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Picosecond photofragment spectroscopy. II. The overtone initiated unimolecular reaction H_2O_2(v_(OH)=5)→2OH

Scherer, Norbert F. and Zewail, Ahmed H. (1987) Picosecond photofragment spectroscopy. II. The overtone initiated unimolecular reaction H_2O_2(v_(OH)=5)→2OH. Journal of Chemical Physics, 87 (1). pp. 97-114. ISSN 0021-9606. https://resolver.caltech.edu/CaltechAUTHORS:20160906-090612592

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Abstract

This paper, second in the series, reports on the picosecond time‐resolved photofragmentation of the overtone (v_(OH)=5) initiated reaction: HOOH+hν→2OH. The hydrogen peroxide is initially excited by way of a picosecond laser pulse to the fourth overtone level of the OH‐stretch local mode. The subsequent unimolecular reaction behavior is obtained by monitoring the laser‐induced fluorescence, caused by the picosecond probe–pulse electronic excitation of the OH radical photoproduct (in a given rotational state). The two pulses are scanned relative to one another in time thereby mapping out the product yield for the given delay‐time interval. The resultant product formation behavior is found to be nonexponential, and may be modeled as a biexponential rise. Furthermore, the quasibiexponential behavior is sensitive to the exact excitation wavelength—slight variations of which result in large changes in the two time constants and the relative amplitudes of the fast and slow components. These experiments give direct evidence for the inhomogeneous nature of the overtone transition on the picosecond time scale, and provide the dissociation rate contribution to the homogeneous width (0.05–0.15 cm^(−1)). The apparent width for the main band feature is about 200 cm^(−1). The rate of product formation (magnitude and form) is interpreted in terms of statistical and nonstatistical theories. The limitations of the applicability of each model is discussed. The fluctuations of the fitting parameters as a function of excitation wavelength may be simulated by a statistical model which considers all possible discrete optical transitions within the simulated laser bandwidth and the details of product formation from each state. For a nonstatistical interpretation, the biexponential form reflects a division of the vibrational phase space, and this is discussed in the spirit of a kinetic model. Finally, experimental results are reported for direct UV initiated photofragmentation. The observed dynamics indicate that a very different type of potential surface (repulsive) is involved, in contrast to the overtone initiated dissociation, which takes place on the ground state surface.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1063/1.453529DOIArticle
http://scitation.aip.org/content/aip/journal/jcp/87/1/10.1063/1.453529PublisherArticle
Additional Information:© 1987 American Institute of Physics. Received 09 December 1986 Accepted 11 February 1987. This research was supported by a grant from the National Science Foundation (CHE-8512887). Some additional support was provided by the Presidential Fund and the Keck Foundation. We thank Professor Rudy Marcus for many useful discussions concerning RRKM theory. We would like to thank Lutfur Khundkar for enlightening conversations regarding statistical theories. We thank Dr. Joseph Perry for his invaluable assistance during the formative period of this project and for his continued interest, and Dr. Fuad Doany for his help in the initial state of this research. We are also grateful to Professor F. Crim for his help and interest throughout these studies, and Professor J. Troe for communicating results prior to publication. Finally, the early attempts to study overtone dynamics on the picosecond time scale (1981) involved the dedicated efforts and hard work of Mr. Joseph Perry and Dr. Eugene Ryabov, and this must be acknowledged here.
Funders:
Funding AgencyGrant Number
NSFCHE-8512887
Caltech President’s FundUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Arthur Amos Noyes Laboratory of Chemical Physics7514
Issue or Number:1
Record Number:CaltechAUTHORS:20160906-090612592
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20160906-090612592
Official Citation:Picosecond photofragment spectroscopy. II. The overtone initiated unimolecular reaction H2O2(vOH=5)→2OH Scherer, Norbert F. and Zewail, Ahmed H., The Journal of Chemical Physics, 87, 97-114 (1987), DOI:http://dx.doi.org/10.1063/1.453529
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70170
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:09 Sep 2016 17:50
Last Modified:03 Oct 2019 10:28

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