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Journal of Physics B Atomic Molecular and Optical Physics

Multiphoton population transfer between rovibrational states of HF: adiabatic rapid passage in a diatomic molecule

Türker Topçu, F. Robicheaux

Published Sep 27, 2010• 5 citations
Publisher

Abstract

Efficient population transfer by adiabatically chirping through a multiphoton resonance in microwave driven and kicked Rydberg atoms has recently been reported both experimentally and theoretically. Here we report on our simulations in which we have exploited this mechanism to vibrationally excite a diatomic molecule up to |ν = 4, J from the ground state by chirping through a four-photon resonance condition. This is an efficient means of population transfer, which is an alternative to the ladder climbing scheme requiring chirping through a sequence of states in the correct order. We discuss and compare one-dimensional quantum and classical models where there is no rotational degree of freedom. This comparison suggests that for the lowest laser intensity we consider, the process is classically forbidden and the transition occurs through tunnelling. We show that for larger peak intensities, the transfer can be looked upon as a classical transition in phase space, similar to that observed in the atomic case. We extend our simulations to fully three-dimensional quantum calculations and investigate the effect of coupling between different rotational pathways. We finally discuss the effect of thermal averaging over the initial J-states using a temperature for which the first few rotational levels inside the ν = 0 manifold are populated.

DOI
10.1088/0953-4075/43/20/205101

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Efficient population transfer by adiabatically chirping through a multiphoton resonance in microwave driven and kicked Rydberg atoms has recently been reported both experimentally and theoretically. Here we report on our simulations in which we have exploited this mechanism to vibrationally excite a diatomic molecule up to |ν = 4, J from the ground state by chirping through a four-photon resonance condition. This is an efficient means of population transfer, which is an alternative to the ladder climbing scheme requiring chirping through a sequence of states in the correct order. We discuss and compare one-dimensional quantum and classical models where there is no rotational degree of freedom. This comparison suggests that for the lowest laser intensity we consider, the process is classically forbidden and the transition occurs through tunnelling. We show that for larger peak intensities, the transfer can be looked upon as a classical transition in phase space, similar to that observed in the atomic case. We extend our simulations to fully three-dimensional quantum calculations and investigate the effect of coupling between different rotational pathways. We finally discuss the effect of thermal averaging over the initial J-states using a temperature for which the first few rotational levels inside the ν = 0 manifold are populated.
Following an experimental observation, a recent simulation has shown that efficient population transfer can be achieved through adiabatic chirping of a microwave pulse through a 10-photon resonance connecting two Rydberg states with n = 72, ℓ = 1 and n ∼ 82. These simulations have revealed that this population transfer is essentially a classical transition caused by separatrix crossing in the classical phase space. Here, we present the results of our fully three-dimensional quantum and classical simulations of coherent multiphoton population transfer in a kicked Li atom in a Rydberg state. We were able to achieve ∼76% population transfer from the 40p to 46p state in Li through a 6-photon resonance and contrast our results with those when the transition is driven by microwaves. We further discuss the case when the atom starts out from a Stark state in conjunction with the ℓ-distribution of the transferred population. We use a one-dimensional classical model to investigate the classical processes taking place in the phase space and find that the same separatrix crossing mechanism observed in microwave transitions is also responsible for the transition when the atom is kicked.
Population transfer in Hamiltonian systems, such as between Rydberg states of atoms and vibrational modes in diatomic molecules, has been both experimentally and theoretically demonstrated. All these systems have the property that the classical frequency of the motion changes monotonically with energy. Referred to as the twist condition, this property makes it possible to transfer population both by chirping a driving field within a ladder climbing scheme and by adiabatically chirping through a multiphoton resonance connecting two bound states. In this paper, we demonstrate that the latter method for population transfer can still be made to work in a system which violates the classical twist condition. We use classical rigid rotor as an example system where the frequency changes monotonically with energy and compare the responsible separatrix crossing mechanism in phase space to that in a system which exhibits an extremum in the frequency-energy profile. We perform both one-dimensional classical and quantum simulations and find that the quantum phase-space distribution mirrors its corresponding classical counterpart.
The population of atoms in Rydberg states is efficiently transferred with a change in principal quantum number nnn of up to ten via multiphoton adiabatic rapid passage through a single multiphoton resonance using a frequency-chirped microwave pulse. A quantum-mechanical picture of multiphoton adiabatic rapid passage in a one-dimensional atom using a Floquet approach provides a good description of most, but not all, of the observed phenomena.
Generally, high optical intensity is required for effective multiphoton excitation of quantum systems to highly excited states. In certain situations, however, lower-intensity, chirped pulses can provide more efficient transfer of population to the upper states by the process of adiabatic passage. We have studied the relative importance of these two mechanisms in the anharmonic $5s\ensuremath{-}5p\ensuremath{-}5d$ quantum ladder system of rubidium using frequency chirped laser pulses from an amplified Ti:sapphire laser (\ensuremath{\lambda}=780 and \ensuremath{\Delta}\ensuremath{\lambda}=10 nm). We measure simultaneously the three-photon ionization signal due to the Ti:sapphire and the population that remains in the 5d5d5d state with a postionizing 532-nm Nd:YAG pulse (where YAG denotes yttrium aluminum garnet). At low infrared fluences (80 \ensuremath{\mu}J/cm2{\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}J/cm2), the transfer to the 5d5d5d state is significantly enhanced when the pulse frequency is swept from the red to the blue, such that it follows the frequency spacing of the rubidium ladder. Counterintuitively, population is also transferred efficiently for the blue-to-red chirp at high fluences (>5 mJ/cm2{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}mJ/cm2). We attribute both of these effects to adiabatic passage from the 5s5s5s state to the 5d5d5d state. Even at the highest fluences, more efficient transfer occurs for either direction of chirp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magnitude and functional form. These results have important implications for the understanding of population transfer in complex ladder systems, such as molecular anharmonic vibrational ladders.
Using the concept of light-induced potentials introduced by Garraway and Suominen [Phys. Rev. Lett. 80, 932 (1998)], we study alternative schemes of adiabatic population transfer in a diatomic molecule from the lowest vibrational level of the ground electronic state to the lowest vibrational level of a third electronic state via allowed optical transitions from the ground and third electronic states to a second electronic state. Our chirped-pulse rapid adiabatic passage scheme is implemented with short-duration optical pulses, and can be understood in terms of ``road design engineering'' of the light-induced time-dependent adiabatic potential upon which the dynamics occurs. We explicitly show that complete adiabatic vibronic population transfer occurs using chirped picosecond-duration pulses in the sodium dimer example.
Coherent interactions between light fields and simple quantum systems, such as $\ensuremath{\pi}$-pulses and adiabatic passage through frequency chirps and stimulated Raman interactions, generally use one- or two-photon processes. This limits the use of such interactions to energy scales close to the photon energy of the control field. However, recent calculations have shown that a 3-level system, generic to ionized diatomic molecules, shows such a strong multiphoton coupling that $\ensuremath{\pi}$-pulses are possible on at least 12-photon transitions. In this paper, we show that adiabatic transfer of population is also possible on such high-order multiphoton transitions. Full one-dimensional quantum mechanical two-electron calculations confirm this effect even in a system with many excited states and an ionization continuum.