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Physical Review A

Photoassociative spectroscopy of ultracold K39 atoms in a high-density vapor-cell magneto-optical trap

H. Wang, P. L. Gould, William C. Stwalley

Published Mar 1, 1996• 88 citations
Publisher

Abstract

We have performed photoassociative spectroscopy of ultracold potassium atoms in a high-density vapor-cell magneto-optical trap (MOT) in which atoms are confined predominantly in "dark" hyperfine levels that do not interact with the trapping light. Three well-resolved vibrational series (one strong and two weak) of the $^{39}\mathrm{K}_{2}$ molecule are observed within 8 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the $4^{2}P_{\frac{3}{2}}$ atomic limit. One of the series is identified as the ${0}_{g}^{\ensuremath{-}}$ pure long-range state and the other two are assigned to the ${0}_{u}^{+}$ and ${1}_{g}$ states. These photoassociative spectra are obtained by measuring reductions in the trap fluorescence due to trap loss induced by an intense photoassociation laser. By using a vapor-cell-dark MOT technique, we achieve atomic densities greater than ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, increasing our trap loss signals by an order of magnitude compared to a normal MOT.

DOI
10.1103/physreva.53.r1216

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We have performed photoassociative spectroscopy of ultracold potassium atoms in a high-density vapor-cell magneto-optical trap (MOT) in which atoms are confined predominantly in "dark" hyperfine levels that do not interact with the trapping light. Three well-resolved vibrational series (one strong and two weak) of the 39K2^{39}\mathrm{K}_{2}39K2​ molecule are observed within 8 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}cm\ensuremath−1 below the 42P324^{2}P_{\frac{3}{2}}42P23​​ atomic limit. One of the series is identified as the 0g\ensuremath−{0}_{g}^{\ensuremath{-}}0g\ensuremath−​ pure long-range state and the other two are assigned to the 0u+{0}_{u}^{+}0u+​ and 1g{1}_{g}1g​ states. These photoassociative spectra are obtained by measuring reductions in the trap fluorescence due to trap loss induced by an intense photoassociation laser. By using a vapor-cell-dark MOT technique, we achieve atomic densities greater than 1011{10}^{11}1011 cm\ensuremath−3{\mathrm{cm}}^{\ensuremath{-}3}cm\ensuremath−3, increasing our trap loss signals by an order of magnitude compared to a normal MOT.
PHOTOASSOCIATIVE SPECTROSCOPY OF Cs2Cs_{2}Cs2​ MOLECULE
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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 (Re=28R_{e} = 28Re​=28 and 39 {\\AA} in 39K2^{39}K_{2}39K2​) as well as long-range levels of known short range states with outer classical turning points of sim20−250AA\\sim 20 - 250 {\\AA}sim20−250AA. 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.
Rovibrational levels (v=0--30) of the ${0}_{\mathrm{g}}^{\mathrm{\ensuremath{-}}}$ ``pure long-range'' state of 39K2^{39}\mathrm{K}_{2}39K2​ are observed by photoassociative spectroscopy of ultracold potassium atoms prepared in a high-density magneto-optical trap. The measured molecular binding energies are used to precisely determine the long-range dipole-dipole potential constant ${\mathrm{C}}_{3}^{\mathrm{\ensuremath{\Pi}}}$ of the K(4s)+K(4p) asymptote and the radiative lifetime of the 4p state. We determine ${\mathrm{C}}_{3}^{\mathrm{\ensuremath{\Pi}}}$=-5.456(9) zJ nm3{\mathrm{nm}}^{3}nm3 (-8.445(14) a.u.), \ensuremath{\tau}(42{\mathrm{}}^{2}2P3/2{\mathrm{P}}_{3\mathrm{/}2}P3/2​)=26.34(5) ns, and \ensuremath{\tau}(42{\mathrm{}}^{2}2P1/2{\mathrm{P}}_{1\mathrm{/}2}P1/2​)=26.69(5) ns. Our result is a significant improvement in precision over previous lifetime measurements and agrees well with the recent fast-beam experiment [U. Volz and H. Schmoranzer, Phys. Scr. T65, 48 (1996)].
Creating ultracold molecules has attracted considerable interest in the last decade. Once created, such molecules can be used for precision spectroscopy or to study chemical reactions at ultracold tem peratures. Several techniques have been developed to produce ultracold molecules; the most common is photoassociation where two ultracold atoms collide in the presence of light that induces a free-bound transition to an excited molecular state. Photoassociation can also be used to perform spectroscopy in order to map out the ro-vibrational levels of a molecular state. In this dissertaiotn, we report on our Photoasociative Spectroscopy (PAS) studies conducted separately in argon and krypton. For each species, we have studied transitions near two different atomic limits ns[3/2]2 —> np[5/2]2 and ns[3/2]2 —» np[5/2]3where n = 4 for argon and n = 5 for krypton. The former atomic transition is called the “quench transition” since it forms a strong coupled channel to the ground state. As a result for this strong coupling the original atomic sample is degraded and quenched. The latter transition is the one used to cool and confine the atoms and is known as the “trapping transition.” Spectroscopy near the trapping transition in argon was studied previously in our group [1] and specific features were observed in the spectrum. At the time, the features were not definitively identified, but more than one explanation was suggested for further testing. One possible explanation was that these features were a result of resonances happening at “doubly-excited” molecular levels. The population of such states could take place as a result of absorbing two photons of the same frequency which causes photoassociation. In this dissertation we report on a series of experiments that were performed to study and conclusively identify the origin of those features. These experiments led to the conclusion that the features in the spectra were an artifact of otherwise undetectable frequency sidebands on our semiconductor diode laser. Once identified as such, a new laser was constructed to repeat the spectroscopy measurements in argon and take new measurements in krypton that would be free from laser artifacts. In those spectra, no specific vibrational features were observed and resolved in either species. Results were consistent with published results obtained by other groups using other noble gases. Finally, we report on our attempts to perform PAS on a quench transition in order to confirm results obtained for krypton by another group [2]. We were not able to observe any photoassociation signal for both argon and krypton on this transition. We were able to observe similar effects to those reported in Ref. [2] but we attribute these to artifacts from an acousto-optic modulator and not as arising from molecular structure.
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.