Bruce H. Mahan

ENERGETICS OF GAS PHASE PROTON SOLVATION BY NH3

The absolute proton affinity of NH3 (203.6±1.3 kcal/mole at 298 K) and the proton solvation energies by more than one NH3 have been determined by the molecular beam–photoionization method. In addition, the NH3+–NH3 interaction energy (0.79±0.05 eV) has been measured by photoionization of the neutral van der Waals dimer. These experiments have shown that photoionization of van der Waals clusters is a very powerful method for determining the energetics of gas phase proton solvation.

Photoionization of dimeric polyatomic molecules: Proton affinities of H2O and HF

Photoionization studies of (H2O)2 and (HF)2 producing H3O+ and H2F+ yield 7.18±0.08 eV (165.8±1.8 kcal/mole) and 4.09±0.06 eV (94.3±1.4 kcal/mole) as the proton affinities of H2O and HF, respectively. The measured ionization potential of (H2O)2, 11.21±0.09 eV, along with the known ionization potential of H2O, 12.615±0.001 eV, allow the deduction of a lower bound for the dissociation energy of (H2O)2:1.58±0.13 eV (36±3 kcal/mole). The experiments have demonstrated that photoionization of dimers is one of the most useful general methods for the determination of proton affinities.

Refined Impulse Approximation for the Collisional Excitation of the Classical Anharmonic Oscillator

A modification of the conventional classical theory of vibrational–translational energy exchange is made which leads to an expression of increased accuracy and which has the proper behavior at high energy. It is pointed out that the refined impulse approximation can be used to calculate the energy transferred to an anharmonic oscillator in configurations where the potential energy curve has positive curvature. The case in which the oscillator potential has negative curvature is analyzed and is shown to lead to energy exchanges which exceed the predictions of the impulse approximation. Expressions are given for the energy transferred in grazing collisions which can be used to estimate total inelastic cross sections.

Photoionization studies of the Kr2 and Ar2 van der Waals molecules

The photoionization efficiency curves of the Kr2 and Ar2 van der Waals dimers were obtained with the molecular beam technique in the wavelength ranges 850–965 A (12.848–14.586 eV) and 750–855 A (14.501–16.531 eV), respectively. The ionization potential of Kr2 was found to be 12.87±0.015 eV (963.7±1.2 A), which agrees with the value obtained by Samson and Cairns. The ionization potential of Ar2 was found to be 14.54±0.02 eV (852.7±1.2 A). Using the known ground state dissociation energies of Kr2 and Ar2, the dissociation energy of Kr+2, D0(Kr+2), is deduced to be 1.15±0.02 eV and that for Ar+2, D0(Ar+2), is 1.23±0.02 eV. The photoion yield curves of Kr2 and Ar2 are compared with that of Xe2. Prominent autoionization structure was observed to correspond to Rydberg molecular states which are derived from the combination of a normal and an excited atom in the 4p5ns (or 4p5nd) configuration for Kr and 3p5ns (or 3p5nd) configuration for Ar.

Resonant Transfer of Vibrational Energy in Molecular Collisions

The cross section for resonant vibration—vibration energy exchange between infrared‐active molecules due to the long‐range dipole potential is formulated. It is found that despite certain restrictions imposed by rotational selection rules, energy‐exchange cross sections may be as much as 0.01 to 0.1 times as large as the molecular gas‐kinetic cross section and are greater than those estimated using only an exponential repulsive interaction between molecules.

Dynamics of the Reaction of N2+ with H2, D2, and HD

Product‐velocity‐vector distributions have been determined for the reactive and inelastic scattering of N2+ by H2, D2, and HD. These distributions show that the reaction proceeds by a direct short‐lived interaction rather than by a long‐lived collision complex. Most products are scattered in the original direction of the N2+ projectile at a speed somewhat greater than calculated from the ideal stripping model. The internal excitation of N2D+ and N2H+ is very high and decreases somewhat with increasing scattering angle. For HD there is an isotope effect that favors N2H+ by large factors at small scattering angles, and N2D+ by smaller factors at large angles. The N2+ scattered from D2 shows very little elastic component, but does reveal an inelastic process which is probably the collisional dissociation of D2.

Molecular Orbital Correlations and Ion–Molecule Reaction Dynamics

We examine the application of molecular orbital correlation diagrams to ion–molecule collision processes. In situations where the reactant and product orbitals are few in number and well spaced in energy, the correlation diagrams give a clear picture of how the reactant electron configuration evolves to that of the products. The diagrams lead to a simple explanation of why the endothermic H2+(He, H)HeH+ and H2+(Ne, H)HeH+ reactions occur, whereas the exothermic processes He+(H2, H)HeH+ and Ne+(H2, H)NeH+ have undetectably small cross sections. Similarly, it is possible to use the diagrams to explain why the CO+(H2, H)HCO+ and N2+(H2, H)N2H+ reactions proceed by direct interaction mechanisms despite the substantial stability of the intermediate ions H2CO+ and N2H2+. Several other applications of the diagrams are discussed.

Gaseous Thermal Electron Reactions: Attachment to SF6 and C7F14

The rates of attachment of thermal gaseous electrons to sulfur hexafluoride and perfluoromethylcyclohexane have been measured using a microwave cavity resonance technique. The measurements are compared to the results of recent electron‐beam work. The rate constants for attachment to SF6 and C7F14 are, respectively, 3.1×10−7 and 9.8×10−8 cc/molecule·sec. The former rate constant decreases and the latter slightly increases as the electron temperature is raised.

Photoionization studies of the diatomic heteronuclear rare gas molecules XeKr, XeAr, and KrAr

The photoionization efficiency curves of the heteronuclear rare gas van der Waals molecules XeKr, XeAr, and KrAr were obtained with the molecular beam technique in the wavelength range from 790 to 1065 A (11.64–15.69 eV). The ionization potentials were found to be 11.757±0.017 eV for XeKr, 11.985±0.017 eV for XeAr, and 13.425±0.020 eV for KrAr. From the known dissociation energies of the ground state XeKr, XeAr, and KrAr van der Waals molecules as determined by low energy molecular beam elastic scattering experiments, the binding energies for the ground state of the heteronuclear rare gas molecular ions were deduced to be 0.37±0.02 eV for XeKr+, 0.14±0.02 eV for XeAr+, and 0.59±0.02 eV for KrAr+. The photoion spectra of the heteronuclear rare gas dimers R1R2 exhibit prominent autoionization structure, which is found to correlate very well with the excited molecular Rydberg states R1*R2 and R1R2* formed by the interaction of a normal ground state rare gas atom R1 (or R2) and an excited atom R2* (or R1*), i...

Photolysis of Carbon Dioxide

The vacuum ultraviolet photolysis of carbon dioxide has been investigated. Kinetic data and quantum yield measurements indicate that electronically excited (1D) oxygen atoms are produced in the primary process at 1470 A and 1236 A. Such a primary process is consistent with the tentative spectral assignments which have been made for carbon dioxide.