- Email: [email protected]
Atomic vibrations at the (0001) graphite surface studied by He atom scattering
A72 phases exist for azulene overlayers as a function of coverage. Azulene overlayers with C / P t atom ratios in the range 1-1.3 exhibit a reversible transition with temperature. The orientational ordering of neighboring molecules appears to be governed by intermolecular forces rather than the molecule-Pt interaction in both azulene and naphthalene overlayers.
Surface Science 114 (1982) 471-484 North-Holland Publishing Company SPECULAR GRAPHITE: P. C A N T I N I
47 I
INELASTIC SELECTIVE ADSORPTION OF He ON A DIFFERENT W A Y TO GO IN A B O U N D S T A T E and R. TATAREK
Gruppo Nazionale di Struttura della Materia del CNR, and lstituto di Scienze Fisiche dell'Unit~er~itit, Viale Benedetto X V 5, 1-16132 Genova, Italy
Received 7 May 1981; accepted for publication I October 19gl Structures in the angular distribution of helium atoms inelastically scattered by the graphite (0001) surface were recently explained as inelastic selective adsorption processes. The resonant transition in a bound state of the gas-surface potential, reached after the exchange of one phonon, can occur also without the exchange of a reciprocal lattice vector. This "specular" inelastic selective adsorption is observed as large maxima in the inelastically scattered intensity. Detailed measurements of this process are reported here for the first time. They are taken both at different incident angles and at different surface temperatures T~. The experimental observations are compared with one-phonon inelastic calculations. Good agreement is found at low T,, while the disagreement obtained at higher temperatures seems to indicate the failure of the one-phonon approximation already at 7~>100K. Specular inelastic selective adsorption associated with out-of-plane phonons is suggested to be a possible reason for the sizeable drop of elastic intensity often observed near grazing incidence.
Surface Science 114 (1982) 485-497 North-Holland Publishing Company ATOMIC STUDIED
485
VIBRATIONS AT TIIE (0001) GRAPHITE BY He ATOM SCATrERING
G. BOATO, S. T E R R E N I
P. C A N T I N I ,
C. S A L V O
*, R . T A T A R E K
SURFACE and
Gruppo Nazionale di Struttura della Materia det CNR, and lstituto di Scienze Fisiche dell'Umtwr~tti~. Viale Benedetto X V 5, 1-16132 Genova, Italy
Received 7 May 1981 : accepted for publication 1 October 1981 He atom scattering has been used to study inelastic bound state resonances and the DebyeWaller factor for the basal plane of graphite. The angular location of the resonant structures allows a dispersion curve to be obtained for the phonons involved in the inelastic process; this dispersion curve is very near that pertaining to TA± transverse acoustic modes of bulk graphite. The attenuation of the specular peak in the surface temperature range from 20 to 300 K is used to derive the mean-square displacement { u ~ ) of surface atoms, perpendicular to the (0001) graphite plane. The temperature dependence of ~ u 2 ) cannot be explained by using a simple Debye model. It is shown that this anomalous behaviour is due to the layered structure and strong anisotropy of the crystal and can be explained in terms of the known low-frequency phonon spectrum of graphite.
Surface Science 114 (1982) 471-484 North-Holland Publishing Company SPECULAR GRAPHITE: P. C A N T I N I
47 I
INELASTIC SELECTIVE ADSORPTION OF He ON A DIFFERENT W A Y TO GO IN A B O U N D S T A T E and R. TATAREK
Gruppo Nazionale di Struttura della Materia del CNR, and lstituto di Scienze Fisiche dell'Unit~er~itit, Viale Benedetto X V 5, 1-16132 Genova, Italy
Received 7 May 1981; accepted for publication I October 19gl Structures in the angular distribution of helium atoms inelastically scattered by the graphite (0001) surface were recently explained as inelastic selective adsorption processes. The resonant transition in a bound state of the gas-surface potential, reached after the exchange of one phonon, can occur also without the exchange of a reciprocal lattice vector. This "specular" inelastic selective adsorption is observed as large maxima in the inelastically scattered intensity. Detailed measurements of this process are reported here for the first time. They are taken both at different incident angles and at different surface temperatures T~. The experimental observations are compared with one-phonon inelastic calculations. Good agreement is found at low T,, while the disagreement obtained at higher temperatures seems to indicate the failure of the one-phonon approximation already at 7~>100K. Specular inelastic selective adsorption associated with out-of-plane phonons is suggested to be a possible reason for the sizeable drop of elastic intensity often observed near grazing incidence.
Surface Science 114 (1982) 485-497 North-Holland Publishing Company ATOMIC STUDIED
485
VIBRATIONS AT TIIE (0001) GRAPHITE BY He ATOM SCATrERING
G. BOATO, S. T E R R E N I
P. C A N T I N I ,
C. S A L V O
*, R . T A T A R E K
SURFACE and
Gruppo Nazionale di Struttura della Materia det CNR, and lstituto di Scienze Fisiche dell'Umtwr~tti~. Viale Benedetto X V 5, 1-16132 Genova, Italy
Received 7 May 1981 : accepted for publication 1 October 1981 He atom scattering has been used to study inelastic bound state resonances and the DebyeWaller factor for the basal plane of graphite. The angular location of the resonant structures allows a dispersion curve to be obtained for the phonons involved in the inelastic process; this dispersion curve is very near that pertaining to TA± transverse acoustic modes of bulk graphite. The attenuation of the specular peak in the surface temperature range from 20 to 300 K is used to derive the mean-square displacement { u ~ ) of surface atoms, perpendicular to the (0001) graphite plane. The temperature dependence of ~ u 2 ) cannot be explained by using a simple Debye model. It is shown that this anomalous behaviour is due to the layered structure and strong anisotropy of the crystal and can be explained in terms of the known low-frequency phonon spectrum of graphite.