Beam-foil spectroscopy decay

C.A. Nicolaides, D.R. Beck, On the possibility of observing nonexponential decay in autoionizing states. In Beam-Foil Spectroscopy, I. SelHn, D. Pegg, (Eds.), Plenum Press, New York, 1976, p. 77. 

Fig.6.5. Examples of decay curves obtained with beam-foil spectroscopy [6.32]. Curve a presents the decay of a Bel level unaffected by cascades. In (b) a level in CII is repopulated by a short-lived level, while in (c) a level in BII is repopulated by a longer-lived level, denotes the half-life of the level feeding the level of interest...

Quantum beats have been extensively studied in beam foil spectroscopy, but only recently have results been reported for their observation following electron impact excitation.For beats to be observed the excitation to the participating levels must be coherent and the modulation period of the decay must be greater than the instrumental resolution of about 10 s obtainable in electron photon coincidence experiments. Since the internal relaxation times from fine or hyperfine interactions of coherently excited states are T 10" s and beats between... 

We have already discussed quantum-beat spectroscopy (QBS) in connection with beam-foil excitation (Fig.6.6). There the case of abrupt excitation upon passage through a foil was discussed. Here we will consider the much more well-defined case of a pulsed optical excitation. If two close-lying levels are populated simultaneously by a short laser pulse, the time-resolved fluorescence intensity will decay exponentially with a superimposed modulation, as illustrated in Fig. 6.6. The modulation, or the quantum beat phenomenon, is due to interference between the transition amplitudes from these coherently excited states. Consider the simultaneous excitation, by a laser pulse, of two eigenstates, 1 and 2, from a common initial state i. In order to achieve coherent excitation of both states by a pulse of duration At, the Fourier-limited spectral bandwidth Au 1/At must be larger than the frequency separation ( - 2)/ = the pulsed excitation occurs at... 

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