R. Gorden

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.

Hydrogen Formation in the γ Radiolysis of Ethylene

The radiolysis of ethylene‐d2 and C2H4–C2D4 mixtures has been investigated in the gas, liquid, and solid phases. The data indicate that hydrogen may be formed by two distinct molecular‐elimination processes: CH2CH2→CH2C+H2 and CH2CH2→CH≡CH+H2. The effect of xenon and pressure on the yields of H2, HD, and D2 in the gas‐phase radiolysis of CH2CD2 has been investigated. The results for the (2P1)Hg‐sensitized and the 1237 A‐photochemical decomposition have been compared with those for radiolysis.

Effect of Electrical Fields in the Gamma Radiolysis of Propane

The γ‐ray radiolysis of C3H8–C3D8 mixtures and of CD3CH2CD3 has been investigated as a function of applied electrical field up to voltages necessary to cause secondary ionization. In the saturation current region, those products which in previous studies were ascribed to ethyl and vinyl ions remain constant with a change in field strength, while products which were originally attributed to the decomposition of neutral excited propane molecules are seen to increase about fivefold before the onset of secondary ionization. The inferred yields of C2H5+ and C2H3+ per ion pair at a propane pressure of 3 cm are 0.40 and 0.08, respectively. These values compare favorably with those deduced from the fragmentation of propane within 10—10 sec as calculated by Vestal, Wahrhaftig, and Johnston on the basis of the quasiequilibrium theory of mass spectra.The modes of decomposition of the neutral excited propane molecules formed by electron impact are the same as those observed in the vacuum‐ultraviolet photolysis at 147...

Effect of Electrical Fields and Density in the Radiolysis of Ethane

The gamma‐ray radiolysis of C2H6–C2D6–NO and CH3CD3−NO mixtures has been investigated as a function of applied electrical field. In the saturation current region, a number of products resulting from the decomposition of neutral excited molecules increase about fivefold before the onset of secondary ionization while several other products, which can be ascribed to ion molecule reactions, remain essentially unchanged. The modes of decomposition of the excited ethane molecules formed by electron impact, as derived from the applied field experiments, are the same as those observed in the vacuum ultraviolet photolysis at 1470 and at 1236 A. From the isotopic composition of the products it could be deduced that the average energy transferred to ethane by electron impact is greater than 10 eV. A value of 0.5 was derived for the number of neutral excited molecule decompositions per ion pair.A change in density has a pronounced effect on the product distribution, both in the photolysis at 1236 A and the radiolysis...

Photolysis of Ethylene at 1048–1067 Å. Reactions of C2H4+ and Fragmentation of the Superexcited Ethylene Molecule

C2H4–NO, C2H4–O2 and C2H4–(CH3)2NH–O2 mixtures have been irradiated with an argon resonance lamp emitting only the 1048‐ and 1067‐A lines. The ionization efficiency ratios: ηC2D4/ηNO, ηC2H4/ηNO, ηC2D4/ηC2H4 were determined to be 0.30±0.003, 0.27±0.003 and 1.12, respectively. trans‐2‐Butene and cis‐2‐butene are major products when NO (I.P.=9.25 eV) or (CH3)2NH (I.P.=8.24 eV) are added to C2H4. Radiation of pure C2H4 or of C2H4–O2 mixtures yielded only minor traces of 2‐C4H8. In accord with earlier radiolysis studies we ascribe the formation of 2‐C4H8 to the charge‐transfer process C4H8++NO (or (CH3)2NH)→C4H8+NO+ (or (CH3)2NH+), where C4H8+ may possess some excess internal energy carried over from the exothermic condensation reaction (C2H4*)++C2H4→(C4H8*)+. For a constant C2H4–NO mixture M(C4H8)/N increases drastically with an increase of the pressure of C2H4 from 0.1 to 6 torr. Also, in pure C2H4, M(CH3)/N which is close to unity at 0.5 torr is seen to diminish to 0.2 when the pressure is raised to 29.8 to...

