INTERFEROMETRIC MEASUREMENT OF CHARGED-PARTICLE SPECTRA (FOURIER-SPECTROSCOPY)

Fourier spectroscopy, the most fundamental and, at least in principle, the most precise spectroscopy method, has been introduced by A.A. Michelson at the end of the last century. The method was restricted to electromagnetic waves until an optical component to shift the interfering wave packets longitudinally was invented [G. Mollensted and G. Wohland, Electron Microscopy 1980, eds. P. Bredoro and G. Boom, 7th Europ. Congr. on Electron Microscopy Foundation, Leiden, Vol. 1, p. 28] and refined [H. Gauch, Diploma thesis, University Tubingen (1983); I. Daberkow et al., Joint Meeting on Electron Microscopy, Antwerp, Belgium, 1983, Program and Abstract Book, p. 100] to such a degree of perfection that the longitudinal shift could be controlled to a fraction of a percent of the wavelength. The Fourier spectrometer for charged matter waves is made up by a wave front splitting electron optical biprism in combination with crossed electric and magnetic fields (Wien-filter) as a wave-packet-shifting device. The contrast of the interference fringes as a function of the longitudinal shift is recorded by a CCD-line-camera, transferred to a PC and Fourier analyzed subsequently. As an example the spectrum of a field emission electron source was analyzed. At an energy of the electrons of 1 keV about 12 000 fringes with a contrast exceeding 10% were recorded. The measured full width at half maximum was 0.6 eV +/- 80 meV. The total error of 80 meV contains 40 meV error due to insufficient sampling of data and 40 meV since about 2000 low contrast fringes were neglected. The action of crossed electric and magnetic fields on electron wave packets including the major problems arising from their weak focusing action for the new method of spectroscopy is discussed extensively.