1 4N spin relaxation studies of N2 in buffer gases. Cross sections for molecular reorientation and rotational energy transfer

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Abstract
Nuclear spin–lattice relaxation times (T1) have been measured as functions of temperature for 14N in N2 gas and in N2–Ar, Kr, Xe, CO, CO2, HCl, CH4, CF4, and SF6 mixtures. The relaxation is dominated by the quadrupolar mechanism so that empirical values of the collision cross sections for molecular reorientation (σθ,2) could be obtained as functions of temperature. The values of σθ,2/Å2 at 300 K are 29.6±0.9 (N2–N2), 33±1 (N2–Ar), 41±2 (N2–Kr), 44±2 (N2–Xe), 32±1 (N2–CO), 59±3 (N2–CO2), 46±1 (N2–HCl), 31±1 (N2–CH4), 59±2 (N2–CF4), and 73±2 (N2–SF6). For all 14N2 –buffer pairs, the temperature dependence of the cross section deviates from T−1, which is not very different from that of the collision cross section (σJ) for changes in the rotational angular momentum vector. This is the first molecule for which the collision cross sections σθ,2 and σJ have both been measured for a series of collision partners. The ratio (σθ,2/σJ) is found to be nearly constant, 2.1±0.2 for the N2 molecule with the ten collision partners. Based on Kouri’s IOS factorization scheme, (σθ,2/σJ)>1 may be true in general. The data for N2 are compared with the theoretical reduced correlation times based on existing mathematical models for molecular reorientation in fluids.