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Physical Review A

Laser-pulse photoassociation in a thermal gas of atoms

Emanuel F. de Lima, Tak-San Ho, HERSCHEL A. RABITZ

Published Dec 19, 2008• 21 citations
Publisher

Abstract

A nonperturbative treatment is presented for laser-pulse-driven formation of heteronuclear diatomic molecules in a thermal gas of atoms. Based on the assumption of full controllability, the maximum possible photoassociation yield is obtained. A one-dimensional model is used for calculating the photoassociation probability as a function of the laser parameters as well as for different temperatures. The dependence of the photoassociation yield on the laser frequency and amplitude reveals complex patterns of one- and multiphoton transitions. The photoassociation yield induced by subpicosecond pulses of a priori fixed shape is very low compared to the maximum possible yield.

DOI
10.1103/physreva.78.063417

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Physical Review A•Oct 16, 2012•12 citations
A nonperturbative treatment is presented for laser-pulse-driven formation of heteronuclear diatomic molecules in a thermal gas of atoms. Based on the assumption of full controllability, the maximum possible photoassociation yield is obtained. A one-dimensional model is used for calculating the photoassociation probability as a function of the laser parameters as well as for different temperatures. The dependence of the photoassociation yield on the laser frequency and amplitude reveals complex patterns of one- and multiphoton transitions. The photoassociation yield induced by subpicosecond pulses of a priori fixed shape is very low compared to the maximum possible yield.
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.
We consider the formation of cold diatomic molecules in the electronic ground state by photoassociation of atoms of dissimilar species. A combination of two transition pathways from the free colliding pair of atoms to a bound vibrational level of the electronic molecular ground state is envisioned. The first pathway consists of a pump-dump scheme with two time-delayed laser pulses in the near-infrared frequency domain. The pump pulse drives the transition to a bound vibrational level of an excited electronic state, while the dump pulse transfers the population to a bound vibrational level of the electronic ground state. The second pathway takes advantage of the existing permanent dipole moment and employs a single pulse in the far-infrared domain to drive the transition from the unbound atoms directly to a bound vibrational level in the electronic ground state. We show that this scheme offers the possibility to coherently control the photoassociation yield by manipulating the relative phase and timing of the pulses. The photoassociation mechanism is illustrated for the formation of cold LiCs molecules.
We investigate femtosecond photoassociation of thermally hot atoms in the gas phase and its coherent control. In the photoassociation process, formation of a chemical bond is facilitated by light in a free-to-bound optical transition. Here, we study free-to-bound photoassociation of a diatomic molecule induced by femtosecond pulses exciting a pair of scattering atoms interacting via the van-der-Waals-type electronic ground state potential into bound levels of an electronically excited state. The thermal gas of reactants is at temperatures in the range of hundreds of degrees. Despite this incoherent initial state, rotational and vibrational coherences are observed in the probing of the created Mg2 molecules.
The formation of the NaH molecule from unaligned ultracold atoms via photoassociation (PA) is theoretically investigated by numerically solving the time-dependent Schrödinger equation including the vibrational and rotational degrees of freedom. The PA processes via one- and two-photon excitations are described. The calculated results show that the associated molecule can be prepared to a desired rovibrational state of the ground electronic state for NaH. Compared with the one-photon association probability, the two-photon association probability is obviously enhanced.
We investigate theoretically the photoassociation (PA) of two Cs\mathrm{Cs}Cs atoms using a slowly-turned-on and rapidly-turned-off (STRT) laser pulse at the temperature of $54\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ by taking the thermal average of the initial continuum states into account. For comparison, the PA probabilities under the action of the STRT and typical Gauss-type pulses are calculated, respectively. The rising and falling times of the STRT pulse are set to be ${\ensuremath{\tau}}_{r}=10$ ps and ${\ensuremath{\tau}}_{f}=0.2$ ps, and the two Gauss-type pulses are set to be ${\ensuremath{\tau}}_{r}={\ensuremath{\tau}}_{f}=10$ ps and ${\ensuremath{\tau}}_{r}={\ensuremath{\tau}}_{f}=5.1$ ps, respectively. Compared to the calculation considering a single initial continuum state (non-thermal-average calculations), the PA probabilities with the consideration of the thermal average are reduced to different extents according to different laser pulses. The PA process induced by the STRT pulse is more robust than that by the two Gauss-type pulses. For the STRT pulse, the PA probability is reduced by roughly 30%, while for the two Gauss-type pulses, it is reduced by roughly 97% and 64%, respectively. Because of the larger bandwidth and the asymmetric time profile of the STRT pulse, the PA probability induced by this pulse is related to both the off-resonant and the near-resonant transitions in a much broader phase space formed by the initial continuum states and the final vibrational states. Consequently, the thermal average of the initial continuum states has relatively smaller influence on the PA probability induced by the STRT pulse. By changing the pulse shape of the standard Gaussian to the sin2{\mathrm{sin}}^{2}sin2 and the Lorentz ones, the STRT pulse still results in the highest PA probability and is the most robust to the thermal average.
The formations of 85Rb2{}^{85}{\text{Rb}}_{2}85Rb2​ molecules via photoassociation (PA) steered by two-color laser fields are explored theoretically in order to find an efficient and robust PA scheme. The PA processes steered by the PA pulses modulated by two Gaussian pulses and by two chirped pulses are discussed and compared in detail. The two pulses are coherent in the picosecond range and reach their maxima at the same time. The influences of the linear chirp rate, the frequency difference between two pulses, and the phase shift of the modulated envelope with respect to the maximum of the Gaussian envelope on the PA process are investigated. The yield of photoassociated molecules on vibrational levels with a binding energy of $>1.0cm1.0 cm1.0cm{}^{\ensuremath{-}1}$ with respect to the 5S+5P1/25S+5{P}_{1/2}5S+5P1/2​ dissociation limit can apparently be enhanced by choosing proper pulse parameters. Especially, the two-color laser field modulated by two chirped pulses can raise the PA efficiency on one side, and weaken the dependence of the PA process on phase shift on the other side.