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Physical Review A

Unitary photoassociation: One-step production of ground-state bound molecules

Shimshon Kallush, Ronnie Kosloff

Published Feb 29, 2008• 17 citations
Publisher

Abstract

Bound-state molecules can be photoassociated directly from ultracold free-atom pairs by excitation to a purely repulsive electronic state. The process is explained on the basis of quantum unitarity: the initially free-scattering state is transformed by an impulsive light pulse to a deformed superposition which contains bound-state components. For pulse durations which are short compared to the ultracold dynamics, the maximal rate of photoassociation was found to be determined by the initial stationary distribution of scattering states of the atom pairs. The process was simulated for an ultracold gas of $^{87}\text{R}\text{b}$ with a temperature of $T=44\text{ }\ensuremath{\mu}\text{K}$ and a density of $\ensuremath{\approx}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}3}$. Transform-limited pulses maximize the photoassociation, yielding $\ensuremath{\sim}1$ bound molecule per pulse. Coherent control calculated by a local control scheme can increase the photoassociation yield by two orders of magnitude.

DOI
10.1103/physreva.77.023421

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Effects of rotation in the laser-pulse photoassociation in a thermal gas of atoms

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Photoassociation is a possible route for the formation of chemical bonds. In this process, the binding of colliding atoms can be induced by means of a laser field. Photoassociation has been studied in the ultracold regime and also with temperatures well above millikelvins in the thermal energy domain, which is a situation commonly encountered in the laboratory. A photoassociation mechanism can be envisioned based on the use of infrared pulses to drive a transition from free colliding atoms on the electronic ground state to form a molecule directly on that state. This work takes a step in this direction, investigating the laser-pulse-driven formation of heteronuclear diatomic molecules in a thermal gas of atoms including rotational effects. Based on the assumption of full system controllability, the maximum possible photoassociation yield is deduced. The photoassociation probability is calculated as a function of the laser parameters for different temperatures. Additionally, the photoassociation yield induced by subpicosecond pulses of a priori fixed shape is compared to the maximum possible yield.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 18, 2013•0 citations
Bound-state molecules can be photoassociated directly from ultracold free-atom pairs by excitation to a purely repulsive electronic state. The process is explained on the basis of quantum unitarity: the initially free-scattering state is transformed by an impulsive light pulse to a deformed superposition which contains bound-state components. For pulse durations which are short compared to the ultracold dynamics, the maximal rate of photoassociation was found to be determined by the initial stationary distribution of scattering states of the atom pairs. The process was simulated for an ultracold gas of 87Rb^{87}\text{R}\text{b}87Rb with a temperature of T=44 \ensuremathμKT=44\text{ }\ensuremath{\mu}\text{K}T=44 \ensuremathμK and a density of \ensuremath≈1011 cm\ensuremath−3\ensuremath{\approx}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}3}\ensuremath≈1011 cm\ensuremath−3. Transform-limited pulses maximize the photoassociation, yielding \ensuremath∼1\ensuremath{\sim}1\ensuremath∼1 bound molecule per pulse. Coherent control calculated by a local control scheme can increase the photoassociation yield by two orders of magnitude.
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 1014W∕cm2{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}1014W∕cm2 and pulse duration of 10ps10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}10ps.
Photoassociation of ultracold atoms with pulsed laser light is shown to lead to alignment of the product molecules along the excitation laser polarization axis. We theoretically investigate pulsed two-photon photoassociation of 87Rb^{87}\mathrm{Rb}87Rb atoms into a specific weakly bound level of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ metastable electronic state and find both stationary and time-dependent field-free alignment. Although a transform-limited pulse yields significant alignment, a frequency-chirped pulse dramatically enhances the ultracold molecular formation rate at the cost of a slight decrease in the alignment. Employing multiple pulses synchronized with the vibrational and rotational periods leads to coherent enhancement of both population and alignment of the target state. Inclusion of the rotational degree of freedom in the model gives rise to processes with multiple quantum paths, which lead to quantum effects such as interference and coherence revivals.
