Historical Background of Quantum Mechanics

The development of quantum mechanics began in 1900 with Planck s study of the light emitted by heated solids, so we start by discussing the nature of light. 

In 1888, Heinrich Hertz detected radio waves produced by accelerated electric charges in a spark, as predicted by Maxwell s equations. This convinced physicists that light is indeed an electromagnetic wave. 

All electromagnetic waves travel at speed c = 2.998 X 10 m/s in vacuum. The frequency v and wavelength A of an electromagnetic wave are related by 

Planck s hypothesis that only certain quantities of light energy can be emitted 

In 1905, Einstein showed that these observations could be explained by regarding light as composed of particlelike entities (called photons), with each photon having 

In the 1890s, physicists measured the intensity of light at various frequencies emitted by a heated blackbody at a fixed temperature, and did these measurements at several temperatures. A blackbody is an object that absorbs all light falling on it. A good 

In 1899-1900, measurements of blackbody radiation were extended to lower frequencies than previously measured, and the low-frequency data showed significant deviations from Wien s formula. These deviations led the physicist Max Planck to propose in October 1900 the following formula / = av / 1), which was found to give an excellent 

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Historical Background.—Relativistic quantum mechanics had its beginning in 1900 with Planck s formulation of the law of black body radiation. Perhaps its inception should be attributed more accurately to Einstein (1905) who ascribed to electromagnetic radiation a corpuscular character the photons. He endowed the photons with an energy and momentum hv and hv/c, respectively, if the frequency of the radiation is v. These assignments of energy and momentum for these zero rest mass particles were consistent with the postulates of relativity. It is to be noted that zero rest mass particles can only be understood within the framework of relativistic dynamics. 

Although it is the modern theory of quantum mechanics in which we are primarily interested because of its applicationsjto chemical problems, it is desirable for us first to discuss briefly the background of classical mechanics from which it was developed. By so doing we not only follow to a certain extent the historical development, but we also introduce in a more familiar form many concepts which are retained in the later theory. We shall also treat certain problems in the first few chapters by the methods of the older theories in preparation for their later treatment by quantum mechanics. It is for this reason that the student is advised to consider the exercises of the first few chapters carefully and to retain for later reference the results which are secured. 

CFT and LFT have a historical background from the end of the 1920s and therefore we should not be surprised if the exact definitions of the two models vary between different textbooks. CFT and LFT together form the background knowledge for all discussions on the electron structure of transition metal systems. They serve to interpret experimental spectra as well as the results of quantum mechanical ab initio calculations. 

Local Thermodynamic Equilibrium (LTE). This LTE model is of historical importance only. The idea was that under ion bombardment a near-surface plasma is generated, in which the sputtered atoms are ionized [3.48]. The plasma should be under local equilibrium, so that the Saha-Eggert equation for determination of the ionization probability can be used. The important condition was the plasma temperature, and this could be determined from a knowledge of the concentration of one of the elements present. The theoretical background of the model is not applicable. The reason why it gives semi-quantitative results is that the exponential term of the Saha-Eggert equation also fits quantum-mechanical expressions. 

In this chapter, the most important quantum-mechanical methods that can be applied to geological materials are described briefly. The approach used follows that of modern quantum-chemistry textbooks rather than being a historical account of the development of quantum theory and the derivation of the Schrodinger equation from the classical wave equation. The latter approach may serve as a better introduction to the field for those readers with a more limited theoretical background and has recently been well presented in a chapter by McMillan and Hess (1988), which such readers are advised to study initially. Computational aspects of quantum chemistry are also well treated by Hinchliffe (1988). 

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