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Multiphoton dissociation of polyatomic molecules

Description: The dynamics of infrared multiphoton excitation and dissociation of SF/sub 6/ 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 SF/sub 6/, the pulse duration of the CO/sub 2/ laser and the frequency in both one and two laser experiments. Translational energy distributions of the SF/sub 5/ dissociation product measured by time of flight and angular distributions and the dissociation lifetime of excited SF/sub 6/ as inferred from the observation of secondary dissociation of SF/sub 5/ into SF/sub 4/ 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.
Date: October 1, 1979
Creator: Schulz, P.A.
Partner: UNT Libraries Government Documents Department

THREE AND FOUR CENTER ELIMINATION OF HC1 IN THE MULTIPHOTON DISSOCIATION OF HALOGENATED HYDROCARBONS

Description: Three and four center unimolecular elimination reactions of HCl have been investigated for CHF{sub 2}Cl, CHFCL{sub 2}, CH{sub 3}CCl{sub 3}, CH{sub 3}CF{sub 2}Cl and in a molecular beam experiment using infrared multiphoton absorption to energize the molecule. The translational energy distributions obtained in this work show that the average translational energy released to the fragments is around 8-12 kcal/mole, except for the three center elimination reaction from CHClCF{sub 2}, which gives a value of 1 kcal/mole. In four center eliminations, the translational energy released is less than 20% of the potential energy barrier of the back reaction. This is somewhat less than previous indications that approximately 30% of the potential energy barrier of the exit channel in four center reactions should be released into translation.
Date: April 1, 1978
Creator: Sudbo, Aa. S.; Schulz, P.A.; Shen, Y.R. & Lee, Y.T.
Partner: UNT Libraries Government Documents Department

MULTIPHOTON DISSOCIATION PRODUCTS FROM HALOGENATED HYDROCARBONS

Description: The recent interest in multiphoton dissociation (MPD) of polyatomic molecules has revealed quite a bit of confusion as to what the major dissociation channels of some molecules are, and whether the dissociation process can be described by a statistical, e.g., RRKM, theory of unimolecular reactions. In a gas cell experiment an experimenter often finds great difficulties in answering questions such as: do vibrationally excited molecules react with one another, how do reactions involving free radicals produced in the dissociation proceed, can these radicals decompose further in the presence of the laser field, and does the detection method used actually probe the primary dissociation channel. Using the molecular beam setup previously described, we have overcome most of these difficulties. We have studied MPD of a number of compounds, mainly halogenated methanes, ethanes and ethylenes, and identified their major dissociation channels. To identify an MPD product, we realize that a unique set of ratios between mass spectrometer signals of various ion fragments must first be established for a given molecular species or radical, and preferably the velocity distributions of the dissociation products should be measured and checked for consistency. In our setup, we could measure both angular and time-of-flight distributions for the fragments, and pereorm this check. The low density of molecules in the beam and the extremely low duty cycle in our experiment prevented us from observing the competing minor channels which have very small branching ratios (< 0.1).
Date: November 1, 1977
Creator: Sudbo, Aa. S.; Schulz, P.A.; Grant, E.R & Lee, Y.T.
Partner: UNT Libraries Government Documents Department

Crossed laser and molecular beam study of multiphoton dissociation of C/sub 2/F/sub 5/Cl. [Rate constants, angular and velocity distributions]

Description: Rate constants for the photodissociation of C/sub 2/F/sub 5/Cl as well as the yield of C/sub 2/F/sub 4//sup +/ were measured. The dynamics of the two dissociation channels was studied by measuring the angular and velocity distributions of the products. 2 references. (JFP)
Date: September 1, 1978
Creator: Krajnovich, D.J.; Giardini-Guidoni, A.; Sudboe, A.S.; Schulz, P.A.; Shen, Y.R. & Lee, Y.T.
Partner: UNT Libraries Government Documents Department