Empirically adjusted ab initio calculations for the valence and rydberg excited states of formic acid

Abstract

We report ab initio calculations on the valence and Rydberg excited states of formic acid which we find below 11.0 eV. Energies and wave functions have been obtained from a frozen core single excitation configuration mixing. Single excitations from the four highest MOs have been used and their energies shifted to match the computed ionization potentials with the experimental values. Our results support the assignment by Bell et al.of the three Rydberg series of formic acid as n 0 - ns, n p and n d, and suggest that 1 (n 0 -3s) is below 1 (Π 2 -Π * ). We find 1 (Π 2 -3s) near the expected position, but its intensity should be very low. We do not find low lying valence n 0 -σ * states, and suggest an alternative explanation for the broad band under C assigned by Robin as 1 (n 0 -σ * ). In agreement with a recent suggestion, we assign the bands at 9.91 and 11.08 eV as Π 2 -3pa″ and 4pa″. Our triplet state excitation energies stand as predictions for future electron impact experiments.