A study of the excited electronic states of normal and fully deuterated furan by photoabsorption spectroscopy and high-level ab initio calculations

Abstract The photoabsorption spectra of C 4 H 4 O and C 4 D 4 O have been measured between ∼5.5 and 17.7 eV using a synchrotron radiation-based Fourier transform spectrometer. In addition to several broad bands due to transitions into valence states, the spectra exhibit numerous sharp bands associated with Rydberg states belonging to series converging onto the X 2 A 2 or the A 2 B 1 state limits. Vertical excitation energies and oscillator strengths have been computed using the second- and third-order algebraic-diagrammic construction polarisation propagator methods (ADC(2) and ADC(3)), and the equation-of-motion coupled-cluster method at the level of singles and doubles model (EOM-CCSD). Adiabatic excitation energies have been estimated using previously computed corrections. The theoretical predictions have allowed assignments to be proposed for the Rydberg series observed in the present single-photon absorption spectra and for some additional series, mainly of A 2 symmetry, reported in previous multiphoton excitation studies. The assignments of some of the Rydberg series converging onto the A 2 B 1 state limit have been revised and, guided by our calculations, the principal series is ascribed to the 2b 1  →  n da 2 1 B 2 and 2b 1  →  n db 1 1 A 1 transitions. f-type Rydberg series, previously observed only in the multiphoton absorption spectrum of furan, have been observed and assigned. Such f-type series, converging onto either the X 2 A 2 or the A 2 B 1 state thresholds, contribute significantly to the single-photon absorption spectrum. Many of the absorption bands associated with Rydberg states display vibrational progressions which resemble those in the corresponding photoelectron band. It appears that some of the structure associated with the 1a 2  → 3pb 2 1 B 1 and 1a 2  → 3pb 1 1 B 2 transitions involves excitation of non-totally symmetric vibrational modes.