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Physical Review A

Cooperative fluorescence in nonadiabatic dissociation of alkali diatoms

Gershon Kurizki, Gabriel Hose, A. Ben‐Reuven

Published Dec 1, 1988• 7 citations
Publisher

Abstract

We predict a new type of time-resolved cooperative fluorescence (CF) obtainable when a homonuclear alkali-metal diatom dissociating via a $^{1}\mathrm{\ensuremath{\Sigma}}^{\mathrm{*}}$ state evolves (by nonadiabatic radial coupling) into a superposition of two excited adiabatic states correlated to different doublet levels. CF intensity then combines ``ringing'' oscillations, caused by time variation of the interference phase between the receding emitting atoms, with quantum beats associated with fine-structure splitting. The calculated CF patterns for ${\mathrm{Li}}_{2}$ are sensitive to the amplitudes and relative phase of the two superposed adiabatic states.

DOI
10.1103/physreva.38.6433

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Time-resolved cooperative fluorescence (TRCF) from electronically excited dissociating homonuclear diatoms is suggested as a new technique for studying orbital and spin-polarization effects in fragmented diatoms. The method is based on a theoretical prediction of a novel feature named here ‘superbeats’. This feature results from a combination of two oscillatory effects, namely, the ringing oscillations caused by the time variation of the interference phase between the receding atoms during emission (P. Grangier, A. Aspect and J. Vigué, Phys. Rev. Lett., 1985, 54, 418) and the quantum beats associated with the fine-structure splitting. The time-resolved superbeat pattern is shown to be sensitive to the amplitudes and phases of the superposed states, with different orbital polarization and spin, mixed by adiabatic and non-adiabatic couplings in the outcome of the dissociation process. Quantitative estimates are made, based on a detailed analysis, for the photodissociation of Li2. Under conditions in which rotational effects can be neglected, the two states 1Σu(g) and 3Πu(g) of Li2 are superposed in the final outcome of the reaction. This should lead to a superbeat pattern dependent on the amplitude and phase of the superposition. Conditions for the experimental observability of these features have been studied, showing that a time resolution of ≲0.1 ns is required.

