Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physical Review A

Computational study of radiative transitions in molecular collisions induced by short laser pulses

Hai-Woong Lee, Thomas F. George

Published Jun 1, 1987• 15 citations
Publisher

Abstract

A theoretical study of radiative transitions in molecular collisions occurring in the presence of short laser pulses is carried out within the semiclassical two-state formalism. Transition probabilities are calculated by numerically integrating the Bloch equation for two model systems and the Na-Ar system. At high laser intensities where a short pulse is more effective in inducing transitions than cw radiation, the transition probability exhibits nonlinear character with respect to the pulse intensity. At such high intensities, noncollisional atomic transitions may also be present, which need to be eliminated for an observation of collisional transitions.

DOI
10.1103/physreva.35.4977

Related papers

Atomic and molecular collisions in pulsed laser fields

Dane K. Madsen, Yiwen Xiao, Larry Eno

We examine collisions in pulsed laser fields within the framework of multimode Floquet theory; the approach as outlined in a recent paper allows us to deal with collisional problems in pulsed fields without having to explicitly follow the varying field amplitude. Our focus here is on pulses which are on a time scale of the order of the collision time or less; the interest follows from an analysis made by Lee and George suggesting that unusually strong radiative transitions are possible in such fields. We examine transitions in a model problem based on the Na–Ar collision system and conclude, as others have done for this system, that a short, intense pulse is indeed effective in promoting transitions. Our assessment is, however, only partly based upon the traditional measure of examining how transition probabilities vary as the pulse length is shortened/lengthened. This traditional measure, we argue, is not sufficient for investigation of the mechanism of Lee and George because the measure includes varying contributions to the pulse effect as the pulse frequency profile changes—and this is not accounted for in the analysis of Lee and George. We combine, then, the traditional measure with an examination of the effect of changes in pulse-frequency profile. More particularly, results are determined which embody the same frequency profile as the pulse train, but are generated from a series of fixed-amplitude sources and these results are, in turn, compared with ones generated from a single fixed-amplitude source operating at the central frequency of the train. While, as noted above, we conclude for the Na–Ar system that a short, intense pulse is indeed effective in promoting transitions, we find that combining the traditional measure with an investigation of the role of frequency-profile changes significantly alters assessment of the pulse effect.

The Journal of Chemical Physics•Dec 15, 1998•0 citations

Nonlinear optical pulse propagation in molecular media

Y. B. Band, Raanan Bavli

We model the response of a molecular medium to an intense pulse of light. We begin by reducing the Maxwell-Bloch equations for a multilevel molecular model system to a set of simplified propagation equations. We thereby obtain expressions for the frequency, temperature, and field intensity dependence of the ground and excited electronic state absorption cross sections, and the emission cross section, as well as the temperature dependence of the decay rate of the excited state. The phenomena of saturation, excited-state absorption, optically induced dichroism and birefringence, vibronic energy transfer, induced molecular alignment, decay of the matter-field interaction energy, and rotational diffusion are considered within the general formalism for describing the propagation of an intense pulse of light in the molecular medium. We find that the cross section for absorption (emission) of a photon is not identical to the cross section for molecular excitation (deexcitation) via the matter-field interaction. This is explained in terms of an energy balance between the matter, radiation, the matter-radiation interaction, and the bath responsible for the broadening of the molecular transitions. The inequality of these cross sections determines whether the system supplies or draws energy from the thermal bath. The rise in the vibrational temperature of molecules subjected to an intense short pulse is shown to significantly affect propagation of the pulse. The alignment of the molecular system resulting from saturation of the ground-state transition by a polarized laser pulse and the effects of the orientational dependence of the matter-field interaction including effects of rotational diffusion are determined. As an example of the formalism we consider pulse propagation in a Rhodamine 6G solution. We fit the parameters of the model to determine the frequency dependence of the ground- and excited-state absorption cross sections and the emission cross section and compare with experimentally determined cross sections. We describe the propagation of a pulse whose temporal width is comparable to the rotational diffusion time of the molecules in the dye solution and calculate polarization pulse probe signals.

Physical Review A•Oct 1, 1987•28 citations

Transitions in Presence of Short Laser Pulses

Fabio Bagarello, Emilio Fiordilino

Accurate numerical calculations are carried out to investigate the validity of the two-state approximation in the case of resonant interactions between electromagnetic radiation and atoms. Short pulses are considered and the presence of the atomic spectrum is modelled by introducing a third, non-resonant, state. We show that the harmonics of the pulse profile may play a significant role in the dynamics of the process and may cause energy non-conserving transitions between the atomic states.

Journal of Modern Optics•Feb 1, 1990•2 citations

Two-photon laser-induced radiative collisions

Munir Hasan Nayfeh, G. Barry Hillard

We present a semiclassical treatment of two-photon laser-induced radiative collisions. We treat the two-photon---one-collision and two-photon---two-collision cases. We find a new intensity-induced collisional shift which makes the two-photon line shape highly sensitive to the intensity, and may make the line shape symmetric.

