Pulse train interference

Fig. 7.19 Measurements of the hfs splittings in the 7 5 /2 ground state of Cs atoms with the pulse-train interference method. Excitation occurs on the D2 line aiX = 852.1 nm with a repetition rate f = Av/q with = 110. (a) Experimental arrangement (b) transmission of the probe pulse as a function of / (c) fluorescence intensity 7fi(/) as a function of / with a modulated small external magnetic field and (d) as a function of the delay time At at a fixed repetition frequency / [902]...

Finally, a few words remain to be said about case (d) (S>l/2,I>l/2). At the present stage of knowledge, this case remains seemingly intractable for quantitative applications outside the non-selective excitation limit Vj/Vq l. The chief problem in applying pulse sequence at Fig. 8b) arises from the nuclear electric quadrupolar interactions affecting the S spins, which interfere with the ability of the 71-pulse trains to refocus the transverse magnetization. In contrast, pulse sequence at Fig. 8a) is able to produce experimental REDOR curves, which are, however, difficult to interpret quantitatively. As a best-effort solution, one may resort to sample-to-sample comparisons on a relative basis and the use of empirical calibration procedures with closely related model compounds [32]. Alternatively, the SEDOR approach, which does not suffer from these restrictions, would be preferable. 

To measure the shape of the dye pulse requires optical techniques, since the 10 -Hz bandwidths involved far exceed the capabilities of conventional circuitry. While a variety of techniques has been developed, the standard today is background-free autocorrelation —an average (over interference terms) of the overlap of the pulse train... 

The hardest part of the experiments will likely be the preparation of coherent pseudorotation of large amplitude (see Appendix A). One preliminary experiment could be to apply a train of short nonresonant pulses, spaced at the ground state vibrational period, to a diatomic molecule such as iodine and test whether the interference signal produced with a subsequent phase locked pulse-pair is detectably altered. 

Chemiluminescence and photoluminescence are other forms of interference that can reduce the accuracy of LS techniques. Chemiluminescence describes the emission within the scintillation cocktail of photons that result from a chemical reaction common initiators are samples with an alkaline pFl or the presence of peroxides. Photoluminescence can occur when the scintillation cocktail is exposed to ultraviolet light. Some substances in the cocktail, notably the scintillator, are excited and then emit light when the species return to ground state. The effect of photoluminescence is reduced in LS systems by decay when the sample train is held in a dark environment for a few minutes prior to counting. On the other hand, chemiluminescence may have a slow decay rate that requires a change in sample preparation to eliminate the chemical that causes it. Some LS systems identify luminescence by pulse shape and indicate its relative extent. 

If several resonator modes within the bandwidth of the laser pulse are excited, beat signals are superimposed onto the exponential decay curve. These beats are due to interference between the different modes with differing frequencies. They depend on the relative phases between the excited resonator modes. Since these phase differences vary from pulse to pulse when the cavity is excited by a train of input pulses, averaging over many excitation pulses smears out the interference pattern, and again a pure exponential decay curve is obtained. 

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