We report results of a fully ab initio treatment of electronic excitation in a polyatomic molecule by electron impact. We have calculated total and differential cross sections for exciting the two lowest (n→${\pi }^{\mathrm{*}}$${)}^1$,3${\mathrm{A}}_2$ excited states of ${\mathrm{H}}_2$CO for collision energies between 5 and 20 eV. The calculations were carried out in a three-state close-coupling approximation using the complex Kohn variational method. The absence of any backward peaking in the cross sections calculated for the ${}_{}{}^3{}_{}{}^{}\mathrm{A}_2^{}{}_{}{}^{}$ state is reminiscent of a rigorous selection rule for ${\Sigma }^{+}$-${\Sigma }^{\mathrm{-}}$ transitions in linear molecules. However, with no experimental studies available for comparison, the present results must be regarded as predictive.

• Received 6 November 1989

DOI:https://doi.org/10.1103/PhysRevA.41.2462