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Physical Review A

Purely-long-range krypton molecules in singly and doubly excited binding potentials

Z. S. Smith, A. Harmon, J. Banister, R. Norman, Kathleen M. Hoogeboom-Pot, M. Walhout

Published Jan 14, 2010• 6 citations
Publisher

Abstract

Diatomic potentials for krypton are computed and also probed experimentally. For a probe-laser wavelength near $811$ nm, several strong dipole-dipole interactions produce purely-long-range potential wells in the singly excited manifold of ($s+p$) potentials and in the doubly excited manifold of ($p+p$) and ($s+d$) potentials. Evidence of resonant photoassociation into bound states of these potential wells is observed in the emission of ions and ultraviolet photons from a magneto-optically trapped krypton cloud.

DOI
10.1103/physreva.81.013407

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Photoassociation spectra for purely long-range (PLR) krypton molecules are both computed and measured. The photoassociation laser is tuned just red of the 5s[3/2]2-5p[5/2]2 transition at 810.6 nm, over a frequency range in which fourteen different PLR potential wells support hundreds of vibrational bound states. In the experiment, laser light is directed into a cloud of magneto-optically trapped 84Kr atoms, and the emission of both ultraviolet photons and ions is monitored. Peaks in these signals are observed for laser frequencies within 200 MHz of the atomic resonance. Ongoing work is aimed at improving the experimental signal and understanding the degree of agreement between empirical data and computed molecular spectra.

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•Jan 1, 1998•0 citations
Diatomic potentials for krypton are computed and also probed experimentally. For a probe-laser wavelength near 811811811 nm, several strong dipole-dipole interactions produce purely-long-range potential wells in the singly excited manifold of (s+ps+ps+p) potentials and in the doubly excited manifold of (p+pp+pp+p) and (s+ds+ds+d) potentials. Evidence of resonant photoassociation into bound states of these potential wells is observed in the emission of ions and ultraviolet photons from a magneto-optically trapped krypton cloud.
Photoassociation spectra for purely long-range (PLR) krypton molecules are both computed and measured. The photoassociation laser is tuned just red of the 5s[3/2]2-5p[5/2]2 transition at 810.6 nm, over a frequency range in which fourteen different PLR potential wells support hundreds of vibrational bound states. In the experiment, laser light is directed into a cloud of magneto-optically trapped 84Kr atoms, and the emission of both ultraviolet photons and ions is monitored. Peaks in these signals are observed for laser frequencies within 200 MHz of the atomic resonance. Ongoing work is aimed at improving the experimental signal and understanding the degree of agreement between empirical data and computed molecular spectra.
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.
The photoassociation spectrum of Kr-F collision pairs in thermal equilibrium (293$\sp\circ$K) to KrF(B) has been observed using a laser-induced fluorescence technique with a tunable probe over the wavelength range 219 nm ≤\leq≤ λ\lambdaλ ≤\leq≤ 250 nm. The spectrum was then analyzed by numerical calculation of the Franck-Condon factors for the bound ←\gets← free transitions. The numerical calculation provided information on the KrF(X) and KrF(B) potential as well as the KrF(B →\to→ X) electronic dipole moment. Saturation intensities for photoassociation produced population were determined at 248 nm for gas mixtures with Kr partial pressures in the range 125 Torr ≤\leq≤ p$\sb{\rm Kr}$ ≤\leq≤ 1000 Torr and 1 Torr of F$\sb2$. Two-photon absorption of Kr/F$\sb2$ mixtures was also observed over the wavelength range 220 nm ≤\leq≤ λ\lambdaλ ≤\leq≤ 250 nm. The mechanism of the two-photon absorption is determined to be photodissociation of F$\sb2$ followed by photoassociation of non-thermalized Kr-F collision pairs.
The work presented in this dissertation represents a comprehensive study on long-range interactions between alkali Rydberg atoms. As a consequence of their size, Rydberg atoms possess very large dipole moments, making them highly sensitive to external fields and to other Rydberg atoms. The strong dipole-dipole interaction between two Rydberg atoms results in a mixing of their angular momentum characters, which is the cornerstone of the work presented here. We describe the ℓ-mixing between various local molecular asymptotes through a residual Coulomb potential, assuming no electron cloud overlap. We then directly diagonalize an interaction Hamiltonian consisting of these Rydberg-Rydberg interactions and spin-orbit (fine structure) coupling in the Hund's case (c). In this manner, we calculate potential energy curves for various molecular symmetries and find that some of these curves exhibit deep potential wells, capable of supporting very extended bound molecular states ( macrodimers). We analyze the specific structure of the ℓ-mixing for these potential wells and derive convenient n-scaling relations for both their equilibrium separations Re and well depths De. We also explore various properties of the macrodimers, including their response to small electric fields and stability with respect to predissociation. We note throughout the dissertation that the detection of such extended dimers could help progress studies in a variety of areas such as exotic, ultracold chemistry and quantum information and computing. To facilitate detection, we propose a scheme to form and study the macrodimers via photoassociation (PA). We present calculated PA lineshapes from two different electronic ground states based on a harmonic trapping model. We find that both PA signals vary significantly with the laser detunings, indicating that different ℓ-characters could be probed at different equilibrium lengths. Finally, we extend the two-body formalism to the case of three interacting Rydberg atoms and present preliminary results for triply excited 58p rubidium atoms along a common axis. We provide a few examples of surface plots corresponding to different local asymptotes, each with a unique three-dimensional "landscape" and discuss the relevance of the physical features. The highlight of our analysis is the appearance of a potential well, which indicates the formation of a linear trimer. ^
We explore long-range interactions between two atoms excited into high principal quantum number n Rydberg states, and present calculated potential energy curves for various symmetries of doubly excited n s and n p rubidium atoms. We show that the potential curves for these symmetries exhibit deep (∼ GHz) potential wells, which can support very extended (∼ µm) bound vibrational states ( macrodimers ). We present n -scaling relations for both the depth D e of the wells and the equilibrium separations R e of these macrodimers, and explore their response to small electric fields and stability with respect to predissociation. Finally, we present a scheme to form and study these macrodimers via photoassociation, and show how one can probe the various ℓ-character of the potential wells.
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.