the absorption spectrum. A molecule excited into a Rydberg state may decay either by autoionization or by predissociation into neutral products, and this competition between de-excitation processes causes variations to be observed in the photoionization quantum efficiency. The results indicate that many of the Rydberg states undergo rapid predissociation. A sum rule analysis has been carried out by combining the present absolute photoabsorption cross section with similar data covering the remaining wavelength regions.

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 AA, 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.