Low-energy electron-impact excitation of the 3,1A2(n--> pi *) states of formaldehyde.

A three-state calculation of electron-impact excitation of formaldehyde to the a ' A 2 and A ' A 2 states is carried out using the Schwinger multichannel variational method. The integral and differential cross sections so obtained agree fairly well with theoretical results obtained using the complex Kohn method. Though agreement between the calculated integral cross section and the single available experimental measurement is qualitative, similar conclusions regarding the excitation mechanism are reached. A generalization of the selection rule for (X+~X ) electron-impact excitation of diatomic molecules is used to explain the shape of the differential cross sections for the a A z and A ' A 2 excitations.