MULTIPHOTON IONIZATION SPECTROSCOPY OF THE 3S(2+2,1) AND 4S(3+1) RYDBERG STATES OF ACETONE - EVIDENCE FOR A MOLECULAR VALENCE STATE AT 153-NM

The resonance-enhanced multiphoton ionisation spectra of a molecular beam of acetone seeded in helium in the region of the 3s Rydberg state shows clear evidence of rotational and vibrational cooling of the compound. The very large spectroscopic width of the 3s adsorption band in photoabsorption spectroscopy is entirely due to thermal effects and not to interaction with an underlying molecular valence state. The vibrational modes active in the spectrum are all localised on the methyl groups of the molecule, which is explained in terms of the location of the 3s Rydberg orbital of the molecule. The REMPI spectra of the 4s Rydberg state shows no enhancement of resolution when the sample is cooled and the presence of a valence state centred at 153 nm is established. This is the first valence state found at energies higher than the UV (n(o) pi-*) state. Very sharp structure at 185 nm is attributed to a triplet state, probably associated with one of the 3p Rydberg states.