High Resolution Measurements of Atomic and Molecular Lifetimes Using the High Frequency Deflection Technique

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Abstract
The basic principles of the High Frequency Deflection (HFD) technique for the determination of atomic and molecular lifetimes as well as the first experimental tests were presented a couple of years ago. In principle this technique is a further refinement of the conventional multichannel delayed coincidence technique with periodic electron excitation of free molecules. The differences are mainly that the HFD technique utilizes a continuous electron beam from a high-power gun operating in the kilovolt range and that the periodic excitation is accomplished by sweeping the beam at an optimal rate, i.e. with a period equal to 3-10 times the lifetime to be studied. With these two refinements the HFD technique enables lifetime measurements in all kinds of atoms, molecules, ions and ion-molecules with several orders of magnitude higher efficiency than other methods permit, thus opening possibilities for high resolution work in the absence of Doppler broadening. Thus a number of lifetimes have been determined at a spectral resolution of 0.1 Å FWHM or better, which is 20-50 times higher a resolution than what is normally achieved in lifetime measurements. The variable sweep frequency and the multichannel registration make possible measurements over a large lifetime range (up to about 104 ns) which in turn offers unique possibilities for analysing multi-exponential decay curves. With a time scale of arbitrary precision given by the sweep frequency, lifetime measurements have so far been performed with a total error down to 0.5% in favourable cases. The HFD technique is now currently in use at this laboratory and several hundreds of lifetimes have been studied and reported in a number of publications. In this paper we shall discuss more in detail the technique itself, its properties and possibilities compared to other methods for lifetime measurements. A review of some of the experimental results obtained so far, with particular emphasis on abundance determinations of atoms and molecules of fundamental astrophysical interest, is also included as well as a critical discussion of given error limits in lifetime measurements in general.