We theoretically explore photoassociation by adiabatic passage of two colliding cold
85Rb atoms in an atomic trap to form an ultracold
Rb2 molecule. We consider the incoherent thermal nature of the scattering process in a trap and show that coherent manipulations of the atomic ensemble, such as adiabatic passage, are feasible if performed within the coherence time window dictated by the temperature, which is relatively long for cold atoms. We show that a sequence of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ pulses of moderate intensities, each lasting $\ensuremath{\sim}750\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, can photoassociate a large fraction of the atomic ensemble at temperature of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and density of
1011atoms∕cm3. Use of multiple pulse sequences makes it possible to populate the ground vibrational state. Employing spontaneous decay from a selected excited state, one can accumulate the molecules in a narrow distribution of vibrational states in the ground electronic potential. Alternatively, by removing the created molecules from the beam path between pulse sets, one can create a low-density ensemble of molecules in their ground rovibrational state.
We propose a scheme of 'R-type' photoassociative adiabatic passage (PAP) to create polar molecular condensates from two different species of ultracold atoms. Due to the presence of a quasi-coherent population trapping state in the scheme, it is possible to associate atoms into molecules with a low-power photoassociation (PA) laser. One remarkable advantage of our scheme is that a tunable atom–molecule coupling strength can be achieved by using a time-dependent PA field, which exhibits larger flexibility than using a tunable magnetic field. In addition, our results show that the PA intensity required in the 'R-type' PAP could be greatly reduced compared to that in a conventional 'Λ-type' one.
This dissertation presents the study of how two laser pulses can bind sodium and cesium atoms at ultracold temperature (
T=200muK) into an ultracold, polar, diatomic molecule with a definite quantum state. \nA single-channel scattering model represents the initial continuum state, and two different models represent the intermediate state: one excluding spin-orbit coupling effects in the intermediate state, the other accounting for such effects. \nWe calculate the
A1Sigma+−b3Pi spin-orbit coupled wave functions using a basis expansion technique, and validate the results by comparing to experimentally obtained, spin-orbit coupled energy levels. \nThe computation of photoassociation rates between the continuum state and the intermediate states reveals the crucial importance of spin-orbit coupling. \nFurthermore, this study shows how the spectral bandwidth (narrow \\vs broad), the chirping (chirped vs. unchirped), the detunings, the intensities, and the pulse delay (intuitive \\vs counter-intuitive sequence) of the lasers affect the transfer of population from the continuum scattering state to a comparatively low-lying (
vX=32,JX=0) rovibrational state of the
X1Sigma+ ground electronic state of NaCs. \nThe transfer process relies either on a sequence of
pi-pulses, or uses stimulated Raman adiabatic passage (STIRAP). \nLasers with narrow spectral bandwidth (
0.5GHz) always yield a final population in
ketX1Sigma+,vX=32,JX=0 higher than 95\\% in less than 4 ns.
This theoretical paper presents numerical calculations for the photoassociation of ultracold cesium atoms with a chirped laser pulse and a detailed analysis of the results. In contrast with earlier work, the initial state is represented by a stationary continuum wave function. In the chosen example, it is shown that an important population transfer is achieved to $\ensuremath{\approx}15$ vibrational levels in the vicinity of the
v=98 bound level in the external well of the ${0}_{g}^{\ensuremath{-}}(6s+6{p}_{3∕2})$ potential. Such levels lie in the energy range swept by the instantaneous frequency of the pulse, thus defining a ``photoassociation window.'' Levels outside this window may be significantly excited during the pulse, but no population remains there after the pulse. Finally, the population transfer to the last vibrational levels of the ground $a\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\sum}_{u}^{+}(6\mathrm{s}+6\mathrm{s})$ state is significant, making stable molecules. The results are interpreted in the framework of a two-state model as an adiabatic inversion mechanism, efficient only within the photoassociation window. The large value found for the photoassociation rate suggests promising applications. The present chirp has been designed in view of creating in the excited state a vibrational wave packet that is focusing at the barrier of the double-well potential.
We investigate theoretically the preparation of ultracold photoassociated Cs 2 molecules in the lowest vibrational level of the ground electronic state via the stimulated Raman adiabatic passage (STIRAP) by solving the time-dependent Schrödinger equation using the mapped Fourier grid method. A negative chirped laser pulse is used to produce the unstable photoassociated molecules in the excited electronic state. A dump pulse is utilized to transfer a partial population of the unstable photoassociated molecules to the vibrational v″ = 18 level of the ground electronic state. This part of population is then transferred to the v″ = 0 level of the ground electronic state by the pump and Stokes laser pulses via an intermediate state which is taken to be the v′ = 7 level of the excited electronic state, forming the stable photoassociated molecules. The population transfer efficiency from v″ = 18 to v″ = 0 in the ground electronic state reaches 96.2% via the STIRAP.
We develop a theoretical description of photoassociative stimulated Raman adiabatic passage (STIRAP) near a Feshbach resonance in a thermal atomic gas. We show that it is possible to use low-intensity laser pulses to directly excite pairs of atoms in the continuum near a Feshbach resonance and to transfer most of the atomic cloud to the lowest rovibrational level of the molecular ground state. For a broad resonance, commonly found in several mixtures of alkali atoms, our model predicts a transfer efficiency up to 97% for a given atom pair, and up to 70% when averaged over an atomic ensemble. The laser intensities and pulse durations needed for such an optimal transfer, 10 2 –10 3 W cm −2 and several microseconds, are easily achievable experimentally. A single pair of STIRAP pulses converts an estimated fraction f ∼10 −6 –10 −4 of atoms in an atomic ensemble, leading to the production of 10–1000 molecules in a large sample of 10 7 atoms. A total of ∼10 4 –10 6 pulse pairs are thus required to transfer most atoms into molecules. Such an efficiency compares with or surpasses currently available techniques for creating stable diatomic molecules, and the versatility of this approach simplifies its potential use for many molecular species.