F. Masnou-seeuws | Université Paris-Saclay / UPSaclay (original) (raw)
Uploads
Papers by F. Masnou-seeuws
Laser Spectroscopy - Proceedings of the XVIII International Conference on ICOLS 2007, 2008
While ultracold matter brought quantum effects onto the macroscopic scale, ultrafast lasers made ... more While ultracold matter brought quantum effects onto the macroscopic scale, ultrafast lasers made quantum dynamical phenomena observable in real-time. Bringing the two together seems natural and holds the promise of employing quantum interferences in an unprecedented way. Photoassociation provides an optimal framework for the merger since in principle it relies only on the presence of optical transitions. Combining it with coherent control where the potential energy surfaces governing the dynamics can be `shaped', a general route toward stable ultracold molecules is obtained.
A description of photoassociation by CW laser is formulated in the framework of grid methods. The... more A description of photoassociation by CW laser is formulated in the framework of grid methods. The Hamiltonian describing one or several bound states coupled to a multiple of continuum manifolds via a radiative field is written in the energy representation and diagonalized. The generality of the treatment allows to compute accurately and efficiently physical properties such as non-linear high-intensity energy shifts, line shapes, and photoassociation rates both for isolated and non-isolated resonances. Application is given to sodium photoassociation in the experimental conditions of Mc Kenzie et al [Phys. Rev. Lett. 88, 090403 (2002)]. Inverted region for the dependency of the rate vs. the intensity and non-symmetric lineshapes were predicted to occur above the saturation limit. Comparison with the model of Bohn and Julienne [Phys. Rev. A 60, 414 (1999)] is discussed. PACS numbers:
Frontiers in Optics, 2006
ABSTRACT
 photoassociation spectrum](https://attachments.academia-assets.com/45157736/thumbnails/1.jpg)
](The Journal of Chemical Physics, 1999
Numerical calculations of vibrational levels of alkali dimers close to the dissociation limit are... more Numerical calculations of vibrational levels of alkali dimers close to the dissociation limit are developed in the framework of a Fourier Grid Hamiltonian method. The aim is to interpret photoassociation experiments in cold atom samples. In order to avoid the implementation of very large grids we propose a mapping procedure adapted to the asymptotic R Ϫn behavior of the long-range potentials. On a single electronic potential, this allows us to determine vibrational wave functions extending up to 500a 0 using a minimal number of grid points. Calculations with two electronic states, A 1 ⌺ u ϩ and b 3 ⌸ u states, both correlated to the Rb(5s)ϩRb(5 p) dissociation limit, coupled by fine structure are presented. We predict strong perturbation effects in the Rb 2 (0 u ϩ ͒ spectrum, manifested under the 5s, 5p 2 P 1/2 dissociation limit by an oscillatory behavior of the rotational constants.
Physical Review A - Atomic, Molecular, and Optical Physics, 2007
Photoassociation of a pair of cooled atoms by excitation with a short chirped laser pulse creates... more Photoassociation of a pair of cooled atoms by excitation with a short chirped laser pulse creates a dynamical hole in the initial continuum wavefunction. This hole is manifested by a void in the pair wavefunction and a momentum kick. Photoassociation into loosely bound levels of the external well in Cs 2 0 − g (6S+6P 3/2 ) is considered as a case study. After the pulse, the free evolution of the ground triplet state a 3 Σ + u wavepacket is analyzed. Due to a negative momentum kick, motion to small distances is manifested and a compression effect is pointed out, markedly increasing the density of atom pairs at short distance. A consequence of the hole is the redistribution of the vibrational population in the a 3 Σ + u state, with population of the last bound level and creation of pairs of hot atoms. The physical interpretation makes use of the time dependence of the probability current and population on each channel to understand the role of the parameters of the photoassociation pulse.
We propose to create ultracold ground state molecules in an atomic Bose-Einstein condensate by ad... more We propose to create ultracold ground state molecules in an atomic Bose-Einstein condensate by adiabatic crossing of an optical Feshbach resonance. We envision a scheme where the laser intensity and possibly also frequency are linearly ramped over the resonance. Our calculations for 87 Rb show that for sufficiently tight traps it is possible to avoid spontaneous emission while retaining adiabaticity, and conversion efficiencies of up to 50% can be expected.
