A study of the valence-shell photoabsorption, photodissociation and photoionisation cross-sections of allene

The absolute photoabsorption cross-section and the photoionisation quantum efficiency of allene have been measured from the ionisation threshold to 35 eV using a double ion chamber and monochromated synchrotron radiation. New structure has been observed in the 10.5-14.6 eV range and attributed to autoionisation from states belonging to Rydberg series converging onto the (A) over tilde(2)E ionisation threshold. Two prominent broad absorption features located at 11.3 and 14.1 eV have been associated, tentatively, with valence-shell transitions into the 3e orbital. It is noticeable that at excitation energies coinciding with these two valence transitions the photoionisation quantum efficiency exhibits local minima. This indicates that predissociation plays a prominent role in the decay of these super-excited states. (C) 1999 Elsevier Science B.V. All rights reserved.