Photoelectron spectra of acetone and acetone dimer

In order to study the ionic states of acetone produced in a supersonic jet, we have carried out threshold photoelectron spectroscopy (TPES) with synchrotron radiation in the region 122.0-130.0 nm. Acetone has also been re-investigated by 58.4 nm He(I) photoelectron spectroscopy (PES). It has been found from a comparison of the PES and TPES spectra that there is an autoionizing state at 9.893 ± 0.015 eV. Furthermore, a threshold-photoelectron photoion coincidence (TPEPICO) spectrum due to the acetone dimer has been obtained for the first time in the region 124.0-135.0 nm with an interval of 0.2 nm. The dimer intensity in each spectrum was plotted against the excitation wavelength, giving rise to intensity distribution corresponding to a TPES spectrum of the non-dissociative acetone dimer. In contrast to the PES and TPES spectra of the acetone monomer, no vibrational structure appears in the dimer spectrum. From this spectrum the appearance ionization potential of the acetone dimer has been evaluated as being 9.210 ± 0.015 eV. Two bands appear in the dimer spectra, one of which may be assigned to a Rydberg state of the neutral dimer, and the other to the electronic ground state of the dimer cation.