Experimental investigation of the valence orbital momentum distributions and ionization energies of the noble gases by binary (e, 2e) spectroscopy

Abstract

A binary (e. 2e) spectrometer has been modified to obtain significantly improved momentum resolution ▪. Using this apparatus a systematic study of the ionization of all the valence orbitals of the noble gases: He, Ne, Ar, Kr and Xe has been carried out at an impact energy of 1200 eV plus the binding energy. Binding-energy spectra have been obtained at a series of azimuthal angles (corresponding to different electron momenta) for each target gas. In Ar, Kr and Xe the pole strength for ns ionization is severely split among several final ion states and the intensities are compared with earlier results from (e, 2e) spectroscopy. X-ray photoelectron spectroscopy and theoretical calculations. The ns and np atomic orbital (AO) electron-momentum distributions have been measured for the noble gases and the results, which give directly the squares of the AO momentum-spade wavefunctions, are compared with the square of Fourier transformed position-space wavefunctions of Hartree-Fock (HF), double-zeta (DZ) and single-zeta (SZ) quality. Excellent agreement is obtained between the experiment and Hartree-Fock calculations. Atomic orbital momentum density and position density contour maps have also been generated using Hartree-Fock wavefunctions. The maps have been used to provide a complementary discussion of position-and momentum-space properties of the noble-gas group of elements in the periodic table. Trends are discussed in the momentum distributions, density maps in position and momentum space and also the binding-energy spectra with reference to the electronic structure of the noble gases.