Optical studies of liquid formamide in the vacuum ultraviolet

Optical and dielectric functions of liquid formamide have been measured between 2 and 24.5 eV. Below 7.8 eV a semicylinder technique was used, whereas above 10.4 eV a new ionization method was employed. Peaks in the absorption spectrum at approximately 6.8 and 16.0 eV are attributed to excitation of π and sigma electrons in the formamide molecule. The latter electrons participate in a collective oscillation as evidenced by a shift in the peak in the energy loss function from the corresponding peak in the absorption spectrum. Sum rule calculations give 11.8 and 4.3 of the eighteen formamide valence electrons participating in optical absorption and energy loss processes, respectively, out to 24.5 eV