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Collisions and half-collisions with lasers

N. K. Rahman, Carla Guidotti

Published Jan 1, 1984• 42 citations
OpenAlex

Abstract

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.

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Abstract Rather different problems can be lumped together under the general term 'laser control of atoms and molecules'. They include the laser selection of atomic and molecular velocities for the purpose of Doppler-free spectroscopy, laser control of the position and velocity of atoms (i.e. laser trapping and cooling of atoms), and laser control of atomic and molecular processes (ionization, dissociation) with a view of detecting single atoms and molecules and particularly separating isotopes and nuclear isomers. Over the last decades the principal problems posed have been successfully solved, and many of them have evolved remarkably in the subsequent investigations of the international research community. For example, the solution of the problem of laser cooling and trapping of atoms has given birth to the new field of the physics of ultracold matter, i.e. quantum atomic and molecular gases. The laser non-coherent control of uni-molecular processes has found an interesting extension in the field of laser coherent control of molecules. The concept of laser control of position has been successfully demonstrated with microparticles (optical tweezers), concurrently with investigations into atomic control. The laser photo-ionization of molecules on surfaces has led to the development of novel techniques of laser-assisted mass spectrometry of macromolecules, and so on. The aim of this book is to review these topics from a unified or 'coherent' point of view. It will be useful for many readers in various fields of laser science and its applications.

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Penning and associative ionization in crossed-beam Na/Na collisions assisted by strong resonant laser fields

J. Weiner, Penelope Polak-Dingels

We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.

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Observation of Structure in Laser-Induced Penning and Associative Ionization in Crossed-Beam Na + Na Collisions

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Photoassociation in laser-assisted collisions of ultracold alkali atoms has been used recently to perform high-precision spectroscopy of very-long-range states. Rovibrational spectra close to the Na(3 $^{2}$S)+${\mathrm{Na}}^{\mathrm{*}}$(3 $^{2}$${\mathit{P}}_{3/2}$) dissociation limit have been obtained by monitoring an enhanced ${\mathrm{Na}}_{2}^{+}$ ion signal. We propose the use of pump-probe techniques to study the dynamics of the ionization process. At very large distances (typically 70 a.u.) two colliding Na atoms absorb a photon and form a coherent vibrational wave packet. We follow the packet in time and probe two possible mechanisms that are proposed to yield molecular ions: photoionization, where the second photon is absorbed at a small internuclear distance R directly ionizing the molecule, and associative ionization with excitation at intermediate R to the nuclear continuum of a doubly excited state followed by a half collision and autoionization at small R. The latter event can also lead to dissociation of the complex. The two mechanisms lead to distinct differences in the ion signals as a function of delay time between the pump and the probe pulses. Estimates of the ion rates to be expected in actual pump-probe experiments are given, which tend to favor the autoionization mechanism as the dominant reaction path.

Physical Review A•Dec 1, 1994•50 citations
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.
A theory is given of radiative collisions, i.e., of elementary events in which the emission of a photon occurs in the course of a collision. Special attention is paid to radiative collisions stimulated by a laser field. Simple but clear approaches formulated in the forties or fifties are described briefly. This theory applies, in particular, to collisions in a strong electromagnetic field which affect the collision dynamics. Chemical radiative collisions, i.e., chemical reactions stimulated by photons absorbed in the course of a collision, are reviewed. An analysis is made of several recent experiments in which the theoretically predicted effects were observed: these include the radiative-collisional transfer of excitation from atom to atom, nonlinear effects due to the influence of the field on the collision dynamics, and photosubstitution of one of the atoms in a halogen molecule with an inert gas atom.
Recent years have seen tremendous progress in research on cold and controlled molecular collisions, both in theory and in experiment. The advent of techniques to prepare cold and ultracold molecules and ions, to store them in optical lattices or in charged quasicristalline structures, and to use them in crossed or merged beam experiments have opened many new possibilities to study the most fundamental aspects of molecular interactions. At the same time, theoretical work has made progress in tackling these problems and accurately describing quantum effects in complex systems, and in proposing viable options to control chemical reactions at ultralow energies. Through tutorials on both the theoretical and experimental aspects of research in cold and ultracold molecular collisions, this book provides advanced undergraduate students, graduate students and researchers with the foundations needed to understand this exciting field.
Abstract Rather different problems can be lumped together under the general term 'laser control of atoms and molecules'. They include the laser selection of atomic and molecular velocities for the purpose of Doppler-free spectroscopy, laser control of the position and velocity of atoms (i.e. laser trapping and cooling of atoms), and laser control of atomic and molecular processes (ionization, dissociation) with a view of detecting single atoms and molecules and particularly separating isotopes and nuclear isomers. Over the last decades the principal problems posed have been successfully solved, and many of them have evolved remarkably in the subsequent investigations of the international research community. For example, the solution of the problem of laser cooling and trapping of atoms has given birth to the new field of the physics of ultracold matter, i.e. quantum atomic and molecular gases. The laser non-coherent control of uni-molecular processes has found an interesting extension in the field of laser coherent control of molecules. The concept of laser control of position has been successfully demonstrated with microparticles (optical tweezers), concurrently with investigations into atomic control. The laser photo-ionization of molecules on surfaces has led to the development of novel techniques of laser-assisted mass spectrometry of macromolecules, and so on. The aim of this book is to review these topics from a unified or 'coherent' point of view. It will be useful for many readers in various fields of laser science and its applications.
We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.
The results of double-laser experiments in which Na2+{\mathrm{Na}}_{2}^{+}Na2+​ and Na+{\mathrm{Na}}^{+}Na+ are produced in crossed alkali beams under single-collision conditions in the presence of strong optical fields are reported. Structure in the mass-selected product ion intensity as a function laser frequency is observed when the optical field is strongly focused and tuned far off atomic or dimer transitions. These measurements are the first to show that the nuclear motion of the quasimolecular collision intermediate plays an important role in laser-induced collisional ionization.
Photoassociation in laser-assisted collisions of ultracold alkali atoms has been used recently to perform high-precision spectroscopy of very-long-range states. Rovibrational spectra close to the Na(3 2^{2}2S)+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗(3 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​) dissociation limit have been obtained by monitoring an enhanced Na2+{\mathrm{Na}}_{2}^{+}Na2+​ ion signal. We propose the use of pump-probe techniques to study the dynamics of the ionization process. At very large distances (typically 70 a.u.) two colliding Na atoms absorb a photon and form a coherent vibrational wave packet. We follow the packet in time and probe two possible mechanisms that are proposed to yield molecular ions: photoionization, where the second photon is absorbed at a small internuclear distance R directly ionizing the molecule, and associative ionization with excitation at intermediate R to the nuclear continuum of a doubly excited state followed by a half collision and autoionization at small R. The latter event can also lead to dissociation of the complex. The two mechanisms lead to distinct differences in the ion signals as a function of delay time between the pump and the probe pulses. Estimates of the ion rates to be expected in actual pump-probe experiments are given, which tend to favor the autoionization mechanism as the dominant reaction path.