The role of relaxation in the nuclear spin conversion process

The nuclear spin conversion rate depends on the collisions, which break the coherence created by magnetic intramolecular interactions between pairs of quasi degenerate levels belonging to the different spin isomers. The collisions act similarly to break the coherence created by a radiation field between two levels inducing pressure broadening of molecular transitions. Collisional relaxation rates have been extensively studied in this last situation using semi-classical approach and rectilign trajectory for collisional path. Taking advantage of the analogy, the present paper shows that calculations can be efficiently adapted for the collisional relaxation terms present in the 'quantum relaxation' model of nuclear spin conversion. For ¹³CH₃F, numerous experimental measurements of spin conversion rates in the presence of an electric field have allowed to derive directly relaxation rates. Our calculation appears to agree satisfactorily with these experimental values. For ¹²CH₃F, calculated relaxations rates are also given for the pairs involved in nuclear spin conversion.