Photoabsorption measurements and theoretical calculations of the electronic state spectroscopy of propionic, butyric, and valeric acids

Absolute photoabsorption cross sections of propionic (C2H5COOH), butyric (C3H7COOH), and valeric (C4H9COOH) acids have been measured from the dissociative π* ← nO transition (beginning around 5.0 eV) up to 10.7 eV. This constitutes the first study of the neutral electronic states of propionic and butyric acids at energies above the π* ← nO band, while no previous spectroscopic data is available for valeric acid in the present range. The present assignments are supported by the first theoretical calculations of electronic transition energies and oscillator strengths for these organic acids. In addition, the excitation energies of the vibrational modes of propionic acid in its neutral electronic ground state and the vertical ionisation energies of all three molecules have been calculated for the first time. The He(I) photoelectron spectroscopy of propionic acid has been measured from 10 to 16 eV, revealing new fine structure in the first ionic band.