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Physical Review Letters

Laser-Catalyzed Production of Ultracold Molecules: The Li6+Li6Li7→ℏωLi6−Li6+Li7 Reaction

Xuan Li, Gregory A. Parker, Paul W. Brumer, Ioannis Thanopulos, Moshe Shapiro

Published Jul 25, 2008• 14 citations
Publisher

Abstract

We show that by using laser catalysis, we can employ translationally cold (${T}_{r}\ensuremath{\approx}1.75\text{ }\text{ }\mathrm{K}$) collisions to produce ultracold ($0.01\text{ }\text{ }\mathrm{mK}<{T}_{p}<1\text{ }\text{ }\mathrm{mK}$) (homonuclear) molecules. We illustrate this approach by studying the laser catalysis of the $^{6}\mathrm{Li}+^{6}\mathrm{Li}^{7}\mathrm{Li}\stackrel{\ensuremath{\hbar}\ensuremath{\omega}}{\ensuremath{\rightarrow}}(^{6}\mathrm{Li}^{6}\mathrm{Li}^{7}\mathrm{Li}{)}^{*}({1}^{4}{A}^{\ensuremath{'}\ensuremath{'}})\stackrel{\ensuremath{\hbar}\ensuremath{\omega}}{\ensuremath{\rightarrow}}^{6}\mathrm{Li}^{6}\mathrm{Li}+^{7}\mathrm{Li}$ reaction in the collinear approximation. Ultracold $^{6}\mathrm{Li}^{6}\mathrm{Li}$ product molecules are shown to be produced at an extraordinary yield of up to 99.97%, using moderate laser intensities of $I=100\text{ }\text{ }\mathrm{kW}/{\mathrm{cm}}^{2}\ensuremath{-}10\text{ }\text{ }\mathrm{MW}/{\mathrm{cm}}^{2}$.

