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Journal of Physics B Atomic Molecular and Optical Physics

Laser-assisted ultracold lithium-hydride molecule formation: stimulated versus spontaneous emission

Elizabeth Juarros, K. Kirby, Robin Côté

Published Sep 25, 2006• 23 citations
Publisher

Abstract

We investigate the feasibility of forming ultracold LiH from a mixture of the ultracold atomic gases, by using B 1 Π as an intermediate state in the photoassociation process. Using accurate molecular potential energy curves and dipole transition moments, we calculate and compare two possible schemes to populate vibrational levels of the ground electronic state, X 1 Σ + : (1) two-photon stimulated radiative association and (2) excitation to bound levels of the B 1 Π state, followed by spontaneous emission to the X 1 Σ + state. With laser intensities and atomic densities that are easily attainable experimentally, we find that significant quantities of molecules can be formed in various v , J levels of the electronic ground state. We examine the spontaneous emission cascade which takes place from the upper vibrational levels on a time scale of milliseconds. We discuss the issue of back-stimulation for the two-photon process and ways to mitigate it. Because photon emission in the cascade process does not contribute to trap loss, a sizable population of molecules in v = 0 can be achieved.

DOI
10.1088/0953-4075/39/19/s11

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We investigate the feasibility of forming ultracold LiH from a mixture of the ultracold atomic gases, by using B 1 Π as an intermediate state in the photoassociation process. Using accurate molecular potential energy curves and dipole transition moments, we calculate and compare two possible schemes to populate vibrational levels of the ground electronic state, X 1 Σ + : (1) two-photon stimulated radiative association and (2) excitation to bound levels of the B 1 Π state, followed by spontaneous emission to the X 1 Σ + state. With laser intensities and atomic densities that are easily attainable experimentally, we find that significant quantities of molecules can be formed in various v , J levels of the electronic ground state. We examine the spontaneous emission cascade which takes place from the upper vibrational levels on a time scale of milliseconds. We discuss the issue of back-stimulation for the two-photon process and ways to mitigate it. Because photon emission in the cascade process does not contribute to trap loss, a sizable population of molecules in v = 0 can be achieved.
Alkali hydride molecules are polar, exhibiting large ground-state dipole moments. As ultracold sources of alkali atoms, as well as hydrogen, have been created in the laboratory, we explore theoretically the feasibility of forming such molecules from a mixture of the ultracold atomic gases, employing various photoassociation schemes. In this work we use lithium and sodium hydride as benchmark systems to calculate molecule formation rates through stimulated one-photon radiative association directly from the continuum as well as two-photon stimulated radiative association (Raman transfer) and excitation to bound levels of an excited state followed by spontaneous emission to the ground state. Using accurate molecular potential energy curves and dipole transition moments and with laser intensities and MOT densities that are easily attainable experimentally, we have found that substantial molecule formation rates can be realized even after the effect of back-stimulation has been accounted for. We examine the spontaneous emission cascade which takes place from the upper vibrational levels of the singlet ground state on a time scale of milliseconds. Because photon emission in the cascade process does not contribute to trap loss, a sizable population of molecules in the lowest vibrational level can be achieved. The triplet ground electronic state is of particular interest for experimental efforts since, although it has never been observed experimentally, molecular structure calculations of the a3Σ+ state for LiH and NaH predict a small van der Waals attraction, with a potential energy well so shallow that it can support only one bound rotational-vibrational level. Any molecule formed in the triplet ground state would then be immediately in the lowest and most stable level of that state and would be quite long-lived, unlike molecules in high vibrational levels which have significantly shorter lifetimes. As an analysis of our method of calculating molecule formation rates, we investigate more thoroughly the two-photon stimulated photoassociation process by employing the exact treatment of Dalgarno & Lewis to solve the Raman scattering problem. ^
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Photoassociation is a process by which two cold, trapped atoms can be optically excited during a collision to make a vibrational level of an excited electronic state of a molecular dimer. High-resolution photoassociation spectra are now well-known for most alkali dimers. The upper level normally decays by spontaneous emission to a number of final molecular levels, bound and unbound. A laser of a second color can be used to stimulate a transition from the upper level to a particular ground state vibrational level. Such a Raman photoassociation process can be used to form translationally cold molecules in a cold atomic gas or a Bose-Einstein condensate. Coupled channels scattering calculations show that the collisional rate coefficient for coherent production of molecules in specific target states, >10−10cm3/s,> 10^{-10} cm^{3}/s,>10−10cm3/s, can greatly exceed the rate coefficient for forming products due to excited state spontaneous decay. This implies the feasibility of rapid coherent conversion of atom pairs in a condensate to molecules on a time scale short compared to the trap oscillation time. The untrapped molecules can form a coherent molecular beam. We will examine the prospects for the experimental realization of photoassociative molecule formation in cold alkali gases or condensates.
We present a theoretical analysis of optical pathways for formation of cold ground-state (NaCa)+{(\mathrm{NaCa})}^{+}(NaCa)+ molecular ions via an intermediate state. The formation schemes are based on ab initio potential energy curves and transition dipole moments calculated using effective-core-potential methods of quantum chemistry. In the proposed approach, starting from a mixture of cold trapped Ca+{\mathrm{Ca}}^{+}Ca+ ions immersed into an ultracold gas of Na atoms, (NaCa)+{(\mathrm{NaCa})}^{+}(NaCa)+ molecular ions are photoassociated in the excited $E^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ electronic state and allowed to spontaneously decay either to the ground electronic state or an intermediate state from which the population is transferred to the ground state via an additional optical excitation. By analyzing all possible pathways, we find that the efficiency of a two-photon scheme, via either the $B^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ or $C^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ potential, is sufficient to produce significant quantities of ground-state (NaCa)+{(\mathrm{NaCa})}^{+}(NaCa)+ molecular ions. A single-step process results in lower formation rates that would require either a high-density sample or a very intense photoassociation laser to be viable.
We propose a method for copiously producing ultracold ground electronic-state molecules by two-photon absorption to a Rydberg molecular level (that spontaneously radiates to the ground molecular state) during collisions of ultracold atoms in a laser trap. Using the Na2{\mathrm{Na}}_{2}Na2​ molecule as an example, the sequential two-photon absorption is via an intermediate high-lying vibrational level of the excited 1g{1}_{\mathit{g}}1g​ state and results in the formation of a given vibrational level of a $^{1}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ Rydberg molecular state. Realistic calculations demonstrating the validity of the method are presented.
We investigate the production of ultracold ground state X 1 σ + (v = 0) RbCs molecules in the lowest vibrational level via short-range photoassociation followed by spontaneous emission. The starting point is the laser cooled 85 Rb and 133 Cs atoms in a dual species, forced dark magneto-optical trap. The special intermediate level (5)0 + (v = 10) correlated to the (2) 3 π electric state is achieved by the photoassociation process. The formed ground state X 1 σ + (v = 0) molecule is resonantly excited to the 2 1 π intermediate state by a 651 nm pulse laser and is ionized by a 532 nm pulse laser and then detected by the time-of-flight mass spectrum. Saturation of the photoionization spectroscopy at large ionization laser energy is observed and the ionization efficiency is obtained from the fitting. The production of ultracold ground state 85 Rb 133 Cs molecules is facilitative for the further research about the manipulation of ultracold molecules in the rovibrational ground state.