Ultracold Photoassociation Research Articles

Observation of Photoassociation Resonances in Ultracold Atom-Molecule Collisions.

We report on the observation of photoassociation resonances in ultracold collisions between ^{23}Na^{40}K molecules and ^{40}K atoms. We perform photoassociation in a long-wavelength optical dipole trap to form deeply bound triatomic molecules in electronically excited states. The atom-molecule Feshbach resonance is used to enhance the free-bound Franck-Condon overlap. The photoassociation into well-defined quantum states of excited triatomic molecules is identified by observing resonantly enhanced loss features. These loss features depend on the polarization of the photoassociation lasers, allowing us to assign rotational quantum numbers. The observation of ultracold atom-molecule photoassociation resonances paves the way toward preparing ground-state triatomic molecules, provides a new high-resolution spectroscopy technique for polyatomic molecules, and is also important to atom-molecule Feshbach resonances.

Hyperfine structure of the NaCs b3Π2 state near the dissociation limit 3S1/2 + 6P3/2 observed with ultracold atomic photoassociation.

We report new observations of the hyperfine structure in three ro-vibrational levels of the b3Π2 state of NaCs near the dissociation limit 3S1/2 + 6P3/2. The experiment was done via photoassociation of ultracold atoms in a dual-species dark-spot magneto-optical trap, and the spectra were measured as atomic trap losses. The simulation of the hyperfine structure showed that the greater part of the observed structure belongs to almost isolated levels of the b3Π2 state, but there are other parts of mixed character where the contribution from the 1Σ symmetry dominates.

Electromagnetically induced transparency in two-colour ultracold photoassociation

We show how to apply the density matrix approach, including radiative relaxation processes, to calculate the linear optical susceptibility due to a probe laser beam propagated through a photoassociation (PA) system of ultracold atoms. We describe the absorptive and dispersive properties of a two-colour PA system where one laser drives a continuum–bound photoassociative transition and the other laser a bound–bound transition. Taking either of the lasers as a probe and the other one as a control field, we discuss the physical conditions under which electromagnetically induced transparency is possible in such continuum systems. We illustrate our numerical results using two-colour PA in ultracold 174Yb as an example.

Short-range photoassociation from the inner wall of the lowest triplet potential of 85Rb2

Ultracold photoassociation is typically performed at large internuclear separations, where the scattering wavefunction amplitude is large and Franck–Condon overlap is maximized. Recently, work by this group and others on alkali-metal diatomics has shown that photoassociation can efficiently form molecules at short internuclear distance in both homonuclear and heteronuclear dimers. We propose that this short-range photoassociation is due to excitation near the wavefunction amplitude maximum at the inner wall of the lowest triplet potential. We show that Franck–Condon factors (FCFs) from the highest-energy bound state can almost precisely reproduce FCFs from a low-energy scattering state, and that both calculations match experimental data from the near-zero positive-energy scattering state with reasonable accuracy. We also show that the corresponding photoassociation from the inner wall of the ground-state singlet potential at much shorter internuclear distance is weaker and undetectable under our current experimental conditions. We predict from FCFs that the strongest of these weaker short-range photoassociation transitions are one order of magnitude below our current sensitivity.

A renormalized potential-following propagation algorithm for solving the coupled-channels equations.

We derive a general renormalized potential-following propagation method that efficiently solves the coupled-channels equations. The step size is variable, the method is compatible with reactive boundary conditions, and the algorithm may be combined with other renormalized algorithms, such as renormalized Numerov. We diagonalize the coupling matrix and consider piece-wise constant and linear reference potentials. The constant reference potential algorithm is very simple to implement, yet for multichannel problems almost as accurate as the linear reference potential method. The applicability of the proposed algorithms to realistic problems is demonstrated for cold collisions of NH radicals. The renormalized approach has the advantage of producing wave functions in a straightforward way, which is illustrated for a shape resonance in NH-NH collisions. These scattering wave functions can be used to study ultracold photoassociation and near-threshold photodissociation.

