VACUUM ULTRAVIOLET PHOTOIONIZATION MASS-SPECTROMETRIC STUDY OF C-60

Gaseous C60 has been studied by photoionization mass spectrometry between the ionization threshold and 40.8 eV. An adiabatic threshold of 7.57 +/- 0.01 eV is observed, which may be slightly low due to hot bands. The energy derivative of the photoin yield curve is in rough agreement with the He I photoelectron spectrum on the positions of some peaks, but others are weak or absent. The discrepancy is not attributed to autoionization, but rather to selection rules governing the ejection of low energy electrons into high angular momentum waves. C60+ + is observed at higher energies, and becomes approximately 0.6 as intense as C60+ at 40.8 eV. The photoion yield curve of C60+ +, approximately linear well above threshold, appears to exhibit curvature near threshold, thwarting an attempt to make a distinction between two alternative values of the second ionization potential. Fragmentation to form C58+ is only observed at the highest energy, 40.8 eV. The unimolecular decay is modelled by quasiequilibrium theory. In this model, the kinetic shift is of the order of 30 eV, and the minimum energy for dissociation into C58+ + C2 seems to be 6.0-6.5 eV.