Photoassociation of cold atoms with chirped laser pulses: Time-dependent calculations and analysis of the adiabatic transfer within a two-state model (original) (raw)
Related papers
Optimizing the photoassociation of cold atoms by use of chirped laser pulses
The European Physical Journal D, 2004
Photoassociation of ultracold atoms induced by chirped picosecond pulses is analyzed in a nonperturbative treatment by following the wavepackets dynamics on the ground and excited surfaces. The initial state is described by a Boltzmann distribution of continuum scattering states. The chosen example is photoassociation of cesium atoms at temperature T=54 µK from the a 3 Σ + u (6s, 6s) continuum to bound levels in the external well of the 0 − g (6s + 6p 3/2 ) potential. We study how the modification of the pulse characteristics (carrier frequency, duration, linear chirp rate and intensity) can enhance the number of photoassociated molecules and suggest ways of optimizing the production of stable molecules. PACS. 33.80.Ps Optical cooling of molecules; trapping; 33.80 -b Photon interactions with molecules 33.90 +h New topics in molecular properties, interaction with photons 33.80.Gj diffuse spectra; predissociation, photodissociation
Physical Review A, 2001
Enhancement of the production of cold molecules via photoassociation is considered for the Cs 2 system. The employment of chirped picosecond pulses is proposed and studied theoretically. The analysis is based on the ability to achieve impulsive excitation which is given by the ultracold initial conditions where the nuclei are effectively stationary during the interaction with a field. The appropriate theoretical framework is the coordinate-dependent two-level system. Matching the pulse parameters to the potentials and initial conditions results in full Rabi cycling between the electronic potentials. By chirping the laser pulse, adiabatic transfer leading to the population inversion from the ground to the excited state is possible in a broad and tunable range of internuclear distance. Numerical simulations based on solving the time-dependent Schrödinger equation ͑TDSE͒ were performed. The simulation of the photoassociation of Cs 2 from the ground 3 ⌺ u ϩ to the excited 0 g Ϫ state under ultracold conditions verifies the qualitative picture. The ability to control the population transfer is employed to optimize molecular formation. Transfer of population to the excited 0 g Ϫ surface leaves a void in the nuclear density of the ground 3 ⌺ u ϩ surface. This void is either filled by thermal motion or by quantum ''pressure'' and it is the rate-determining step in the photoassociation. The spontaneous-emission process leading to cold-molecules is simulated by including an optical potential in the TDSE. Consequently, the rate of cold molecule formation in a pulsed mode is found to be larger than that obtained in a continuous-wave mode.
2006
The total number of molecules produced in a pulsed photoassociation of ultracold atoms is a crucial link between theory and experiment. A calculation based on first principles can determine the experimental feasibility of a pulsed photoassociation scheme. The calculation method considers an initial thermal ensemble of atoms. This ensemble is first decomposed into a representation of partial spherical waves. The photoassociation dynamics is calculated by solving the multichannel time-dependent Schrödinger equation on a mapped grid. The molecules are primarily assembled in a finite region of internuclear distances, the 'photoassociation window'. The ensemble average was calculated by adding the contributions from initial scattering states confined to a finite volume. These states are Boltzmann averaged where the partition function is summed numerically. Convergence is obtained for a sufficiently large volume. The results are compared to a thermal averaging procedure based on scaling laws which leads to a single representative initial partial wave which is sufficient to represent the density in the 'photoassociation window'. For completeness a third high-temperature thermal averaging procedure is described which is based on random phase thermal Gaussian initial states. The absolute number of molecules in the two first calculation methods agree to within experimental error for photoassociation with picosecond pulses for a thermal ensemble of rubidium or caesium atoms in ultracold conditions.
Resonant Coupling in the Formation of Ultracold Ground State Molecules via Photoassociation
Physical Review Letters, 2001
We demonstrate the existence of a new mechanism for the formation of ultracold molecules via photoassociation of cold cesium atoms. The experimental results, interpreted with numerical calculations, suggest that a resonant coupling between vibrational levels of the 0 1 u ͑6s 1 6p 1͞2 ͒ and ͑6s 1 6p 3͞2 ͒ states enables formation of ultracold molecules in vibrational levels of the ground state well below the 6s 1 6s dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.
