James A. R. Samson

Absolute Intensity Measurements in the Vacuum Ultraviolet

Photo-ionization of the rare gases is proposed as a method to measure the absolute intensities of radiation below 1022 A, the ionization potential of xenon. The method is based on the assumption that the photo-ionization yield of the rare gases is unity. Experimental evidence is presented to justify this assumption. The relative quantum yield of sodium salicylate is measured and shown to be constant between 400–900 A. Finally, the photoelectric yield of gold “black” has been measured and found to be about 1% at 1215 A rising to 4% at 850 A and remaining constant from there down to 500 A.

Precision measurements of the total photoionization cross-sections of He, Ne, Ar, Kr, and Xe

Abstract Absolute photoionization cross-sections for the rare gases have been measured from threshold to 125 eV with an accuracy of ±1 to 3% and are presented in both tabular and graphical form. These data are compared with the optical oscillator strength measurements of Brion and co-workers, who used the Dipole (e, e) collision technique.

Total and partial photoionization cross-sections of O2 from 100 to 800 Å

Abstract The branching ratios and absolute values of the total and partial photoionization cross-sections for producing O 2 + in its various electronic states are given as a function of wavelength in the range 100–800 A. Cross-sections for dissociative ionization are given based on the premise that some of the bound ionic states predissociate. Photoelectron spectroscopy and absorption techniques utilizing a double ionization chamber were used to obtain the data.

Calibration of electron-energy spectrometers

Abstract Relative photoionization cross-sections of strong peaks in the He(I) photoelectron spectra of N2, CO, CO2, and O2, are tabulated. These data have been measured with an electron energy analyzer whose relative luminosity has been calibrated to an accuracy of ± 5 ‰ Thus, the tables will be useful for calibrating the transmission of other analyzers for electron energies below 9 eV. Correction for angular distribution effects is discussed.

Vacuum‐Ultraviolet Absorption Cross Sections of CO, HCl, and ICN between 1050 and 2100 Å

Absorption cross sections have been measured under intermediate resolution (0.25 A) for CO, HCl, and ICN in the spectral region from 1050 to 2100 A. An attempt has been made to analyze the spectra in terms of the reactions responsible for the observed features. A Rydberg series has been identified in the absorption spectrum of ICN with a series limit of 1140.6 ± 0.1 A.

Ionization yields, total absorption, and dissociative photoionization cross sections of CH4 from 110 to 950 Å

Absolute absorption and photoionization cross sections of methane have been measured with an accuracy of about 2% or 3% over most of the wavelength range from 950 to 110 A. Also, dissociative photoionization cross sections were measured for the production of CH+4 , CH+3 , CH+2 , CH+, and C+ from their respective thresholds to 159 A, and for H+ and H+2 measurements were made down to 240 A. Fragmentation was observed at all excited ionic states of CH4.

Total and partial photoionization cross sections of N2 from threshold to 100 AA

The total photoionization cross section of N2 has been obtained from the ionization threshold to 100 AA with an accuracy of +or-3%. In addition, partial photoionization cross sections for transitions into the x2 Sigma g+, A2 Pi u and B2 Sigma u+ electronic states have been measured using the technique of photoelectron spectroscopy over the wavelength range from 745 to 304 AA. Transitions to higher electronic states are also observed at 304 AA.

Photoionization of atoms and molecules

A literature review on the present state of knowledge in photoionization is presented. Various experimental techniques that have been developed to study photoionization, such as fluorescence and photoelectron spectroscopy, mass spectroscopy, are examined. Various atoms and molecules were chosen to illustrate these techniques, specifically helium and xenon atoms and hydrogen molecules. Specialized photoionization such as in positive and negative ions, excited states, and free radicals is also treated. Absorption cross sections and ionization potentials are also discussed.

Precision measurements of the absolute photoionization cross sections of He

Precision photoionization cross sections for He have been measured from threshold to 120 eV with an accuracy ranging from 1 to 2%. In addition, a critical review of the published data has been made and recommended photoionization cross sections from the ionization threshold to 8 keV are presented with an estimated uncertainty of +or-10% above 500 eV. The effect of coherent (Rayleigh) and incoherent (Compton) scattering is discussed.

Line Broadening in Photoelectron Spectroscopy

The factors causing line broadening in photoelectron spectroscopy are discussed. Experimental results are given for the half‐width of the 584 A helium line emitted from a variety of sources. Typical half‐width values lie between 1 and 6.5 mV, which corresponds to 0.027 and 0.18 A, respectively. Curves are presented showing the contribution of the thermal motion of the photoionized gas to the spread in the energy of the ejected photoelectrons.