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Papers by Renato Spadacini

Chemical Physics, 1998

The Kramers problem in non-separable periodic potentials is studied solving the 2D Fokker-Planck ... more The Kramers problem in non-separable periodic potentials is studied solving the 2D Fokker-Planck equation (FPE), by the matrix-continued-fraction method, directly obtaining the dynamic structure factor Ss. Ss is numerically evaluated, in a wide friction and coupling range, for the egg-carton potential depending on two parameters g0 and g1 which give the amplitude of the decoupled and coupled part respectively. By

Chemical Physics Letters, 1998

The diffusion coefficient D in a periodic potential with exponentially decaying memory friction i... more The diffusion coefficient D in a periodic potential with exponentially decaying memory friction is studied by the matrix-continued-fraction method. It is shown that, at sufficiently high static friction, D presents a maximum as a function of the memory decay parameter. This maximum is due to the interplay between two oscillatory motions, the first induced by the periodic potential and the second by the memory. The maximum appears when the frequencies of these motions are in resonance. q

Journal of Physics C: Solid State Physics, 1973

The diffractive scattering of nearly monochromatic helium and neon beams from a (001) cleaved LiF... more The diffractive scattering of nearly monochromatic helium and neon beams from a (001) cleaved LiF surface at low temperatures was experimentally studied. A quantum theory of surface rainbow based on a corrugated hard surface model was developed. The calculated relative intensities of diffraction peaks agree with the measured values.

Physical Review E, 1996

... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. .... more ... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. ... results. As said in the Introduction, the DPA is not restricted a priori to a particular range of the friction or of the potential amplitudes. ...

Springer Series in Surface Sciences, 1988

NATO Science Series II: Mathematics, Physics and Chemistry, 2001

Surface Science, 1991

The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokke... more The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokker-Planck equation in the high friction limit (Wilemski equation) with a simple cosine potential. The quasi-elastic dynamic structure factor is expressed as a series of Lorentzian functions and its FWHM is calculated for an extended range of the surface momentum transfer hQ. Jump and free diffusion are recovered as limiting cases dependent on the temperature.

Surface Science Letters, 1991

The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokke... more The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokker-Planck equation in the high friction limit (Wilemski equation) with a simple cosine potential. The quasi-elastic dynamic structure factor is expressed as a series of Lorentzian functions and its FWHM is calculated for an extended range of the surface momentum transfer hQ. Jump and free diffusion are recovered as limiting cases dependent on the temperature.

Surface Science Letters, 1989

Physical Review E, 1996

... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. .... more ... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. ... results. As said in the Introduction, the DPA is not restricted a priori to a particular range of the friction or of the potential amplitudes. ...

Surface Science Letters, 1982

of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe ho... more of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment p=(0.18-C0.02)X10-30C m and polarizability a=(6.36*l.3)X10-40 J V2 m2. Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2X2), ~(2x2)

Surface Science Letters, 1981

A computational method is described, which, starting from given difraction intensities, approache... more A computational method is described, which, starting from given difraction intensities, approaches effectively the best-fit corrugation function ~'(R). Because of the approximations involved, the procedure works well for smooth corrugations with amplitudes not exceeding ~ 10% of the lattice constant. The method rests on two crucial observations: (i) With the full knowledge of the scattering amplitudes A G = IA G I exp(i~0G) (absolute values plus phases), the corrugation function can be calculated to a high degree of accuracy from ~'(R) = (2iki) -1 lnl -A G exp(iG .R) I which is derived easily from the hard corrugated wall scattering (HCWS) equation by approximating k G by -k i (k i and k G being the wavevectors of the incoming and diffracted beams, respectively). (ii) With only the IA G I's (or intensities) known, approximate solutions of the HCWS equation can be obtained with a rough estimate of the relative phases of only a few intense diffraction beams: the estimate is readily performed by investigating systematically a coarse mesh of phases. In this way, approximate corrugations are found with which a full set of phases can be generated, which allows the calculation of an improved ~'(R); this step is repeated in a loop, until optimum agreement between calculated and given intensities is obtained. The effectiveness of the procedure is demonstrated for three one-dimensional model corrugations described by several Fourier coefficients. The method is finally applied to the case of H2 diffraction from the quasi-one-dimensional adsorbate corrugation Ni(110) + H(1 x 2).

Physical Review E, 1993

The Kramers problem in periodic potentials is solved separating the intrawell and interwell dynam... more The Kramers problem in periodic potentials is solved separating the intrawell and interwell dynamics. Both the jump rate and the probability distribution of the jump lengths are obtained by a Fourier analysis of the decay function f(q); at high and intermediate potential barriers, in the first Brillouin zone, f(q) essentially coincides with the energy half-width of the quasielastic peak of

Surface Science, 1982

of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe ho... more of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment p=(0.18-C0.02)X10-30C m and polarizability a=(6.36*l.3)X10-40 J V2 m2. Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2X2), ~(2x2)

Surface Science, 1976

The quantum theory of atomic scattering from hard surfaces, previously developed, is extended to ... more The quantum theory of atomic scattering from hard surfaces, previously developed, is extended to cover molecular scattering, taking into account rotational transitions. Two methods (a perturbative and a nonperturbative approach) are considered: the former is applied to Hz scattering, the latter to HD scattering from LiF. A comparison with recent experimental data gives satisfactory results when a potential well of 440 K and a corrugation amplitude of 0.17 A are assumed for the Hz -LiF interaction.

