Electron-impact ionization of H2O at low projectile energy: Internormalized triple-differential cross sections in three-dimensional kinematics

We report a combined experimental and theoretical study of the electron-impact ionization of water (H2O) at the relatively low incident energy of E-0 = 81 eV in which either the 1b(1) or 3a(1) orbitals are ionized leading to the stable H2O(+) cation. The experimental data were measured by using a reaction microscope, which can cover nearly the entire 4 pi solid angle for the secondary electron emission over a range of ejection energies. We present experimental data for the scattering angles of 6(circle) and 10(circle) for the faster of the two outgoing electrons as a function of the detection angle of the secondary electron with energies of 5 and 10 eV. The experimental triple-differential cross sections are internormalized across the measured scattering angles and ejected energies. The experimental data are compared with predictions from two molecular three-body distorted-wave approaches: one applying the orientation- averaged molecular orbital (OAMO) approximation and one using a proper average (PA) over orientation- dependent cross sections. The PA calculations are in better agreement with the experimental data than the OAMO calculations for both the angular dependence and the relative magnitude of the observed cross-section structures.