Rotational relaxation in ultracoldCO+Hecollisions

Abstract

Cold and ultracold collisions involving rotationally hot CO molecules are investigated using quantum me- chanical coupled channel, coupled states, and effective potential scattering formulations. Quenching rate co- efficients are given for initial rotational levels near the dissociation threshold. The stability of the CO super rotors against collisional decay is compared to previous investigations involving homonuclear molecules. It is found that quasiresonant transitions provide a significantlystronger contribution to the total relaxation rate than in the comparable case of O2. As in the case of H2, sharp structures in the distribution of total quenching rate coefficients are found at rotational levels where quasiresonant scattering is not allowed. Experimental schemes to cool and trap neutral polar mol- ecules using a time-varying electric fieldhave been proposed 1,2 and recently realized 3 . Meijer and co-workers have applied the so-called Stark decelerator to slow down meta- stable CO molecules 1 . Schemes to produce diatomic mol- ecules in highly excited rotational states have also been pro- posed 4,5 and recently realized 6 . Theoretical studies have suggested that the collisional dynamics of such rota- tionally hot molecules would be particularly interesting at low temperatures 7-1 1 . These studies concentrated on homonuclear diatomic molecules where it was found that quasiresonant vibration-rotation QR VR transitions contrib- uted significantly in quenching the highly rotationally ex- cited molecules. Another recent investigation showed that QR VR transitions have a neglible effect on the relaxation of rotationally hot oxygen molecules and that the quenching is very efficient for all rotational levels and dominated by pure rotational deexcitation 12 . Each of the theoretical investi- gations 7-12 assumed a helium atom collision partner. He- lium buffer gas cooling 13 has proven to be an effective technique for loading molecules into a magnetic trap 14 and it has recently been shown that the cooling technique may be applied to a beam of molecules 15 . Therefore, col- lisions involving helium atoms are of considerable interest in ultracold molecular physics. In this work, we perform relax- ation studies for helium collisions with CO. We compute an extensive amount of collisional data that may be used as a point of comparison to the molecular hydrogen and oxygen systems studied previously and investigate whether hetero-