VUV optical-absorption and electron-energy-loss spectroscopy of formamide

Absolute absorption cross-sections for formamide have been measured using a synchrotron radiation source (5-11.2 eV; 250-110 nm), along with electron-energy-loss (EEL) spectra at (i) high incident electron energies and low scattering angles and (ii) near-threshold incident energies and large scattering angles. In the optical- and high-energy EEL data, the excitation energies of the historical amide absorption bands (W, R₁, V₁, R₂ and Q) are in agreement with expectation. Vibrational structure is assigned to the V₁ (¹p p*) band. It is proposed that the Q band (~9.2 eV) arises from superposition of transitions to several Rydberg states, with the second ¹p p* state (V₂, once related to the Q band) lying at a higher energy; the V₂ state may be visible in the EEL data. A number of Rydberg series converging to the lowest ionisation potential are suggested and the possibility of using these Rydberg data to assign the two first (closely spaced) ionisation potentials is discussed. The near-threshold EEL data fail to resolve the low-lying ³p p* state (the V₁ state triplet) because of spectral congestion. Dissociative electron attachment occurs in formamide upon impact with electrons of energy around 6.3 eV. © 1997 Elsevier Science B.V.