Study of resonances in formic acid by means of vibrational excitation by slow electrons

Absolute differential elastic and vibrational excitation cross sections have been measured for formic acid at 135° from threshold to 5 eV. Most vibrationally inelastic cross sections have a narrow peak at threshold, followed by a broadband with a boomerang structure due to the known π* shape resonance. The cross section for the excitation of the O–H stretch vibration behaves differently, it also peaks at threshold, but then drops only slowly, with narrow cusp structures, and only a very weak influence of the π* shape resonance. The cusp structures are even more pronounced in the cross section for the excitation of the O–H stretch overtone. The elastic cross section rises steeply at low energies. The π* shape resonance decays also by the ejection of very slow electrons, exciting a vibrational quasicontinuum at large energy losses.