This thesis describes the formation of electronically excited but translationally cold molecules formed from rubidium atoms and two isotopes of ytterbium (176Yb and 174Yb) by means of photoassociation. The experiments were performed in a combined MOT with 1e9 rubidium atoms and 2e6 ytterbium atoms at temperatures of less than 1mK.
Photoassociation lines were found by trap loss spectroscopy throughout a wavelength range of 2nm near the 795nm D1 transition in rubidium. The majority of lines belong to two vibrational series in the excited YbRb* molecule, converging on a system of a ground state ytterbium atom and an excited rubidium atom. The strong variation of line strength between different vibrational lines is explained through the Franck-Condon principle. An improved version of the Leroy-Bernstein equation was used to extract the leading dispersion coeffcient of the potential from the vibrational progression.
Most of the observed lines show a resolved rotational structure as expected from a basic quantum mechanical model. The series
terminates with the third or forth rotational component due to the ground state centrifugal barrier.The measured rotational constants agree very well with calculations based on the C6 coefficient.
The discovery of a splitting of the rotational components into subcomponents indicates an uncommon angular momentum coupling described by Hund's case (e). Variations in the depth of the subcomponents indicates a similar splitting in the ground state, with the energies of the substates based on the alignment of the rubidium atom's magnetic dipole moment relative to the angular momentum carried by an approaching ytterbium
atom. This creates an additional ground state barrier, partially suppressing some of the subcomponents.
Using a rate equation model developed for this purpose, a maximum formation rate of 2.5e6 molecules per second was calculated over the volume of the entire trap.
The work presented here is an important step on the way to the formation of molecules that are not only translationally ultracold, but also in the electronic and rovibrational ground state.
This thesis describes the formation of electronically excited but translationally cold molecules formed from rubidium atoms and two isotopes of ytterbium (176Yb and 174Yb) by means of photoassociation. The experiments were performed in a combined MOT with 1e9 rubidium atoms and 2e6 ytterbium atoms at temperatures of less than 1mK.
Photoassociation lines were found by trap loss spectroscopy throughout a wavelength range of 2nm near the 795nm D1 transition in rubidium. The majority of lines belong to two vibrational series in the excited YbRb* molecule, converging on a system of a ground state ytterbium atom and an excited rubidium atom. The strong variation of line strength between different vibrational lines is explained through the Franck-Condon principle. An improved version of the Leroy-Bernstein equation was used to extract the leading dispersion coeffcient of the potential from the vibrational progression.
Most of the observed lines show a resolved rotational structure as expected from a basic quantum mechanical model. The series
terminates with the third or forth rotational component due to the ground state centrifugal barrier.The measured rotational constants agree very well with calculations based on the C6 coefficient.
The discovery of a splitting of the rotational components into subcomponents indicates an uncommon angular momentum coupling described by Hund's case (e). Variations in the depth of the subcomponents indicates a similar splitting in the ground state, with the energies of the substates based on the alignment of the rubidium atom's magnetic dipole moment relative to the angular momentum carried by an approaching ytterbium
atom. This creates an additional ground state barrier, partially suppressing some of the subcomponents.
Using a rate equation model developed for this purpose, a maximum formation rate of 2.5e6 molecules per second was calculated over the volume of the entire trap.
The work presented here is an important step on the way to the formation of molecules that are not only translationally ultracold, but also in the electronic and rovibrational ground state.
Within the scope of this thesis, the production of ultracold molecules at a temperature of a few µK with various isotopes of rubidium (Rb) and ytterbium (Yb) was examined by means of photoassociation spectroscopy and magnetic Feshbach resonances in combined conservative traps. The long-term goal of this experiment is the production of ultracold RbYb molecules in the rovibronic ground state.
It was possible to produce electronically excited ^{87}Rb ^{176}Yb molecules in a novel hybrid trap (HT) at a combined temperature of 1.7 µK by means of 1-photon photoassociation close to the Rb D1 line at 795 nm. This HT takes advantage of the different magnetic properties of Rb and Yb and allows for independent trapping and manipulation of the atomic species. It combines an Ioffe-Pritchard type magnetic trap for Rb and a near-resonant optical dipole trap for Yb. The excited molecular ^2 Pi_1/2 state could be characterized further extending previous works in a combined MOT and vibrational levels reaching binding energies up to E_b=-h x 2.2 THz could be assigned by trap-loss spectroscopy.