DOI
10.1103/physrevlett.101.043003

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We present a new theory of population transfer by adiabatic passage. This theory relates laser catalysis to adiabatic passage, enhancing chemical reactions with the freedom to choose the translational energies of the reactants and products separately. The process, A+BC<-->(Planck's over omega(p) )ABC*(v)<-->(Planck's over omega(s))AB+C, involves two laser fields that are slowly varying so the process is adiabatic, and sufficiently intense so the population of the intermediate bound complex (ABC) is minimized. We apply this theory to the collinear exchange reaction (6)Li+(7)Li(2)(T(r))<-->(Planck's over omega(p))((6)Li(7)Li(7)Li)*<-->(variant Planck's over 2piomega(s) ) (6)Li(7)Li(T(p))+(7)Li. We show that at translational energies T(p)=T(r)=1 mK with a narrow energy bandwidth of delta(E)=0.01 mK, we can obtain nearly total (> or =98%) population transfer from the reactant to the product states. This can be done with a pump laser and a Stokes laser in an "intuitive" sequence (t(p)<t(s)) at a low intensity (I(p)< or =600 MW/cm(2)) and a "coincident" sequence (t(p)=t(s)) at a higher intensity.
The recent development of a range of new methods for producing samples of gas-phase molecules that are translationally cold ( K) or ultracold ( mK) is driving efforts to study reactive and inelastic collisional processes in these temperature regimes. In this review article the new methods for cold/ultracold molecule production are reviewed in the context of their potential or current use in collisional studies and progress in the application of these methods is highlighted. In these sub-Kelvin temperature ranges, where the de Broglie wavelength is long compared with molecular dimensions, quantum effects may play a crucial role in the collision dynamics. Reactions with no potential energy barrier are of greatest importance, and this review article summarizes some of the principal theoretical approaches to understanding quantum effects in these barrierless processes.
The realization of rovibrationally stable dense samples of ultracold diatomic molecules remains one of the main stepping stones to achieve the next slate of major goals in the field of atomic and molecular physics. Though obtaining diatomic alkali molecules was seen as a logical next step following the optical cooling of atoms, many of the possible applications currently under investigation extend beyond atomic and molecular physics. For example, spectroscopy of ultracold molecules can help in testing extensions of the Standard Model via the search for a permanent electric dipole moment of the electron (1; 2), or the energy difference between enantiomers of chiral molecules (3). Various molecular transitions can be utilized to track the time dependence of fundamental constants, including the fine structure constant and the proton to electron mass ratio (4). They also open the way for cold and ultracold chemistry, where the interacting species and products are in a coherent quantum superposition state (5) and reactions can happen via quantum tunneling. Dipolar ultracold quantum gases promise to show a plethora of new phenomena due to anisotropic long-range dipole-dipole interactions (6). Dipolar molecules in optical lattices can be employed as a quantum simulator of condensed matter systems, and they are predicted to demonstrate new quantum phases such as a dipolar crystal, supersolid, checkerboard and collapse phases (7; 8). Ultracold polar molecules also represent an attractive platform for quantum computation (9). They offer a variety of long-lived states for qubit encoding, including rotational, spin and hyperfine (if electronic and nuclear spins are non-zero), Λ and Ω-doublet states (10) and scalability to a large number of qubits. Polar molecules can be easily controlled by DC electric and magnetic fields, as well as bymicrowave and optical fields, allowing the design of various traps (11; 12). The main appeal of polar molecules for quantum information processing, however, comes from their permanent electric dipole moment, permitting them to interact via a long-range dipole-dipole interaction. The dipole-dipole interaction offers a tool to construct two-qubit gates, required for universal quantum computation (9; 13). Ultracold molecules in their ground vibrational state v = 0 (and even in specific rotational, hyperfine or Zeeman states) are required formany of these applications since they have a large permanent electric dipole moment and are stable with respect to collisions and spontaneous emission. Currently translationally ultracold (100 nK 1 mK) molecules are produced by magneto(14) and photo-association (15) techniques. In a typical photoassociation scheme, 3
Summary form only given. Although there has been stunning progress in the production of ultracold atoms (defined loosely as atoms with T<1 mK), efforts to produce translationally ultracold molecules were unsuccessful until very recently. Conventional laser cooling of atoms relies on the existence of a cycling transition, generally not an option in molecules because of their rotational and vibrational multiplicity. Various proposals have been offered to overcome this difficulty, including ultracold molecule production by photoassociation (PA) of ultracold atoms, buffer gas cooling, and cooling by a multiple-frequency laser. The first and second of these approaches have now been demonstrated successfully. This talk will describe some of these new experimental results, focusing particularly on our work at the University of Connecticut on ultracold K/sub 2/ molecules.
In the past two decades, the revolutionary technologies of creating cold and ultracold molecules have provided cutting-edge experiments for studying the fundamental phenomena of collision physics. To a large degree, the recent explosion of interest in the molecular collisions has been sparked by dramatic progress of experimental capabilities and theoretical methods, which permit molecular collisions to be explored deep in the quantum mechanical limit. Tremendous experimental advances in the field have already been achieved, and the authors, from an experimental perspective, provide a review of these studies for exploring the nature of molecular collisions occurring at temperatures ranging from the Kelvin to the nanoKelvin regime, as well as for applications of producing ultracold molecules.
We present a nonperturbative time-dependent quantum mechanical theory of the laser catalysis and control of a bifurcating A+BC<-->(variant Planck's over 2pi omega(0))ABC*(v)<-->(variant Planck's over 2pi omega(0) )AB+C reaction, with ABC*(v) denoting an intermediate, electronically excited, complex of ABC in the vth vibrational state. We apply this theory to the low collision energy fermion-boson light-induced exchange reaction, (6)Li((2)S)+(7)Li(2)((3)Sigma(u)(+))<-->(variant Planck's over 2pi omega(0))((6)Li(7)Li(7)Li)*<-->(variant Planck's over 2pi omega(0))(6)Li(7)Li((3)Sigma(+))+(7)Li((2)S). We show that at very low collision energies and energetically narrow (approximately 0.01 cm(-1)) initial reactant wave packets, it is possible to tune the yield of the exchange reaction from 0 to near-unity (yield >or=99%) values. Controllability is somewhat reduced at collisions involving energetically wider (approximately 1 cm(-1)) initial reactant wave packets. At these energetic bandwidths, the radiative reactive control, although still impressive, is limited to the 0%-76% reactive-probabilities range.