Chemical Physics, 1998

The Kramers problem in non-separable periodic potentials is studied solving the 2D Fokker-Planck ... more The Kramers problem in non-separable periodic potentials is studied solving the 2D Fokker-Planck equation (FPE), by the matrix-continued-fraction method, directly obtaining the dynamic structure factor Ss. Ss is numerically evaluated, in a wide friction and coupling range, for the egg-carton potential depending on two parameters g0 and g1 which give the amplitude of the decoupled and coupled part respectively. By

Chemical Physics Letters, 1998

The diffusion coefficient D in a periodic potential with exponentially decaying memory friction i... more The diffusion coefficient D in a periodic potential with exponentially decaying memory friction is studied by the matrix-continued-fraction method. It is shown that, at sufficiently high static friction, D presents a maximum as a function of the memory decay parameter. This maximum is due to the interplay between two oscillatory motions, the first induced by the periodic potential and the second by the memory. The maximum appears when the frequencies of these motions are in resonance. q

Journal of Physics C: Solid State Physics, 1973

The diffractive scattering of nearly monochromatic helium and neon beams from a (001) cleaved LiF... more The diffractive scattering of nearly monochromatic helium and neon beams from a (001) cleaved LiF surface at low temperatures was experimentally studied. A quantum theory of surface rainbow based on a corrugated hard surface model was developed. The calculated relative intensities of diffraction peaks agree with the measured values.

Physical Review E, 1996

... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. .... more ... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. ... results. As said in the Introduction, the DPA is not restricted a priori to a particular range of the friction or of the potential amplitudes. ...

Springer Series in Surface Sciences, 1988

NATO Science Series II: Mathematics, Physics and Chemistry, 2001

Surface Science, 1991

The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokke... more The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokker-Planck equation in the high friction limit (Wilemski equation) with a simple cosine potential. The quasi-elastic dynamic structure factor is expressed as a series of Lorentzian functions and its FWHM is calculated for an extended range of the surface momentum transfer hQ. Jump and free diffusion are recovered as limiting cases dependent on the temperature.

Surface Science Letters, 1991

The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokke... more The diffusion of a classical particle in a 2D periodic field of force is studied, solving a Fokker-Planck equation in the high friction limit (Wilemski equation) with a simple cosine potential. The quasi-elastic dynamic structure factor is expressed as a series of Lorentzian functions and its FWHM is calculated for an extended range of the surface momentum transfer hQ. Jump and free diffusion are recovered as limiting cases dependent on the temperature.

Surface Science Letters, 1989

Physical Review E, 1996

... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. .... more ... holds. Symbols as in Fig. 7. 54 NOISE-ACTIVATED DIFFUSION IN THE EGG-CARTON . . . 4717 FIG. ... results. As said in the Introduction, the DPA is not restricted a priori to a particular range of the friction or of the potential amplitudes. ...

Surface Science Letters, 1982

of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe ho... more of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment p=(0.18-C0.02)X10-30C m and polarizability a=(6.36*l.3)X10-40 J V2 m2. Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2X2), ~(2x2)

Surface Science Letters, 1981

A computational method is described, which, starting from given difraction intensities, approache... more A computational method is described, which, starting from given difraction intensities, approaches effectively the best-fit corrugation function ~'(R). Because of the approximations involved, the procedure works well for smooth corrugations with amplitudes not exceeding ~ 10% of the lattice constant. The method rests on two crucial observations: (i) With the full knowledge of the scattering amplitudes A G = IA G I exp(i~0G) (absolute values plus phases), the corrugation function can be calculated to a high degree of accuracy from ~'(R) = (2iki) -1 lnl -A G exp(iG .R) I which is derived easily from the hard corrugated wall scattering (HCWS) equation by approximating k G by -k i (k i and k G being the wavevectors of the incoming and diffracted beams, respectively). (ii) With only the IA G I's (or intensities) known, approximate solutions of the HCWS equation can be obtained with a rough estimate of the relative phases of only a few intense diffraction beams: the estimate is readily performed by investigating systematically a coarse mesh of phases. In this way, approximate corrugations are found with which a full set of phases can be generated, which allows the calculation of an improved ~'(R); this step is repeated in a loop, until optimum agreement between calculated and given intensities is obtained. The effectiveness of the procedure is demonstrated for three one-dimensional model corrugations described by several Fourier coefficients. The method is finally applied to the case of H2 diffraction from the quasi-one-dimensional adsorbate corrugation Ni(110) + H(1 x 2).

Physical Review E, 1993

The Kramers problem in periodic potentials is solved separating the intrawell and interwell dynam... more The Kramers problem in periodic potentials is solved separating the intrawell and interwell dynamics. Both the jump rate and the probability distribution of the jump lengths are obtained by a Fourier analysis of the decay function f(q); at high and intermediate potential barriers, in the first Brillouin zone, f(q) essentially coincides with the energy half-width of the quasielastic peak of

Surface Science, 1982

of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe ho... more of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment p=(0.18-C0.02)X10-30C m and polarizability a=(6.36*l.3)X10-40 J V2 m2. Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2X2), ~(2x2)

Surface Science, 1976

The quantum theory of atomic scattering from hard surfaces, previously developed, is extended to ... more The quantum theory of atomic scattering from hard surfaces, previously developed, is extended to cover molecular scattering, taking into account rotational transitions. Two methods (a perturbative and a nonperturbative approach) are considered: the former is applied to Hz scattering, the latter to HD scattering from LiF. A comparison with recent experimental data gives satisfactory results when a potential well of 440 K and a corrugation amplitude of 0.17 A are assumed for the Hz -LiF interaction.