PHOTOIONIZATION CROSS SECTION OF ATOMIC AND MOLECULAR OXYGEN

Photoionization cross sections of atomic oxygen and dissociative photoionization cross sections of molecular oxygen were measured from their respective thresholds to 120 (ANGSTROM) by use of a photoionization mass spectrometer in conjunction with a spark light source. The photoionization cross sections for the O(,2)(\u27+) parent ion and O(\u27+) fragment ion from neutral O(,2) were obtained by a technique that eliminated the serious problem of identifying the true abundances of O(\u27+) ions. These ions are generally formed with considerable kinetic energy and because most mass spectrometers discriminate against energetic ions true O(\u27+) abundances are difficult to obtain. In the present work the relative cross sections for producing O(,2)(\u27+) ions are obtained and normalized against the total cross sections in a spectral region where dissociative ionization is not possible. The fragmentation cross sections for O(\u27+) were then obtained by subtraction of O(,2)(\u27+) cross sections from the known total photoionization cross sections. The results are compared with the previously published measurements. The absolute photoionization cross section of atomic oxygen, (sigma)(,0+), was measured at 304 (ANGSTROM). The actual number density of oxygen atoms within the ionization region was obtained by measuring the fraction of O(,2) molecules dissociated. This (sigma)(,0+) at 304 (ANGSTROM) was used to convert the relative photoionization cross sections, measured as a function of wavelength using a calibrated photodiode, to absolute cross sections. The results are compared with previous measurements and calculated cross sections. A Rydberg series converging to the OII (\u274)P state was observed