Rydberg States of Diatomic and Polyatomic Molecules

Abstract

Rydberg states clearly play an important role in the study of the electronic structure of atoms and molecules in the vacuum ultraviolet. Moreover many molecules, including diatomic and simpler polyatomic types of particular importance in astrophysics, aeronomy and photochemistry—such as H_2, O_2, NO, N_2, CO, CO_2, H_2O, C_6H_6—have their most important and strongest electronic bands in the vacuum ultraviolet. Interest in this spectral region has been stimulated both by the improved resolution of optical absorption studies and by the development of techniques such as electron impact spectroscopy. From electron impact spectroscopy one can now obtain accurate spectra of molecules well into the vacuum ultraviolet quite readily. In the analysis of such electron impact spectra one must know the location and nature of several Rydberg series. From several recent experiments it has also become obvious that the Rydberg series in many molecules can perturb the intravalence region of the spectrum and lead to some unusual features in the term values and intensity distribution. We will return to this point shortly. Rydberg series also lead to several important features in photoionization cross sections. With the increased resolution and activity in the measurement of photoionization cross sections we can expect a renewed interest in this role of Rydberg series. Finally a knowledge of the quantum defects in a Rydberg series can provide useful information on the phase shifts for electron-molecule scattering.