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 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. ^
Ultracold heteronuclear polar KRb molecules have enjoyed tremendous attention for many years now. The presence of a large molecule fixed dipole moment in the absolute rovibrational ground state (v''=0, J'' =0) makes external control very convenient, which opens up the prospect of studying various physical phenomenon in many areas of physics and chemistry. The methods used so far to produce the absolute ground state of KRb have involved very complicated techniques. Molecules formed in weakly bound levels were transferred to the absolute ground state through the multiple steps of transfer processes involving several lasers with extremely narrow linewidths. My thesis research demonstrates an alternate simpler method of formation of the lowest vibrational levels of the ground state utilizing PA combined with radiative decay, which has been so far useful to form ultracold molecules in the higher vibrational levels of the ground state. This thesis presents two main topics of research. The first part of the thesis consists of results describing the scheme which provides a single-step, continuous, and irreversible method of forming KRb molecules in their absolute ground state. The second topic involves spectroscopy on the two excited electronic states of KRb which may provide alternative pathways for the formation of the absolute ground state (X 1Σ+, (v''=0, J'' =0)) and suitable pathways for efficient formation of the lowest metastable state (a 3Σ+, (v''=0, N'' =0)). I believe this research will lay the foundation for many new exciting experiments in the future.
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. ^
We present spectroscopic studies of ultracold LiRb molecules using ionization detection. The molecules are created by cooling and trapping Li and Rb atoms in overlapping magneto-optical traps (MOTs) and using light resonant with a free-bound transition to create weakly bound excited state molecules in a process known as photoassociation (PA). We explore weakly bound vibrational levels of LiRb with PA spectroscopy using ionization detection and, where possible, compare our results with earlier measurements performed in our lab using trap-loss spectroscopy. In addition, we also probe vibrational levels of the ground triplet electronic state and excited electronic states using resonantly enhanced multiphoton ionization (REMPI) spectroscopy. We identify several vibrational levels of the α3Σ+, (3)3ΠΩ and (4)3Σ + states and compare our observations with theoretical calculations. As LiRb is one of the least studied heteronuclear diatomic molecules, these studies are aimed towards exploring the molecular structure. The spectroscopic work is also in line with the long-term project goals of transferring ultracold LiRb molecules into the lowest rotational and vibrational levels of the ground singlet electronic state. Molecules in this rovibronic ground state possess a large electric dipole moment, which is essential for application of ultracold molecules in various quantum computation schemes. The rovibronic ground state will also be an ideal starting point for investigating molecular entangled states.
We report on the use of depletion spectroscopy to detect ultracold ground-state KRb molecules with rotational resolution. The population of a specific ground-state vibrational level ${v}^{\ensuremath{''}}$, produced by photoassociation of ultracold atoms, is monitored by one-color two-photon pulsed-laser ionization. When a cw laser is resonant with a rovibrational transition to an excited state, the ground-state population, and hence the ion signal, is depleted. This narrow-band spectroscopic technique allows the individual rotational levels in both ground and excited states to be resolved, and thus the population of a single ground-state rovibrational level to be monitored. These resolved transitions are a necessary first step in transferring population from high vibrational levels of the ground state, such as produced by photoassociation, to the absolute molecular ground state $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ (${v}^{\ensuremath{''}}=0$, ${J}^{\ensuremath{''}}=0$). This technique can also be used to determine binding energies of high-${v}^{\ensuremath{''}}$ molecules. Combining our binding energy measurement with the results of previous spectroscopy yields an improved dissociation energy of the X state: ${D}_{e}=4217.822\ifmmode\pm\else\textpm\fi{}0.003\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We also report on the two-color pulsed-laser ionization used to locate the depletion transitions.
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.
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.