G. Chiaia

Comparative study of the preparation of negative electron affinity GaAs photocathodes with O2 and with NF3

We discuss the possibility of replacing oxygen with nitrogen trifluoride in the preparation of stable GaAs negative electron affinity (NEA) photocathodes. The different role of O2 versus NF3 exposure during the activation is examined by means of quantum yield and photocurrent lifetime measurements. Quantum yield results clearly demonstrate that NEA conditions can be attained also with NF3. Measurements performed on the same samples and in the same experimental conditions, show a remarkably different time evolution of the activation process in the two cases. However, as a result of a huge number of activations, no evidence of significantly larger lifetimes for NF3 prepared photocathodes can be deduced.

Spin‐polarized electron gun for electron spectroscopies

A spin‐polarized electron gun for electron spectroscopies is described in detail. The gun consists of an electron source based on a negative electron affinity GaAs photocathode coupled to an appropriate transport electron optics. The gun has been designed with the aid of ray‐tracing analysis and then accurately tested. It produces a transversely polarized (P0∼27%) electron beam at variable energy with a small spot size and angular spread (less than 2 mm and 5°, respectively). Such performances are attained up to sample currents as high as 10 μA for the whole beam energy range (8–50 eV). As an application we present data on spin‐dependent absorbed current spectroscopy from bcc Fe films epitaxially grown on Ag(100).

Spin resolved empty energy bands in Fe/Ag(1 0 0) films

Abstract We present a room temperature study of the empty electronic states in thin Fe films expitaxially grown on Ag(1 0 0). The spin resolved band structure of the magnetic film is determined vs film thickness in the range from 1 to 100 bcc Fe monolayers by means of spin- and angle-resolved Inverse Photoemission Spectroscopy. Measurements on films thicker than 5 monolayers point out the presence of two peaks in the spectra exhibiting opposite polarization and related to majority and minority empty bands split-off by the exchange interaction. No inplane spin polarization is observed in thinner films.

Calibrating the spin polarization in electron beams from NEA GaAs photocathodes by means of electron spectroscopy

Abstract The interaction of polarized electrons with magnetic targets gives rise to large and well measurable effects in electron spectroscopy. Such effects can be used to test and calibrate the spin polarization of an electron beam impinging on a reference sample. We present data on the spin polarization of electron beams produced by negative electron affinity (NEA) GaAs photocathodes measured by means of spin dependent absorbed current and inverse photoemission spectroscopies on Fe films epitaxially grown on the Ag(100) surface.

Cs and NF3 coadsorption on GaAs(100) and GaAs0.62P0.38(100) for photocathodes preparation

Abstract Negative electron affinity (NEA) photocathodes have been prepared by Cs and NF 3 coadsorption on heat-cleaned GaAs(100) and GaAs 0.62 P 0.38 (100) surfaces. The use of NF 3 as oxidizing gas leads to a simple and reliable activation procedure. The NEA photocathodes are studied by means of quantum yield and photocurrent lifetime measurements. Studies performed at low temperature (120 K) suggest that Cs release from the activated surface is the main reason of photocathode degradation.

Photon energy dependence of inverse photoemission from LaPd3 in the ultraviolet range

Abstract We present a set of angle integrated inverse photoemission data on LaPd3 taken in the u.v. region (10–25 eV) in the isochromat mode. The photon energy dependence of the spectra gives direct insight on the assignments of the spectral features. A strong solid state effect is found in transitions having as final states the La 4f and 5d-derived states; in particular the cross sections for the two channels become much more similar than in the atomic case. This effect is qualitatively discussed in terms of the modifications of the La 5d-states upon the formation of the solid.

Spin-resolved inverse photoemission from Fe films epitaxially grown on Ag(100)

Abstract Very thin Fe films epitaxially grown on Ag(100) have been studied by means of spin-resolved inverse photoemission spectroscopy. Results for film thicknesses in the range 1–100 bcc Fe monolayers are reported. It is shown that the exchange splitting δEex at the H point of the bcc Fe Brillouin zone reaches the bulk value of 1.8 eV for film thicknesses larger than 15 ML; for thinner films, the exchange splitting is reduced. Moreover, it is found that films thinner than 5 ML do not show any in-plane remanent magnetization.