Introduction

Abstract

Spectroscopy as a special technique of scientific research is almost as old as the scientific research itself. Originally it consisted of an analysis of the wavelength and intensity of light after absorption or emission from atoms, molecules, or even from condensed matter. This analysis was, however, restricted basically to the visible spectral range. Today the range of a spectrum is much wider. Any analysis of the energy of radiation is considered as spectroscopy, no matter whether the radiation is electromagnetic, mechanical, or embodied in particles. The spectral range of the electromagnetic radiation extends from radio frequencies to γ radiation and the particle radiation includes electrons, neutrons, positrons, muons, and even neutral or charged atoms. The energy range to be considered covers at least ten orders of magnitude. The importance of such an extensive energy range to condensed matter and, in particular, to solids needs to be investigated. Information on the matter is obtained from the radiation spectrum modified by its interaction with the electronic and magnetic configuration of molecules or crystals. This interaction can proceed either by two-particle interaction, such as the processes of absorption or emission of radiation, or by three-particle interaction as in the process of scattering.¹ In fact, many electronic transitions in condensed matter are in the energy range of 10^-7 eV which allows absorption of radio waves in the MHz region.

1 In a quantum-mechanical notation absorption is a three-particle process and scattering is a four-particle process.