Excitation of the symmetric and antisymmetric stretch vibrations of H2O by electron impact

Electron energy loss spectra recorded at 135degrees at constant scattered electron energies of 0.05, 0.6 and 3.0 eV, and with 10 meV resolution, reveal that the antisymmetric stretch vibration (001) is excited much less than the symmetric stretch vibration (100). Spectra subtraction indicates that the (001) level is excited about 5x less than (100) at I eV. Band profiles recorded 0.6 eV above threshold show that the DeltaJ = 0 branch dominates the excitation of (010) and (100) vibrations but is absent for the (001) vibration. This indicates that resonant excitation is decisive for the (010) and (100) states but direct dipole mechanism for (001). The R-matrix adiabatic nuclei theory with closure approximation reproduces well the cross section for the (010) state, but dramatically underestimates the cross section for the symmetric stretch vibration and overestimates the cross section for the asymmetric stretch vibration.