Colliding neutral atoms, confined in a laser trap, are photoassociated to bound excited states of the dimer molecule by absorbing a photon from a tunable laser.

The technique can probe long range and "purely long range" molecular states that are difficult or impossible to detect by traditional means and, because of the extremely low energy of the colliding atoms, is capable of high resolution (< 0.001 cm^-1)

The spectra are useful for atomic lifetime measurements, determination of atomic ground-state scattering information and measurement of curve-crossing probabilities. Theoretical and experimental work in the field, including multiple resonance techniques and photoassociation line shapes, are pursued at NIST.

By using light to combine two colliding cold, trapped atoms into a molecule, a new kind of high precision molecular spectroscopy is now possible for probing the long range forces between the atoms. This photoassociation spectroscopy has been used to make the most precise measurements to date of the atomic lifetimes of the first excited state of the Na and Li atoms, and to observe the influence of relativistic corrections, often called retardation corrections, to the long range potential between one ground state and one excited state atom due to the finite speed of light. A laser of variable frequency n is tuned so that its energy hn matches the difference in energy between a bound vibrational level in the excited state potential of the diatomic molecule formed from the atom pair and the energy of the ground state colliding atoms. The formation of the excited state is observed either by ionizing it with a second photon and detecting the ions or by detecting the loss of trapped atoms due to decay of the excited state to untrapped states. Tuning the laser frequency gives an excitation spectrum for production of molecular excited states. The spectral line shapes are very sharp, rivaling those of conventional Doppler-free bound state spectroscopy, because the kinetic energy of the cold ground state atoms is sharply defined, usually to less than a natural linewidth of the atomic line used to do the laser cooling of the atoms. A careful consideration of the actual spectral line shapes is necessary for the most precise use of these spectra, since they are strongly influenced by the quantum nature of the ground state wavefunction of the cold colliding atoms [1]. The number of rotational lines in the spectra is also limited by the low collision energy.

A combined experimental-theoretical effort at NIST [2] has concentrated on studying a very special excited state of the sodium diatomic molecule. This state is a "pure long range state" [3] which correlates at large internuclear separation with a Na ground state atom and a Na atom in its first excited state. The long range form of the potential between two atoms of the same species, one in the ground state and one excited, is analogous to the potential due to two dipole antennas, varying as the inverse cube of the separation between them and proportional to the product of dipole strengths. Here, this product is proportional to the decay rate of the excited state atom, that is, to the inverse of the atomic lifetime. The state is a very shallow one, with a potential energy curve only about 55 GHz (2 cm^-1) deep with a minimum energy near 71 a₀ and a repulsive wall that never lets the atoms get closer together than about 55 a₀. This state is entirely determined from the atomic properties of the separated atoms. Careful measurement of the photoassociation spectrum of this state, along with theoretical modeling of the line shapes, determines the positions of the rotational lines of the lowest 7 vibrational levels to approximately 5 MHz. By determining the strength of the dipolar potential, these molecular spectra determine the Na lifetime to 0.1 %. In addition, the 121 MHz shift in binding energy of the ground vibrational level due to the retardation corrections to the interatomic potential is evident from the measurements. Atomic lifetimes determined from molecular spectra are the most accurate lifetimes to date for Li [4,5] and Na [2]. It is gratifying that they agree within error limits with other recent determinations of the atomic lifetimes for these species, thereby resolving longtime questions and discrepancies about the lifetimes of these simple atoms. Photoassociation spectroscopy has also determined an atomic Rb lifetime at the 1 % level [6], and work is in progress on potassium [7]. In addition, conventional molecular spectroscopy on long range excited states of the sodium molecule has also been used to determine an accurate atomic sodium lifetime at 0.3 % accuracy [8].

References:

1. R. Napolitano, J. Weiner, C. J. Williams, and P. S. Julienne, Phys. Rev. Lett. 73, 1353 (1994).
2. K. M. Jones, P. S. Julienne, P. D. Lett, W. D. Phillips, E. Tiesinga, and C. J. Williams, Europhys. Lett. 35, 85 (1996). (See pdf file below for reprint.)
3. W. C. Stwalley, Y.-H. Uang, and G. Pichler, Phys. Rev. Lett. 41, 1164 (1978).
4. W. I. McAlexander, E.R.I. Abraham, and R. G. Hulet, Phys. Rev. A, 54(1) R5-R8 (1996).
5. C. Linton, F. Martin, I. Russier, A. J. Ross, P. Crozet, S. Churassy, and R. Bacis, J. Mol. Spect. 175, 340 (1996).
6. H.M.J.M. Boesten, C. J. Tsai, J. R. Gardner, D. J. Heinzen, and B. J. Verhaar, Z. Phys. 37, 323 (1996); a 3 percent determination is reported in J. R. Gardner, et al., Phys. Rev. Lett. 74, 3764 (1995).
7. W. C. Stwalley, private communication (1996).
8. E. Tiemann, H. Knoeckel, and H. Richling, Z. Phys. D 37, 323-332 (1996).
   

Papers/Preprints:
(PDF documents readable with free Adobe Acrobat Reader.)

(498 kB) A Spectroscopic Determination of Scattering Lengths for Sodium Atom Collisions. Eite Tiesinga, Carl J. Williams, Paul S. Julienne, Kevin M. Jones, Paul D. Lett, and William D. Phillips, Journal of Research of the National Institute of Standards and Technology, 101(4), (July-August 1996).

(681 kB) Measurement of the atomic Na(3P) lifetime and of retardation in the interaction between two atoms bound in a molecule. K. M. Jones, P. S. Julienne, P. D. Lett, W. D. Phillips, E. Tiesinga, and C. J. WIlliams, Europhys. Lett. 35, 85 (1996).

Contact:

Paul Lett
National Institute of Standards and Technology
PHYS A167
100 Bureau Drive, Stop 8424
Gaithersburg, MD 20899-8424
(301) 975-6559
e-mail: paul.lett@nist.gov


Kevin Jones
Dept. of Physics
Williams College
Williamstown, MA 01267
(413) 597-2123
e-mail: Kevin.Jones@williams.edu