Photodecomposition of Chloromethanes Adsorbed on Silica Surfaces.

Irradiation of CCl4, CFCl3, and CF2Cl2 in the presence of C2H6 in vessels containing silica sand or fused quartz tubing results in the formation of chlorine-containing products. The formation of these compounds occurs at wavelengths extending up to approximately 400 nm, that is, at wavelengths well beyond the absorption threshold of the chloromethanes in the gas phase. It is suggested that CCl4 adsorbed on silica surfaces photodissociates to yield CCl3 and CCl2 species. The poor material balance obtained in these experiments indicates that several of the chlorine-containing fragments are strongly adsorbed on the surface. At a CCl4 pressure of 13 Pa (0.1 torr), photolysis with 366 nm light in the presence of sand results in the decomposition of one molecule for every 104 photons striking the surface. Under otherwise identical conditions, the photon-induced breakdown of CFCl3 and CF2Cl2 is respectively only 10 percent or 3 percent as efficient.

Photochemistry of Methane in the Photoionization Region.

Methane was irradiated with microwave operated helium (21.2 eV) and neon (16.7-16.8 eV) resonance lamps which were separated from the reaction vessel by an aluminum window. The quantum yields of the stable end products have been determined at methane pressures ranging from 1 to 20 torr. Over this pressure range the abundances of the primary ions, determined through various diagnostic experiments, are within experimental error the same at 21.2 eV as at 16.7-16.8 eV ( CH 4 + ≃ CH 3 + ≃ 0.5 , CH 2 + ⩽ 0.02 ), and are in good agreement with the primary mass spectra obtained in a photoionization mass spectrometer under collision free conditions (P < 10-5 torr). The C 2 H 5 + which is formed by the reaction: CH 3 + + CH 4 → C 2 H 5 + + H 2 loses a proton by an undetermined mechanism to give C2H4 as a product. There is no evidence for the formation of neutral fragments such as H2, C, CH or CH2 at 16.7-16.8 eV. The fact that the ionization quantum is equal to unity in this energy range accounts for the absence of these intermediates. At 21.2 eV where (Φionization = 0.95) there is concrete evidence for the formation of carbon atoms (Φ(C) ⩾ 0.002). In an attempt to demonstrate the usefulness of enclosed neon and helium resonance light sources in the ion-molecule kinetic studies, the relative probabilities of transfer of H- over D- from various partially deuterium labeled hydrocarbons to C 2 H 5 + (or C 2 D 5 + ) has been determined. The results presented in this article resolve the existing disagreements between previous helium resonance photolysis studies on CH4.

Gas Phase Far Ultraviolet Photolysis and Radiolysis of Vinyl Chloride.

Quantum yields of the major products formed in the far ultraviolet photolysis of CH2CDCl (C2H2, C2HD, C2H3D, C2H2D2, and 1,3-C4H4D2) have been determined at 147 nm (8.4 eV), 123.6 nm (10.0 eV), and 104.8-106.7 nm (11.6-11.8 eV). The quantum yield of the stable vinyl radicals which can be unambiguously ascribed to the primary process (CH2CDCl + hv → CH2CD + Cl) is 0.3 and 0.05 at 147 and 123.6 nm, respectively. The sharp decrease in the yield of vinyl radicals with the increase in energy of the incident photon beam is in part attributed to the decomposition of internally excited vinyl radicals to give acetylene as a product. At 147 nm, the combined yield of acetylene plus vinyl radicals is 0.95 ±0.05. At the shorter wavelengths, approximately one acetylene molecule is formed per electronically excited vinyl chloride molecule. It is concluded that the dissociative process: C2H3Cl* → C2H2 + H + Cl, occurring via a C2H3 or C2H2Cl intermediate adequately accounts for the reactive neutral species formed at higher photon energies. Isotopic labeling experiments show that the hydrogen atoms are detached from both positions of the parent molecule. Ethylene which is a product over the entire wavelength range is in part formed via the reaction: H* + C2H3Cl → C2H4 + Cl, where H* represents a translationally excited hydrogen atom. The C2H2DC1+ ions formed at 104.8 - 106.7 nm with a quantum yield of 0.47 do not contribute to the formation of acetylene or vinyl radicals. In the gas phase radiolysis of vinyl chloride, acetylene (G ~ 1.5) is mainly formed in the dissociation of neutral electronically excited vinyl chloride molecules. From this value, we may estimate that the ratio of neutral excited molecules formation to ionization in the radiolysis of vinyl chloride is 0.39. Vinyl ions, which are also produced (G ~ 0.28-0.35) react mainly by addition to vinyl chloride.

Pulse Radiolysis of Methane.

