Nitrogen-14 line widths

Fig. 3.8. The Q-branch Raman width alteration with condensation of nitrogen. The theoretical results for the strong (A) and weak (B) collision limits are shown together with experimental data for gaseous [89] ( ) and liquid nitrogen [145] ( ) (point a is taken from the CARS experiment of [136]). The broken curves in the inset are A and B limits whereas the intermediate solid curve presents the rotational contribution to line width at y = 0.3. The straight line estimates the contribution of vibrational dephasing [143], and the circles around it are the same liquid data but without rotational contribution.

The electron spin resonance of the nitroxalkylcorrinoids can be readily observed in aqueous solution at room temperature. Both the cobalamin and cobinamide show nitrogen hyperfine coupling constants of 17.2 gauss. A typical spectrum is shown in Fig. 20. The line widths for the low, intermediate, and high field peaks are 1.87, 1.87, and 2.20... 

The width of the peaks in LETS depends upon the sharpness of the onset of the inelastic process, which in turn depends upon the thermal distribution of electron energies about EP. Thus, the IETS line width depends strongly on temperature and as shown by (3) [75]. Because of this, vibrational IETS provides infrared-quality resolution only when performed below 5 K. Electronic transitions are usually much broader than vibrational transitions therefore, electronic IETS is usually performed at liquid nitrogen temperature and slightly above (>77 K). An example of a system showing both vibrational and electronic IETS is presented in Fig. 5 [19]. 

The matrix isolation procedure relies on the condensation of H20 and C02 on a liquid-nitrogen-cooled cold finger to form a stable matrix for radicals and other atmospheric species (Figure 9). Typically about 20 L of air are required to achieve the desired sensitivity for ambient measurements. A matrix of deuterated water (D20) narrowed the EPR line widths and improved the signal-to-noise ratio, and thus this matrix has been used for measurements since October 1982. Recent improvements in the use of this matrix isolation-EPR technique have been in the analysis of the spectra. 

Nearly all the efforts toward the application of double-resonance NQR to explosives detection have been driven by the problem of TNT detection [91,96,97], although reports on its application to RDX [98] and PETN [99] detection have appeared recently. Both of the double-resonance detection schemes described earlier have been applied to TNT detection, as it does not fit neatly into either category around the H frequency of 1 MHz, the proton and nitrogen T1 s are similar, with the proton T1 becoming much longer at higher frequencies there are multiple 14N NQRs the proton line width is greater than 10 000 Hz. 

The other isotope of nitrogen, l5N, also has an inherent low sensitivity, which, when multiplied by a very low natural abundance, leads to an extremely low absolute sensitivity. Modern instrumentation has largely overcome the problem of sensitivity (for labeled samples or by indirect detection) and we focus our attention on 15N largely because its spin number is one-half and its line-widths are quite narrow. 

Owing to the special form of the eigenwave functions for t] f=0, and in accordance with the absence of first-order Zeeman effect, it may be shown that the magnetic dipolar contribution to nitrogen resonance line width is very small33,34). Lines are consequently narrow for many of the compounds studied, a very convenient feature when weak effects, like the Stark effect, are to be studied35). 

As the temperature increases, the line width may be reduced by an averaging of the dipolar magnetic field at the nitrogen nuclei when a motion of the neighboring protons takes place 17). 

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