Collisionless Dissociation of Polyatomic Molecules by Multiphoton Infrared Absorption PDF Download
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Languages : en
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The physical theory of dissociation of an isolated molecule of SF6 by a CO2 laser pulse is reviewed from both a quantum mechanical and classical point of view. The characteristics of the physical process are compared with the experimental data on collisionless dissociation.
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Languages : en
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The physical theory of dissociation of an isolated molecule of SF6 by a CO2 laser pulse is reviewed from both a quantum mechanical and classical point of view. The characteristics of the physical process are compared with the experimental data on collisionless dissociation.
Author: Cyrus D. Cantrell
Publisher: Springer Science & Business Media
ISBN: 3642822924
Category : Science
Languages : en
Pages : 300
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Book Description
In the early 1970s, researchers in Canada, the Soviet Union and the United States discovered that powerful infrared laser pulses are capable of dissociating mole cules such as SiF4 and SF6' This result, which was so unexpected that for some time the phenomenon of multiple-photon dissociation was not recognized in many cir cumstances in which we now know that it occurs, was first publicized at a time when the possibility of using lasers for the separation of isotopes had attracted much attention in the scientific community. From the mid-1970s to the early 1980s, hun dreds of experimental papers were published describing the multiple-photon absorp tion of C02 laser pulses in nearly every simple molecule with an absorption band in the 9 - 11 jJm region. Despite this impressive volume of experimental results, and despite the efforts of numerous theorists, there is no agreement among re searchers in the field on many fundamental aspects of the absorption of infrared laser light by polyatomic molecules. This book is devoted to reviells of the experimental and theoretical research that provides the foundations for our current understanding of molecular multiple photon exc itat i on, and to rev i ews of research that is pert i nent to the 1 aser sep aration of isotopes.
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Languages : en
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The dynamics of infrared multiphoton excitation and dissociation of SF6 was investigated under collision free conditions by a crossed laser-molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments were carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF6, the pulse duration of the CO2 laser and the frequency in both one and two laser experiments. Translational energy distributions of the SF5 dissociation product measured by time of flight and angular distributions and the dissociation lifetime of excited SF6 as inferred from the observation of secondary dissociation of SF5 into SF4 and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer on a nanosecond timescale, and can be adequately described by RRKM theory. An improved phenomenological model including the initial intensity dependent excitation, a rate equation describing the absorption and stimulated emission of single photons, and the unimolecular dissociation of excited molecules is constructed based on available experimental results. The model shows that the energy fluence of the laser determines the excitation of molecules in the quasi-continuum and the excess energy with which molecules dissociate after the laser pulse. The role played by the laser intensity in multiphoton dissociation is more significant than just that of overcoming the intensity dependent absorption in the lowest levels. 63 references.
Author: Jerry Glomph Black
Publisher:
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Category :
Languages : en
Pages :
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Author: M. R. Humphries
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Category :
Languages : en
Pages : 0
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Author: Jim-son Johnson Chou
Publisher:
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Category : Infrared sources
Languages : en
Pages : 412
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![Dynamics of Infrared Multiphoton Dissociation of SF6 by Molecular Beam Method. [Nonrandomize Excitation Energy]. PDF](https://books.google.com/books/content/images/frontcover/vEk4nQAACAAJ?fife=w80-h100)
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The decomposition of polyatomic molecules by infrared multiphoton absorption is a subject which is under extensive investigation in many laboratories. The process has been shown to be efficient, selective and collisionless. The physical principles which are mainly responsible for the absorption of a large number of photons by an isolated molecule under intense infrared laser radiation have been elucidated through many experimental and theoretical investigations. However, one very important question which remains to be answered is the relation between the dynamics of molecular decomposition and the degree of vibrational excitation, i.e., the question of whether the excitation energy is completely randomized before molecular decomposition. The production of electronically excited fragments in the dissociation of halogenated hydrocarbons and the observations of SF4 fragment in the decomposition of SF6 without the evidence of the formation of lower energy SF5 fragment provided some basis of speculation that the excitation energy might not be randomized before the dissociation of excited molecules. A crossed molecular beam apparatus has been adapted to study the dynamics of excitation and dissociation of polyatomic molecules in intense IR laser fields. Initial experiments have involved the study of the dissociation of SF6 by CO2 laser radiation at 10.6 .mu.m. a molecular beam of SF6 was formed by supersonic expansion using three stages of differential pumping. A grating tuned pulsed CO2 TEA laser was used as the excitation source. The laser beam was focused by a 25 cm focal length ZnSe lens, and crossed the molecular beam near its focal point. The fragments produced by multiphoton dissociation of SF6 within the small interaction region were detected as a function of recoil angle and velocity.
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Languages : en
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A study is made of the multiphoton dissociation of SF6 and other molecules under the collisionless molecular beam conditions used routinely. The results are unambiguously consistent with a far more orderly picture of the excitation and dissociation process than that suggested previously. It is concluded that although the initial excitation is selective, nearly complete energy randomization must take place in the molecular vibrational degrees of freedom before decomposition. 26 references. (JFP).
Author: Michael J. Coggiola
Publisher:
ISBN:
Category :
Languages : en
Pages : 4
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The dynamics of multiphoton absorption and dissociation of polyatomic ions will be studied to characterize the details of the process. Ion fragments will be identified and the energy released into translation will be measured using a coaxial laser ion beam spectrometer. Single photon absorption studies of positive and negative polyatomic ions failed to reveal any vibrational absorptions in the infrared region investigated. A single photon electronic transition absorption in O2(+) was found which resulted in dissociation. (Author).
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Languages : en
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The procedure of maximal entropy is applied to characterize the three distributions over energy states which are of direct interest for studies of multiple photon excitation of polyatomic molecules. These distributions are the original population of the different energy states given the mean number of photons absorbed, the distribution over the final energy states after a single collision, given a well defined initial energy state and the mean energy transfer per collision, and the time evolution of the population of the different energy states due to collisional deactivation, given the mean energy transfer per collision. Good agreement with experimentally determined values of (.delta.E), the average amount of energy removed in a collision, are obtained for deactivation of sec-butyl, cyclohexane, .beta.-hexyl, and .beta.-naphthylamine by structureless collision partners such as He or H/sub 2/. The vibrational relaxation surprisal parameter is found to be lambda/sub 1/ approximately equal to 0.1 for all these systems. This is much closer to a statistical, or strong-collision limit, than vibrational deactivation of diatomics by atoms, for which lambda/sub 1/ is approximately equal to 1.0. Deactivation most likely proceeds through a sequence of maximal-entropy distributions.