Physical Review A•Sep 1, 1981•10 citations

Theory of electronic-state coherences produced in combined laser-field—collisional reactions

P. R. Berman, E. Giacobino

A method for producing electronic-state coherences using either collisionally-aided radiative excitation (or "optical collision") or laser-induced collisional energy transfer (or "radiative collision") is proposed. Two atoms, $A$ and ${A}^{\ensuremath{'}}$, collide in the presence of two pulsed-laser fields having frequencies $\ensuremath{\Omega}$ and ${\ensuremath{\Omega}}_{1}$. It is shown that by choosing ${\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}$ such that an energy-conserving transition can occur in the composite $A{A}^{\ensuremath{'}}$ system, one can create an electronic-state coherence in the $A$ or ${A}^{\ensuremath{'}}$ atoms. The coherence can be produced between states of the same or of opposite parity; if it is between states of opposite parity, coherent emission at frequency ${\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}$ may be generated. The relationship of this work to theories of "pressure-induced extra resonances" in four-wave mixing is discussed.

Physical Review A•Nov 1, 1983•13 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
A theoretical study of radiative transitions in molecular collisions occurring in the presence of short laser pulses is carried out within the semiclassical two-state formalism. Transition probabilities are calculated by numerically integrating the Bloch equation for two model systems and the Na-Ar system. At high laser intensities where a short pulse is more effective in inducing transitions than cw radiation, the transition probability exhibits nonlinear character with respect to the pulse intensity. At such high intensities, noncollisional atomic transitions may also be present, which need to be eliminated for an observation of collisional transitions.
We examine collisions in pulsed laser fields within the framework of multimode Floquet theory; the approach as outlined in a recent paper allows us to deal with collisional problems in pulsed fields without having to explicitly follow the varying field amplitude. Our focus here is on pulses which are on a time scale of the order of the collision time or less; the interest follows from an analysis made by Lee and George suggesting that unusually strong radiative transitions are possible in such fields. We examine transitions in a model problem based on the Na–Ar collision system and conclude, as others have done for this system, that a short, intense pulse is indeed effective in promoting transitions. Our assessment is, however, only partly based upon the traditional measure of examining how transition probabilities vary as the pulse length is shortened/lengthened. This traditional measure, we argue, is not sufficient for investigation of the mechanism of Lee and George because the measure includes varying contributions to the pulse effect as the pulse frequency profile changes—and this is not accounted for in the analysis of Lee and George. We combine, then, the traditional measure with an examination of the effect of changes in pulse-frequency profile. More particularly, results are determined which embody the same frequency profile as the pulse train, but are generated from a series of fixed-amplitude sources and these results are, in turn, compared with ones generated from a single fixed-amplitude source operating at the central frequency of the train. While, as noted above, we conclude for the Na–Ar system that a short, intense pulse is indeed effective in promoting transitions, we find that combining the traditional measure with an investigation of the role of frequency-profile changes significantly alters assessment of the pulse effect.
We model the response of a molecular medium to an intense pulse of light. We begin by reducing the Maxwell-Bloch equations for a multilevel molecular model system to a set of simplified propagation equations. We thereby obtain expressions for the frequency, temperature, and field intensity dependence of the ground and excited electronic state absorption cross sections, and the emission cross section, as well as the temperature dependence of the decay rate of the excited state. The phenomena of saturation, excited-state absorption, optically induced dichroism and birefringence, vibronic energy transfer, induced molecular alignment, decay of the matter-field interaction energy, and rotational diffusion are considered within the general formalism for describing the propagation of an intense pulse of light in the molecular medium. We find that the cross section for absorption (emission) of a photon is not identical to the cross section for molecular excitation (deexcitation) via the matter-field interaction. This is explained in terms of an energy balance between the matter, radiation, the matter-radiation interaction, and the bath responsible for the broadening of the molecular transitions. The inequality of these cross sections determines whether the system supplies or draws energy from the thermal bath. The rise in the vibrational temperature of molecules subjected to an intense short pulse is shown to significantly affect propagation of the pulse. The alignment of the molecular system resulting from saturation of the ground-state transition by a polarized laser pulse and the effects of the orientational dependence of the matter-field interaction including effects of rotational diffusion are determined. As an example of the formalism we consider pulse propagation in a Rhodamine 6G solution. We fit the parameters of the model to determine the frequency dependence of the ground- and excited-state absorption cross sections and the emission cross section and compare with experimentally determined cross sections. We describe the propagation of a pulse whose temporal width is comparable to the rotational diffusion time of the molecules in the dye solution and calculate polarization pulse probe signals.
A method for producing electronic-state coherences using either collisionally-aided radiative excitation (or "optical collision") or laser-induced collisional energy transfer (or "radiative collision") is proposed. Two atoms, AAA and ${A}^{\ensuremath{'}}$, collide in the presence of two pulsed-laser fields having frequencies $\ensuremath{\Omega}$ and ${\ensuremath{\Omega}}_{1}$. It is shown that by choosing ${\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}$ such that an energy-conserving transition can occur in the composite $A{A}^{\ensuremath{'}}$ system, one can create an electronic-state coherence in the AAA or ${A}^{\ensuremath{'}}$ atoms. The coherence can be produced between states of the same or of opposite parity; if it is between states of opposite parity, coherent emission at frequency ${\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}$ may be generated. The relationship of this work to theories of "pressure-induced extra resonances" in four-wave mixing is discussed.
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.