Edwin N. Lassettre

Power Series Representation of Generalized Oscillator Strengths

Expansion of generalized oscillator strengths in powers of the momentum change K (of the colliding electron) generally leads to series with finite radii of convergence. From a study of singularities of the function for complex values of K it is possible to transform the series into one which converges for all physically attainable values of K. The transformation depends on the ionization potential and the excitation energy, both of which can be determined experimentally. The transformed series should be useful in fitting experimental data and in the extrapolation of oscillator strengths. It is especially useful, however, when applied to transitions for which a few terms of the series in inverse powers of K can be obtained. As an example the 11S→21S transition in helium is treated. The mere determination of the degree of the first nonvanishing term in the inverse power series is enough to severely restrict the form of series and, in fact, to sum the first six terms. These terms, which involve only one adju...

Intensity Distribution in the Electron‐Impact Spectrum of Carbon Monoxide at High‐Resolution and Small Scattering Angles

An electron spectrometer, which provides velocity selection before scattering, is described; and the results of a study of relative intensities in the carbon monoxide spectrum are reported. The vibrational levels of the fourth positive band system have been resolved and relative intensities determined for the first nine. The relative intensities are compared with calculated Franck—Condon factors. The agreement is good at low vibrational quantum numbers but noticeable discrepancies are found for high‐vibrational levels.The problem of calculating relative oscillator strengths from a fully resolved spectrum is considered and applied to data obtained in the present research. Relative oscillator strengths are compared with those obtained previously from unresolved spectra.If the Born approximation is valid, then the electron‐impact and ultraviolet absorption spectra should be closely similar. This is actually observed for most of the spectrum but the transition at 12.79 V is an outstanding exception. The relat...

Quadrupole‐Allowed Transitions in the Electron‐Impact Spectrum of N2

The forbidden transitions observed in the nitrogen spectrum below 12.5 V have been studied by electron‐impact methods. Since the theory indicates that electric quadrupole transitions are preferred to singlet—triplet in electron‐impact spectra at intermediate energies, the observed transitions are probably electric quadrupole allowed. This has been demonstrated for the 12.25‐V transition by studying its relative intensity as a function of the scattering angle. The data on the 11.86‐V transition are also compatible with an electric quadrupole transition. In addition, new data on the Lyman—Birge—Hopfield bands in the 60–400‐V energy range indicate that the intensity distribution among vibrational levels is independent of energy.

Intensity Distribution in the Energy‐Loss Spectrum of Ethylene

The electron‐impact spectrum of ethylene has been determined using a high‐resolution electron spectrometer. The intensity distribution obtained at 200‐V electron kinetic energies agrees well with previous measurements made with 33‐kV electron kinetic energies, and ultraviolet‐absorption measurements except in the region of the 7.45‐V transition. This discrepancy is attributed to a quadrupole transition which occurs in this region.

RELATIVE INTENSITIES OF TWO RYDBERG TRANSITIONS IN THE ELECTRON-IMPACT SPECTRUM OF WATER

The electron‐impact spectrum of water has been determined using a higher‐resolution electron spectrometer. The intensity distribution of five peaks in the spectrum which belong to two different Rydberg transitions has been investigated at zero scattering angle and electron kinetic energy of 200 V. Quantities proportional to the optical oscillator strengths for these excitations have been compared with absorption coefficients from ultraviolet spectra. The entire electron‐impact spectrum in the region below the first ionization potential shows good agreement with ultraviolet absorption.

Intensity Variation with Scattering Angle of Electronic Transitions in H2O Excited by Electron Impact

Electron‐impact spectra of water vapor have been investigated at 400‐ and 500‐V electron kinetic energy. The relative intensities of the five peaks associated with the A and B Rydberg transitions have been studied as a function of the scattering angle. The results suggest a new method of identification of vibrational levels where a high‐resolution study of rotational structure is not feasible.