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Physical Review A

Inversion of polarization by light-induced stabilization

H.G. Weber, F. Bylicki, G. Miksch

Published Jul 1, 1984• 20 citations
Publisher

Abstract

We report on a new effect which is observed when the small polyatomic molecule ${\mathrm{NO}}_{2}$ is prepared under collision-free conditions into an electronically excited state. As the intensity of the exciting laser light is varied, the degree of polarization of the fluorescence light may undergo a change in sign. The analysis of the experimental results confronts us with an interesting problem concerning the time evolution of an isolated polyatomic system. In this paper we present the experimental results and a simple model which successfully describes these results. According to this model, the inversion of the polarization of the fluorescence light is a consequence of light-induced stability of the optically prepared state.

DOI
10.1103/physreva.30.270

Related papers

Magnetic resonances verified in agreement with an assumed time-asymmetric evolution

H.G. Weber, G. Miksch

With use of a narrow-band laser, the free ${\mathrm{NO}}_{2}$ molecule is prepared in a well-defined excited state under conditions that correspond to an optical-frequency--radiofrequency double-resonance experiment. We observe a resonant increase or decrease (dependent on the divergence of the light beam) of the total fluorescence intensity when the radiofrequency field is in resonance with the Zeeman splitting in the ground or excited state of the laser-induced transition. These resonances differ significantly from well-known optical-rf double-resonance signals and laser saturation effects. The resonances are, however, well described with use of a model which assumes an irreversible evolution in the free ${\mathrm{NO}}_{2}$ molecule.

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Effect of inversion of polarization in laser-excited molecular beams

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In the paper by Weber, Bylicki, and Miksch [Phys. Rev. A 30, 270 (1984)] the ``inversion effect'' of the Hanle and rf double-resonance signals in ${\mathrm{NO}}_{2}$ molecules is explained with the questionable assumption of a ``light-induced stability of the optically prepared state.'' We give an alternative explanation of the ``inversion effect'' which is based on the nonstationarity of molecular perturbation by a laser and which is consistent with the present knowledge of interaction of laser light with matter.

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Laser-induced fluorescence and polarized light in the collision regime: the ) case

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Laser-induced fluorescence experiments carried out using linearly polarized photons from a pulsed laser on a excitation - detection scheme in electrical discharges at 0.04 - 0.6 Torr, show that light emitted during the interaction and in the subsequent decay manifests anisotropy in its angular distribution and polarization. A rate equation approach is used to discuss the general features of the interaction between polarized light and atoms observed experimentally. The incident electromagnetic field is decomposed into multipolar components that may interact with the atom through an appropriate coupling. Defining the Einstein coefficients for each coupling, it is possible to introduce the equations for the dynamics of the atomic populations. For each direction and polarization, the temporal profile of the intensity during the transition is calculated. The theoretical prediction explains the outcome of the He experiment satisfactorily, providing that the role of the quenching and of the collision mixing within the manifold of the state is duly taken into account. Fitting of the model calculations to the experimental data then allows one to deduce the value of the `thermally averaged' cross section for collision-induced transitions between fine-structure and degenerate components of the sublevel, , for each J,M in the manifold.

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Saturation effects on polarized-light-pulse propagation and the resultant laser-induced fluorescence

Y. B. Band

We consider the nonlinear propagation of an intense short pulse of polarized light through a molecular medium with ground- and excited-state absorption, excited-state emission, and fluorescence from the excited state to the ground state and possibly to a lower-lying excited state. We incorporate the effects of saturation, field-induced alignment, and rotational diffusion of the molecules in the medium. Our approach uses a slowly varying envelope approximation for the light propagation and rate equations for the effective parallel and perpendicular (and higher-multipole) population components of the molecular states. The time dependence of the anisotropy of the fluorescence occurring after preparation of the medium by the intense short pulse is determined. We can treat propagation through an optically nonthin medium wherein the intensity of the pulse decreases while propagating through the medium. An example of a 100-ps pulse propagating through a sample of Rhodamine-6G is presented. We compare our method with others used to describe intense-short-pulse propagation in molecular media.

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Far wing depolarisation of collision-induced fluorescence

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Redistribution and depolarisation of light scattered by atoms undergoing collisions outside the impact region of the line profile has been studied experimentally and theoretically. In a cell experiment intensity and linear degree of polarisation of the fluorescence components 32S1/2 to or from 32P1/2,3/2 of Na atoms induced by collisions with Ar buffer gas atoms were measured as functions of the laser frequency omega L. The decrease of polarisation of the D2 component observed when omega L is shifted away from the D2 toward D1 resonance disagrees with calculations on the basis of the factorisation approximation of the theory of redistribution. Using Baylis (1975) potentials for the Na/Ar system, the effect is interpreted in terms of adiabatic coupling of the 32P3/2 electronic angular momentum to the collision axis during 'strong' collisions, giving rise to a greater than average depolarisation.

