Origins of quantum theory

A good account of the historicai deveiopment of quantum mechanics. Whiie much of the book deais with quantum fieid theory and particie physics, the first third of the book focuses on the period 1850-1930 and the origins of quantum theory. An admirabie job is done in piacing events in a proper historicai context. 

The origins of quantum theory blackbody radiation and the photoelectric effect... 

The Origins of Quantum Theory Blackbody Radiation and the Photoelectric Effect... 

Why should a C—H bond in methane be so remarkably similar to a C—H bond in sugar Quantum mechanical calculations can confirm the similarities but cannot provide a simple explanation neither for this case nor for the general principle. The simple bonding calculus, which had already become quite sophisticated and recognizably modern half a century before the origins of quantum theory, takes this generalization as axiomatic and marches on from there. 

In this chapter we give a brief review of some of the basic concepts of quantum mechanics with emphasis on salient points of this theory relevant to the central theme of the book. We focus particularly on the electron density because it is the basis of the theory of atoms in molecules (AIM), which is discussed in Chapter 6. The Pauli exclusion principle is also given special attention in view of its role in the VSEPR and LCP models (Chapters 4 and 5). We first revisit the perhaps most characteristic feature of quantum mechanics, which differentiates it from classical mechanics its probabilistic character. For that purpose we go back to the origins of quantum mechanics, a theory that has its roots in attempts to explain the nature of light and its interactions with atoms and molecules. References to more complete and more advanced treatments of quantum mechanics are given at the end of the chapter. 

The orthodox or Copenhagen interpretation of quantum theory originated with three seminal papers published in 1925-26 by Heisenberg, Born and Jordan and an independent paper by Dirac (1926) all of these are available in English (translation) in a single volume [13]. A detailed summary was published by Heisenberg [9]. The primary aim of these studies was to formulate a mathematical system for the mechanics of atomic and electronic motion, based entirely on relations between experimentally observable quantities. An immediate consequence of this stipulation was that the motion of electrons could no longer be described in terms of the familiar concepts of space and time, but rather in terms of state functions constructed from matrix elements that relate to the Fourier sums over observed spectroscopic frequencies. The procedure became known as matrix mechanics. 

Apart from detail, reformulation of quantum theory to be consistent with chemical behaviour, requires the recognition of molecular structure. In this spirit, it may be introduced as an essential assumption, or emergent property, without immediate expectation of retrieving the concept from first principles. Medium-sized molecules, especially in condensed phases, are assumed to have a characteristic three-dimensional distribution of atoms, which defines a semi-rigid, flexible molecular frame. The forces between the atoms are of quantum-mechanical origin, but on a macro scale, are best described in terms of classical forces. 

The causal interpretation of quantum theory as proposed by De Broglie and Bohm is an extension of the hydrodynamic model originally proposed by Madelung and further developed by Takabayasi [36]. In Madelung s original proposal R2 was interpreted as the density p(x) of a continuous fluid with stream velocity v= VS/rri. Equation (5) then expresses conservation of fluid, while (6) determines changes of the velocity potential S in terms of the classical potential V, and the quantum potential... 

The physics of condensed phases is commonly formulated as of infinite extent. However, solid and liquid objects in the laboratory are of finite size and terminate discontinuously in a surface (in vacuum) or an interface, under all other conditions. Atoms or molecules at the surface or interface of the condensed object find themselves in a completely different environment, compared to those in the interior of the body. They are less confined in at least one direction, which means that the wave function looks different in this direction - it is less classical. It is implied that surface or interfacial species show more quantum-mechanical behaviour, compared to the bulk. This is the basic reason for the special properties of surfaces and the origin of all interfacial phenomena. Surface chemistry should therefore be formulated strictly in terms of quantum theory, but this has never been attempted. In its present state of development it still is an empirical science, although many physico-chemical concepts are introduced to rationalize the behaviour of interfaces. 

This quotation is from J.S. Bell [5]. The event referred to, marks the beginning of a slow, but certain return to the original spirit of quantum theory to understand the stability and structure of ponderable matter. Bohm s ideas have been resisted by the physics community but, as pointed out by Bell,... 

Quantum defect theory (QDT) was developed by Seaton [111] and his collaborators, from ideas which can be traced to the origins of quantum mechanics, through the work of Hartree and others. They relate to early attempts to extend the Bohr theory to many-electron systems (see e.g. [114]). 

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