Quantifying transition radiation by employing
CL
and
EELS
Authors: Stöger‐Pollach Michael 
Published: December 2016

The excitation probability per incoming electron for transition radiation (TR) is measured by employing electron energy losses spectrometry (EELS) and cathodoluminescence (CL) in a transmission electron microscope (TEM) using beam energies varying from 20 – 200 keV. We further demonstrate that TR is excited only at the sample surfaces, because the emitted intensity and the respective energy loss are independent of the sample thickness. As specimen we use an aluminum single crystal, because TR is known to be strongest on metals.

For the CL experiments we use a GATAN VULCAN system with upper and lower mirror. The recorded light spectra are corrected for the system response of the spectrometer and glass fiber cables. Then the spectra were integrated over the whole visible range (400‐900 nm) and finally the conversion coefficient of counts to emission probability was done for the 20 keV spectra as described in [1]. Once the conversion coefficient was found, this unique value (depending on the collection efficiency of the optical mirrors in the VULCAN system) was applied to all other spectra thus giving a beam energy dependent emission probability (Fig 1 right). The behavior is perfectly in agreement with the Ginzburg‐Frank theory [2]. The experiment was performed also with varying sample thicknesses in order to demonstrate that TR excitation is a surface effect only (Fig 1 left).

Using EELS we perform three different experiments: (i) an angle resolved measurement where we see the angular distribution of the transition radiation losses (TRLs) having an angular resolution of 7.5 µrad (see Fig 2), (ii) a thickness dependent experiment using 40 keV electrons showing the surface loss character of the TRLs (Fig 3 left), and (iii) an energy dependent experiment for determining the TR emission probability (Fig 3 right). The TR emission probability is the ratio between the TRL intensity and the total electron intensity.

For the angular resolved experiment we are raising the specimen out of the eucentric position by 278 µm and are choosing the largest possible magnification. Thus we achieve an effective camera length in the diffraction pattern of 288 m. The angular resolution of the electron beam is measured to be 7.5 µrad. Subsequently we move the beam 400 µrad across the spectrometer entrance aperture while recording a set of 200 single spectra, all of them 2 µrad apart (Fig 2 center).

For the experiments determining the emission probability per incoming electron we simply divide the integrated intensity of the TRLs after subtraction of the zero loss peak and the intraband transition intensities by the total spectral intensity. At the respective experimental conditions (collection semi‐angle of 0.6 mrad) only appr. 70% of the angular distribution of the EELS spectrum falls into the spectrometer entrance aperture, whereas all of the TRL intensity is collected. This is because the angular distribution is extremely narrow (see Fig 2). Consequently we have to correct for the not collected intensity, too. Finally we find that the integrated emission probabilities measured with CL and EELS are consistent to each other. The results are given in Fig 3 (right hand side).

As expected from theory and from the CL experiment, a thickness dependence cannot be observed in EELS, too.

Summing up it is the first time that within a TEM transition radiation of aluminum was studied by means of EELS and CL. We proved experimentally that the angular dispersion of the TRLs is within the light cone, that TR is emitted from the surfaces only, and we determined the photon emission probability with respect to the beam energy. Due to the fact, that the probability for the emission of TR is very low, even for aluminum, it is even smaller in the case of semiconductors, where low loss EELS is used for determining the local dielectric behavior.