Ultracold atomic physics offers an experimentalist the opportunity to perform sensitive tests and measurements that are not possible at higher temperatures. In this case it allows for access to molecular states, through photoassociation, that are inaccessible to conventional techniques. We use photoassociation to measure quasi-bound levels of the 2 1Σg+ state of 85Rb2 . With these data, we examine a coupling between the short-range Hund’s case (c) 2 (0g+ ) component and the long-range 2 (1g ) state, which corresponds to the 1 1Πg state. A second experiment studies the mechanism of short-range photoassociation and finds our simple model to have strong predictive power. In a third experiment, we photoassociatively form vibrationally excited a 3 Σu+ molecules. We then excite these molecules using a frequency-doubled pulse-amplified CW laser to create low-n long-range Rydberg states of 85Rb2 . Autoionization and ion detection of these states gives high-resolution spectra, which are compared to theoretical potential curves.
We present analysis of low-
n long-range molecular Rydberg states in
85Rb
2, based on high-resolution spectra. The weakly bound states are accessed by bound-bound transitions from high-
v levels of the
a,3Sigmau+ state, which are prepared by photoassociation of laser-cooled atoms. Single-photon transitions to target states near the
5s+7p asymptote are excited by a frequency-doubled pulse-amplified CW laser with a narrow linewidth, under 200 MHz. The long-range portion of the bonding potential is dominated by the elastic scattering interaction of the Rydberg electron of a perturbed
7p atom and a nearby ground-state atom, in much the same manner as trilobite states. We use time of flight to selectively measure molecular ions, which are formed via autoionization. This technique gives a two orders-of-magnitude improvement in linewidth over our previous work, reported in Ref. [1]. We also present calculations of a proposed scheme for STIRAP transfer from the current
v′′=35 level of the
a,3Sigmau+ state to the
v′′=39 level. The long-range states accessible to us are defined in large part by the Franck-Condon factors, which are dominated by the outer lobe of the wavefunction. Thus, choosing a
v′′ sets
R, and determines the Franck-Condon window. The proposed
v′′=39 level has a classical outer turning point at
sim72,a0, and will provide access to higher-
n states with longer-range wells. This work is supported by the NSF and AFOSR.\ noindent [1] M. A. Bellos textit{et al.}, Phys. Rev. Lett. textbf{111}, 053001 (2013)
In the first part of this dissertation we develop and successfully demonstrate two methods, based on photoassociation of ultracold potassium atoms, for producing ultracold K2 molecules, with sub mK temperatures, in deeply bound vibrational levels of the ground XS+g 1 state. The principal objective has been to develop an “ultracold molecule factory” that can be used to support experiments on spectroscopy, collisions, and chemistry of ultracold molecules. ^ In the first experiment the ground-state molecules are produced by the radiative decay of excited, n≅191 AS+u 1 state molecules, photoassociated from ultracold potassium atoms in a vapor-cell MOT. The decay is primarily into the vibrational continuum, but a small fraction decays into bound levels of the X state near n = 36. The experimentally observed molecule production rate is about 103 per second. ^ In our second experiment, we use two-step laser excitation to effect an “R-transfer” from long-range to intermediate internuclear separations, greatly increasing the efficiency for radiative production of low- n molecules in the X state. Molecules are formed in the Pg1 state by photoassociation, then excited by a second laser to the 51Pu or 61Pu Rydberg states, in selected vibrational levels from n = 13–18. Finally, radiative decay produces very large numbers of X state molecules with n≅25 and significant numbers of molecules in very low vibrational levels, including n = 0. We have observed production rates up to about 105 molecules/second per vibrational level. ^ In both experiments the ground-state molecules are detected with a sensitive and selective resonant two-color photoionization with nanosecond pulses. ^ In the second part of this dissertation we demonstrate a novel method for high sensitivity absorption measurements, that combines a phase modulated, high-resolution pulsed laser with an actively stabilized optical cavity to achieve a sensitivity of ∼10−9 per cm. This is comparable to the best existing methods for pulsed absorption measurements, but unlike most other methods, it can also be readily extended to the UV region and to Doppler-free spectroscopy. ^
In recent years several groups of researchers have succeeded in making samples of cold molecules, at temperatures below 1 K, and of ultracold molecules, at temperatures below 1 mK. We concentrate on the formation of Cs2 dimers. Starting with laser-cooled atoms, photoassociation forms molecules in an excited electronic state. Various mechanisms are presented for stabilizing these molecules by spontaneous emission into a bound level of the ground electronic state. The design of new formation schemes requires precise knowledge of molecular potential curves that is well beyond the accuracy of ab initio calculations. We show that analytical long-range potentials can be fitted on photoassociation spectra and yield, through the determination of the C3 long-range coefficient, a value of the radiative lifetime of the 6P3/2 cesium level with an unprecedented accuracy. For short-range potentials it is possible to avoid explicit calculations by using asymptotic methods that generalize to long-range molecules the quantum-defect concepts developed for Rydberg atoms. Considering two coupled channels, we show how generalized Lu–Fano plots can extract precise information from experimental spectra.
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.
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.
Very high-lying vibrational levels of the 1g∼1 1Πg electronic state of K2 have been observed in a photoassociation experiment using an ion detection scheme. The photoassociation measurements have been treated together with laser induced fluorescence data recorded by Fourier transform spectrometry to construct a pointwise potential curve for this electronic state. Several vibrational levels were observed by both techniques, so the dissocation energy can be deduced without extrapolation, from the sum of binding energies measured in photoassociation and vibrational energies established with respect to the potential minimum (defined by the fluorescence data), giving De=1290.292±0.002 cm−1 (1 standard deviation). The potential curve reproduces over 2000 term values (up to v=138) with a root mean square error of 0.0025 cm−1. Nevertheless, small differences are found between the rotational constants generated from the potential curve and the effective rotational constants deduced from binding energy measurements at very high v. The smoothness of the outermost part of the pointwise potential energy curve has been investigated through fits to a Hund’s case (c) asymptotic model.