A new experimental method is described, applicable to beam-foil spectroscopy or, more generally, to the spectroscopy of light emitted from excited ions moving with uniform velocity in a narrow beam. In the new method the ordinary entrance slit of the spectrometer is replaced by an arrangement comprised of a special coupling lens and an intermediate-image aperture. It is shown that the interplay between the parameters describing the point of emission (θ1), the angle of emission (θ2), and the Doppler shift of the light emitted from the beam can be made such that simultaneous first-order focusing in θ1 and θ2 is obtained at the exit slit. For a discrete atomic transition this implies an image brightness much greater than that of the object (i.e., the beam), and a considerable advantage in detector counting rate without loss in spectrometer resolution. Although the employed portion of the beam is not infinitesimal in length in the new method, the spatial resolution along the beam can be maintained to the degree generally required in beam-foil applications. In a practical test of the new method, employing He ions of 225 keV energy, an intensity gain by a factor of 70 has been observed on the 4686 Å line in He ii, with a spatial resolution width along the beam of 0.2–0.3 mm. The resolution in wavelength in the test recording was 2.8–3.0 Å. The relative merit of the new method improves with the resolution in wavelength required, but will ultimately be limited by beam divergence, spread in beam energy, and by imaging properties higher than the first order in the coupling lens and the spectrometer.
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