The ionic and ground states of gamma-pyrone. The photoionization spectrum studied by synchrotron radiation and interpreted by configuration interaction and density functional calculations.

A synchrotron based photoionization spectrum represents a considerable improvement in resolution over early He(I) and He(II) spectra. The spectral onset shows a complex profile where two states, later shown to be the 12B2 and 12B1 states, are strongly overlapping. Symmetry-adapted coupled cluster calculations of the ionic state sequence up to 27 eV, give the sequence of state vertical ionization energies (VIE) as: 12B2 < 12B1 < 12A2 < 12A1 < 22B1 < 32B1 < others. Density functional calculations of the corresponding adiabatic terms (AIE) were also performed using the long range-corrected version of the Becke three-parameter hybrid functional (B3LYP), through the Coulomb-attenuating method (CAM-B3LYP) functional. Complete active space self-consistent field (CASSCF) methods were used to determine the wave-functions for the 22B1 and 32B1 states. Selected ionic state vibrational profiles, including all p-ionizations, were interpreted by Franck-Condon (FC) procedures, and compared with experiment for the above group of states. Although the lowest IE shows considerable overlap of the 12B2 and 12B1 states, there is no apparent vibrational interference between the two states. This is in marked contrast to our previous work on the halogeno-benzenes, where overlapping state envelopes led to degradation of the higher energy states, which showed considerable widening of the line-width at half-height.