J.A.D. Stockdale

Production of negative ions from CH3X molecules (CH3NO2, CH3CN, CH3I, CH3Br) by electron impact and by collisions with atoms in excited Rydberg states

Thermal electron attachment to nitromethane, methylcyanide, methyliodide, and methylbromide is compared with capture of electrons by these molecules from highly excited Rydberg states of atoms. Data on thermal electron attachment to CH3NO2 are consistent with a three‐body attachment process, with the nature of the third body being important. The thermal energy electron attachment rate constant for CH3CN is ≤1.24×10−14 cm3 sec−1. Some results on dissociative electron attachment and ion pairing processes in CH3NO2, CH3CN, CH3I, and CH3Br are also presented.

Dissociative Electron Attachment to Molecules

Dissociative‐electron‐attachment cross sections (some unpublished) for 30 molecules are summarized, evaluated, and discussed within the framework of the resonance scattering theory. The dissociative‐attachment peak cross section, σc(emax), is found to be a strong function of the peak (resonance) energy, emax, with a break in this dependence at the energy where electronic excitation of the neutral molecule begins to occur. Based on the experimental data, three groups of molecules have been distinguished: (i) those where emax is less than the energy, eN, of known electronic excited states of the neutral molecule, and the negative‐ion state is purely repulsive in the Franck‐Condon region, (ii) those where emax ≥ eN, and (iii) those with exceptionally small σc(emax) for which a vertical onset for dissociative attachment occurs. For the molecules in group (i) σc(emax) varies almost as (emax)−1, while for group (ii) σc(emax) is a much stronger decreasing function of the resonance energy emax. For group (i) the ...

Negative Ion Formation in Selected Hexafluoride Molecules

Negative ion formation in gaseous SeF6, TeF6, MoF6, ReF6, and UF6 has been studied as a function of incident electron energy in the region from 0 to ∼10 eV. Negative hexafluoride ions formed by direct electron attachment were observed only in the cases of MoF6 and ReF6. SeF6−, TeF6−, and UF6− were produced by charge exchange with thermal SF6−* and rates for these reactions and for the reaction UF5− + UF6→UF6− + UF5 were measured and are reported here. Electrons having energies close to zero are known to attach to SF6 with a cross section approaching the maximum for s‐wave capture. The excited negative ion SF6−* formed in this way has a lifetime of 26 μsec against autodetachment of the electron. An argument based on molecular orbital theory is given for the existence of a Jahn–Teller effect in SF6−. This could provide a means for the coupling between electronic and nuclear motion needed to trap the low‐energy electron. Selenium and tellurium lie directly under sulfur in the same column of the periodic tabl...

Formation of gas-phase negative ions in Fe(CO)5 and Ni(CO)4

Abstract The formation and structure of negative ions from iron and nickel carbonyl has been studied in gas-phase collision experiments using both low-energy (≦ 10 eV) electrons and fast (≦40 eV) potassium atoms as projectiles. Dissociative attachment of low-energy electrons to Fe(CO)5 and Ni(CO)4 occurs with a high probability to produce Fe(CO)4− and Ni(CO)3−, respectively. Structure is found in the dissociative attachment cross section for producing Fe(CO)3− and Fe(CO)2− from Fe(CO)5 in the energy region from 3 eV to 6 eV. This structure is attributed to predissociation from an electronically excited state of the Fe(CO)4− ion.

Collisions of Monoenergetic Electrons with NO2: Possible Lower Limits to Electron Affinities of O2 and NO

The yields of O−, NO−, and O2− ions produced by dissociative attachment of electrons to NO2 have been studied as a function of the electron energy from 0 to 6 eV. A modulated retarding‐potential‐difference technique permitted electron‐energy resolution of approximately 0.1 eV. The measured appearance potentials for NO− and O2− were found to be consistent with an electron affinity of ≥0.65 eV for NO and ≥1.1 eV for O2. These lower limit measurements to the electron affinities of O2 and NO are discussed in relation to previous studies of electron attachment to these molecules.

