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

Abstract

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, R 1, V 1, R 2 and Q) are in agreement with expectation. Vibrational structure is assigned to the V 1 ( 1ππ ∗ ) band. It is proposed that the Q band (∼ 9.2 eV) arises from superposition of transitions to several Rydberg states, with second 1ππ ∗ state (V 2, once related to the Q band) lying at a higher energy; the V 2 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 3ππ ∗ state (the V 1 state triplet) because of spectral congestion. Dissociative electron attachment occurs in formamide upon impact with electrons of energy around 6.3 eV.