Interband contribution to transition radiation from metals

2014 Emission spectra from aluminium and copper targets excited by fast electrons have been measured. The ratio of these two measurements can be directly compared with theoretical transition radiation calculations. A well pronounced shoulder is observed at 2.1 eV giving the first experimental evidence of interband contribution to transition radiation. J. Physique LETTRES 43 (1982) L-733 L-737 Classification Physics Abstracts 78.90 1 er NOVEMBRE 1982, -1 Light emission from metal targets excited by fast electrons is produced via several physical processes erg transition radiation, Bremsstrahlung, and radiative decay of surface plasmong induced by incident electrons. Transition radiation is emitted when charged particles pass through a dielectric discontinuity with constant velocity. The light is fully polarized in the incident electron plane. The spectrum is continuous with a significant dip near the volume plasma frequency (wp) of the metal, and there is a low energy background varying roughly as the inverse square of the wavelength. This radiation has been theoretically investigated by several authors [1] using Maxwell’s equations, assuming the metal can be described by a local dielectric constant. Bremsstrahlung is produced by electron scattering in the metal. Its intensity is stronger the higher the atomic number of the target, and has an energy dependence ~ ~’ ~ [2]. The spectrum and polarization depend on the microscopic model used to describe the interaction between the incident electron and the target. Radiative decay of electron excited surface plasmons is only allowed if the surface is rough, the radiation being nearly unpolarized [3]. In tnis letter, we give experimental evidence of an interband contribution to the transition radiation spectrum of copper. Two experimental difficulties are generally encountered : a 1) The radiation detected in the incident electron plane is not fully polarized and the identify cation of the component polarized perpendicular to this plane (5 polarization is necessary for interpretation of the measured intensity. (*) ERA C.N.R.S. 070373. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:019820043021073300 L-734 JOURNAL DE PHYSIQUE LETTRES Boersch and Sauerbrey [4] have shown that it is possible to separate the transition radiation and Bremsstrahlung contributions to the light emitted by electron excited bulk silver targets using the different dependences of these two components on the electron energy E. Light observed was separated into one component proportional to E (identified as transition radiation) and another component proportional to E -1 (identified as Bremsstrahlung). With the assumption that the emission components which are not transition radiation are unpolarized, several authors [5] have compared the measured intensity difference (Is Ip) with transition radiation calculations. In this work, we report that light emitted by aluminium and copper targets excited by fast electrons, is quite fully p-polarized (P ~ 80 %). This means that the p-component of this emission can be directly compared to theoretical transition radiation calculations. 2) To study a wide energy spectrum, it is necessary to have knowledge of the efficiency of the detection system. For example, figure 2 shows that the measured copper experimental spectrum is very different from a calculated transition radiation spectrum. In particular, in the experimental spectrum, the small interband contribution structure near 2.1 eV is smoothed out. In the following, we show that by measuring instantaneously the light intensity emitted by alternatively excited copper and aluminium targets, we were able to eliminate the detection system efficiency. This is achieved by comparing experimental and theoretical intensity ratios. All the drifts of the experimental parameters occurring during the time of spectrum measurements are also eliminated. Aluminium has been chosen as reference metal because in this energy range its transition radiation spectrum is smooth and Bremsstrahlung intensity is very weak for this low atomic number metal. A 5 000 A thick copper film was first evaporated in an ultra vacuum chamber on an optical polished quartz window. The effective part of the film was an 8 mm diameter disc half the lower part of which was then covered by an aluminium film about 1 000 A thick. These two materials can be considered as semi-infinite media for a study of transition radiation. The chemical cleanness was controlled by Auger spectroscopy. Electrons have a bent trajectory to prevent sample pollution by filament. The energy was 45 keV. The electron spot was scanned from the copper to the aluminium target at 100 Hz frequency. Electronic gates allowed photon counting only while spot position was either high (copper) or low (aluminium). The validity of our method was controlled on an homogeneous sample (copper or aluminium) and the counting rate obtained was then the same when the spot was either high or low. The mean electron beam current was I = 1 ~A. The radiation intensity was measured with a cooled photomultiplier through a grating monochromator, with a dark count of about 5 cp/s. On the experimental set up shown in figure 1, one can see that the angle of electron incidence and of observation direction are simultaneously changed by a sample rotation. Figure 3 shows the experimental ratio Ip Cu/Ip Al as a function of spectral energy for three difference observation angles (0 = 32.5~ ; 47.5~ ; 57.5~) along with the corresponding theoretical