Level-crossing resonances in nuclear spin conversion of molecules

The first observation of level-crossing resonances in ortho-para conversion is presented. A sharp increase in the conversion rate of gaseous (H3F)-H-13 is observed when an applied electric field produces crossing of molecular ortho and para levels. The experimental results are described well by the spin conversion theory based on intramolecular ortho-para state mixing. The results provide direct identification of the levels being mixed and the interaction responsible. The observed spectrum may be considered as an example of Doppler-free spectroscopy.