Using light-induced drift as a tool to separate nuclear spin isomers, we have studied the ortho-para conversion process for gaseous 13 CH 3 F interacting with O 2 , N 2 , CH 3 Cl, and CH 3 F itself. It is found that the conversion rate is significantly smaller for collisions with O 2 or N 2 than with CH 3 Cl or CH 3 F. The data show that intermolecular magnetic interactions do not play a leading role in the conversion. In order to test an alternative model based on mixing of states due to intramolecular interactions, pressure broadening has been measured for these systems. The observed correlation between conversion rates and broadening coefficients supports this mixing-of-states model.
Experimental data on the temperature dependence of the ortho-para nuclear spin conversion rate of gaseous CH 3 F are presented. In the range 300-750 K, the 12 CH 3 F conversion rate increases by one order of magnitude. By contrast, the conversion rate for 13 CH 3 F decreases with increasing temperature ͑for TϽ600 K͒. These results are in qualitative agreement with a theoretical model based on mixing of states due to intramolecular spin-spin interactions between the molecular nuclei. Above 600 K, the rate in 13 CH 3 F starts to increase again, indicating a possible additional mechanism for the conversion. ͓S1050-2947͑98͒07505-2͔
Ortho–para conversion for gaseous 13CH3F is measured in mixtures with O2. As collision partner, O2 is found to be 4 times less efficient for conversion that CH3F itself. This demonstrates that intramolecular rather than intermolecular magnetic interactions provide the main pathway leading to nuclear spin conversion for such molecules.
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