A study of the Rydberg states of furan (C4H4O) and deuterated furan (C4D4O) using mass-resolved (2+1) and (3+1) resonance enhanced multiphoton ionization

A combination of (i) two- and three-photon resonance enhanced multiphoton ionization techniques with linear and circular polarization, (ii) a comparison with the single-photon absorption spectrum and (iii) isotopic substitution has been used to study the Rydberg states of furan (C4H4O). The three X[(2)A(2)](c); 3p states have been unambiguously assigned from the identification of the promoting modes involved in the strong false origins observed in the two- photon spectrum. The frequencies of the A(1) vibrational modes of the ion-core that are Franck - Condon active are reported. The l values of the higher Rydberg states are assigned from their quantum defects and a comparison of the single-, two- and three-photon spectra, up to the ionization limit, reveals a strong propensity for excitation of d series with two photons and p and f series with one and three photons.