Michael A. Dillon

High‐Resolution Study of Electron‐Impact Spectra at Kinetic Energies between 33 and 100 eV and Scattering Angles to 16°

An electrostatic lens system which compensates for chromatic aberration has been tested in an electron spectrometer. The results indicate that this lens is suitable for comparisons of peak intensities in electron‐impact spectra. Relative intensities in vibrational progressions that belong to a single electronic transition have been studied in N2, CO, and NH3 and found to be nearly independent of the scattering angle. Electron‐impact spectra have been reported for helium, nitrogen, oxygen, argon, nitric oxide, nitrous oxide, ammonia, water vapor, carbon dioxide, ethylene, acetylene, and benzene at electron kinetic energies between 33 and 100 eV. Spectral regions of special interest are encountered in CO2 and C6H6. At excitation energies of 7–10 eV in CO2 a change in intensity distribution, attributed to transition from an electric‐quadrupole to an electric‐dipole spectrum, is observed as the kinetic energy is raised. In the case of C6H6 a change in the spectrum with angle is encountered which strongly sugg...

Generalized Oscillator Strength for 11S→21P Transition of Helium. Theory of Limiting Oscillator Strengths

Relative generalized oscillator strengths have been determined for the 11S→21P transition in helium as a function of momentum change of the colliding electron and normalized to the theoretical oscillator strength {calculated by Schiff and Pekeris [Phys. Rev. 134, A638 (1964) ]} at zero momentum change. The validity of the normalizing procedure is investigated theoretically and it is shown that the limit of the generalized oscillator strength at zero momentum change is equal to the optical oscillator strength for any atom or molecule at any incident energy regardless of whether the first Born approximation holds. The normalizing procedure is therefore justified. The results are compared with the experimental and theoretical oscillator strengths obtained by other investigators. The limiting relation for oscillator strengths is established under much more general conditions than in any previous research.

Electron Impact Spectra of Ethane and Other Saturated Hydrocarbons

Electron impact spectra for ethane are reported at accelerating voltages from 50 to 180 V and scattering angles to 9°. Spectra of methane at 0° and 6° and for propane and butane at 0° are reported at an accelerating voltage of 50. Spectra of perdeutero ethane are reported at 0° and accelerating voltages of 50 and 100. A normal coordinate analysis of an excited electronic state of ethane is carried out using the frequencies observed in C2H6 and C2D6. A model, suggested by semiempirical molecular orbital theory, leads to the conclusion that the C–C internuclear distance decreases on excitation. The model is tested by means of an application of the Franck–Condon principle based on the normal coordinate analysis and the model proves to be self consistent if (a) the C–C internuclear distance decreases to 1.39 A and (b) the bond angle C–C–H is increased 10° above the tetrahedral. The significance of this result in the theory of chemical bonding is discussed.

Excitation by Electron Impact of Vibrational Transitions in Water and Carbon Dioxide at Kinetic Energies between 30 and 60 eV

Vibrational excitation by electron impact, in the kinetic‐energy range 30–60 eV, has been studied for water vapor and carbon dioxide. Intensities relative to elastic scattering have been determined at scattering angles to 15°. Fundamental vibrations are excited with much greater intensity than either overtones or combination bands, although the latter appear (weakly) in the spectra. Weak scattering in the energy‐loss region 4.5–7.0 eV and its interpretation is discussed.

Forbidden Vibrational Transitions in the Electron‐Impact Spectra of Molecular Oxygen and Nitrogen

The excitation of forbidden vibrational transitions by electron impact has been studied at 45‐eV electron kinetic energy. The relative intensities of these transitions in N2 and O2 referred to the elastic peak, are independent of the scattering angle between 3° and 14°. For comparison we have also included data on vibrational transitions in CO and NO at one angle (4°). These are allowed by electric‐dipole selection rules. In molecular oxygen we have also observed the forbidden electronic transitions a 1Δg ← X 3Σg− and b 1Σg+ ← X 3Σg− although the latter can be barely detected above background.

Electron impact excitation of the 1 1S→2 3S transition in helium; differential cross‐section determinations at high incident kinetic energies

Differential cross sections have been determined to within ±8% accuracy for the 1 1S→2 3S transition in helium at 200, 300, 400, and 500 eV incident electron energies and scattering angles out to 35°. Relative measurements were made absolute by both direct and indirect normalization to highly precise elastic differential cross sections. Serious deviations from the predictions of first‐order plane wave theory are found throughout the range of experimental parameters employed in the present work. Further consideration suggests that such discrepancies persist to incident energies as high as 1000 eV raising serious doubts about the validity of the Born–Oppenheimer approximation.

Singlet‐triplet energy differences calculated from generalized oscillator strengths

Using a one‐electron model, the difference in energy, ES — ET, between a singlet state (obtained by exciting one electron from a nondegenerate closed‐shell ground state) and the corresponding triplet is shown to be (in atomic units) equal to (π gW)−1 ∫000K2f dK where f is the generalized oscillator strength for excitation to the singlet state, W is the excitation energy, g is the degeneracy of the excited singlet state, and K is the change in wave vector on collision. This relationship is tested for several states in helium and carbon monoxide for which both ES — ET and f are known. Calculated and observed values of ES — ET differ, on the average, by about 15%, the discrepancy being due no doubt to the one‐electron approximation. Where high accuracy is not required (and singlet‐triplet energy differences are difficult to obtain in other ways) the method may be of value since the position of the triplet is obtained from experimental measurements performed exclusively on the singlet excitation at high kinet...

Effect of the reverse reaction on the experimental determination of energy transfer rate constants

The effect of the reverse reaction in the experimental determination of energy transfer rate constants is re‐emphasized. It is shown that the pressure independence of the measured rate constant, frequently used experimentally as an indicator of the absence of reverse reactions, may have more complicated sources like the presence of a third channel of dissipating the excited species. Thus, pressure independence alone is not a reliable indicator of the absence of reverse reactions. The quenching of Ar 3P1 by H2 is used as an illustrative example with the energy transfer rate constant and the rate constant of the reverse reaction estimated using the Born approximation [W. M. Huo, J. Chem. Phys. (preceding paper)].

Test of Born approximation on the 60 eV autoionized transition in helium

The 60.3 eV autoionization transition of helium has been studied by electron spectroscopy. The discrepancy between experiment and a theory based on the Born approximation greatly exceeds experimental error. (AIP)

Generalized differential oscillator strengths for the electron impact ionization of helium determined for large and intermediate momentum transfers at 300 to 500 eV incident energies

Differential oscillator strengths for the electron impact ionization (ejected electron energy=1 a.u.) of helium have been determined to ±6% accuracy for incident electron energies of 300 to 500 eV and