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. Mic helson at the end of the last century. The method was restricted to el ectromagnetic waves until an optical component to shift the interferin g wave packets longitudinally was invented [G. Mollensted and G. Wohla nd, Electron Microscopy 1980, eds. P. Bredoro and G. Boom, 7th Europ. Congr. on Electron Microscopy Foundation, Leiden, Vol. 1, p. 28] and r efined [H. Gauch, Diploma thesis, University Tubingen (1983); I. Daber kow et al., Joint Meeting on Electron Microscopy, Antwerp, Belgium, 19 83, Program and Abstract Book, p. 100] to such a degree of perfection that the longitudinal shift could be controlled to a fraction of a per cent of the wavelength. The Fourier spectrometer for charged matter wa ves is made up by a wave front splitting electron optical biprism in c ombination with crossed electric and magnetic fields (Wien-filter) as a wave-packet-shifting device. The contrast of the interference fringe s as a function of the longitudinal shift is recorded by a CCD-line-ca mera, transferred to a PC and Fourier analyzed subsequently. As an exa mple 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 contr ast exceeding 10% were recorded. The measured full width at half maxim um was 0.6 eV +/- 80 meV. The total error of 80 meV contains 40 meV er ror due to insufficient sampling of data and 40 meV since about 2000 l ow 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 spectro scopy is discussed extensively.