Making Molecules at MicroKelvin*
William C. Stwalley, University of Connecticut

The cooling and trapping of atoms and atomic ions is a rapidly advancing field of fundamental science (e.g. Bose-Einstein condensation).  We are attempting to extend the field to neutral molecules (and also molecular ions) as well, e.g. for study of elastic, inelastic and reactive collisions in the highly quantum mechanical regime at extremely low energies.  As a first step, we are employing single- and multicolor photoassociation to produce translationally ultracold 39K2 molecules from ultracold (~300µK) 39K atoms confined in a magneto-optical trap.1-7  Photoassociation of ultracold atoms (as opposed to thermal atoms) includes sharp resonances with wavelength as long-range rovibrational levels (outer turning points of tens or hundreds of Bohr) are accessed from colliding atomic pairs with  10 MHz of relative kinetic energy and only a few partial waves ( = 0, 1, 2).  Potential energy curves derived from these spectra test electronic structure and long-range perturbation theory calculations of interatomic potentials.  The molecules formed are translationally ultracold and rotationally cold.

Recently we have used two-color resonance enhanced multiphoton ionization to directly detect translationally ultracold molecules.8  These molecules are formed in v? = 36 of the ground X1Sg+ state of 39K2 following spontaneous emission from v¢ ª 191 of the A1Su+ state, formed in turn by one-color photoassociation of ultracold 39K atoms.  In the near future, we will seek to produce translationally ultracold molecules in low rovibrational levels (v?   0, J?   3) of the X1Sg+ state via two-color photoassociation as proposed by Band and Julienne.9  We plan as a further step to cool the rovibrational distribution of ground state translationally ultracold molecules produced by two-color photoassociation using laser cooling.10

We also plan to directly study free Æ bound Æbound stimulated Raman photoassociation to directly produce state-selected translationally ultracold K2 molecules as recently proposed.11  Note the application of this technique to an atomic Bose-Einstein Condensate may yield a coherent beam of state-selected molecules12 (a “molecule laser”).

Finally we note that translationally ultracold molecules can also be produced in metastable electronic states (e.g. the a3Su+ and b3Pu states of the alkali dimers).  Indeed Fioretti et al. have recently observed translationally ultracold Cs2 a3Su+ molecules using one-color resonance enhanced multiphoton ionization.13

* In collaboration with Professors Phil Gould and Ed Eyler, Drs. He Wang, John Bahns, Paul Julienne, Eite Tiesinga and Carl Williams, and Jing Li, Xiaotian Wang and Anguel Nikolov.  Supported in part by the National Science Foundation.

1.     H. Wang, P. L. Gould and W. C. Stwalley, “Photoassociative Spectroscopy of Ultracold 39K Atoms in a Vapor Cell
        Magneto-Optical Trap”, Phys. Rev. A. 53, R1216 (1996).
2.     H. Wang, P. L. Gould and W. C. Stwalley, “Photoassociative Spectroscopy of Pure Long-Range Molecules”, Z. Phys. D
        36, 317 (1996).
3.     H. Wang, J. Li, X. T. Wang, C. J. Williams, P. L. Gould and W. C. Stwalley, "Precise Determination of the Dipole
        Matrix Element and Radiative Lifetime of the 39K 4p State by Photoassociative Spectroscopy", Phys. Rev. A 55, R1569
4.     H. Wang, P. L. Gould and W. C. Stwalley, "The Long-Range Interaction of 39K(4s) + 39K(4p) Asymptote by
        Photoassociative Spectroscopy:  Part I:  The   Pure Long-Range State and the Long-Range Potential Constants", J. Chem.
        Phys. 106, 7899 (1997).
5.     H. Wang, X. T. Wang, P. L. Gould and W. C. Stwalley, "Optical-Optical Double Resonance Photoassociative
        Spectroscopy of Ultracold 39K Atoms near Highly-Excited Asymptotes", Phys. Rev. Letters 78, 4173 (1997).
6.     H. Wang, P. L. Gould and W. C. Stwalley, “Fine Structure Predissociation of Ultracold Photoassociated 39K2 Molecules
        Observed by Fragmentation Spectroscopy,” Phys. Rev. Letters 80, 476 (1998).
7.     X. T. Wang, H. Wang, P. L. Gould, W. C. Stwalley, E. Tiesinga and P. S. Julienne, “First Observation of the Pure
        Long-Range 1u State of an Alkali Dimer by Photoassociative Spectroscopy,” Phys. Rev. A57, 4600 (1998).
8.     A. N. Nikolov, E. E. Eyler, X. Wang, H. Wang, J. Li, W. C. Stwalley and P. L. Gould, “Observation of Translationally
        Ultracold Ground State Potassium Molecules,” to be submitted to Phys. Rev. Letters.
9.     Y. B. Band and P. S. Julienne, “Ultracold Molecule Production by Laser-Cooled Atom Photoassociation,” Phys. Rev.
        A51,  R4317 (1995).
10.   J. T. Bahns, W. C. Stwalley and P. L. Gould, “Laser Cooling of Mole-cules:  A Sequential Scheme for Rotation,
        Translation and Vibration”, J. Chem. Phys., 104, 9689 (1996).
11.   A. Vardi, D. Abrashkevich, E. Frishman and M. Shapiro, “Theory of Radiative Recombination with Strong Laser Pulses
        and the Formation of Ultracold Molecules via Stimulated Photo-Recombination of Cold Atoms,” J. Chem. Phys. 107,
        6166 (1997).
12.   P. S. Julienne, K. Burnett, Y. B. Band and W. C. Stwalley, “Stimulated Raman Molecule Production in Bose-Einstein
        Condensates, “ submitted to Phys. Rev. A (Rapid Communication).
13.   A. Fioretti, D. Comparat, A. Crubellier, O. Dulieu, F. Masnou-Seeuws and P. Pillet, “Cs2 Cold Molecules Formation
        through Photoassociative Scheme in a Cs Vapor-Cell Magneto-Optical Trap,” Phys. Rev. Letters, in press (1998).