When a gaseous sample of ${}^{13}\mathrm{CH}_3\mathrm{F}$ is prepared with a spin-isomer population ratio (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon, called nuclear spin conversion, is mainly governed by intramolecular spin-spin and spin-rotation interactions which couple two pairs of quasidegenerate ortho-para levels ($J=9,K=3;$ $J^{\prime }=11,K^{\prime }=1$) and ($J=20,K=3;$ $J^{\prime }=21,K^{\prime }=1$). The presence of a static electric field can induce the degeneracy for Stark sublevels and yields an increase of the conversion rate. Such a “conversion spectrum” has been recorded experimentally. The intensities of the peaks are directly related to the intramolecular magnetic interaction strengths, and their widths depend on how the collisions break the coherence between ortho and para levels which is created by the interactions. Such collision-induced rates are directly determined and compared to the rate of rotationally inelastic molecular collisions.

• Received 17 October 2003

DOI:https://doi.org/10.1103/PhysRevA.69.032704