Spectrometer pulse

Designs of line-narrowing m.p. including CRAMPS spectrometers have been published by several groups (Ellett et al., 1971 Maciel et al., 1990). Here we present an overview of our current 270-MHz spectrometer and describe four of the more important components in some detail. This spectrometer has been developed stepwise over many years at the MPI in Heidelberg. Major contributors in recent years were Rainer Umathum, Wolfgang Scheubel, and Volker Schmitt. 

The transmitter amplifier chain consists of a linear, three-stage transistor amplifier from Amplifier Research (10 W), a class C single-stage field effect transistor (FET) amplifier (120 W) custom-built by H. Bonn GmbH, Munich, and a final tube amplifier with two tetrodes 4CX 350A that deliver more than 1.5 kW of pulse power. A special effort was made to match the input and output impedances of this tube amplifier to the characteristic impedance (50 ( ) of the cables connecting it with the probe and the driver, respectively. This impedance matching resulted in the virtually complete disappearance of antisymmetric phase transients (for a discussion of the effects of such phase transients on m.p. spectra, see Haeberlen, 1976, Appendix D). 

The transmitter is followed by a circulator that guarantees that the transmitter sees a resistive 50-f load during all states of the duplexer, in particular while this component disconnects the probe from the transmitter. The circulator suppresses effectively any ringing (of transients) between the transmitter and the probe. The bidirectional coupler is used to monitor the rf pulses (forward direction) and probe matching (backward direction). Two concepts for the duplexer—a crucial part of every m.p. spectrometer—are discussed in Section III.B.3. 

This preamplifier unit adds only a negligible amount of time to the total deadtime of the spectrometer, which is short enough to allow the observation of the NMR signal between two 90° pulses delayed by not more than 

rf Gates, Leak-Compensated Double-Balanced Mixers 

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Technology developments are revolutionizing the spectroscopic capabilities at THz frequencies. While no one teclmique is ideal for all applications, both CW and pulsed spectrometers operating at or near the fiindamental limits imposed by quantum mechanics are now within reach. Compact, all-solid-state implementations will soon allow such spectrometers to move out of the laboratory and into a wealth of field and remote-sensing applications. From the study of the rotational motions of light molecules to the large-amplitude vibrations of... 

There was no special chapter devoted to NMR in the last edition in 1982, but such is the utility of the technique that many references were made to NMR studies in other chapters. Our main concern here will be work published after that date. Although commercial FT pulse spectrometers had been available for ten or more years, the techniques of spectroscopy in two frequency dimensions, 2D-NMR, were in their infancy11, whereas now they are almost commonplace. 

Signals obtained with both cw and pulse spectrometers are usually weak and their improvement is often desirable. 

The problem is quite different with pulse spectrometers, for which the prospects are good free precession and echo signals are rather long as compared to those of protons in solids and they can readily be fed into a standard digital memory oscilloscope with an access time of about 40 microseconds. The gain is considerable 15> 161 provided good sample temperature and spectrometer frequency stabilizations are achieved. 

The conducting phase of TMQ /16/. Microwave conductivity experiments, performed at low temperature on the samples used for the pressure experiment, have succeeded in showing an increase by a factor 10 from 300 K down to loo K, /41/. The possibility of susceptibility measurement via low field method is limited by the EPR line broadening occurring at low temperature and under pressure. The spin susceptibility was derived from a fit of the EPR absorption line shape with a Lorentzian curve. The proton relaxation time was measured under pressure at low field lOe- with a pulse spectrometer. Figures... 

The sample, inside of the nuclear coil, is put in the microwave cavity working in the T.E. 112 mode at 9.6 GHz. The nuclear signal is observed with a pulse spectrometer at about 14.6 MHz. By saturating the electron resonance, the nuclear signal is enhanced and we measure the temperature dependence of this enhancement. 

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... 

Despite its utility, off-resonance decoupling is now considered an old-fashioned technique. It has been replaced by more modem methods, the most important of which is Distortionless Enhancement by Polarization Transfer, better know as DEPT. The DEPT technique requires an FT-pulsed spectrometer. It is more complicated than off-resonance decoupling and it requires a computer, but it gives the same information more reliably and more clearly. Chapter 10 will discuss the DEPT... 

To do this, measurements of relaxation time (spin-spin) and T2 (spin-spin) were accomplished by means of a Bruker type SXP 4/100 pulsed spectrometer. Measurements of were carried out by the method of iii5)ulse application II-T-n/2. Impulse length H/2 was 2-3 ys, and field frequency - 90 MHz. The relaxation time T2 spin-spin was determined by the "solid echo" method, when the relaxation time was in the range of 10-200 ps, and if the relaxation time was of the order of 1 ms, the Gill-Meiboom>s method was used. Separation of relaxation time components was effected using the graphical method described by Me Brierty. In order to study the specific interactions between PAN and unsaturated elastomers the mixtures of model substances, i.e. n-dodecene-1, n-dodecane and n-butyronitrile in infrared and ultraviolet were investigated by means of Pye-Unicam spectrophotometer of the SP-700 type. 

The only safe way of measuring values of T2 is with a pulse spectrometer. For broad lines, where field inhomogeneity broadening constitutes only a small part of the line width, the relationship ... 

The technique can be carried out using either a continuous wave (CW) or a pulsed spectrometer. The RE energy is used to excite the nuclear magnetization. The measurement is the response of the spin system to this excitation. In CW the nuclear magnetization is irradiated at a... 

Figure 4 shows our recent measurements of the FT-NMR of similar samples. Sample preparation was similar to that described earlier, except that we added the free radical, di-t-butyl nitroxide in an attempt to obtain narrower lines.29 The NMR measurements were made with a Bruker pulse spectrometer operating at a frequency of 13.004 MHz. 

There are several obvious applications that are analogous to those encountered in NMR spectroscopy. Probably the most important of these is the measurement of relaxation times that are of the order of microseconds. In NMR, these times are of great structural value and lead to conclusions about molecular motion. No doubt these benefits will also apply to ESR as commercial pulse spectrometers become more widespread. If the relaxation times of two paramagnetic centers are sufficiently different, their spectra can be separated easily using the... 

Relaxation data on quadrupolar halogen nuclei may be obtained either from pulsed NMR studies or from line width measurements. The lower limit of T.j and T2 values that can be measured with a pulse spectrometer is set by the effective dead time of the receiver system after a pulse and by the width and amplitude of the pulse. At present... 

The phase equilibria of the surfactant systems have been studied by observing the water deuteron ( H) NMR spectra at a resonance frequency of 15.351 MHz with a modified Varian XL-100-15 pulsed spectrometer. The experimental details and the analysis of NMR spectra are as reported previously [1]. 

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