Line Shape Analysis of Ultracold Heteronuclear Molecular Photoassociation Spectroscopy by Resonance-Enhanced Two-Photon Ionization

Resonance-enhanced two-photon ionization (RETPI) is a powerful tool in ultracold molecular photoassociation spectroscopy measurement. Using the RETPI, the line shape of ultracold RbCs molecular photoassociation spectroscopy, which contains intensity and full width at half maximum, is measured and analyzed systematically with varied photoassociation laser intensity and ionization laser energy. The saturation of photoassociation spectroscopy at a high ionization laser energy is observed and the suppression phenomenon is observed at high photoassociation laser intensity. The saturation intensity of RbCs molecular photoassociation, the linewidths of excited state molecules in different rovibrational levels at zero photoassociation laser intensity, and two-photon ionization efficiency have been derived from the line shape of photoassociation spectroscopy. The line shape analysis of photoassociation spectroscopy is important for precision spectroscopic analysis.

Observation and analysis of resonant coupling between nearly degenerate levels of the21Σg+and11Πgstates of ultracold85Rb2

We report on the anomalously high line strength of a single rotational level in the ultracold photoassociation of two ${}^{85}$Rb atoms to form ${}^{85}$Rb${}_{2}$. The ${v}^{\ensuremath{'}}=111$, ${J}^{\ensuremath{'}}=5$ level belongs to the Hund's case (c) $2\phantom{\rule{0.16em}{0ex}}({0}_{g}^{+})$ state, which correlates to the Hund's case (a) $2{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ state. Its strength is caused by coupling with a very near-resonant long-range state. The long-range component is the energetically degenerate ${v}^{\ensuremath{'}}=155$, ${J}^{\ensuremath{'}}=5$ level of the case (c) $2\phantom{\rule{0.16em}{0ex}}({1}_{g})$ state, correlating to the case (a) $1{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{g}$ state. The line strength is enhanced by an order of magnitude through this coupling, relative to nearby vibrational levels and even to nearby rotational levels of the same vibrational level. This enhancement is in addition to the enhancement seen in all ${J}^{\ensuremath{'}}=3$ and 5 levels of the $2\phantom{\rule{0.16em}{0ex}}({0}_{g}^{+})$ state due to an $l=4$ shape resonance in the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state continuum, which alters the distribution of levels formed by photoassociation.

Vacuum-induced coherence in ultracold photoassociative rovibrational excitations

We show that coherence between two excited ro-vibrational states belonging to the same molecular electronic configuration arises quite naturally due to their interaction with electromagnetic vacuum. For initial preparation of a molecule in the desired ro-vibrational states, we propose to employ the method of ultracold photoassociation. Spontaneous decay of the excited molecule then gives rise to vacuum induced coherence between the excited ro-vibrational states. We demonstrate theoretically an interesting interplay of effects due to vacuum induced coherence and photoassociation. We apply our theory to photoassociation of bosonic Ytterbium (^{174}Yb) atoms which appear to be a promising system for exploring such interplay. The effects discussed here can be important for controlling decoherence and dissipation in molecular systems.

Experimental investigation on ionized ultracold molecules formed in a magneto-optical trap by time-of-flight mass spectroscopy

We use time-of-flight mass spectroscopy to detect the ultracold ground state Cs2 molecules formed directly by a photoassociation due to the trapping laser beams and repumping laser beam in a magneto-optical trap. We investigate the dependences of the Cs2+ intensity on operating voltage of micro-channel-plate, accelerating field intensity and duration time of the accelerating field. The experimental results are consistent with theoretical fitting. We obtain the best optimal parameters suited to our experimental condition, which is important for the future experiment on ultracold molecule photoassociation and photoionization spectroscopy.

Resonant enhancement of the ultracold photoassociation rate by an electric field-induced anisotropic interaction

We study the effects of a static electric field on the photoassociation of a heteronuclear atom pair into a polar molecule. The interaction of permanent dipole moment with a static electric field largely affects the ground state continuum wavefunction of the atom pair at short separations where photoassociation transitions occur according to the Franck–Condon principle. Electric field-induced anisotropic interaction between two heteronuclear ground state atoms leads to scattering resonances at some specific electric fields. Near such resonances the amplitude of the scattering wavefunction at short separation increases by several orders of magnitude. As a result, the photoassociation rate is enhanced by several orders of magnitude near the resonances. We discuss in detail electric field-modified atom–atom scattering properties and resonances. We calculate the photoassociation rate that shows giant enhancement due to electric field tunable anisotropic resonances. We present selected results among which particularly important are the excitations of higher rotational levels in ultracold photoassociation due to electric field tunable resonances.

