Theoretical study of the X-ray absorption spectra of small formic acid clusters

X-ray absorption spectra of small formic acid clusters, (HCOOH)n, n = 1–4, were examined theoretically within the framework of density functional theory. For monomer, assignment of the first peak around 532 eV was consistent with the experiment, whereas the second peak around 535 eV was assigned to a mixture of three bands, O 1s(CO) → σ*(OH), O 1s(OH) → π*(OH), and O 1s(OH) → σ*(OH) excitations. For the dimer, relative intensities of the oscillator strengths of O 1s(CO) and O 1s(OH) → σ*(OH) excitations decrease due to strong hydrogen bond formation, whereas those of O 1s(CO) and O 1s(OH) → π*(CO) excitations are insensitive to the dimerization.