Author: David Lewis OsbornPublisher:
ISBN:
Category :
Languages : en
Pages : 842
Book Description
Photodissociation Dynamics and Spectroscopy of Free Radical Combustion Intermediates
Author: David Lewis OsbornPhotodissociation Dynamics and Spectroscopy of Free Radical Combustion Intermediates
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 422
Book Description
The photodissociation spectroscopy and dynamics of free radicals is studied by the technique of fast beam photofragment translational spectroscopy. Photodetachment of internally cold, mass-selected negative ions produces a clean source of radicals, which are subsequently dissociated and detected. The photofragment yield as a function of photon energy is obtained, mapping out the dissociative and predissociative electronic states of the radical. In addition, the photodissociation dynamics, product branching ratios, and bond energies are probed at fixed photon energies by measuring the translational energy, P(E{sub T}), and angular distribution of the recoiling fragments using a time- and position-sensitive detector. Ab initio calculations are combined with dynamical and statistical models to interpret the observed data. The photodissociation of three prototypical hydrocarbon combustion intermediates forms the core of this work.
Photodissociation Spectroscopy and Dynamics of Free Radicals, Clusters, and Ions
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 169
Book Description
The photodissociation spectroscopy and dynamics of free radicals and ions is studied to characterize the dissociative electronic states in these species. To accomplish this, a special method of radical production, based on the photodetachment of the corresponding negative ion, has been combined with the technique of fast beam photofragment translational spectroscopy. The photofragment yield as a function of photon energy is obtained, mapping out the dissociative and predissociative electronic states. Branching ratios to various product channels, the translational energy distributions of the fragments, and bond dissociation energies are then determined at selected photon energies. The detailed picture of photodissociation dynamics is provided with the aid of ab initio calculations and a statistical model to interpret the observed data. Important reaction intermediates in combustion reactions have been studied: CCO, C2H5O, and linear C{sub n} (n = 4--6).
Photoisomerization and Photodissociation Dynamics of Reactive Free Radicals
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 198
Book Description
The photofragmentation pathways of chemically reactive free radicals have been examined using the technique of fast beam photofragment translational spectroscopy. Measurements of the photodissociation cross-sections, product branching ratios, product state energy distributions, and angular distributions provide insight into the excited state potential energy surfaces and nonadiabatic processes involved in the dissociation mechanisms. Photodissociation spectroscopy and dynamics of the predissociative {tilde A}2A1 and {tilde B}2A2 states of CH3S have been investigated. At all photon energies, CH3 + S(3P{sub j}), was the main reaction channel. The translational energy distributions reveal resolved structure corresponding to vibrational excitation of the CH3 umbrella mode and the S(3P{sub j}) fine-structure distribution from which the nature of the coupled repulsive surfaces is inferred. Dissociation rates are deduced from the photofragment angular distributions, which depend intimately on the degree of vibrational excitation in the C-S stretch. Nitrogen combustion radicals, NCN, CNN and HNCN have also been studied. For all three radicals, the elimination of molecular nitrogen is the primary reaction channel. Excitation to linear excited triplet and singlet electronic states of the NCN radical generates resolved vibrational structure of the N2 photofragment. The relatively low fragment rotational excitation suggests dissociation via a symmetric C{sub 2V} transition state. Resolved vibrational structure of the N2 photofragment is also observed in the photodissociation of the HNCN radical. The fragment vibrational and rotational distributions broaden with increased excitation energy. Simple dissociation models suggest that the HNCN radical isomerizes to a cyclic intermediate (c-HCNN) which then dissociates via a tight cyclic transition state. In contrast to the radicals mentioned above, resolved vibrational structure was not observed for the ICNN radical due to extensive fragment rotational excitation, suggesting that intermediate bent states are strongly coupled along the dissociation pathway. The measurements performed in this Thesis have additionally refined the heats of formation and bond dissociation energies of these radicals and have unambiguously confirmed and added to the known electronic spectroscopy.
