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Journal of Physics B Atomic Molecular and Optical Physics

Time-independent and time-dependent photoassociation of spin-polarized rubidium

H. M. J. M. Boesten, Chin‐Chun Tsai, Daniel J. Heinzen, A J Moonen, BOUDEWIJN J. VERHAAR

Published Jan 1, 1999• 20 citations
Publisher

Abstract

We extract information about collisions of ultra-cold ground-state rubidium atoms from observations of a g-wave shape resonance in the 85Rb + 85Rb system via time-independent and time-dependent photoassociation. The shape resonance arises from a quasi-bound state inside a centrifugal barrier that enhances the excitation to the bound electronically excited state by the photoassociation laser in the time-independent experiment. The shape resonance is sufficiently long-lived that its build-up through the barrier can be observed by first depleting it via a photoassociation laser pulse and then measuring the rate of photoassociation by a second laser pulse with a variable delay time. A combined method of analysis of the time-independent and time-dependent experiments is presented. We discuss the spectroscopy of states of two particles with spin trapped inside a centrifugal barrier, interacting via direct and indirect spin-spin interactions.

DOI
10.1088/0953-4075/32/2/012

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We extract information about collisions of ultra-cold ground-state rubidium atoms from observations of a g-wave shape resonance in the 85Rb + 85Rb system via time-independent and time-dependent photoassociation. The shape resonance arises from a quasi-bound state inside a centrifugal barrier that enhances the excitation to the bound electronically excited state by the photoassociation laser in the time-independent experiment. The shape resonance is sufficiently long-lived that its build-up through the barrier can be observed by first depleting it via a photoassociation laser pulse and then measuring the rate of photoassociation by a second laser pulse with a variable delay time. A combined method of analysis of the time-independent and time-dependent experiments is presented. We discuss the spectroscopy of states of two particles with spin trapped inside a centrifugal barrier, interacting via direct and indirect spin-spin interactions.
We observe the time dependence of a cold-atom collision in a pulsed photoassociation experiment. For a ggg-wave shape resonance in the 85Rb+85Rb{}^{85}\mathrm{Rb}{+}^{85}\mathrm{Rb}85Rb+85Rb system we measure the time needed to build up the resonant state by tunneling through the centrifugal barrier. Combining this with time-independent 85Rb{}^{85}\mathrm{Rb}85Rb and 87Rb{}^{87}\mathrm{Rb}87Rb photoassociation we determine the resonance energy and find evidence for the decay of the shape resonance into inelastic channels. We also determine the 85Rb+85Rb{}^{85}\mathrm{Rb}+{}^{85}\mathrm{Rb}85Rb+85Rb and 87Rb+87Rb{}^{87}\mathrm{Rb}{+}^{87}\mathrm{Rb}87Rb+87Rb C6{C}_{6}C6​ coefficient and triplet scattering lengths without relying on ab initio calculations.
We observe a shape resonance in the scattering of two ultracold 87Rb^{87}\mathrm{Rb}87Rb atoms, causing the colliding atoms to form a long-living compound system inside an l=2 centrifugal barrier. Its existence follows from a photoassociation experiment in a gas sample of doubly polarized 87Rb^{87}\mathrm{Rb}87Rb atoms. Using it we are able to carry out direct determinations of the triplet scattering length for 87Rb^{87}\mathrm{Rb}87Rb, relevant to Bose-Einstein condensation experiments, and of the Rb+Rb C6{\mathrm{C}}_{6}C6​ dispersion coefficient. Consequences for the 85Rb^{85}\mathrm{Rb}85Rb scattering length are discussed.
We experimentally investigate various processes present in the photoassociative interaction of an ultracold atomic sample with shaped femtosecond laser pulses. We demonstrate the photoassociation of pairs of rubidium atoms into electronically excited, bound molecular states using spectrally cut femtosecond laser pulses tuned below the rubidium D1 or D2 asymptote. Time-resolved pump-probe spectra reveal coherent oscillations of the molecular formation rate, which are due to coherent transient dynamics in the electronic excitation. The oscillation frequency corresponds to the detun-ing of the spectral cut position to the asymptotic transition frequency of the rubidium D1 or D2 lines, respectively. Measurements of the molecular photoassociation signal as a function of the pulse energy reveal a non-linear dependence and indicate a non-perturbative excitation process. Chirping the association laser pulse allowed us to change the phase of the coherent transients. Furthermore, a signature for molecules in the electronic ground state is found, which is attributed to molecule formation by femtosecond photoassociation followed by spontaneous decay. In a subsequent article [A. Merli et al., submitted] quantum mechanical calculations are presented, which compare well with the experimental data and reveal further details about the observed coherent transient dynamics.
