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PHOTOASSOCIATION OF ULTRACOLD ATOMS: A NEW TOOL FOR MOLECULAR SPECTROSCOPY

William C. Stwalley

Published Jan 1, 1998• 0 citations
OpenAlex

Abstract

The field of laser cooling and trapping of atoms has had a profound impact on many areas of atomic, molecular and optical science. In particular, ultracold atoms colliding in magneto-optical traps typically have relative kinetic energies of $300\\mu K (\\sim 7 MHz)$, so free-bound absorption lines are as sharp as bound-bound lines. Moreover, because of very long range centrifugal barriers, only a few collisional partial waves are important ($J^{\\prime\\prime} = 0, 1$ and 2 in our work on $^{39}K$). Such photoassociative free-bound absorption allows observation of ``pure long-range molecule'' states ($R_{e} = 28$ and 39 {\\AA} in $^{39}K_{2}$) as well as long-range levels of known short range states with outer classical turning points of $\\sim 20 - 250 {\\AA}$. Such spectra have been observed by ``trap loss'' (decrease in atomic fluorescence), molecular ionization, and fragment ionization. Optical-optical double resonance (OODR), e.g. through the ``Franck-Condon windows'' of the ``pure long-range'' states, produces readily assignable spectra at higher asymptotes as well. Further extensions of ultracold photoassociative spectroscopy and applications to determination of long range potentials, atomic properties, and Bose-Einstein condensate stability will be presented.

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The relationship of photoassociative spectroscopy of ultracold atoms to atomic line broadening

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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 as well. As a first step, we are studying single- and multicolor photoassociation of ultracold (∼300 μK) 39K atoms confined in a magneto-optical trap. 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 (l=0,1,2). Potential energy curves derived from these spectra test electronic structure and long-range perturbation theory calculations of interatomic potentials. In particular, such studies yield the full set of potential energy curves needed for fully quantum calculations of atomic line broadening at any temperature.

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Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold $(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

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High-resolution photoassociation spectroscopy

Joseph D. Miller, Robert A. Cline, D. J. Han +1 more

With the ultracold temperatures (T 1012 cm-3) of atoms that can be produced with laser-cooling and trapping techniques, it is now possible to carry out a new kind of high-resolution spectroscopy.1 This photoassociation is based on optical absorption resonances between the initially free states of colliding atoms and specific, bound excited molecular states. High resolution can be obtained because of the narrow energy distribution of the ultracold gas.1 Because the initial states are formed by collisions, it becomes possible to probe unique, excited molecular states with very large interatomic separations that are impossible to probe with conventional molecular spectroscopy. The feasibility of this new spectroscopy has been demonstrated in two recent experiments.2,3

•May 8, 1994•0 citations
The field of laser cooling and trapping of atoms has had a profound impact on many areas of atomic, molecular and optical science. In particular, ultracold atoms colliding in magneto-optical traps typically have relative kinetic energies of 300muK(sim7MHz)300\\mu K (\\sim 7 MHz)300muK(sim7MHz), so free-bound absorption lines are as sharp as bound-bound lines. Moreover, because of very long range centrifugal barriers, only a few collisional partial waves are important (Jprimeprime=0,1J^{\\prime\\prime} = 0, 1Jprimeprime=0,1 and 2 in our work on 39K^{39}K39K). Such photoassociative free-bound absorption allows observation of pure long-range molecule'' states ($R_{e} = 28$ and 39 {\\AA} in $^{39}K_{2}$) as well as long-range levels of known short range states with outer classical turning points of $\\sim 20 - 250 {\\AA}$. Such spectra have been observed by trap loss'' (decrease in atomic fluorescence), molecular ionization, and fragment ionization. Optical-optical double resonance (OODR), e.g. through the Franck-Condon windows'' of the pure long-range'' states, produces readily assignable spectra at higher asymptotes as well. Further extensions of ultracold photoassociative spectroscopy and applications to determination of long range potentials, atomic properties, and Bose-Einstein condensate stability will be presented.
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 as well. As a first step, we are studying single- and multicolor photoassociation of ultracold (∼300 μK) 39K atoms confined in a magneto-optical trap. 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 (l=0,1,2). Potential energy curves derived from these spectra test electronic structure and long-range perturbation theory calculations of interatomic potentials. In particular, such studies yield the full set of potential energy curves needed for fully quantum calculations of atomic line broadening at any temperature.
Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold (<1mK)(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})(<1mK) temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.
Advances in laser cooling of neutral atoms have made possible a new form of high-resolution laser spectroscopy: photoassociation of ultracold atoms. 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 (<1 mK), 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 photo association line shapes, are reviewed.
We report on the observation of photoassociation resonances in ultracold collisions between ^{23}Na^{40}K molecules and ^{40}K atoms. We perform photoassociation in a long-wavelength optical dipole trap to form deeply bound triatomic molecules in electronically excited states. The atom-molecule Feshbach resonance is used to enhance the free-bound Franck-Condon overlap. The photoassociation into well-defined quantum states of excited triatomic molecules is identified by observing resonantly enhanced loss features. These loss features depend on the polarization of the photoassociation lasers, allowing us to assign rotational quantum numbers. The observation of ultracold atom-molecule photoassociation resonances paves the way toward preparing ground-state triatomic molecules, provides a new high-resolution spectroscopy technique for polyatomic molecules, and is also important to atom-molecule Feshbach resonances.
We report the first observation of optical-optical double resonance photoassociative spectroscopy of ultracold 39{}^{39}39K atoms near the highly excited 39K(4s)+39K(4d,5d,6d,6s,7s)^{39}K(4s)+^{39}K(4d,5d,6d,6s,7s)39K(4s)+39K(4d,5d,6d,6s,7s) asymptotes in a high density magneto-optical trap. Using the ${v}^{\ensuremath{'}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ level of the ${0}_{g}^{\ensuremath{-}}$ ``pure long-range'' state (a localized long-range Franck-Condon window) as the intermediate state, we observe simple and assignable rovibrational progressions dissociating to these asymptotes. The dominant long-range potential constants are determined. Line broadening, presumably due to autoionization, and rotational excitations due to strong field effects are observed.
With the ultracold temperatures (T 1012 cm-3) of atoms that can be produced with laser-cooling and trapping techniques, it is now possible to carry out a new kind of high-resolution spectroscopy.1 This photoassociation is based on optical absorption resonances between the initially free states of colliding atoms and specific, bound excited molecular states. High resolution can be obtained because of the narrow energy distribution of the ultracold gas.1 Because the initial states are formed by collisions, it becomes possible to probe unique, excited molecular states with very large interatomic separations that are impossible to probe with conventional molecular spectroscopy. The feasibility of this new spectroscopy has been demonstrated in two recent experiments.2,3