P. Ausloos

The fluorescence and photofragmentation of liquid saturated hydrocarbons at energies above the photoionization threshold

Fluroescence quantum yields (φF) are reported for saturated hydrocarbons excited by photon absorption in the liquid phase at energies below and above the photoionization threshold. The quantum yields of fluorescence obtained in the subionization region are in excellent agreement with those reported by Lipsky and colleagues. In the phtotionization region, emission occurs as a result of both charge recombination processes and deactivation of the superexcited molecule to the vibrationally relaxed first excited singlet state. The presence of these two populations of fluorescing species above the ionization threshold is manifested in differences in the slopes of plots of φF as a function of energy above and below the ionization onset. Since, for saturated hydrocarbons, photofragmentation is the only nonradiative channel, the variations in the relative importances of the major modes of fragmentation have been examined as a function of energy for selected molecules. It is observed that the two main dissociative ...

Photolysis of Ethane at 11.6–11.8 eV

The photolysis of ethane, carried out with an argon resonance lamp, has been reinvestigated with the related purposes of (1) measuring quantum yields of all fragments formed in the dissociation of excited ethane and (2) associating these fragments with the primary processes occurring in the photolysis of ethane. These and their relative abundances are C2H6*→C2H6+, 5% →CH4+CH2, 16% →C2H5+H, 41% →C2H4+H2, 26% →CH3+CH3, 15%. These results are compared with conclusions reached in earlier studies on the photolysis of ethane with xenon and krypton lamps in order to determine the effect of energy on the relative probabilities of the primary processes. It is found that direct bond scission increases in importance with increasing energy, while processes involving rearrangement decrease in importance. The radical and molecular fragments formed in the dissociation of excited ethane were determined by (a) analyzing the products formed in C2H6–C2D6–NO (1:1:0.1) mixtures and (b) C2D6in the presence of H2S, which scaven...

Gas-phase photodecomposition of carbon tetrachloride

Abstract The gas-phase photolysis of CCl 4 was investigated at 213.9, 163.3 and 147.0 nm in the presence of HCl, HBr, and C 2 H 6 . Quantum yields of the products measured in these mixtures at a temperature of 300 K led to the conclusion that at 213.9 nm over 90% of the photodecomposition can be attributed to the photodissociative process: independent of pressure (5 - 60 Torr). At 163.3 nm, CCl 2 is formed via the photodecomposition process: Contrary to earlier suggestions, CCl 2 is unreactive towards CCl 4 . Combination with other radicals and insertion into HCl are the major modes of reaction of CCl 2 . Experiments carried out at 313.0 nm show evidence for the occurrence of photodissociation of CCl 4 . On the assumption that absorption of a photon by CCl 4 invariably leads to the detachment of a chlorine atom, the absorption cross-section at 313.0 nm is /s 3.7 ± 0.4 X 10 −26 cm 2 molecule −1 (300 K). This result indicates that photodecomposition of CCl 4 in the troposphere is of minor importance as compared to other processes including diffusion up to the earths stratosphere.

Hydride transfer reactions involving saturated hydrocarbons and CCl3+, CCl2H+, CCl2F+, CF2Cl+, CF2H+, CF3+, NO+, C2H5+, sec-C3H7+ and t-C4H9+☆

Abstract The compounds CCl4, CCl3H, CCl3F, CCl2F2, CF3H, CF4, NO, C2H6, neo-C5H12, and C3H8 were ionized by low energy electrons in a pulsed ion cyclotron resonance spectrometer to produce CCl3+, CCl2H+, CCl2F+, CF2Cl+, CF2H+, CF3+, NO+, C2H5+, t-C4H9+, and sec-C3H7+ ions, respectively. In these pure compounds, the respective ions formed are all unreactive, and can be trapped with essentially no loss in the ICR analyzer cell for up to 0.2 s after their formation, at a pressure of 10−7–10−5 torr. When alkanes or cycloalkanes are added to these compounds, the predominant ions formed in each case undergo a hydride transfer reaction with the added alkane. This is the only reaction channel: A+ + RH → AH + R+ (where A+ is the initial ion formed and RH is the added alkane or cycloalkane). By following the abundances of the reactant ions as a function of time in these dilute mixtures, approximately one hundred rate coefficients of hydride transfer reactions of thermal ions (in the range 10−12–10−9 cm3 molecule−1 s−1) have been determined. For several of these ions, it is demonstrated that even when reaction is very exothermic at every site in the molecule the reacting ion exhibits positional selectivity. Detailed examination of the reaction probabilities for these ions reacting with alkanes of different chain lengths and structures leads to the conclusion that the rates of hydride transfer reactions depend in part on the exothermicity of the reaction and in part on the lifetime of the ion-molecule complex. For large reactant ions such as t-C4H9+, steric effects may also play a role. The product ions formed in these hydride transfer reactions undergo further fragmentation to an extent which depends on the exothermicity of the reaction; the most important fragmentation paths correspond to loss of C2C4 olefins from the product alkyl ions.

