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Effects of buffer gases on the infrared multiphoton dissociation of C 2 H 4 DCl: A chemical clock to explore highly excited molecules
Author(s) -
Papagiannakopoulos P. J.,
Kosnik Ken,
Benson S. W.
Publication year - 1982
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550140402
Subject(s) - chemistry , excited state , photodissociation , buffer gas , dissociation (chemistry) , torr , branching fraction , infrared , molecule , thermal decomposition , excitation , wavelength , atomic physics , analytical chemistry (journal) , laser , photochemistry , optics , physics , electrical engineering , organic chemistry , chromatography , thermodynamics , engineering
Abstract Observations are reported of the effect of the buffer gases He, Ne, and CF 4 , in the pressure range of 0–30 torr, on the branching ratio [HCl]/[DCl] of the unimolecular decompositionThe ratio R = k H / k D has been measured in high‐pressure thermal decomposition (670–1100 K) and was shown to give a unique measure of the internal energy of the decomposing molecules and hence, with RRKM theory and pressure fall‐off data, a time scale for their decomposition. Applying the thermal data to the photolysis leads to the conclusion that excitation and decomposition are produced by the laser spike (high intensity, 70 ns FWHM) and also at a slower rate by the larger, less intense tail (1.6 μs). Added buffer gases quench the latter, leaving the former which, from measurements of R , is shown to correspond to excitations of 115 ± 15 kcal/mol and lifetimes of ∼30 ps. No bond breaking is seen despite the high energies, in accord with theoretical expectations. The results require an enhanced rate of photon absorption by the highly excited molecules, which are about hundredfold greater than that observed for 300 K molecules. Data are also reported for C 2 H 2 F 2 and the secondary multiphoton photolysis of the ethylenes produced. Effects of beam geometry and wavelength are explored.