D.R. Frankl

Selective adsorption of 3He and 4He on the basal plane surface of graphite

Abstract Atomic beam scattering was used to measure the binding energies of selectively adsorbed states of two isotopes of helium on the basal plane of graphite. The results are used to determine semi-empirical potential parameters, and the ground state energies are compared with values derived from adsorption studies for both isotopes.

Atomic and molecular beam scattering from crystal surfaces in the quantum regime

Abstract Atomic beam scattering from surfaces is rapidly evolving as a valuable technique for exploring atom-surface interactions, manifested in both elastic and inelastic processes. We review this field with emphasis on the regime of elastic scattering of low mass particles, where quantum effects are apparent. The discussion focuses on recent developments including both experimental and theoretical methods and results. Information about the potential that can be deduced from selective adsorption observations is described.

Theory of the small-signal photovoltage at semiconductor surfaces

Abstract An explicit expression is derived for the surface photovoltage under small-signal conditions, when the quasi-equilibrium approximation will be valid in the space-charge layer. The theory reduces to that of Garrett and Brattain in the absence of surface states, but differs in the presence of such states through the use of more accurate trapping statistics. Computed results give a qualitative explanation of some recent data, and also predict several possibly observable new effects.

Selective adsorption of 3He and 4He on clean surfaces of NaF and LiF

Abstract Atomic beam scattering was used to study the gas-surface interaction between helium and ionic crystals. In particular, the binding energies of the selectively adsorbed states were measures for two isotopes of helium on the (001) surfaces of NaF and LiF cleaved in vacuum. Consistent results were obtained by avoiding incident conditions where band structure effects invalidate the free particle approximation. The energy levels can be fit to a 9-3 model potential.

Selective adsorption of 4He on clean LiF (001) surfaces

Abstract The selective adsorption of 4 He on in-situ cleaved LiF surfaces has been studied under improved resolution. The main results are as follows: (1) There are four bound states in the surface potential well, at energies of −5.8, −2.2, −0.6 and −0.1 meV. The lowest three levels were reported previously. (2) Most of the structure previously designated as “fine structure” is due either to transitions to these four levels via various small reciprocal lattice vectors or to the opening of diffraction channels. (3) The transitions involving the [01] and [01] reciprocal lattice vectors (i.e., the ones nearly perpendicular to the incident wave vector) are strong; as much as 85% of the specular intensity may be removed. Transitions via the other small reciprocal lattice vectors are much weaker. (4) The widths of the lines are consistent with the velocity distribution, which has a half-width of about 2%. (5) The observed energies agree fairly well with those calculated by Tsuchida for a zeta-function potential, but are not consistent with a Morse potential.

Conditions for quasi-equilibrium in a semiconductor surface space-charge layer

Abstract The transport equations in a surface space-charge layer are solved approximately. The results show that the quasi-equilibrium approximation is generally invalid for the bulk minority carriers in an accumulation layer or in an inversion layer under extraction, and for the bulk majority carriers in an inversion layer under injection. The possibility of complete depletion of carriers of one or the other type at the surface imposes a finite upper limit on the effective surface recombination velocity.

Atomic beam scattering from single crystal surfaces

Abstract Application of atom-scattering to a variety of surface problems is expanding rapidly, owing in large part to the extreme surface- sensitivity of this probe. Helium is particularly useful because of its low mass and chemical inertness. Beams with velocity spreads of less than one percent and wavelength of the order of one Angstrom can be formed by nozzle expansion. The scattered flux from a clean, well-ordered crystal surface contains elastic and inelastic, coherent and incoherent, components. The coherent elastic component (i.e., the specular and diffracted beams) contains information about the crystallographic structure of the outer- most atomic layer of the crystal and about the interaction potential between the crystal and the scattered particle. The latter manifests itself in the form of resonances between the incoming free-particle state, and the two-dimensional Bloch states bound in the potential well at the surface. Elastic scattering theory has reached the point where the resonance signatures in the various diffracted beams can be predicted accurately. Crystallographic information resides in the diffracted beam intensities. Theoretical interpretation is less well advanced, though some progress has been made with “hard-wall” models. Experimental studies of reconstructed surfaces and chemisorbed overlayers appear very promising. In inelastic scattering, energy resolution has been achieved by both time-of-flight and diffraction methods. High-resolution studies on alkali halide surfaces have led to experimental determination of Rayleighwave dispersion relations over the full Brillouin zone. Preliminary results have also been obtained on some metals.

Study of level crossings in the selective adsorption of 4He on graphite (0001)

Abstract Strongly interacting selective adsorption resonances were examined for 4 He/graphite at both 17 and 4.7 meV incident energies. Weak second-order resonances were seen by means of their admixture with stronger resonances. Matrix elements were measured at 4.7 meV and found to agree with those determined by Boato et al., at higher incident energies.

Debye-Waller effects in atom-surface scattering☆

The temperature sensitivity of helium atom reflection from NaF ranges from slight in the quantum regime (long wavelength, near-glancing incidence) to considerable in the classical regime (short wavelength, near-normal incidence). The latter is describable by the Debye-Waller theory only over a limited range of angles. In this range, no Beeby well-depth correction is required. The surface Debye temperature of the NaF is found to be 425 ± 20 K.

Selective adsorption of 4He on the NaCl(001) surface

The diffraction of 4He nozzle beams at several energies from cleaved (001) surfaces of NaCl has been studied. Resonance features are identified and give binding energies of 4.1 ± 0.1, 1.5 ± 0.1 and 0.3 ± 0.15 meV. One feature suggests an energy of 5 meV, but this is unsubstantiated. The resonance signatures are in disagreement with elastic scattering theories.