New access to very weak interactions in molecules

It is predicted that nuclear spin conversion in molecules can be efficiently controlled by strong laser radiation resonant to rovibrational molecular transition. The phenomenon can be used for substantial enrichment of spin isomers, or for detection of very weak (10-100 Hz) interactions in molecules.Comment: REVTEX, 11 pages + 4 eps figure

Coherent control of enrichment and conversion of molecular spin isomers

A theoretical model of nuclear spin conversion in molecules controlled by an external electromagnetic radiation resonant to rotational transition has been developed. It has been shown that one can produce an enrichment of spin isomers and influence their conversion rates in two ways, through coherences and through level population change induced by radiation. Influence of conversion is ranged from significant speed up to almost complete inhibition of the process by proper choice of frequency and intensity of the external field.Comment: REVTEX, 13 pages + 6 eps figure

Enrichment of CH3F nuclear spin isomers by resonant microwave radiation

Theoretical model of the coherent control of nuclear spin isomers by microwave radiation has been developed. Model accounts the M-degeneracy of molecular states and molecular center-of-mass motion. The model has been applied to the 13CH3F molecules. Microwave radiation excites the para state (J=11,K=1) which is mixed by the nuclear spin-spin interaction with the ortho state (9,3). Dependencies of the isomer enrichment and conversion rates on the radiation frequency have been calculated. Both spectra consist of two resonances situated at the centers of allowed and forbidden (by nuclear spin) transitions in the molecule. Larger enrichment, up to 7%, can be produced by strong radiation resonant to the forbidden transition. The spin conversion rate can be increased by 2 orders of magnitude at this resonance.Comment: REVTEX, 14 pages + 6 eps figure

Theory of nuclear spin conversion in ethylene

First theoretical analysis of the nuclear spin conversion in ethylene molecules (13^CCH4) has been performed. The conversion rate was found equal approx. 3x10^{-4} 1/s*Torr, which is in qualitative agreement with the recently obtained experimental value. It was shown that the ortho-para mixing in 13^CCH4 is dominated by the spin-rotation coupling. Mixing of only two pairs of ortho-para levels were found to contribute significantly to the spin conversion.Comment: 20 pages, 5 eps figure

Coherent control of nuclear spin isomers of molecules: The role of molecular motion

Molecular center-of-mass motion is taken into account in the theory of coherent control of nuclear spin isomers of molecules. It is shown that infrared radiation resonant to the molecular rovibrational transition can substantially enrich nuclear spin isomers and speed up their conversion rate.Comment: REVTEX, 13 pages + 3 eps figure