Almost every detected vibrational state consists of two resonances that could be assigned to the molecular analogue of the hyperfine structure of ^{87}Rb. An important experimental observation is a decrease in hyperfine splitting with increasing binding energy of a vibrational level. For the deepest found vibrational state the hyperfine splitting amounts only 70 % of the atomic value (817 MHz) which emphasizes a gradual passage from weakly to tightly bound molecules.
Furthermore, detailed attempts were undertaken to induce magnetic Feshbach resonances in ^{85}Rb and different Yb isotopes, especially ^{171}Yb in a crossed optical dipole trap at 1064 nm at temperatures of 10 µK. For this purpose, a homogeneous magnetic field was applied and scanned in small steps over the range of 495 G... 640 G. Unfortunately, our efforts were without success. Additionally, well known as well as not yet reported Rb_2 Feshbach resonances in ^{85}Rb and ^{87}Rb were detected. These resonances allowed for a detailed analysis of the typical behaviour of Feshbach resonances.
The first experimental realization of photoassociation spectroscopy in a conservative is also a major step towards the ultimate project goal of producing ultracold RbYb molecules in the electronic ground state. A spatial confinement, which is in principle possible in the HT, is a prerequisite for transferring the molecules to the rovibronic ground state, e.g. with the help of a STIRAP process.
Summary form only given. We report results from an efficient new method for producing translationally cold (T<1 mK) potassium molecules in the ground X/sup 1//spl Sigma//sub g//sup +/ electronic state. We use the two-photon R-transfer scheme conceptually similar to a proposal by Band and Julienne (1995). First, ultracold potassium atoms in a MOT are photoassociated at long range to the 1/sub g/ molecular electronic state, which correlates to the 1/sup 1//spl Pi//sub g/ state at short range. The resulting molecules are then excited with a second laser to the 5/sup 1//spl Pi//sub u/ or 6/sup 1//spl Pi//sub u/ Rydberg states. These Rydberg states have favorable Franck-Condon factors for spontaneous decay into low vibrational levels of the ground state. The molecules are detected by use of two-color resonant ionization with pulsed lasers. By scanning one of the detection lasers, we observe transitions from several vibrational levels of the X state. The largest observed molecule formation rate in a single vibrational level is about 10/sup 5/ molecules/s for /spl nu/=25. Molecules in the lowest vibrational levels, including /spl nu/=0, are also observed with slightly lower rates of production. The translational temperature of the ground-state molecules is measured by analyzing their ballistic decay from the detection region. It is approximately 500 /spl mu/K, roughly the same as the estimated temperature of potassium atoms in the MOT.
This thesis describes the spectroscopy work on ultracold 39K 85Rb molecules as part of our effort towards producing ground-state ultracold polar molecules. ^ The starting points of our experiment are laser cooled and trapped K and Rb atoms in dual-species MOTs and the molecule formation method is photoassociation (PA). A fraction of the PA-produced excited-state molecules can spontaneously emit to the two lowest electronic states, the X 1Σ + state and the a 3Σ + state, to form high vibrational level molecules. In order to develop a population transfer scheme to make deeply bound molecules, we have studied the spectroscopy of KRb in several steps. Besides the PA step, we have also carried out a state-selective detection experiment to study the population distribution of the high vibrational level molecules formed via spontaneous emission. The detection method is pulsed-laser based ion time-of-flight mass spectroscopy. Due to the linewidth limitations of the pulsed laser, this detection method can only resolve vibrational structures. A cw laser induced ion depletion spectroscopy technique has been developed in order to obtain rotational resolution. Using this ion depletion method, we have studied a possible stimulated Raman transfer scheme via the 3 1Σ+ state. To this end, we have obtained all the spectroscopic information required to produce deeply bound KRb molecules in a single vibrational and rotational level. ^ Several other spectroscopic studies using the high vibrational level ultracold molecules are also discussed. ^
