Extensions of Beam Foil Spectroscopy

Abstract

The key advantage of beam-foil spectroscopy over alternative methods of studying related atomic properties is its simple, straightforward application to the direct determination of lifetimes of excited states of atoms and ions through measurements of the decay in flight of such particles in beams of well-defined and easily measured velocity. The linear relation between velocity (~1 cm ns⁻¹) and time after excitation in a thin target (thickness ~ a few hundred Å, transit time ~10⁻¹⁴ s) allows good measurements of macroscopic radiation decay lengths ~1 cm in the laboratory and equivalent fractional accuracy in the determination of lifetimes ~1 ns of excited ions undergoing decay in flight. Through such lifetime measurements, related quantities like transition probabilities, or equivalently, oscillator strengths for dominant decay modes, can frequently be established to accuracies limited by various systematic effects (e.g., target aging effects) to errors of a few percent or less. Fig.7.1 displays a typical decay in flight curve for the ~0.54 ns 1¹P₀−2³P₁ inter-combination transition in heliumlike fluorine, measured in our laboratory by MOWAT et al. [7.1] a few years ago. This particular curve illustrates the principle of differential metastability, in that the fast decays of, for example, the unresolved resonance transition 1¹S₀−2¹P₁ had initially died out at the slight decay times after excitation corresponding to the leftmost data point shown. The longer-lived cascade feeding transitions which eventually dominate at points far downstream are sufficiently weak in the primary decay region of interest to permit an approximate background subtraction from the upstream portion of the curve. Establishment of the beam-velocity independence of the fitted lifetime (within experimental errors) for the upstream portion of the curve is an important test of the adequacy of this kind of background subtraction procedure. In this regard the measurement of metastable state decays, which arise through the violation of some selection rule whose magnitude is frequently the object of measurement, is often eased by the tendency of these decays to outlive all but very weak, long-lived cascade transitions which rarely repopulate the metastable state.