Medium electron energy (e,2e) spectroscopy in grazing incidencegeometry is able to reconstruct the band structure (bindingenergy vs electron momentum q) [1] and the electronmomentum density .(q) [2] of the valence electronic states ofsurfaces. (e,2e) experiments reveal in time-coincidence, withdefined energy and angle, the two final electrons generated bythe impact of a primary electron on the surface. By the newlyimplemented spectrometer for surface (e,2e) spectroscopy newexperiments were performed on HOPG in fully-multichanneldetection mode. A model available in literature [3,4] for themeasured cross-section in reflection mode has been specializedfor the specific asymmetric kinematics used in our experiment(10-350eV). This model describes the (e,2e) event taking intoaccount those processes in which the projectile electron undergoesa single scattering from the crystal potential before orafter the interaction with the bound electron: the cross-sectionresults to be factorized in two components, a kinematics factortimes the momentum density of bounded electron states. Tocheck the validity of the model an (e,2e) experiment has beenperformed choosing a condition where the electron momentumdensity was constant. The calculated kinematics factor reproducesthe angular distribution of the fast scattered electronmeasured while keeping fixed the directions of primary andslow electrons and all three electron energies. The enhancedluminosity of the apparatus by energy-multichannel detectionallowed us to map the valence band structure of . states reducingthe acquisition time by a factor of 8 with respect tosingle-channel detection mode without degradation of the energyand the momentum resolutions (1 eV and 0.2 °A €1). Takinginto account the inelastic multiple scattering contributionthe measured intensities correspond to the electron momentumdensities projected into the direction parallel to the surface ina momentum range from the first Brillouin zone to the secondone. The observed minimum near the zone boundary can bean indication that real momentum of . states were measured[5]. We have measured a value of Fermi momentum (1.7 °A €1)which is in agreement with band calculations. The observedmaximum of electron momentum density is in agreement withthe . states, additionally we observe minor structures correspondingto .2 states. The observation of these electron statesinvolves processes assisted by reciprocal lattice vectors, hencefor these states the reconstructed momentum should be thecrystal momentum.
Liscio, A., Iacobucci, S., Ruocco, A., Stefani, G. (2004). Valence band mapping by reflection (e,2e) spectroscopy in grazing angle mode: the HOPG case.
Valence band mapping by reflection (e,2e) spectroscopy in grazing angle mode: the HOPG case
LISCIO, ANDREA;RUOCCO, Alessandro;STEFANI, Giovanni
2004-01-01
Abstract
Medium electron energy (e,2e) spectroscopy in grazing incidencegeometry is able to reconstruct the band structure (bindingenergy vs electron momentum q) [1] and the electronmomentum density .(q) [2] of the valence electronic states ofsurfaces. (e,2e) experiments reveal in time-coincidence, withdefined energy and angle, the two final electrons generated bythe impact of a primary electron on the surface. By the newlyimplemented spectrometer for surface (e,2e) spectroscopy newexperiments were performed on HOPG in fully-multichanneldetection mode. A model available in literature [3,4] for themeasured cross-section in reflection mode has been specializedfor the specific asymmetric kinematics used in our experiment(10-350eV). This model describes the (e,2e) event taking intoaccount those processes in which the projectile electron undergoesa single scattering from the crystal potential before orafter the interaction with the bound electron: the cross-sectionresults to be factorized in two components, a kinematics factortimes the momentum density of bounded electron states. Tocheck the validity of the model an (e,2e) experiment has beenperformed choosing a condition where the electron momentumdensity was constant. The calculated kinematics factor reproducesthe angular distribution of the fast scattered electronmeasured while keeping fixed the directions of primary andslow electrons and all three electron energies. The enhancedluminosity of the apparatus by energy-multichannel detectionallowed us to map the valence band structure of . states reducingthe acquisition time by a factor of 8 with respect tosingle-channel detection mode without degradation of the energyand the momentum resolutions (1 eV and 0.2 °A €1). Takinginto account the inelastic multiple scattering contributionthe measured intensities correspond to the electron momentumdensities projected into the direction parallel to the surface ina momentum range from the first Brillouin zone to the secondone. The observed minimum near the zone boundary can bean indication that real momentum of . states were measured[5]. We have measured a value of Fermi momentum (1.7 °A €1)which is in agreement with band calculations. The observedmaximum of electron momentum density is in agreement withthe . states, additionally we observe minor structures correspondingto .2 states. The observation of these electron statesinvolves processes assisted by reciprocal lattice vectors, hencefor these states the reconstructed momentum should be thecrystal momentum.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
