ELECTRON-IMPACT EXCITATION OF THE NORMAL VIBRATIONAL-MODES OF NH3 IN THE INTERMEDIATE REGION (12-50 EV)

Electron-impact excitation of the normal vibrational modes of NH 3 in its ground electronic state has been studied by electron-energy-loss spectroscopy in the 12-50 eV impact energy range out of resonances. At 25 and 50 eV the differential cross sections (in arbitrary units) for the elastic scattering process and for the vibrationally inelastic v = 0→1 transitions have been measured at various scattering angles ranging from 8° to 60°. The ratio of the vibrational differential cross sections to the elastic one, reported as a function of the impact energy, indicates that for energies above 10 eV, the vibrational excitation occurs mainly through a direct mechanism. At 50 eV the angular shape of the cross sections is strongly dependent on the specific normal mode excited. The v2 differential cross section is forward peaked, whereas that of v4 is isotropic. The differential cross section of v1,3 composite has a minimum at 30°. At 25 eV the angular behavior of the differential cross sections is roughly similar for all the normal modes with a minimum at 30°, but the values (in arbitrary units) below and above 30° are quite different for each normal mode. © 1990 American Institute of Physics.