The pulse radiolysis of methane has been studied in the absence and presence of electron scavengers such as SF6 and CD3I and positive ion scavengers such as i-C4D10 in order to define the role of the intermediates H, C, CH, CH2, CH3, CH 5 + , and C 2 H 5 + in product formation. The dose rate was varied from 0.68 to 15.2 × 1019 eV/g-s, the dose (number of pulses) was varied, and the duration of the pulse was changed from 3 ns to 100 ns. The variation of the yields of the ethylene and ethane products with dose is explained by the reaction of H-atoms with accumulated ethylene product. The fast reacting C, CH, and 1CH2 species insert into methane to form acetylene, ethylene, and ethane products, but all of the reactions of these species cannot be completely specified since they may originate in upper electronic states, whose reactions with methane are unknown. Product formation by the slow reacting 3CH2 and CH3 radicals is also examined; for instance, evidence is presented for the occurrence of the reaction: 3CH2 + CH3 → C2H4 + H. Results indicate that the ions CH 5 + and C 2 H 5 + undergo neutralization mainly through the processes CH 5 + + e → CH 4 + H C 2 H 5 + + e → ( C 2 H 4 ) * + H → C 2 H 2 + H + H 2 ( 2 H ) When i-C4D10 is added, a fraction of the CH 5 + and C 2 H 5 + react with the additive rather than undergo neutralization. A calculation demonstrates that the fraction of ions undergoing reaction with a given concentration of i-C4D10 can be correctly predicted by assuming that the rate constant for neutralization of CH 5 + and C 2 H 5 + is the same as that determined recently for the t-butyl ion.

Pulse Radiolysis of Neopentane in the Gas Phase.

The pulse radiolysis of gaseous neopentane has been investigated in the absence and presence of electron scavengers (SF6, CD3I, CCl4). Deuterium labeling experiments show that the stable product molecules can be accounted for by (a) radical combination reactions involving mainly CH3 and H; (b) hydride ion transfer reactions involving C2H3 +, C2H5 +, and C3H5 +; (c) neutralization reactions of C4H9 + and C5H11 +; and (d) unimolecular dissociation of the parent ion (C5H12 +) and of electronically excited neopentane. Neutralization of the t-C4H9 + ion, which is the major positive ion in the system occurs as follows: (a) t-C4H9 + + e → i-C4H8 + H and (b) t-C4H9 + + e → 2CH3 + C3H6. It is shown that C5H11 + produced in hydride ion transfer reaction C n H m + + neo-C5H12 → C n H m+1 + C5H11 + (where C n H m + = C2H3 +, C2H5 +, and C3H5 +) rearranges to the CH3C+(CH3)CH2CH3 structure prior to neutralization. A detailed accounting of all products produced in the unimolecular and bimolecular reactions led to the conclusion that the ratio of neutral electronically excited molecules to parent ions (Nex/N+) is 0.28.

Ionization Quantum Yields and Absorption Coefficients of Selected Compounds at 58.4 and 73.6-74.4 nm.

The ionization quantum yields and I he extinction coefficients of a number of compounds have been determined at the wavelengths of the helium (58.4 nm) and neon (73.6-74.4 nm) resonance lamps. These are lamps with thin aluminum windows (100-200 nm) which we inserted in a glass cell backed by a second cell. Both cells are provided with parallel plate electrodes and separated from each other by an aluminum window. The ionization quantum yields are based on ionization efficiency of argon which is unity. Hydrogen, which has an ionization quantum yield of 0.94 and 1.00 at 73.6-74.4 and 58.4 nm respectively, was used as a secondary standard because it yielded better defined saturation ion current plateaus. The extinction coefficients were determined in both a double cell and a single cell arrangement. The agreement between the two measurements was excellent. In general an inert diluent was added to the gas of interest in order to improve the plateau of the saturation ion current. These results are compared with the literature values, which were mainly determined in windowless systems with monochromators.

The Photochemistry of Propane at High Photon Energies (8.4-21.2 eV).

Neon and helium resonance lamps, which deliver photons of 16.7-16.8 eV and 21.2 eV energy, respectively, have been used to photolyze C3H8, C3D8, C3H8-C3D8 (1:1) mixtures, and the results obtained at the two energies are compared. In particular, it is noted that although the quantum yield of ionization in propane is unity at 16.7-16.8 eV, when the energy is raised still further to 21.2 eV, the probability of ionization apparently diminishes to 0.93, an observation which suggests that at 21.2 eV, superexcited states may be reached whose dissociation into neutral fragments competes with ionization. The quantum yields of the lower hydrocarbon products formed in the presence of a radical scavenger in C3H8 and C3D8 are reported, and are compared with quantum yields of products formed in the vacuum ultraviolet photolysis at lower energies. (Quantum yields of products formed at 8.4 eV and 10.0 eV are reported here for the first time.) Acetylene is formed as a product in the decomposition of the neutral excited propane molecule, and its yield increases in importance with increasing energy; at 16.7-16.8 eV, where all product formation can be traced to ionic processes, acetylene is formed in negligible yields. It is concluded that ionic processes in propane do not lead to the formation of acetylene, and the observation of this product in radiolytic systems may be a reliable indicator of the relative importance of neutral excited molecule decomposition processes. From the results obtained with the C3H8-C3D8 (1:1) mixture, and with CD3CH2CD3, details of the ion-molecule reaction mechanisms and the unimolecular decomposition of the propane ion are derived.