Journal of Physics B Atomic and Molecular Physics•Feb 14, 1981•14 citations

Collisions and half-collisions with lasers

N. K. Rahman, Carla Guidotti

Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.

•Jan 1, 1984•42 citations
We report on a new effect which is observed when the small polyatomic molecule NO2{\mathrm{NO}}_{2}NO2​ is prepared under collision-free conditions into an electronically excited state. As the intensity of the exciting laser light is varied, the degree of polarization of the fluorescence light may undergo a change in sign. The analysis of the experimental results confronts us with an interesting problem concerning the time evolution of an isolated polyatomic system. In this paper we present the experimental results and a simple model which successfully describes these results. According to this model, the inversion of the polarization of the fluorescence light is a consequence of light-induced stability of the optically prepared state.
With use of a narrow-band laser, the free NO2{\mathrm{NO}}_{2}NO2​ molecule is prepared in a well-defined excited state under conditions that correspond to an optical-frequency--radiofrequency double-resonance experiment. We observe a resonant increase or decrease (dependent on the divergence of the light beam) of the total fluorescence intensity when the radiofrequency field is in resonance with the Zeeman splitting in the ground or excited state of the laser-induced transition. These resonances differ significantly from well-known optical-rf double-resonance signals and laser saturation effects. The resonances are, however, well described with use of a model which assumes an irreversible evolution in the free NO2{\mathrm{NO}}_{2}NO2​ molecule.
In the paper by Weber, Bylicki, and Miksch [Phys. Rev. A 30, 270 (1984)] the ``inversion effect'' of the Hanle and rf double-resonance signals in NO2{\mathrm{NO}}_{2}NO2​ molecules is explained with the questionable assumption of a ``light-induced stability of the optically prepared state.'' We give an alternative explanation of the ``inversion effect'' which is based on the nonstationarity of molecular perturbation by a laser and which is consistent with the present knowledge of interaction of laser light with matter.
Laser-induced fluorescence experiments carried out using linearly polarized photons from a pulsed laser on a excitation - detection scheme in electrical discharges at 0.04 - 0.6 Torr, show that light emitted during the interaction and in the subsequent decay manifests anisotropy in its angular distribution and polarization. A rate equation approach is used to discuss the general features of the interaction between polarized light and atoms observed experimentally. The incident electromagnetic field is decomposed into multipolar components that may interact with the atom through an appropriate coupling. Defining the Einstein coefficients for each coupling, it is possible to introduce the equations for the dynamics of the atomic populations. For each direction and polarization, the temporal profile of the intensity during the transition is calculated. The theoretical prediction explains the outcome of the He experiment satisfactorily, providing that the role of the quenching and of the collision mixing within the manifold of the state is duly taken into account. Fitting of the model calculations to the experimental data then allows one to deduce the value of the `thermally averaged' cross section for collision-induced transitions between fine-structure and degenerate components of the sublevel, , for each J,M in the manifold.
We consider the nonlinear propagation of an intense short pulse of polarized light through a molecular medium with ground- and excited-state absorption, excited-state emission, and fluorescence from the excited state to the ground state and possibly to a lower-lying excited state. We incorporate the effects of saturation, field-induced alignment, and rotational diffusion of the molecules in the medium. Our approach uses a slowly varying envelope approximation for the light propagation and rate equations for the effective parallel and perpendicular (and higher-multipole) population components of the molecular states. The time dependence of the anisotropy of the fluorescence occurring after preparation of the medium by the intense short pulse is determined. We can treat propagation through an optically nonthin medium wherein the intensity of the pulse decreases while propagating through the medium. An example of a 100-ps pulse propagating through a sample of Rhodamine-6G is presented. We compare our method with others used to describe intense-short-pulse propagation in molecular media.
Redistribution and depolarisation of light scattered by atoms undergoing collisions outside the impact region of the line profile has been studied experimentally and theoretically. In a cell experiment intensity and linear degree of polarisation of the fluorescence components 32S1/2 to or from 32P1/2,3/2 of Na atoms induced by collisions with Ar buffer gas atoms were measured as functions of the laser frequency omega L. The decrease of polarisation of the D2 component observed when omega L is shifted away from the D2 toward D1 resonance disagrees with calculations on the basis of the factorisation approximation of the theory of redistribution. Using Baylis (1975) potentials for the Na/Ar system, the effect is interpreted in terms of adiabatic coupling of the 32P3/2 electronic angular momentum to the collision axis during 'strong' collisions, giving rise to a greater than average depolarisation.
Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.