Nuclear spin conversion in polyatomic molecules

Except for ortho- and para-H-2, little is known about nuclear spin isomers (or spin modifications) of molecules. The main reason is the lack of practi cal enrichment techniques. Recently, a few enrichment methods were develope d, which opened up new possibilities in the field. These methods are briefl y reviewed. Substantial progress in the field has been made by the introduc tion of light-induced drift as a gas-phase separation tool. This is illustr ated by extensive data on CH3F, which reveal that the gas-phase ortho-para conversion is governed by intramolecular mixing of the nuclear spin states. The role of direct ortho-para transitions is small. Various aspects of the conversion were investigated in detail: pressure and collision partner dep endence, isotope effect, and temperature dependence. The most decisive info rmation on the spin conversion mechanism is derived from the observation of level-crossing resonances in an electric field and the quantum Zeno effect induced by collisions.