Investigation of the complete valence shell of formic acid by electron momentum spectroscopy and Green's function methods

Abstract

The complete valence shell binding energy spectrum (8–50 eV) of formic acid has been measured by electron momentum spectroscopy at an incident energy of 1500 eV plus binding energy. Many body calculations of binding energies and pole strengths using Green's function methods and an SCF basis set have been carried out, and are compared with the data. Good agreement is found in the outer valence region between the measured and predicted pole strengths and binding energies and between the measured and the predicted spherically averaged SCF orbital momentum distributions. An even better fit to the latter is provided by SCF wavefunctions calculated from a larger basis set which is, however, too large to be employed in the many body calculation. In the inner valence region there are large discrepancies between the calculated and observed distribution of ionization strengths of the 5 a′ and 4 a′ manifolds, indicating that the Green's function calculation in the ADC (3) approximation underestimates the extent to which these manifolds are split through correlation effects. Computed position and momentum density maps for each orbital of oriented formic acid molecules are discussed in relation to the measured and calculated spherically averaged momentum distribution.