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Spectral Line Shapes•Jan 1, 1987•0 citations
We predict a new type of time-resolved cooperative fluorescence (CF) obtainable when a homonuclear alkali-metal diatom dissociating via a 1\ensuremathΣ∗^{1}\mathrm{\ensuremath{\Sigma}}^{\mathrm{*}}1\ensuremathΣ∗ state evolves (by nonadiabatic radial coupling) into a superposition of two excited adiabatic states correlated to different doublet levels. CF intensity then combines ``ringing'' oscillations, caused by time variation of the interference phase between the receding emitting atoms, with quantum beats associated with fine-structure splitting. The calculated CF patterns for Li2{\mathrm{Li}}_{2}Li2​ are sensitive to the amplitudes and relative phase of the two superposed adiabatic states.
Time-resolved cooperative fluorescence (TRCF) from electronically excited dissociating homonuclear diatoms is suggested as a new technique for studying orbital and spin-polarization effects in fragmented diatoms. The method is based on a theoretical prediction of a novel feature named here ‘superbeats’. This feature results from a combination of two oscillatory effects, namely, the ringing oscillations caused by the time variation of the interference phase between the receding atoms during emission (P. Grangier, A. Aspect and J. Vigué, Phys. Rev. Lett., 1985, 54, 418) and the quantum beats associated with the fine-structure splitting. The time-resolved superbeat pattern is shown to be sensitive to the amplitudes and phases of the superposed states, with different orbital polarization and spin, mixed by adiabatic and non-adiabatic couplings in the outcome of the dissociation process. Quantitative estimates are made, based on a detailed analysis, for the photodissociation of Li2. Under conditions in which rotational effects can be neglected, the two states 1Σu(g) and 3Πu(g) of Li2 are superposed in the final outcome of the reaction. This should lead to a superbeat pattern dependent on the amplitude and phase of the superposition. Conditions for the experimental observability of these features have been studied, showing that a time resolution of ≲0.1 ns is required.
The time-resolved cooperative emission from a system of correlated neutral dissociation fragments, or molecular collision products in beams, is investigated. The investigation is focused on emission at large fragment separations (between 1 nm and a few emission wavelengths), exceeding the domain of short-range interactions within the reactive or collisional molecular complex. A master-equation approach is used to obtain a general expression for the cooperative emission rate, which consists of nonexponential decay factors multiplied by temporal ringing patterns. These features result from the time-dependent radiative coupling between the receding fragments; they depend in an essential manner on the initial electronic state of the parent molecular complex and its symmetry which determine the correlations between the fragments. In the model system of a pair of identical two-level fragments two cases are considered separately. (a) A single photon shared by the fragments, where the emission is initially superradiant or subradiant (radiation trapping), depending on the spin and inversion symmetry of the parent molecular system and of the nascent fragments. The ringing pattern depends on the electronic angular momentum state of the parent molecule and on the polarization of the emitted light. (Such a ringing has been observed recently by Grangier, Aspect, and Vigu\'e [Phys. Rev. Lett. 54, 418 (1985)] in the emission of photodissociated Ca2{\mathrm{Ca}}_{2}Ca2​.) (b) Two initially excited fragments, where the ringing pattern is of smaller amplitude, and is weakly dependent on the electronic angular momentum of the parent molecule. All the aforementioned cooperative features generally last until the fragments recede several radiation wavelengths away from each other. The application of this time-resolved analysis to various diagnostic problems is discussed, especially with regard to the identification of excited electronic states of the parent molecular complex, and the stereospecificity of the fragmental orbitals.
The recent experiment by Grangier, Aspect, and Vigu\'e on interference in the emission from fragments of electronically photodissociated molecules is treated as a special case of cooperative fluorescence (CF) from products of various molecular processes. This general analysis relates time-resolved features of the CF to characteristics (such as orbital symmetry) of the dissociating parent molecule (PM) suggests various PM state preparations (including formation of subradiant states), and discusses the persistence of CF in systems of nonidentical fragments. The diagnostic potentialities of such studies are emphasized.
A system of two colliding neutral or ionized atoms with excited states that exhibit fine-structure splitting is shown to emit resonance fluorescence, the quantum features of which can reveal the collisional dynamical correlations between the atoms. Time-resolved measurements of quantum noise (squeezing properties) of the emitted field are shown to yield detailed information on the dynamical correlations. Notably, they disclose the phase between the dynamically correlated two-atom adiabatic states, which is not available from conventional spectroscopy. Such phase information can be a sensitive probe of collision dynamics. It is also important for the achievement of enhanced selectivity in laser-induced reactive collisions or dissociation, based on controllable coherent superpositions of adiabatic states.
Fine-structure transitions of alkali atoms induced by collisions with diatomic molecules are shown to be strongly influenced by a dynamical feature that is completely absent when the collision partner is a rare gas. The effect is a nonadiabatic coupling caused by the anisotropy of the molecule. Calculations for Rb(52P12)\mathrm{Rb}(5^{2}P_{\frac{1}{2}})Rb(52P21​​) + H2{\mathrm{H}}_{2}H2​ and for Rb(52P12)\mathrm{Rb}(5^{2}P_{\frac{1}{2}})Rb(52P21​​) + He are in good agreement with recent experiments and explain the dramatic difference between these two systems.
The fluorescence intensities for the D lines in non-resonant excitation of the alkali-helium pairs under single-collision conditions were obtained from calculations of the optical collision spectra by the quantum coupled-channels method. The close relation of the non-adiabatic transition probability from a molecular level to an atomic state during the final stage of an optical collision and the spectral dependence of the relative fluorescence intensities is investigated. An analysis of the impact region in terms of line-broadening and fine-structure cross sections calculated separately for all systems confirms the factorisation approximation.