Nitrogen relaxation phenomena

Experimental data on nitrogen obtained from spin-lattice relaxation time (Ti) in [71] also show that tj is monotonically reduced with condensation. Furthermore, when a gas turns into a liquid or when a liquid changes to the solid state, no breaks occur (Fig. 1.17). The change in density within the temperature interval under analysis is also shown in Fig. 1.17 for comparison. It cannot be ruled out that condensation of the medium results in increase in rotational relaxation rate primarily due to decrease in free volume. In the rigid sphere model used in [72] for nitrogen, this phenomenon is taken into account by introducing the factor g(ri) into the angular momentum relaxation rate... 

In this section experimental results are described, which are obtained by applying the conventional pump-probe technique to m-LPPP films kept in vacuum at the temperature of liquid nitrogen [25], These results allow the identification of the primary excitations of m-LPPP and the main relaxation channels. In particular, the low and high excitation density regimes are investigated in order to get an insight into the physical processes associated with the emission line-narrowing phenomenon. 

From this point of view, nitrogen-15 exhibits a peculiarity which is characteristic of nuclei with a negative gyromagnetic ratio and is frequently emphasized. Indeed, it is possible for the relative contribution of the dipolar mechanism to be reduced to about 20% and therefore to give values of n = -1 and I/Io = 0. The signal is then cancelled by the Overhauser effect. This phenomenon should be kept in mind, in particular, when using relaxation reagents, since the electronuclear interaction may compete with the nuclear dipolar effect to yield approx-imatively n j g = 1 [Eq. (2.6)] (H 13). 

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