Photoionization of N2, O2, NO, CO, and CO2 by soft x rays

The total and relative abundances of thermal energy ions produced by absorption of 0.28, 1.25, and 1.5 keV x rays in N2, O2, NO, CO, and CO2 and the dissociative ionization cross sections at these energies were measured using a mass-spectrometric technique. The results demonstrate that there is a significant change in the character of the ionization at the K-absorption edges of the atomic constituents. At these energies dissociative ionization predominates for all the molecules accounting for over 95% of the ions formed at energies above the K edge. Quasistable doubly charged molecular ions make a significant contribution to the thermal energy ions at energies above the K edge. Our results for the total abundances of thermal singly and doubly charged molecular ions are compared with recent measurements by Carlson and Krause.