Alexander Dalgarno

COLLISIONS of METASTABLE ATOMS
Personnel, theoretical: Alex Dalgarno (PI)
Working with Robert Forrey, the influence of the fine-structure splitting and the Lamb shift on the quenching collisions of metastable hydrogen atoms has been explored. There occurs an interesting discrepancy between the derived rate coefficient and the measureemtns of David Landhuis.

COLLISIONS OF MOLECULES AT ULTRACOLD TEMPERATURES
Personnel: theoretical: A. Delgarno (PI), N. Balakrishnan, E. Bodo, F. Gianturco, R. Krems
In collaboration with N. Balakrishnan, E. Bodo and F. Gianturco, calculations on the reactions at ultracold temperatures of fluorine atoms with H_2, HD and D_2 have been carried out. The reactions appear to proceed by tunneling. The rate coefficient for H_2 is enhanced by the presence of a virtual state.
With R. Krems, the fine-structure quenching of F(^2P_1/2) by H_2 has been investigated. The rate coefficient is large at zero temperature.

BUFFER GAS COOLING
Personnel: theoretical: A. Delgarno (PI), R. Krems
With R. Krems and others, it has been shown that the rate coefficients for the quenching of ^3P_1 atoms of carbon vanishes at low temperatures. Detailed calculations have been carried out with R. Krems, N. Balakrishnan and G.C. Groenenboom of the rotational and vibrational quenching of CaH by ^3He and of the elastic scatttering. Agreement with measurements of the Doyle group is mixed. Doyle et al. found a non-zero rate for spin-flip. We identified the mechanism and evaluated the rate coefficient. It is consistent with the measurement. With R. Krems, we presented a general discussion of the threshold laws for spin flip in collisions of paramagnetic atoms.

COLLISIONS OF HYDROGEN ATOMS
Personnel: theoretical: A. Delgarno (PI),
With B. Zygelman (CUA visitor) and M. Jamieson, we set up and solved the coupled equations for spin change in the collisions of two ground state hydrogen atoms. We showed conclusively that the discrepancy with experiment cannot be removed by any acceptable change in the interaction potentials.

COLLISIONS IN MAGNETIC FIELDS
Personnel: theoretical: A. Delgarno (PI),
The influence of magnetic fields on inelastic collision processes is being investigated with particular attention to the modifications of threshold laws, with R. Krems and B. Zygelman. B. Zygelman has obtained reliable rate coefficients for the dipolar relaxation of sodium atoms.