Electron-impact excitation of the low-lying electronic states of formaldehyde

Abstract

Electron-impact excitation has been observed at incident electron energies of 10.1 and 20.1 eV to the first five excited electronic states of formaldehyde lying at and below the 1B2 state at 7.10 eV. These excitations include two new transitions in the energy-loss range 5.6–6.2 eV and 6.7–7.0 eV which have been detected for the first time, either through electron-impact excitation or photon absorption. The differential cross sections of these new excitations relative to that of the optically-allowed 1B2 ← X̃ 1A1 transition are given at scattering angles between 15° and 135°. These cross-section ratios peak at large scattering angles—a characteristic of triplet ← singlet excitations. From a comparison of the observed and calculated vertical transition energies, the transitions are assigned as 3A1 ← X̃ (5.6–6.2 eV) and 3B2 ← X̃ (6.7–7.0 eV). The design and performance of the electron-impact spectrometer used in the above observations is outlined and discussed.