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

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.