4 - Diatomic Molecules

Abstract

This chapter focuses on the different properties such as discrete spectrum, the ionization continuum, and analysis of several molecules including molecular hydrogen (H2), molecular nitrogen (N2), molecular oxygen (O2), carbon monoxide (CO), nitric oxide (NO), and hydrogen chloride (HCl). Molecular hydrogen introduces more complications than He, since each np Rydberg is split into npσ and npπ by the molecular field, and each Rydberg member has vibrational structure. In H2, for the first ∼1.2 eV above the IP there is competition between auto ionization, predissociation, direct ionization, and direct photo dissociation. Unlike H2, where high-quality calculations are available to determine the oscillator strengths of the resulting bands, the calculations for N2 are not of sufficiently high quality. The oxygen molecule has two electrons in the valence anti-bonding πg orbital. This results in a reduction of the adiabatic ionization potential relative to N2 and also a diminution in the dissociation energy. The CO molecule is isoelectronic with N2. This implies similarities in orbital aufbau and excitation, but there are noteworthy differences. The dissociation energy (D0) is higher in CO than in N2, but the IP is lower. This latter observation suggests lower excitation energies in CO than in N2. The total discrete oscillator strength is about one unit in both cases, but in CO the lower excitation energies make S(−2) in this domain relatively larger, which accounts in part for the polarizability (α) being larger in CO than in N2.