We experimentally investigate various processes present in the photoassociative interaction of an ultracold atomic sample with shaped femtosecond laser pulses as an detailed extension of previous work [W. Salzmann et al., Phys. Rev. Lett. 100, 233003 (2008)]. We demonstrate the photoassociation of pairs of rubidium atoms into electronically excited, bound molecular states using spectrally cut femtosecond laser pulses tuned below the rubidium D1{D}_{1}D1​ or D2{D}_{2}D2​ asymptote. Time-resolved pump-probe spectra reveal oscillations of the molecular formation rate, which are due to coherent transient dynamics in the electronic excitation. The oscillation frequency corresponds to the detuning of the spectral cut position to the asymptotic transition frequency of the rubidium D1{D}_{1}D1​ or D2{D}_{2}D2​ lines, respectively. Measurements of the molecular photoassociation signal as a function of the pulse energy reveal a nonlinear dependence and indicate a nonperturbative excitation process. Chirping the association laser pulse allowed us to change the phase of the coherent transients. Furthermore, a signature for molecules in the electronic ground state is found, which is attributed to molecule formation by femtosecond photoassociation followed by spontaneous decay. In a subsequent article [A. Merli et al., Phys. Rev. A 80, 063417 (2009)] quantum mechanical calculations are presented, which compare well with the experimental data and reveal further details about the observed coherent transient dynamics.
This thesis comprises two topics in the field of the ultracold atomic and molecular physics, both involving formation of ultracold 85Rb 2 by photoassociation (PA). Typically, these ground-state molecules are formed mainly in the very highest vibrational levels, rather than the low-v levels that will be required for most future investigations of ultracold molecular physics and chemistry. The first topic of my research is a method for circumventing this problem: resonant enhancement (RE) of photoassociative ground-state molecule production. The key idea of RE is to photoassociate pairs of colliding atoms into an excited state that has a bimodal vibrational probability distribution, with one peak at long range to enhance the photoassociation rate and another at shorter range to enhance radiative decay into deeply bound levels of the ground state. This can be arranged thanks to the strong spin-orbit perturbative interaction between the two 0+u states converging to two different asymptotes, (5s + 5p1/2) and (5s + 5 p3/2). This method enhances ground-state molecule formation in the region well below the dissociation limit. The experimental work described here was done at the University of Connecticut in collaboration with C. P. Koch at the Institut für Theoretische Physik, Freie Universität Berlin, Germany, who performed the theoretical analysis [1].^ The second part of my thesis is the investigation of ultracold molecules in an optical dipole trap. We have constructed a quasi-electrostratic optical trap (QUEST) for 85Rb using a focused CO2 laser [2]. This provides a very high density of ultracold atoms, up to 10 12 cm-3, providing much higher photoassociation rates. It also keeps the molecules trapped once they have formed. The QUEST provides an ample trapping lifetime for both trapped atoms and molecules, yielding about 4-5×106 atoms and about 9×105 high-v molecules. The long lifetime of the trapped molecules and atoms, combined with their high density, allows us to study interactions such as atom-molecule or molecule-molecule collisions in the ultracold regime. A preliminary experiment on Rb2 molecular decay is described. ^
We report the first observation of the highest lying bound levels of the ground electronic state of ^85Rb2 by means of optical double resonance (free-bound-bound) photoassociation spectroscopy of colliding, laser-cooled ^85Rb atoms. Twelve levels with binding energies from 0.13 to 20 Ghz are observed. We carry out an inverse perturbation analysis of these levels that takes full account of the hyperfine-induced singlet-triplet mixing with a coupled channels approach. This analysis yields previously undetermined atomic interaction parameters that are crucial for the understanding of Rb ultracold collisions and Bose-Einstein condensates. Complete consistency is obtained with all previously measured cold collision properties of Rb. We gratefully acknowledge the support at Texas of the R. A. Welch Foundation, the NSF, and the Microgravity Science and Applications Division of NASA.