The Journal of Chemical Physics, 1997
A local minimum is found in the long range potential curves of the and alkali dimers. This well-o... more A local minimum is found in the long range potential curves of the and alkali dimers. This well-of magnitude for and foris located above the first dissociation limit and metastable states could be populated using laser light blue detuned compared to the resonance line. ...
The total number of molecules produced in a pulsed photoassociation of ultracold atoms is a cruci... more 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.
Journal of Physics B: Atomic and Molecular Physics, 1984
A method is presented for computing the molecular properties of alkali dimers using correlated wa... more A method is presented for computing the molecular properties of alkali dimers using correlated wavefunctions. The results of model potential calculations for the alkali-dimer cations are used, and a generalisation of the Pluvinage treatment for the two-electron problem is presented. Satisfactory results are obtained using three correlated configurations.
Physical Review A, 2004
In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by p... more In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by photoassociation, Feshbach resonances, and three-body recombination. The created molecules are translationally cold, but vibrationally highly excited. This will eventually lead them to be lost from the trap due to collisions. We propose shaped laser pulses to transfer these highly excited molecules to their ground vibrational level. Optimal control theory is employed to find the light field that will carry out this task with minimum intensity. We present results for the sodium dimer. The final target can be reached to within 99% if the initial guess field is physically motivated. We find that the optimal fields contain the transition frequencies required by a good Franck-Condon pumping scheme. The analysis is able to identify the ranges of intensity and pulse duration which are able to achieve this task before other competing process take place. Such a scheme could produce stable ultracold molecular samples or even stable molecular Bose-Einstein condensates.
Physical Review A, 2004
This theoretical paper presents numerical calculations for photoassociation of ultracold cesium a... more This theoretical paper presents numerical calculations for photoassociation of ultracold cesium atoms with a chirped laser pulse and detailed analysis of the results. In contrast with earlier work, the initial state is represented by a stationary continuum wavefunction. In the chosen example, it is shown that an important population transfer is achieved to ≈ 15 vibrational levels in the vicinity of the v=98 bound level in the external well of the 0 − g (6s + 6p 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 3 Σ + u (6s + 6s) 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 a vibrational wavepacket in the excited state which is focussing at the barrier of the double well potential.
Physical Review A, 2001
This paper aims at studying the time-dependent effects involved in the photoassociation reaction ... more 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.
Physical Review A, 2007
We analyze the formation of Rb2 molecules with short photoassociation pulses applied to a cold 85... more We analyze the formation of Rb2 molecules with short photoassociation pulses applied to a cold 85 Rb sample. A pump laser pulse couples a continuum level of the ground electronic state X 1 Σ + g with bound levels in the 0 + u (5S+5P 1/2 ) and 0 + u (5S+5P 3/2 ) vibrational series. The nonadiabatic coupling between the two excited channels induces time-dependent beatings in the populations. We propose to take advantage of these oscillations to design further laser pulses that probe the photoassociation process via photoionization or that optimize the stabilization in deep levels of the ground state. PACS numbers: 32.80.Qk, 33.80.Ps, 34.50.Rk Making ultracold molecules in the lowest vibrational level v = 0 of the ground electronic state and creating stable molecular condensates is presently an important challenge since it opens the road toward ultracold chemistry . Schemes based on photoassociation (PA) of ultracold atoms [3] with cw lasers, have been very successful to form molecules in an excited electronic state. The latter have been stabilized into excited vibrational levels of the ground electronic state , but not yet into v = 0 except for the case of RbCs . The possibility of controlling PA by use of short laser pulses has been discussed in theoretical papers ] and very recently attempted by two experimental groups , both in the rubidium case. Success in such experiments will create a bridge between the two domains of cold matter and coherent control, where femtosecond (fs) pulses are used to control chemical reactions . Unfortunately, up to now, PA experiments with fs laser pulses have achieved destruction of the molecules already existing in the trap rather than creation of additional molecules .