We discuss the production of ultracold molecules in their electronic ground state by photoassociation employing electronically excited states with ion-pair character and strong spin-orbit interaction. A short photoassociation laser pulse drives a nonresonant three-photon transition for alkali-metal atoms colliding in their lowest triplet state. The excited-state wave packet is transferred to the ground electronic state by a second laser pulse, driving a resonant two-photon transition. After analyzing the transition matrix elements governing the stabilization step, we discuss the efficiency of population transfer using transform-limited and linearly chirped laser pulses. Finally, we employ optimal control theory to determine the most efficient stabilization pathways. We find that the stabilization efficiency can be increased by one and two orders of magnitude when using linearly chirped and optimally shaped laser pulses, respectively.
In this work, the coherence properties of the matter is studied in the frame of photoassociation of ultracold alkali atoms by using short and ultrashort laser pulses. Ultracold can be described with temperatures below 100 microK. When energy approaches the ultracold regime, quantum properties of matter becomes visible. In this regime matter such as atoms and molecules show wave properties. Coherent control employs the wave properties of matter to steer a physical process toward a desired target. It can be implemented with coherent radiation such as laser light. In particular, femtosecond laser pulses can be shaped. They thus have become a standard tool for coherent control. Since matter shows its wave properties most clearly at ultralow temperatures, ultracold matter represents the perfect object for coherent control. In the other hand using incoherent fields on ultracold matter does not give us good opportunities to drive quantum properties of it. In this work, it is aimed to bring together ultracold and ultrafast regimes in the frame of photoassociation. Photoassociation, excitation of two colliding atoms by laser light creates very weakly bound molecules. In the first part of this study, femtosecond photoassociation experiments are simulated. In these experiments and simulations, the broad spectrum of the femtosecond pulses addressing both atomic and molecular transitions is manipulated in a way to prevent the atomic transitions and to promote the molecular transitions. Then the effect of chirp and the effect of additional phase to the pulse are examined. Excited and ground state population are monitored by carrying out quantum dynamical simulations. The driving ultrafast dipole dynamics are presented. The photoassociation rate is limited by the low initial atomic pair density at inter-nuclear separations where photoassociation is feasible. In the second part of this study, a new scheme is proposed in order to manage this low pair density by manipulating the shape resonances of the initial atom pair. A shape resonance which is a metastable state occurs when some part of the wavefunction of partial waves with J > 0 trapped by a rotational barrier shows a bound structure in the closer internuclear distances where the rest of it shows a scattering wavefunction behavior. Most of the shape resonances in alkaline and alkaline earth metals occur typically at temperatures of a few milli-Kelvin. Therefore, the thermal weight of a shape resonance at photoassociation temperatures is quite small. By applying a non-resonant laser to the system the energy of the resonant state is brought close to the energies those corresponding to the photoassociation temperatures. Thus, the thermal weight of a shape resonance is amplified and initial pair density is enhanced. This effect is studied in both adiabatic and non-adiabatic time scales with respect to the rotational period of the system, intensity effects on the resonance energy and lifetime are presented.
Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold (<1mK)(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})(<1mK) temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.
Photoassociation is a possible route for the formation of chemical bonds. In this process, the binding of colliding atoms can be induced by means of a laser field. Photoassociation has been studied in the ultracold regime and also with temperatures well above millikelvins in the thermal energy domain, which is a situation commonly encountered in the laboratory. A photoassociation mechanism can be envisioned based on the use of infrared pulses to drive a transition from free colliding atoms on the electronic ground state to form a molecule directly on that state. This work takes a step in this direction, investigating the laser-pulse-driven formation of heteronuclear diatomic molecules in a thermal gas of atoms including rotational effects. Based on the assumption of full system controllability, the maximum possible photoassociation yield is deduced. The photoassociation probability is calculated as a function of the laser parameters for different temperatures. Additionally, the photoassociation yield induced by subpicosecond pulses of a priori fixed shape is compared to the maximum possible yield.