This thesis presents the development of an experimental apparatus designed to investigate the ultracold collisional properties for mixtures of Cs and Yb, with a long-term view to the creation of ultracold CsYb molecules via indirect cooling methods. The unpaired electron spin that is inherent to molecules of this form gives rise to a magnetic dipole moment in addition to a ground state electric dipole moment. This enables extra control over molecular interactions and should enable the experimental simulation of spin lattice models. We focus on the implementation of a system designed to controllably laser cool and optically trap Yb. The first step in this system is the production of a magneto-optical trap (MOT) on the triplet 1S0 to 3P1 transition of Yb. With careful control over the cooling beam detunings and power, gravitational-assisted Doppler cooling allows samples of Yb to be prepared at 22 uK. This regime of enhanced Doppler cooling is investigated and proves to be a crucial step to ensuring good transfer of cold Yb to optical traps. The construction and characterisation of single and crossed beam optical dipole traps for Yb are discussed. The single beam optical trap has been used to verify a model for the optical trapping of Yb in its ground state. This trap has also been utilised as a tool for the measurement of the light shift on the 1S0 to 3P1 transition at a wavelength of 1070~nm. In the main experimental sequence, Yb atoms are loaded from the magneto-optical trap into the crossed optical dipole trap, allowing evaporative cooling ramps to quantum degeneracy to be performed. This highly-reproducible system typically forms Bose-Einstein condensates with 2 x 10^5 174Yb atoms. This thesis additionally reports on the progress made towards measurements of the interspecies scattering length for 133Cs and Yb isotopes. We present two approaches that are being developed in tandem: rethermalisation in a conservative trap, and two-photon photoassociation. Progress towards rethermalisation measurements has focussed on developing systems for the efficient transfer of Cs to an optical trap. For photoassociative measurements, a laser system has been developed and tested by producing one-photon photoassociation spectra of Cs2.
Heteronuclear polar molecules have recently attracted enormous attention owing to their ground state having a large electric dipole moment. The long range anisotropic dipole-dipole interaction in such systems is the basis for a variety of applications including quantum computing, precision measurements, ultracold chemistry and quantum simulations. Heteronuclear bi-alkali molecules, only a small subset of polar molecules, have received special attention mainly because the constituent alkali atoms are easy to laser cool and can be relatively easily associated to form molecules at ultracold temperatures. Our choice, the LiRb molecule, is motivated by the relatively high dipole moment (4.1 Debye) of the LiRb molecule in its rovibronic ground state. In this thesis, we study the LiRb molecule using laser spectroscopy and report, for the first time, the production of ultracold LiRb molecules by photoassociation (PA). The LiRb molecule is the least studied among all bi-alkali molecules and the first spectroscopic measurements on hot vapor phase LiRb molecules were reported only in 2011. We describe these measurements and their significance in chapter 2, after a brief introduction to the field of ultracold polar molecules in chapter 1. In chapter 3, we discuss our apparatus for simultaneous cooling and trapping of Li and Rb atoms in a dual-species magneto-optical trap (MOT) and report the measurement of interspecies collision-induced losses. In chapter 4, we describe the production of ultracold LiRb molecules in excited electronic states by photoassociation (PA). We report the measurements of the C6 coefficients for the Li (2s 2S1/2) + Rb (5p 2P1/2) and the Li (2s 2S1/2) + Rb (5p 2P3/2) asymptotes. We find a molecule formation rate (PLiRb) as high as 3.5x107 s-1 and a PA rate coefficient (KPA) as high as 1.3x10-10 cm3/s, the highest among heteronuclear bi-alkali molecules. In chapter 5, we discuss results on two-photon PA and we conclude, in chapter 6, with a road roadmap of future experiments for the production and detection of ultracold LiRb molecules in their rovibronic ground state.
Photoassociation of two ultracold rubidium atoms and the subsequent formation of stable molecules in the singlet ground and lowest triplet states is investigated theoretically. The method employs laser pulses inducing transitions via excited states correlated to the 5S+5P1∕2 asymptote. Weakly bound molecules in the singlet ground or lowest triplet state can be created by a single pulse while the formation of more deeply bound molecules requires a two-color pump-dump scenario. More deeply bound molecules in the singlet ground or lowest triplet state can be produced only if efficient mechanisms for both pump and dump steps exist. While long-range 1∕R3 potentials allow for efficient photoassociation, stabilization is facilitated by the resonant spin-orbit coupling of the 0u+ states. Molecules in the singlet ground state bound by a few wave numbers can thus be formed. This provides a promising first step toward ground-state molecules which are ultracold in both translational and vibrational degrees of freedom.