EFFECT OF PRESSURE IN THE RADIOLYSIS AND PHOTOLYSIS OF METHANE

The photolysis and radiolysis of equimolar CH4–CD4 mixtures were investigated as a function of pressure. The fact that, in the presence of NO, the ethane fraction consists entirely of C2D6, C2D4H2, C2H4D2, and C2H6 in comparable amounts indicates that CH2 and CD2 are produced. The relative yield of these ethanes which are formed by insertion of methylene into methane increases with pressure in both the photolysis and radiolysis. In the radiolysis, the G value reaches a value of 0.35±0.1 at pressures above 15 atm. Information about the effect of pressure on the production of the ethyl ion was obtained by investigating the radiolysis of CH4–C4D10 and CD4—C3H8 mixtures from 1.5 cm to 130 atm. The data indicate that there is a gradual decrease of the ethyl ion yield with increase in pressure while the parent ion yield increases with increase in pressure to a pressure of at least 15 atm.

Effect of Additives on the Ionic Reaction Mechanism in the Radiolysis of Methane

This work describes the reactions of the ions C2H5+ and CH5+ with C3D8, C4D10, C5D12, as well as the reactions of C2D5+ and CD5+ with C3H8. It is shown that ethyl ions undergo a hydride transfer reaction with all higher hydrocarbons even though they are present at a concentration of only 0.01%. The scavengers NO and O2 react with ethyl ions effectively, provided the relative concentration of the higher hydrocarbons is less than that of the free radical scavengers. A proton‐transfer reaction between CH5+ and higher hydrocarbons followed by a rapid decomposition of the protonated hydrocarbon into a carbonium ion and a neutral alkane molecule is shown to occur. From the data it can be deduced that at a methane pressure of 48 cm G(C2H5+) =0.9±0.2 and G(CH5+) =1.9±0.2. It is concluded that in earlier published studies of the radiolysis of methane, the C2H5+ and CH5+ ions reacted with the accumulated products or with the added free radical scavengers, thus excluding the neutralization of these two species.

H2‐Transfer Reactions in the Gas‐Phase Radiolysis of Hydrocarbons

The radiolysis of cyclohexane has been investigated in the presence of varying concentrations of acetylene, ethylene, propylene, butene, and cyclopropane. On the basis of a number of observations, it is concluded that, in all cases, the H2‐transfer reaction CnHm+cyclo−C6H12+→CnHm+2+C6H10+ does take place. No extensive rearrangement occurs in the collision complex. For instance, a transfer of H2 to CD3CDCD2 results exclusively in the formation of CD3CDHCD2H, while an H2 transfer to (CD2)3 leads to the formation of CD2HCD2CD2H. Relative rate constants for the transfer of an H2 molecule to CH3CHCH2, 1‐C4H8, iso‐C4H8, 2‐C4H8, C2H4, and C2H2 are, respectively, 1.00, 0.68, 0.27, 0.10, 0.11, and 0.072.Similar variations in the relative rate constants of the H2−‐transfer reaction CnHm++cyclo−C6H12→CnHm+2+C6H10+ are noted.The following additional information was derived in the course of this study:(1) When cyclopentane or n‐pentane is substituted for cyclohexane in the reaction mixture, the H2‐transfer reaction ag...