Interaction of Low‐Energy Electrons with Water Vapor and with Other Polar Molecules

Electron attachment coefficients and drift velocities have been measured in mixtures of H2O vapor with N2, CH4, C2H4, and with CO2. Electron capture takes place only in the H2O–CO2 mixture, and even in this case the attachment coefficient is quite small. Thus, low‐energy (less than 1 eV) electrons do not, in general, form stable negative ions with H2O.Electron drift velocities are very sensitive to the addition of small quantities of H2O vapor in all the mixtures. When certain other polar molecules (acetone, heavy water, methyl alcohol, dimethyl ether, hydrogen sulfide, toluene and nitrous oxide) were mixed with C2H4, it was found that the decreases in drift velocity correlated well with the magnitude of the electrical dipole moments. The drift velocity data were analyzed to obtain the cross sections for momentum transfer. These cross sections are in fair agreement with the theory of Altshuler, except for H2O, D2O, and H2S, where the experimental values are a factor of 2 larger than theory. This discrepan...

Studies of Electron Impact Excitation, Negative Ion Formation, and Negative Ion‐Molecule Reactions in Boron Trifluoride and Boron Trichloride

Electron attachment processes in BF3 and BCl3 have been studied with both electron swarm and electron beam techniques. Thermal electron attachment rates were determined by the drift‐dwell‐drift technique to be 〈5 × 105 sec−1 · torr−1 for BF3 and 9 × 107 sec−1 · torr−1 for BCl3. Beam studies showed that F−, F2−, and BF2− were produced from BF3 by electrons of energy near 11.5 eV while Cl− and Cl2− were produced in BCl3 near 1 eV. The SF6− threshold electron impact excitation spectrum of BF3 exhibited no structure, however, a number of peaks were seen in BCl3, the chief one being near 7.6 eV. A low energy peak was observed in BCl3 at ∼ 2.5 eV. The thermal energy SF6−* ion was found to react readily with both BF3 and BCl3, yielding BF4− and BCl3F−, respectively. BF4− was also produced through the reaction F2−/BF3+BF3→ BF4−+F. Thermal energy rate constants for these reactions determined by a pulsed source method were 1.8 × 10−9, 1.6 × 10−10, and 6.1 × 10−11 cm3 molecules−1 · sec−1 in the order above.

Dissociative ionization of molecules by electron impact. II. Kinetic energy and angular distributions of N+ and N++ ions from N2

An apparatus containing crossed molecular and pulsed electron beams has been used to obtain distributions in kinetic energy and angle of fast (≳1.5 eV) N+ and N++ ions produced through dissociative ionization of N2 by impact of electrons with energies from threshold to 300 eV. Previously unreported structure is found in both N+ and N++ kinetic energy spectra. The angular distributions of N+ (energy resolved) and N++ (total) ions are found to be substantially isotropic for electron beam–ion detector angles from 30°–110°.

Dissociative ionization of molecules by electron impact. I. Apparatus and kinetic energy distributions of D+ ions from D2

A crossed beam apparatus for the study of electron impact dissociative ionization of molecules is described. D+ time‐of‐flight and energy spectra are presented for impact of electrons on D2, from energies near dissociative threshold to 300 eV. In addition to the group of D+ ions peaking near 8 eV kinetic energy from dissociative ionization via the 2Σ+u state of D+2, groups of ions of lower kinetic energy peaking near 5, 2, and possibly near 1 eV are observed. These ions appear to be due to excitation of states of D2 or D+2 lying near to or above the 2Σ+u state of D+2.

Capture of Thermal Electrons by Oxygen

Permanent capture of thermal electrons by O2 to form O2− ions requires the participation of a third body in the collision process to provide a dynamical channel for stabilization. The effectiveness of H2O and C2H4 as stabilizing agents in three‐(C2H4–O2–H2O) and two‐(C2H4–O2) component mixtures has been investigated. The results are analyzed through the following three‐body reaction scheme: e+O2+O2→ lim k1O2−+O2+energy,e+O2+H2O→ lim k2O2−+H2O+energy,e+O2+C2H4→ lim k3O2−+C2H4+energy, and the rate constants k1, k2, and k3 are equal to 0.20±0.02 [J. L. Pack and A. V. Phelps, J. Chem. Phys. 44, 1870 (1966); 45, 4316 (1966)], 1.4±0.5, and 0.34±0.04×10−29 cm6 sec−1, respectively.