Nuclear spin conversion in molecules induced by hyperfine interactions

Abstract

ABSTRACT In this paper the role of hyperfine interactions in molecular ortho—para conversion is considered. The mainexperimental results in the CH3F spin conversion are briefly described. The general theory of nuclear spinconversion in molecules based on intramolecular ortho—para state mixing is given. Two types of ortho—parastate mixing are taken into account: the spin—spin interaction between molecular nuclei and the spin—rotation interaction. The level—crossing spectra for the spin conversion in '3CH3F and '2CH3F molecules are calculatedfor an electric field range of O—iO V/cm. The spin conversion spectrum, discussed here, is an example of a newtype of Doppler—free spectroscopy. 1. INTRODUCTION Hyperfine structure in molecular spectra has its origin in nuclear spins coupling to the internal states ofmolecules. Insight into these tiny forces is obtained by high resolution spectroscopy. On the other hand, it is known that apart from a small hyperfine shift of molecular energy levels (typically iO—iO Hz), there is another,much more pronounced, consequence of the presence of nuclear spin in molecules. That is the line—intensity—alteration effect in molecules which possess identical nuclei. This effect, known from the late 20s, was at that timeone of the challenging puzzles in spectroscopy. Nowadays the line—intensity—alteration effect is considered almost