Photoionization of Propylene at 1236 Å. Reactions of C3D6+ with Added Alkanes

C3H6–C3D6–O2 mixtures have been irradiated at 1236 A (10 eV) in the presence and absence of an applied electrical field with the purpose of obtaining information about the ion—molecule reaction mechanism. On the basis of a number of observations, it was concluded that the product, propane, can be ascribed to the reaction C3mH6m++C3H6→C3mH6m−2++C3H8 (m≥2). When alkanes (RH2) whose ionization energy is larger than 10 eV are added to C3D6—O2 mixtures, the following H2−‐transfer reaction occurs: C3D6++RH2→CD3CDHCD2H+R+ . Relative rates of the reaction of C3D6+ with various alkanes have been determined and compare favorably with values obtained in a recent radiolytic study in which cyclopentane‐d10 was used as a source of C3D6+ ions.The optimum experimental conditions, under which meaningful saturation ion currents can be obtained in the photoionization apparatus, are discribed. On the basis of measurements of the saturation ion current in NO and propylene at 1236 A, values of 0.197 and 0.208 were obtained for...

Formation of dimeric parent cations in aromatic hydrocarbons

Abstract Third-order rate coefficients have been determined at 295 K for the formation of dimeric parent cations in benzene, benzene-d6, toluene, the xylenes, mesitylene, and fluoro- and chlorobenzene using the NBS high pressure photoionization mass spectrometer. The reactions are all quite fast, the rate coefficients falling in the range 0.5–6 · 10−25 cm6 molecule−2 s−1. The differences in rates observed are rationalized in terms of structural (geometric) considerations, and upper limits are established for the binding energies in (C6H6)2+ and (C6H5CH3)2+. The temperature dependence for production of (C6D6)2+ has been determined from 227 to 316 K, and the rate coefficient seems to obey a simple 1/T relationship over this range. The detection of trimeric species in benzene is also reported for the first time. Additional experiments involving benzene-nitric oxide mixtures are also reported, which include detection of an apparent equilibrium of the type NO+ + C6H6 ⇆ C6H6+ + NO.

Ionic polymerization of vinyl halides initiated by photoionization using photons with energies near the ionization threshold

Abstract Thermal bimolecular rate coefficients have been obtained for the reactions of the parent monomer ions in the vinyl halides with the respective neutral molecules when the ions are formed by photoionization at or very near the ionization thresholds. The values obtained are as follows: C 2 H 3 F + , 0.27 eV above threshold, k = 3.2 × 10 −10 cm 3 molecule −1 s −1 , C 2 H 3 Cl + , within kT of threshold, k = 1.78 × 10 −10 cm 3 molecule −1 s −1 , and C 2 H 3 Br + , 0.2 eV above threshold, k = 1.25 × 10 −10 cm 3 molecule −1 s −1 . An increase in photon energy drastically lowers the values of these coefficients. The reactions of C 2 H 3 F + and C 2 H 3 Cl + were also investigated at high pressures in order to elucidate certain aspects of the cationic polymerization scheme. The formation of (C 2 H 3 Cl) 2 + and (C 2 H 3 Cl) 3 + is reported for the first time. The factors controlling the relative importances of various modes of dissociation of the vinyl halide dimer ions are discussed in terms of thermodynamic and structural considerations.

Effects of low concentrations of O2 and CO on the ion-clustering reactions in the lower ionosphere of Mars

Abstract It is demonstrated that under conditions which approximate those of the Martian ionosphere traces of CO and O 2 can be effectively incorporated in ion clusters via ion-molecule reaction schemes initiated by the CO 2 + ion. For example, when 0.3 % CO is added to CO 2 , (CO) 2 + and [(CO) 2 CO 2 ] + appear as the major cations (584 A radiation, 300°K). In mixtures containing O 2 in addition to CO 2 (CO 2 . O 2 ) + and [(CO 2 ) 2 O 2 ] + are important species. A recently proposed mechanism to account for the low abundance of CO and O 2 in the Martian atmosphere is discussed in the light of these observations.