Laser Spectroscopy - Proceedings of the XVIII International Conference on ICOLS 2007, 2008
While ultracold matter brought quantum effects onto the macroscopic scale, ultrafast lasers made ... more While ultracold matter brought quantum effects onto the macroscopic scale, ultrafast lasers made quantum dynamical phenomena observable in real-time. Bringing the two together seems natural and holds the promise of employing quantum interferences in an unprecedented way. Photoassociation provides an optimal framework for the merger since in principle it relies only on the presence of optical transitions. Combining it with coherent control where the potential energy surfaces governing the dynamics can be `shaped', a general route toward stable ultracold molecules is obtained.
A description of photoassociation by CW laser is formulated in the framework of grid methods. The... more A description of photoassociation by CW laser is formulated in the framework of grid methods. The Hamiltonian describing one or several bound states coupled to a multiple of continuum manifolds via a radiative field is written in the energy representation and diagonalized. The generality of the treatment allows to compute accurately and efficiently physical properties such as non-linear high-intensity energy shifts, line shapes, and photoassociation rates both for isolated and non-isolated resonances. Application is given to sodium photoassociation in the experimental conditions of Mc Kenzie et al [Phys. Rev. Lett. 88, 090403 (2002)]. Inverted region for the dependency of the rate vs. the intensity and non-symmetric lineshapes were predicted to occur above the saturation limit. Comparison with the model of Bohn and Julienne [Phys. Rev. A 60, 414 (1999)] is discussed. PACS numbers:
Frontiers in Optics, 2006
ABSTRACT
The Journal of Chemical Physics, 1999
Numerical calculations of vibrational levels of alkali dimers close to the dissociation limit are... more Numerical calculations of vibrational levels of alkali dimers close to the dissociation limit are developed in the framework of a Fourier Grid Hamiltonian method. The aim is to interpret photoassociation experiments in cold atom samples. In order to avoid the implementation of very large grids we propose a mapping procedure adapted to the asymptotic R Ϫn behavior of the long-range potentials. On a single electronic potential, this allows us to determine vibrational wave functions extending up to 500a 0 using a minimal number of grid points. Calculations with two electronic states, A 1 ⌺ u ϩ and b 3 ⌸ u states, both correlated to the Rb(5s)ϩRb(5 p) dissociation limit, coupled by fine structure are presented. We predict strong perturbation effects in the Rb 2 (0 u ϩ ͒ spectrum, manifested under the 5s, 5p 2 P 1/2 dissociation limit by an oscillatory behavior of the rotational constants.
Physical Review A - Atomic, Molecular, and Optical Physics, 2007
Photoassociation of a pair of cooled atoms by excitation with a short chirped laser pulse creates... more Photoassociation of a pair of cooled atoms by excitation with a short chirped laser pulse creates a dynamical hole in the initial continuum wavefunction. This hole is manifested by a void in the pair wavefunction and a momentum kick. Photoassociation into loosely bound levels of the external well in Cs 2 0 − g (6S+6P 3/2 ) is considered as a case study. After the pulse, the free evolution of the ground triplet state a 3 Σ + u wavepacket is analyzed. Due to a negative momentum kick, motion to small distances is manifested and a compression effect is pointed out, markedly increasing the density of atom pairs at short distance. A consequence of the hole is the redistribution of the vibrational population in the a 3 Σ + u state, with population of the last bound level and creation of pairs of hot atoms. The physical interpretation makes use of the time dependence of the probability current and population on each channel to understand the role of the parameters of the photoassociation pulse.
We propose to create ultracold ground state molecules in an atomic Bose-Einstein condensate by ad... more We propose to create ultracold ground state molecules in an atomic Bose-Einstein condensate by adiabatic crossing of an optical Feshbach resonance. We envision a scheme where the laser intensity and possibly also frequency are linearly ramped over the resonance. Our calculations for 87 Rb show that for sufficiently tight traps it is possible to avoid spontaneous emission while retaining adiabaticity, and conversion efficiencies of up to 50% can be expected.