Making ultracold molecules in a two-color pump-dump photoassociation scheme using chirped pulses
Physical Review A, 2006
This theoretical paper investigates the formation of ground state molecules from ultracold cesium atoms in a two-color scheme. Following previous work on photoassociation with chirped picosecond pulses [Luc-Koenig et al., Phys. Rev. A 70, 033414 (2004)], we investigate stabilization by a second (dump) pulse. By appropriately choosing the dump pulse parameters and time delay with respect to the photoassociation pulse, we show that a large number of deeply bound molecules are created in the ground triplet state. We discuss (i) broad-bandwidth dump pulses which maximize the probability to form molecules while creating a broad vibrational distribution as well as (ii) narrow-bandwidth pulses populating a single vibrational ground state level, bound by 113 cm −1 . The use of chirped pulses makes the two-color scheme robust, simple and efficient.
Physical Review A, 2001
This paper aims at studying the time-dependent effects involved in the photoassociation reaction for a sample of cold alkali-metal atoms, within a two-channel model where the vibrational motion in the excited state is coupled by laser light to the continuum state describing two colliding atoms in the lowest triplet electronic state (a 3 ⌺ u ϩ ). Both photodissociation and photoassociation processes are considered at a time scale shorter than the radiative lifetime, so that spontaneous emission does not have to be considered. The characteristic times are then the vibrational period in the excited state, which for alkali-metal dimers can be estimated of the order of a few hundreds of picoseconds, and the Rabi period, depending upon the laser intensity. Numerical calculations using wave-packet propagation are performed for the coupling of the vibrational motion in the Cs 2 1 g (6sϩ6p 3/2 ) and a 3 ⌺ u ϩ (6sϩ6s) channels by a cw laser slightly red detuned relative to the D 2 resonance line. The results show Rabi oscillations in the populations of the two channels during time intervals when the vibrational motion is stopped at the outer turning point. At intensities of Ϸ250 kW cm Ϫ2 , a new characteristic time appears, a factor of 2 larger than the classical vibrational period, which corresponds to vibrational motion in the upper adiabatic potential created by the coupling. Such an effect modifies the scattering length for collisions in the lower state, and it clearly opens a possibility of control by tuning the laser intensity.
Selective excitation of diatomic molecules by chirped laser pulses
The Journal of Chemical Physics, 2000
A new method for the selective excitation of diatomic molecules in single vibrational states on excited electronic potentials by two-photon absorption is proposed. The method implies the use of two chirped strong pulse lasers detuned from the optical transition to an intermediate electronic state. We show under what scenarios the method is successful on the time-energy scale in which the pulses operate. They involved a long-time ͑nanosecond͒ weak-field regime and a short-time ͑picosecond͒ strong-field regime. The adiabatic representation in terms of energy levels or in terms of light-induced potentials is used to interpret the physical mechanism of the excitation. The efficiency and robustness of the scheme are demonstrated by the excitation of the ground vibrational state of the 1 ⌺ g (4s) electronic potential of the Na 2 molecule.
Propagation of frequency-chirped laser pulses in a medium of atoms with a Lambda-level scheme
Physical Review A, 2007
We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a ⌳ level-scheme ͑Lambda atoms͒ and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower ͑metastable͒ levels of the ⌳-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in ⌳-atoms, there is a regime of enhanced transparency of the medium-the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.
Pulsed adiabatic photoassociation via scattering resonances
2011
We develop the theory for the Adiabatic Raman Photoassociation (ARPA) of ultracold atoms to form ultracold molecules in the presence of scattering resonances. Based on a computational method in which we replace the continuum with a discrete set of "effective modes", we show that the existence of resonances greatly aids in the formation of deeply bound molecular states. We illustrate our general theory by computationally studying the formation of 85 Rb 2 molecules from pairs of colliding ultracold 85 Rb atoms. The single-event transfer yield is shown to have a near-unity value for wide resonances, while the ensemble-averaged transfer yield is shown to be higher for narrow resonances. The ARPA yields compare favourably with that of (the experimentally measured) "Feshbach molecule" magneto-association, suggesting that ARPA is a viable alternative for the production of ultracold molecules.