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Journal of Physics B Atomic Molecular and Optical Physics

Formation, detection and spectroscopy of ultracold Rb2in the ground X1Σg+state

Yanyan Huang, J Qi, Hyewon Pechkis, Dajun Wang, Edward E. Eyler, P. L. Gould, William C. Stwalley

Published Sep 25, 2006• 52 citations
Publisher

Abstract

We form ultracold ground-state Rb 2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0 u + state, which decays radiatively into high vibrational levels of the X 1 Σ g + state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2 1 Σ u + state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2 3 Π u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v = 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest- v levels by the same photoassociation laser that produces them.

DOI
10.1088/0953-4075/39/19/s04

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We form ultracold ground-state Rb 2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0 u + state, which decays radiatively into high vibrational levels of the X 1 Σ g + state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2 1 Σ u + state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2 3 Π u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v = 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest- v levels by the same photoassociation laser that produces them.
We investigate the production of ultracold ground state X 1 σ + (v = 0) RbCs molecules in the lowest vibrational level via short-range photoassociation followed by spontaneous emission. The starting point is the laser cooled 85 Rb and 133 Cs atoms in a dual species, forced dark magneto-optical trap. The special intermediate level (5)0 + (v = 10) correlated to the (2) 3 π electric state is achieved by the photoassociation process. The formed ground state X 1 σ + (v = 0) molecule is resonantly excited to the 2 1 π intermediate state by a 651 nm pulse laser and is ionized by a 532 nm pulse laser and then detected by the time-of-flight mass spectrum. Saturation of the photoionization spectroscopy at large ionization laser energy is observed and the ionization efficiency is obtained from the fitting. The production of ultracold ground state 85 Rb 133 Cs molecules is facilitative for the further research about the manipulation of ultracold molecules in the rovibrational ground state.
We have studied resonance-enhanced two-photon ionization of ultracold 39 K 85 Rb molecules in highly excited long-range vibrational levels of the X 1 Σ + and a 3 Σ + states. These molecules are formed by photoassociation (PA) of ultracold 39 K and 85 Rb atoms, followed by spontaneous emission to the X 1 Σ + and a 3 Σ + states. In the range 15 500–17 200 cm −1 , we observe many intermediate and long-range levels of the previously unobserved 4 3 Σ + and 4 1 Σ + states and evidence for the onset of the previously unobserved 3 3 Π state. The observed vibrational spectra of these states are in very good agreement with calculated vibrational levels based on the ab initio potentials of Rousseau, Allouche and Aubert-Frécon. Such experiments illustrate some of the unique advantages of using PA-produced ultracold molecules for investigation of long-range vibrational levels and long-range forces, and for unravelling dense spectra because only a few rotational levels are populated.
The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the 85 Rb(5S 1/2 ) + 133 Cs(6P 3/2 ) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0 + or the 4th 0 − electronic states correlated to the Rb(5P 1/2, 3/2 ) + Cs(6S 1/2 ) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic ground state.
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. ^
Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a (3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].
We report on the state-selective detection of near-dissociation ultracold KRb molecules in the ground $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ state and the metastable $a\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Sigma}^{+}$ state. The molecules are produced by photoassociation of ultracold atoms followed by radiative decay into high vibrational levels of the XXX and aaa states. Detection utilizes resonance-enhanced one-color two-photon ionization, followed by time-of-flight mass spectroscopy. Scanning the detection laser frequency over the range 582−−625nm582--625\phantom{\rule{0.3em}{0ex}}\mathrm{nm}582−−625nm, we observe transitions from the ${v}^{\ensuremath{''}}=86--92$ levels of the XXX state, which are bound by up to $30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and the ${v}^{\ensuremath{''}}=17--23$ levels of the aaa state, which are also bound by up to $30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The measured vibrational spacings are in excellent agreement with those previously measured and those calculated from the relevant potential curves. Relative vibrational populations are also consistent with Franck-Condon factors for decay from the photoassociated levels.