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The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.

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Coincident measurements of projectile energy loss and kinetic electron emission yield for grazing scattering of 150 eV/amu to some keV/amu neutral hydrogen and helium atoms from an atomically clean and flat Al͑111͒ surface allow us to correlate electron emission and inelastic interaction mechanisms at a metal surface. Our data show evidence for a threshold behavior of kinetic electron emission which is interpreted by energy transfer in binary encounters of projectiles in the electron selvage of a quasi-free electron gas. Contributions of electron emission to projectile energy loss are found to be negligibly small.Ionization of atoms, molecules, and complex matter by impact of particles ͑electrons, atoms, and ions͒ is of fundamental interest and relevant for many practical applications. Since first treatments of electron-impact ionization of atoms by Thomson, 1 numerous authors have considered the ionization process in a basically classical way, e.g., Thomas, 2 Gryzinsky, 3 or Kingston. 4 This led to semiempirical ionization formulas used in atomic and plasma physics. 5,6 For ionization of solids, one distinguishes between two mechanisms ͑1͒ kinetic electron emission ͑KE͒ mediated by the kinetic energy of the projectile and ͑2͒ potential electron emission ͑PE͒ induced by the internal energy of excited or ionized projectiles. 7,8 In reference to gas-phase collisions mentioned above, it is tempting to ask to what extent the KE process may be described by classical concepts, apart from the KE threshold that corresponds to the minimum-energy transfer of projectiles to electrons in a solid to reach vacuum. Such a classical treatment is probably most appropriate for metals that can be described as a free-electron system ͑jellium͒. 9 Here the threshold of projectile velocity v th for KE is derived by assuming maximum-energy transfer of atomic projectiles to free electrons of the metal with Fermi energy E F ͑velocity v F ) in order to overcome the surfaceExperimental studies on the threshold behavior of KE for impact of light ions were not conclusive with respect to v th so far. 8 Aside from uncertainties inherent in the separation of contributions from PE to electron yields by using ionized projectiles, KE may be caused by several other mechanisms, 8,10 in particular, for heavier atomic projectiles by electron promotion in close collisions with target atom cores. 11 A specific technical problem for a reliable determination of v th concerns small electron yields as low as ␥ р10 Ϫ3 electrons/projectile which are extremely difficult to obtain from measurements of ion and electron currents. 12 Furthermore, KE induced by impact of atomic projectiles is accompanied by ͑electronic͒ excitation of the target which cannot be elucidated by KE measurements only.In this paper, we report on studies on the threshold behavior of KE for grazing incidence scattering of fast neutral hydrogen and helium atoms from an Al͑111͒ surface. For this specific collision geometry, scattering of projectiles proceeds in the regime of planar surface cha...

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