Chapter 3 - Diatomic Molecules

Abstract

Listen

Translate article

The absolute partial cross-sections (partial and total) of diatomic molecules (H2, N2, O2, CO, NO, HCl) have been studied experimentally and theoretically, drawing conclusions from various sources. Autoionization is the path used for producing protons with higher kinetic energy; Rydberg series describe the energy levels associated with partially removing an electron from the ionic core; and shape resonance generates energetic electrons. The shapes and magnitudes of the absolute partial cross-sections change with instrumental resolution. Line sources, synchrotrons, and electron energy analysers have been used to achieve partition in nitrogen molecules. In hydrogen molecules, the Franck–Condon factors and relative vibrational intensities, and the absolute partial cross-section for each vibrational level in the photoionization of H2 at 21.2 eV have been determined, plotted and tabulated. In other molecules, absolute partial cross-sections have been studied by the photoionization method in the outer and inner valence regions, taking into account the molecular electronic configuration in its ground state. The absolute total ionization cross-section at the peak can be estimated from the relative photoionization spectrum of Berkowitz. In the molecules studied, we found deviations from the independent particle behavior. For example in CO, ionization from the three outermost orbitals complies with the independent particle picture, and this breaks down as we approach the inner valence region above 20 eV. Several shake-up states are reported at lower energies. Fluorescent and line source measurements, branching ratios and synchrotron determinations have been used to correlate, and several pieces of evidence have been combined to come to a final conclusion. From the adiabatic ionization potential (AIP: H2, 23.583; O2, 12.070; NO, 9.2643) of diatomic molecules to the dissociative ionization threshold, the absolute partial cross-section is nearly the same as the absolute total ionization cross-section in the dissociative photoionization cross-sections. The decay mechanism of various absorption features has been studied by Auger electron spectroscopy, mass spectrometry and coincidence methods.