Sweep field

The bipolar junction transistor (BIT) consists of tliree layers doped n-p-n or p-n-p tliat constitute tire emitter, base and collector, respectively. This stmcture can be considered as two back-to-back p-n junctions. Under nonnal operation, tire emitter-base junction is forward biased to inject minority carriers into tire base region. For example, tire n type emitter injects electrons into a p type base. The electrons in tire base, now minority carriers, diffuse tlirough tire base layer. The base-collector junction is reverse biased and its electric field sweeps tire carriers diffusing tlirough tlie base into tlie collector. The BIT operates by transport of minority carriers, but botli electrons and holes contribute to tlie overall current. 

It was suggested above that the field Hq is held constant while the Hi frequency is swept this mode is called frequency sweep. An equivalent result is obtained by holding the rf field at a constant frequency and sweeping the field Ho- NMR spectra obtained by field-sweep measurements are displayed with the field increasing from left to right spectra obtained by frequency sweep are displayed with the frequency increasing from right to left, so that the two presentations are comparable. 

In the case of a field-sweep N.M.D.R. experiment (22, 26), the frequency separation between the two R.F. fields (a>2-i) is set approximately equal to the chemical shift separation between the proton which is to be observed and that which is to be irradiated. Then the spectrum... 

It should be noted that it would have been totally impossible to have effected these decoupling experiments by the field-sweep method a>i has to be scanned ca. 100 Hz to observe the H2 resonances which of course would require < >2 to be scanned by an equal amount. Since it is not possible to obtain a decoupling field of this magnitude, the above experiment would merely result in partial decoupling of the H2 resonances. 

Low-field Direction of field sweep- High-field- ... 

Note that the values of are orders of magnitude larger than the A values given for the hyperfine splitting of an EPR spectrum (or ENDOR spectrum) in units of the field sweep necessary to obtain electron spin resonance. These are usually on the order of only 10 mT or less ... 

Center Field, Sweep Width and Field Offset... 

Suppose we want to compare two spectra—let s call them spectrum-a and spectrum-P—taken over field sweeps that may be identical but with a slight difference in their micro-wave frequency. The spectra are digital arrays corresponding to amplitudes at equidistant field values. The procedure to convert spectrum P taken at frequency vp to frequency va of reference spectrum a is as follows For each field value B of spectrum-a we calculate the corresponding field for vp /ip = (vp/va)5a, and then we search in spectrum-p to the two digital field values that nearly match (that embrace ) the value /ip in order to interpolate the two corresponding amplitudes to an intermediate amplitude value for flp to be stored in a new array of P-amplitudes onto a B(J grid. In pseudo-code... 

Experimentally there are two alternatives. One is to fix the magnetic field that determines the Larmor frequency and to sweep the radiation frequency through this value. Alternatively radiation at a fixed frequency is used while the field sweeps through the resonance value. When the resonance condition is reached absorption of radiation occurs. 

Fig. 9. The continuous wave EPR spectrum of recombinant Rhodnius prolixus NPl-histamine at a microwave frequency of 9.338 GHz (trace 1) and field-sweep ESE spectra at 8.706 GHz (trace 2), 3.744 GHz (trace 3), 3.065 GHz (trace 4). Dashed arrows show the changes in magnetic fields corresponding to principalg -values at the different microwave frequencies. Inset trace 1, the primary ESE decay recorded at 8.706 GHz, Bo = 213 mT (the low-field turning point in the field-sweep ESE spectrum shown by trace 2 in the main panel). Inset trace 2, the primary ESE decay recorded at 3.065 GHz, Bo = 140.8 mT (the high-field turning point in the field-sweep ESE spectrum shown by trace 4 in the main panel). Reproduced with permission from Ref. (89).

The first observation of Zn NMR in zinc metal was by Abart and coworkers in a measurement at 4.2 K, using a field sweeping technique, which yielded a value of = 12 73(4) MHz. No subsequent NMR observation has been reported. A measure of the temperature dependence of the Zn nuclear quadrupole coupling in zinc metal has been obtained from time differential perturbed angular correlation (TDPAC) measurements using an excited state of Zn (/ = 9/2 605 keV). However, the use of liquid helium temperatures and exotic short-lived isotopes precludes the adoption of these techniques for general material characterization. 

Exercise 9-21 Use Figure 9-24 to map the nmr spectrum you would expect for 13CCI31H in a field-sweep spectrometer in which the transmitter frequency is kept constant at 30 MHz and the magnetic field is swept from 0 to 30,000 gauss. Do the same for a frequency-sweep spectrometer when the magnetic field is kept constant at 10,000 gauss and the frequency is swept from 0 to 100 MHz. (For various reasons, practical spectrometers do not sweep over such wide ranges of field or frequency.)... 

Common usage is upheld, more shielding downfield, less shielding and you should remember that field-sweep spectra always are recorded with the field... 

We will use frequency sweep simply because it is easier to talk about energy changes in frequency units. However, the same arguments will hold for a field-sweep experiment. 

In some ways, measurement of the frequency dependence of the EPR line width (1/T2e) is simpler than very accurate g-factor determination. A precise calibration of the magnetic field sweep used to acquire the spectrum is required, which is obtained using field standards like the hyperfine splitting in Fremy s salt, in conjunction with a tracking NMR Gaussmeter. Once the data are obtained at multiple EPR frequencies, they can be analyzed to determine A2 and rv. 

Figure 18. Q-band EPR spectrum of the g perpendicular region of spinach plastocyanin. Field sweep was between 11,275 and 12,275 gauss while the microwave frequency was 34.282 GHz.

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