The Journal of Chemical Physics, 1997
A local minimum is found in the long range potential curves of the and alkali dimers. This well-o... more A local minimum is found in the long range potential curves of the and alkali dimers. This well-of magnitude for and foris located above the first dissociation limit and metastable states could be populated using laser light blue detuned compared to the resonance line. ...
The total number of molecules produced in a pulsed photoassociation of ultracold atoms is a cruci... more 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.
Journal of Physics B: Atomic and Molecular Physics, 1984
A method is presented for computing the molecular properties of alkali dimers using correlated wa... more A method is presented for computing the molecular properties of alkali dimers using correlated wavefunctions. The results of model potential calculations for the alkali-dimer cations are used, and a generalisation of the Pluvinage treatment for the two-electron problem is presented. Satisfactory results are obtained using three correlated configurations.
Physical Review A, 2004
In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by p... more In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by photoassociation, Feshbach resonances, and three-body recombination. The created molecules are translationally cold, but vibrationally highly excited. This will eventually lead them to be lost from the trap due to collisions. We propose shaped laser pulses to transfer these highly excited molecules to their ground vibrational level. Optimal control theory is employed to find the light field that will carry out this task with minimum intensity. We present results for the sodium dimer. The final target can be reached to within 99% if the initial guess field is physically motivated. We find that the optimal fields contain the transition frequencies required by a good Franck-Condon pumping scheme. The analysis is able to identify the ranges of intensity and pulse duration which are able to achieve this task before other competing process take place. Such a scheme could produce stable ultracold molecular samples or even stable molecular Bose-Einstein condensates.
Physical Review A, 2004
This theoretical paper presents numerical calculations for photoassociation of ultracold cesium a... more This theoretical paper presents numerical calculations for photoassociation of ultracold cesium atoms with a chirped laser pulse and detailed analysis of the results. In contrast with earlier work, the initial state is represented by a stationary continuum wavefunction. In the chosen example, it is shown that an important population transfer is achieved to ≈ 15 vibrational levels in the vicinity of the v=98 bound level in the external well of the 0 − g (6s + 6p 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 3 Σ + u (6s + 6s) 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 a vibrational wavepacket in the excited state which is focussing at the barrier of the double well potential.
Physical Review A, 2001
This paper aims at studying the time-dependent effects involved in the photoassociation reaction ... more 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.
Physical Review A, 2007
We analyze the formation of Rb2 molecules with short photoassociation pulses applied to a cold 85... more We analyze the formation of Rb2 molecules with short photoassociation pulses applied to a cold 85 Rb sample. A pump laser pulse couples a continuum level of the ground electronic state X 1 Σ + g with bound levels in the 0 + u (5S+5P 1/2 ) and 0 + u (5S+5P 3/2 ) vibrational series. The nonadiabatic coupling between the two excited channels induces time-dependent beatings in the populations. We propose to take advantage of these oscillations to design further laser pulses that probe the photoassociation process via photoionization or that optimize the stabilization in deep levels of the ground state. PACS numbers: 32.80.Qk, 33.80.Ps, 34.50.Rk Making ultracold molecules in the lowest vibrational level v = 0 of the ground electronic state and creating stable molecular condensates is presently an important challenge since it opens the road toward ultracold chemistry . Schemes based on photoassociation (PA) of ultracold atoms [3] with cw lasers, have been very successful to form molecules in an excited electronic state. The latter have been stabilized into excited vibrational levels of the ground electronic state , but not yet into v = 0 except for the case of RbCs . The possibility of controlling PA by use of short laser pulses has been discussed in theoretical papers ] and very recently attempted by two experimental groups , both in the rubidium case. Success in such experiments will create a bridge between the two domains of cold matter and coherent control, where femtosecond (fs) pulses are used to control chemical reactions . Unfortunately, up to now, PA experiments with fs laser pulses have achieved destruction of the molecules already existing in the trap rather than creation of additional molecules .