Pulse- and gamma ray-radiolysis of cyclohexane: Ion recombination mechanisms

Abstract The products formed in the γ-radiolysis and pulse-radiolysis of gaseous cyclohexane have been interpreted in terms of the ion fragmentation, ion-molecule reaction, and ion recombination mechanisms. It is shown that the fragmentation of the parent ion is partly quenched at a pressure of 55 torr. The products resulting from homogeneous neutralization of the major unreactive ions, c - C 6 H + 12 , c - C 6 H + 11 , and c - C 6 H + 10 , are deduced by comparing product yields at high dose rates, where the ions undergo homogenous neutralization, with yields in the gamma radiolysis, where the unreactive ions disappear mainly by reaction with impurities or neutralization on the wall. Ethylene and 1,3-butadiene are the major products resulting from electron neutralization of these ions. Fragmentation is strongly reduced when the neutralization process involves an atomic- or polyatomic- anion rather than an electron. For instance, addition of CCl 4 to cyclohexane results in a sharp drop of the yield of 1,3-butadiene, and a concurrent rise in the yield of 2-C 4 H 8 . In the gas phase, a value of 0.16±0.08 is suggested for the ratio of neutral excited molecule formation to ionization. In the liquid phase, it is seen that the relative importances of processes observed in the radiolysis are very different from the importances of these same processes in the far ultraviolet photolysis (8-11.6eV). In the radiolysis, where the mean energy taken up by the cyclohexane molecule may be higher than 11.6eV, solvent assisted fragmentation of the parent ion to give c -C 6 H + 11 , and the formation of triplet excited molecules in ion recombination processes are considered as explanations for the discrepancies. In the radiolysis, geminate neutralization of the vibrationally-relaxed parent ion to produce a singlet excited state of cyclohexane accounts for no more than about 25% of the cations.

H2S as a Free‐Radical Interceptor in the Gas‐Phase Radiolysis and Photolysis of Propane

The gas‐phase photolysis (1236 A) and the γ‐ray radiolysis of C3D8 has been investigated in the presence of varying concentrations of H2S. When 10% or more H2S is added to C3D8, the majority of the D, CD3, C2D3, and C2D5 radicals abstract an H atom from H2S to form HD, CD3H, C2D3H, and C2D5H, respectively. The fully deuterated molecules formed in these mixtures result from the unimolecular elimination of a stable molecule from C3D8 or C3D8+ and from fast bimolecular processes such as ion—molecule reactionsThe mechanisms of the radiolysis and the photolysis proposed in earlier studies have been re‐examined in the light of the information derived from the C3D8—H2S experiments and of some additional photolysis experiments on CD3CH2CD3—NO mixtures. The results indicate that the modes of decomposition of the neutral excited propane molecule are as follows: C3D8*→C2D6+CD2,→CD4+C2D4,→D2+C3D6,→CD3+C2D5,→D+C3D7. The internally excited C2D4, C2D5, C3D6, and C3D7 species formed in these primary processes decompose t...

Gas‐Phase Photolysis of Cyclohexane in the Far Ultraviolet: Modes of Decomposition of the Neutral Excited Cyclohexane Molecule and Reactions of the Parent Cyclohexane Ion

The photolysis of cyclohexane—cyclohexane‐d12 (1:1) mixtures and of cyclohexane‐1,1,2,2,3,3‐d6 was investigated using xenon (8.4 eV) and krypton (10, 10.6 eV) resonance radiation. It is demonstrated that the formation of hydrogen by molecular detachment is the major primary process cyclo−C6H12*→C6H10+H2. In this process, the hydrogen is mainly eliminated from a single carbon atom. The resulting C6H10 diradical rearranges mainly to cyclohexene, a fraction of which decomposes to form 1,3‐butadiene and ethylene. It is shown that, because of collisional deactivation, the probability of the latter mode of decomposition decreases with an increase in pressure or an increase in wavelength. Other primary processes such as cyclo−C6H12*→3C2H4 and, to a lesser extent, cyclo−C6H12*→2C3H6 as well as processes leading to the formation of H atoms and CH3 radicals are also shown to occur.In the photolysis at 1236 and 1165 A, C6H12+ ions are formed which undergo H2‐transfer reactions such as C6H12++CD3CDCD2→C6H10++CD3CDHCD...