| Résumé | In this contribution gas-phase 129Xe spin-lattice relaxation time measurements are extended to conditions (pressure, temperature, magnetic-field strength, isotope composition) not previously used. It is shown that wall effects become apparent at densities below ~20 amagat, and that these become dominant below ~3 amagat. A significant new discovery from field-dependent studies is that, in addition to the previously identified field-independent spin�rotation relaxation operative in the bulk gas, there is a contribution from the modulation of the chemical shift that depends on the square of the applied magnetic-field strength. The weak temperature dependence of the relaxation times can be understood in terms of the opposite temperature coefficients of the field-independent and field-dependent contributions to the relaxation. The spin�rotation contribution and its temperature dependence are calculated and found to be in good agreement with the experimental data. The low field, low density limit for the relaxation time is 56.3�2.6 hr�amagat at 295 K in the absence of wall effects. |
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