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The Journal of Chemical Physics

Theory of laser enhancement of ultracold reactions: The fermion-boson population transfer by adiabatic passage of Li6+Li6Li7(Tr=1mK)→Li6Li6+Li7(Tp=1mK)

Xuan Li, Gregory A. Parker

Published May 14, 2008• 13 citations
Publisher

Abstract

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.

DOI
10.1063/1.2920186

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International Conference on Ultrafast Phenomena•Jan 1, 1992•0 citations
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.
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.
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  mK<Tp<1  mK0.01\text{ }\text{ }\mathrm{mK}<{T}_{p}<1\text{ }\text{ }\mathrm{mK}0.01  mK<Tp​<1  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 6Li6Li^{6}\mathrm{Li}^{6}\mathrm{Li}6Li6Li product molecules are shown to be produced at an extraordinary yield of up to 99.97%, using moderate laser intensities of $I=100\text{ }\text{ }\mathrm{kW}/{\mathrm{cm}}^{2}\ensuremath{-}10\text{ }\text{ }\mathrm{MW}/{\mathrm{cm}}^{2}$.
We present the ability to coherently control triatomic chemical reactions with pulsed-laser techniques. We show that one can control the final state probability distribution of triatomic chemical reactions with nearly 100% selectivity. We develop the population transfer by adiabatic passage theory to coherently control the chemical reaction with the ability of choosing the translational energies of the final reaction products. We also show a new way to achieve the creation of both homonuclear and heteronuclear diatomic molecules at an ultracold temperature using laser catalysis.
We report the experimental realization of coherent population transfer from the 23S2{}^{3}S23S state to 23P2{}^{3}P23P fine-structure levels of helium, using the technique of hyper-Raman adiabatic passage, in which the pump-laser coupling involves a two-photon transition. We discuss the dependence of population transfer upon pump and Stokes detunings, and show that by suitably setting the detunings one can transfer population efficiently as either a two-step process to final states or as a single-step two-photon transition to an intermediate state. We use an adiabatic-state picture to interpret these experimental results. Since the optimum detuning depends on the pump laser intensity the transfer efficiency is compromised by the variation of the intensity across the spatial profile of the laser beam. Our experimental data agree well with numerical simulation based on density-matrix equations, including all 15 magnetic sublevels.
In a recent work, we have developed a robust chainwise atom–molecule adiabatic passage scheme to produce ultracold ground–state molecules via photo-associating free atoms [Phys. Rev. A81, 013632 (2010)10.1103/PhysRevA.81.013632PLRAAN1050-2947]. With the help of intermediate auxiliary levels, the pump laser intensity requested in the atomic photo-association process can be greatly reduced. In the present work, we extend the scheme to a more generalized (2n+1)-level system and investigate its efficiency limitations. As the intermediate levels and auxiliary lasers increase, the atom–molecule adiabatic passage would be gradually closed, leading to a poor transfer efficiency. For the purpose of enhancing the efficiency, we present various optimization approaches to the laser parameters, involving order number n, relative strength ratio, and absolute strength. We show that there can remain a limit on the population transfer efficiency given by a three-level Λ system. In addition, we illustrate the importance of selecting an appropriate number of intermediate levels for maintaining a highly efficient transfer under mild experimental conditions.
Optical excitation of material transitions by adiabatic passage with frequency modulated (chirped) laser pulses is a useful spectroscopic tool.1,2 Here we demonstrate that picosecond frequency modulated laser pulses produce more efficient population transfer, and more selective excitation than equivalent (same spectral bandwidth) nearly transform limited laser pulses. We apply frequency modulated pulses to gas-phase and condensed phase systems. We also point out the "robustness", or insensitivity of the properties of the material excitation to the parameters of the laser field, that is inherent to the adiabatic process. The robustness obtainable by adiabatic passage will ultimately prove useful to laboratory applications of theoretical schemes which require frequency modulated ultrashort laser pulses to control chemical reaction dynamics.3