Relativistic calculations of ground and excited states of LiYb molecule for ultracold photoassociation spectroscopy studies

We report a series of quantum-chemical calculations for the ground and some of the low-lying excited states of an isolated LiYb molecule by the spin-orbit multistate complete active space second-order perturbation theory (SO-MS-CASPT2). Potential energy curves, spectroscopic constants, and transition dipole moments (TDMs) at both spin-free and spin-orbit levels are obtained. Large spin-orbit effects especially in the TDMs of the molecular states dissociating to Yb((3)P(0,1,2)) excited states are found. To ensure the reliability of our calculations, we test five types of incremental basis sets and study their effect on the equilibrium distance and dissociation energy of the ground state. We also compare CASPT2 and CCSD(T) results for the ground state spectroscopic constants at the spin-free relativistic level. The discrepancies between the CASPT2 and CCSD(T) results are only 0.01 Å in equilibrium bond distance (R(e)) and 200 cm(-1) in dissociation energy (D(e)). Our CASPT2 calculation in the supermolecular state (R=100 a.u.) with the largest basis set reproduces experimental atomic excitation energies within 3% error. Transition dipole moments of the super molecular state (R=100 a.u.) dissociating to Li((2)P) excited states are quite close to experimental atomic TDMs as compared to the Yb((3)P) and Yb((1)P) excited states. The information obtained from this work would be useful for ultracold photoassociation experiments on LiYb.

Rotational excitations in two-color photoassociation

We show that it is possible to excite higher rotational states J > 2 in ultracold photoassociation by two laser fields. Usually higher J states are suppressed in photoassociation at ultracold temperatures in the regime of Wigner threshold laws. We propose a scheme in which one strong laser field drives photoassociation transition close to either J = 1 or J = 2 rotational state of a particular vibrational level of an electronically excited molecule. The other laser field is tuned near photoassociation resonance with J > 2 rotational levels of the same vibrational state. The strong laser field induces a strong continnum-bound dipole coupling. The resulting dipole force between two colliding atoms modifies the continnum states forming continnum-bound dressed states with a significant component of higher partial waves in the continnum configuration. When the second laser is scanned near the resonance of the higher J states, these states become populated due to photoassociative transitions from the modified continnum.

Momentum control in photoassociation of ultracold atoms

Ultracold photoassociation is a process in which two cold atoms combine to form a molecule. This process is crucially dependent on the atom pair density at close distance termed the photoassociation window. We explore the possibilities for increasing the pair density at the photoassociation window by using a prepulse to accelerate the pairs of atoms at large interatomic separation toward each other. We show that the signal of a subsequent photoassociation pulse could be enhanced by a factor of one to two orders of magnitude with respect to the conventional continuous wave experiment. For neutral encounters on the ground potential surface which scales as ${R}^{\ensuremath{-}6}$ the acceleration becomes negligible. The electronic excited state potentials scale for homonuclear $S\ensuremath{\rightarrow}P$ transitions more favorably as ${R}^{\ensuremath{-}3}$. A possible pump-dump mechanism for acceleration, excites a pair of atoms at large distance employing the natural acceleration on the excited state. Then a dump pulse moves the population back to the ground surface. By controlling the phase and the amplitude of the light field this scenario can be optimized. In addition the momentum partitioned between the ground and excited surfaces can also be controlled. The possibility for pure quantum light-induced acceleration due to a gradient of the transition dipole is analyzed. Significant acceleration can be obtained only for pulses with intensities above ${10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ and pulse duration of $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$.

Light forces in ultracold photoassociation

We study the time-resolved photoassociation of ultracold sodium in an optical dipole trap. The photoassociation laser excites pairs of atoms to molecular states of large total angular momentum at high intensities (above 20 kW/cm$^{2}$). Such transitions are generally suppressed at ultracold temperatures by the centrifugal barriers for high partial waves. Time-resolved ionization measurements reveal that the atoms are accelerated by the dipole potential of the photoassociation beam. We change the collision energy by varying the potential depth, and observe a strong variation of the photoassociation rate. These results demonstrate the important role of light forces in cw photoassociation at high intensities.

