Pulse ringdown

In addition to the effects from the probe there is the electronic deadtime, including pulse ringdown (100 ns), preamplifier recovery (800 ns), filter overdrive recovery (1 p.s) and ADC conversion droop (200 ns) (Hoult 1979). Magnetoacoustic ringing (up to 200 p.s) can be very significant if careful probe design (e.g. coil wire) is not considered. Samples that exhibit peizoelectric behaviour can lead to very long response times of up to 10 ms. 

In addition, backward LP can be utilized for a completely different purpose. The first few data points at the beginning of the FID occasionally become corrupted if acquisition of the signal starts before pulse ringdown has faded to an acceptable level. In such a situation, a large baseline roll appears in the spectrum and renders it essentially useless. This result is especially disheartening when it occurs in a spectrum accumulated over a weekend. Still, the problem can easily be solved by replacing the first 5-10 data points with predicted points. Reweighting and a second FT result in an undistorted spectrum. 

Linear prediction. A mathematical operation that generates new or replaces existing time domain data points with predicted ones. Linear prediction can add to the end of an array of digitized FID data points, can extend the number of t, time domain data points in a 2-D interferogram, or can replace the initial points in a digitized FID that may have been corrupted by pulse ringdown. 

Scherer J J, Paul J B, O Keefe A and Saykally R J 1997 Cavity ringdown laser absorption spectroscopy history, development, and application to pulsed molecular beams Chem. Rev. 97 25-51... 

A promising technique is cavity ringdown laser absorption spectroscopy (307), in which the rate of decay of laser pulses injected into an optical cavity containing the sample is measured. Absorption sensitivities of 5 x 10 have been measured on a ]ls time scale. AppHcations from the uv to the ir... 

Scherer, J.J. et al.. Cavity ringdown laser absorption spectroscopy History, development and applications to pulsed molecular beams, Chem. Rev., 97, 25, 1997. 

The 14N spectra of single crystals of h-GaN (Fig. 3, bottom right) are doublets, corresponding to the two transitions — 1 <-> 0 and 0 +1, and also show the expected orientation dependence for a Cq of 24.14 kHz [49]. A 90 — t — 90v x quadrupolar-echo pulse sequence was used to eliminate probe ringdown. The spin-... 

Scherer, J. J., J. B. Paul, A. O Keefe, and R. J. Saykally, Cavity Ringdown Laser Absorption Spectroscopy History, Development, and Application to Pulsed Molecular Beams, Chem. Rev., 97, 25-51 (1997). 

The fact that the echo decays very nq>idly in the time domain leads to a problem common in solid-state NMR work the appearance of a pulse feedthrong . This artifact results from the recovery of the receiver circuits and a ringdown of the transmitter and probe circuits after the high-power excitation pulses. To reduce this problem a pulse sequence used to eliminate ringdown in quadrupolar echo NMR was employed [10]. In this sequence, the phase of the tc/2 and the n pulses of the Hahn-echo sequence are varied in such a way that the arti cts add destructively and are essentially eliminated. 

The key feature of ReMims (Doan) ENDOR is that Ti can be less than the deadtime of the spectrometer. This is because, in contrast to Mims ENDOR, the stimulated spin echo (which would be distorted by cavity ringdown) is not detected. Instead, an additional tt pulse is applied after time tz, which leads to a standard spin echo at time tz (Hahn echo, since it results from the original Hahn sequence here from the third tt/2 pulse and the following tt pulse). More important, there are two additional spin echoes formed one at time (tz + x ), which is observable for all values of x and xz, and is denoted the RME, which is detected, and one at time (xz — x ), which is observable only for values of t2 > ti, and is denoted the refocused stimulated echo (RSE). The deadtime for the ReMims (Doan) sequence is thus the minimum feasible value of (t2 + Ti), rather than the minimum value of x in a Mims sequence (Figure 6). As mentioned earlier (Section 3.3.4), the maximum undistorted Aiso (MHz) is - 1/(2ti (jls)). The deadtime in the X-band pulsed spectrometer is /d 0.1 J,s... 

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