Fast Beam Photodissociation Spectroscopy and Dynamics of Small Molecules
Author: Alexandra Angelika HoopsPublisher:
ISBN:
Category :
Languages : en
Pages : 600
Book Description
Photoisomerization and Photodissociation Dynamics of Reactive Free Radicals
Author: Ryan Tyler BisePhotodissociation Studies of Combustion Relevant Radical Molecules
Author: Mark Jeffrey ShaperoPublisher:
ISBN:
Category :
Languages : en
Pages : 118
Book Description
The photodissociation of several combustion relevant radical molecules has been carried out. These species behave significantly different from closed shell species and the experiments presented here provide fundamental new insights into the chemical dynamics of free radicals. The experiments were carried out on a molecular beam photofragment translational spectroscopy instrument. Radicals were generated by flash pyrolysis of a suitable precursor and entrained in a pulsed molecular beam. The radicals were then photodissociated and the products were detected with a rotatable mass spectrometer based on electron impact ionization. The benzyl radical photodissociation and the cyclopentadienyl radical photodissociation were investigated at 248 nm. Two dissociation channels were observed for each radical. The benzyl radical dissociates into H + C7H6 and CH3 + C6H4 and the cyclopentadienyl radical dissociates to H + C5H4 and C3H3 + C2H2, where the C5H4 fragment was identified as ethynylallene by its ionization energy. The translational energy distribution determined for each channel suggests that every dissociation mechanism occurs via internal conversion to the ground state followed by intramolecular vibrational redistribution and dissociation. The branching ratio between the two competing channels for each radical was measured experimentally and compared to RRKM (Rice-Ramspberger-Kassel-Marcus) calculations. For both radicals, the dominant experimental channel was corroborated by RRKM calculations.
Production and Photodissociation of Neutral Free Radicals
Author: Neil Charles Cole-FilipiakPublisher:
ISBN:
Category :
Languages : en
Pages : 136
Book Description
The primary photochemistry of several combustion-relevant free radicals have been in- vestigated via photofragment translational spectroscopy. The relevance of radical photo- chemistry will be discussed, along with methodologies and details of each experiment. The experimental apparatus will also be described, especially with regard to the recent installa- tion of a tunable energy electron ionizer. The upgraded ionizer has been a significant advance, allowing for more detailed characterization of the radical source employed in this thesis. The photochemistry of the phenyl radical (c-C6H5), a combustion intermediate and pre- cursor to polycyclic aromatic hydrocarbons, was investigated at 248 and 193 nm. At 248 nm, an H-atom loss pathway was found, while at 193 nm both H-atom loss and C2H2 loss pathways were observed. For both wavelengths, P(ET) distributions suggested internal con- version to the ground electronic state followed by energy randomization and dissociation. The branching ratio between the two 193 nm dissociation pathways was found to be 0.2 ± 0.1 in favor of H-atom loss, in good agreement with statistical Rice-Rampsperger-Kassel-Marcus (RRKM) theory. An initial investigation of the methyl perthiyl radical (CH3SS) at 248 nm suggested the surprising results of both CH3 + SS and CH2S + SH dissociation channels with no evidence for S-atom loss. In both cases, the translational energy distributions were inconsistent with the expected energetics. Upon reinvestigation, the assumption of radical production--and there- fore radical photodissociation--was shown to be incorrect. The new results demonstrated S-loss and CH3 loss pathways, with the former appearing to involve a repulsive electronic excited state.
Photodissociation Spectroscopy and Dynamics of Free Radicals, Clusters, and Ions
Author: Hyeon ChoiInfrared Absorption Spectroscopy and Chemical Kinetics of Free Radicals
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
This research was directed at the detection, monitoring, and study of the chemical kinetic behavior by infrared absorption spectroscopy of small free radical species thought to be important intermediates in combustion. Work on the reaction of OH with acetaldehyde has been completed and published and work on the reaction of O(1D) with CH4 has been completed and submitted for publication. In the course of our investigation of branching ratios of the reactions of O(1D) with acetaldehyde and methane, we discovered that hot atom chemistry effects are not negligible at the gas pressures (13 Torr) initially used. Branching ratios of the reaction of O(1D) with CH4 have been measured at a tenfold higher He flow and fivefold higher pressure.