Publisher's Note: Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering [Rev. Mod. Phys.78, 483 (2006)

Publisher's Note: Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering [Rev. Mod. Phys.78, 483 (2006)

Storage ring to investigate cold unidimensional atomic collisions

In this paper we employ a circulating ring of trapped atoms, that we have named the atomotron, to study cold collisions. The atomotron is obtained from a conventional magneto-optical trap when the two pairs of normally retroreflecting Gaussian laser beams in the $x\text{\ensuremath{-}}y$ plane are slightly offset. Circulating stable atomic orbits then form a racetrack geometry in this plane. The circulating atom flux behaves similarly to an atomic beam with an average tangential velocity much greater than the transverse components, and is therefore suitable for one-dimensional atomic collision studies. Using the atomotron, we have investigated the polarization dependence of ultracold photoassociation collisions between Rb atoms circulating in the racetrack. The ability to investigate collisions in ultracold circulating atomic rings reveals alignment and orientation properties that are averaged away in ordinary three-dimensional magneto-optical trap collision processes.

The photoassociative spectroscopy, photoassociative molecule formation, and trapping of ultracold $\mathsf{^{39}K^{85}Rb}$

We have observed the photoassociative spectra of colliding ultracold 39K and 85Rb atoms to produce KRb* in all eight bound electronic states correlating with the 39K (4s) + 85Rb(5p 1/2 and 5p 3/2) asymptotes. These electronically excited KRb* ultracold molecules are detected after their radiative decay to the metastable triplet (a $^{3}\Sigma^{ + })$ state and (in some cases) the singlet (X $^{1}\Sigma^{ + })$ ground state. The triplet (a $^{3}\Sigma^{ + })$ ultracold molecules are detected by two-photon ionization at 602.5 nm to form KRb + , followed by time-of-flight mass spectroscopy. We are able to assign a majority of the spectrum to three states (2(0 + ), 2(0-), 2(1)) in a lower triad of states with similar C 6 values correlating to the K(4s) + Rb (5p 1/2) asymptote; and to five states in an upper triad of three states (3(0 + ), 3(0-), 3(1)) and a dyad of two states (4(1), 1(2)), with one set of similar C 6 values within the upper triad and a different set of similar C 6 values within the dyad. We are also able to make connection with the short-range spectra of Kasahara et al. [J. Chem. Phys. 111, 8857 (1999)], identifying three of our levels as v = 61, 62 and 63 of the 1 $^{1}\Pi \sim $ 4(1) state they observed. We also argue that ultracold photoassociation to levels between the K(4s) + Rb (5p 3/2) and K(4s) + Rb (5p 1/2) asymptotes may be weakly or strongly predissociated and therefore difficult to observe by ionization of a $^{3}\Sigma^{ + }$ (or X $^{1}\Sigma^{ + })$ molecules; we do know from Kasahara et al. that levels of the 1 $^{1}\Pi \sim $ 4(1) and 2 $^{1}\Pi \sim $ 5(1) states in the intra-asymptote region are predissociated. A small fraction ( $\le $ 1/3) of the triplet (a $^{3}\Sigma^{ + })$ ultracold molecules formed are trapped in the weak magnetic field of our magneto-optical trap (MOT).

Intensity effects in ultracold photoassociation line shapes

We derive ultracold atom-atom photoassociation line shapes valid for intense light fields and investigate laser power effects in sodium and rubidium photoassociation to the purely long-range ${0}_{g}^{\ensuremath{-}}$ symmetry state. We consider intensities up to a few hundreds ${\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ a strongly saturating intensity for typical transitions of experimental interest. For these intensities the ${0}_{g}^{\ensuremath{-}}$ rotational spectrum is still well resolved; however, it is essential to couple the photoassociation resonance to both s- and d-wave ground-state channels. A low-energy d-wave shape resonance can have a profound effect on the line shape. Understanding the line shape is essential for precision spectroscopic analysis and could improve the extraction of ground-state scattering properties such as scattering lengths.

Fine-Structure Predissociation of Ultracold PhotoassociatedK239Molecules Observed by Fragmentation Spectroscopy

We report the first observation of fragmentation spectra of the fine-structure predissociated long-range ${1}_{g}$ and ${0}_{u}^{+}$ $(b^{3}\ensuremath{\Pi}_{{0}^{+}u})$ states of ultracold $(T<1\mathrm{mK})$ ${}^{39}{\mathrm{K}}_{2}$ molecules. The ${1}_{g}$ state predissociates via the curve crossing at long range $(R\ensuremath{\sim}23\AA{})$ with the repulsive ${0}_{g}^{+}$ state. The ${0}_{u}^{+}$ state, however, predissociates predominantly through molecular fine-structure mixing at short range with the $b^{3}\ensuremath{\Pi}_{1u}$ component. Our technique of fragmentation spectroscopy is a sensitive and selective tool for ultracold photoassociation and fine-structure-changing collisions.