Radiation quantum theory

Einstein A 1917 On the quantum theory of radiation Phys. Z. 18 121 Reprinted ter Haar D 1967 The Old Quantum Theory (New York Pergamon)... 

The results of the theory of quantum mechanics require that nuclear states have discrete energies. This is in contrast to classical mechanical systems, which can have any of a continuous range of energies. This difference is a critical fact in the appHcations of radioactivity measurements, where the specific energies of radiations are generally used to identify the origin of the radiation. Quantum mechanics also shows that other quantities have only specific discrete values, and the whole understanding of atomic and nuclear systems depends on these discrete quantities. 

The first known laser was made by Theodore Maiman at Hughes Research Laboratories in Malibu, California, in 1960, but the seeds of this breakthrough were planted years before. In 1917 Albert Einstein, through his work on the quantum theory of light, theorized that stimulated emission of light radiation could occur. The idea was forgotten, though, until the middle of the century. 

W. Heitler, The Quantum Theory of Radiation, Oxford University Press, London, second edition, 1944 page 157. 

Experimental chemists are rarely concerned with quantum effects and it s not unusual to find them ignoring this fundamental theory altogether. Even when an effort is made to explore the topic more deeply traditional quantum phenomena like black-body radiation, Compton scattering and even the photoelectric effect may appear to be of somewhat limited importance. Experimentalists who rely on spectroscopic measurements get by with interpretations based on a few simple semi-classical rules, and without ever appreciating the deep significance of quantum theory. Maybe there is a problem with the rigorous mathematical formulation of quantum theory and too little emphasis on quantum effects routinely encountered in chemistry. 

Schrodinger s equation is widely known as a wave equation and the quantum formalism developed on the basis thereof is called wave mechanics. This terminology reflects historical developments in the theory of matter following various conjectures and experimental demonstration that matter and radiation alike, both exhibit wave-like and particle-like behaviour under appropriate conditions. The synthesis of quantum theory and a wave model was first achieved by De Broglie. By analogy with the dual character of light as revealed by the photoelectric effect and the incoherent Compton scattering... 

The first application of quantum theory to a problem in chemistry was to account for the emission spectrum of hydrogen and at the same time explain the stability of the nuclear atom, which seemed to require accelerated electrons in orbital motion. This planetary model is rendered unstable by continuous radiation of energy. The Bohr postulate that electronic angular momentum should be quantized in order to stabilize unique orbits solved both problems in principle. The Bohr condition requires that... 

Quantum theory started as an explanation of electromagnetic effects (black-body radiation) in terms of corpuscular properties of light, expressed... 

The failure of classical mechanics in the analysis of physical phenomena, such as black-body radiation, is routinely discussed in elementary texts to emphasize the need of a quantum theory. The failure of classical mechanics to deal correctly with simple chemical systems, although rarely stated, is equally dramatic. 

J.H. Jeans, Report on Radiation and the Quantum-theory, 1914, The Physical Society, London. 

Quantum theory considers radiation as a stream of energy packets - photons or quanta - travelling through space at a constant velocity (c when in a vacuum). The energy of a photon is related to the frequency of the radiation, as defined in wave theory, by the expression... 

The correct explanation of black-body radiation was an early triumph of quantum theory. 

The application of the quantum theory predicts that there can be no radiation produced of higher frequency than that given by the Einstein quantum equation for the maximum energy that an electron can have... 

In 1912, Einstein extended the concept of quantum theory of radiation to photochemical processes and stated that each quantum of radiation absorbed by molecule activates one molecule in the primary step of a photochemical process . This is known as Einstein law of photochemical equivalence. 

A broadening in Mark s intellect is shown clearly in his publications of this period. The topics in 1926 and 1927 alone ranged from atomic structure and quantum theory (1), and Compton radiation (2) to the scattering of x-rays by an ideal gas (3) and mineral structure (4). The shear diversity of his contacts and interests made him qualified for expanded responsibilities. 

The Occurrence of Her mite Functions in Wave Mechanics. The Hcrmite functions which wc have discussed in the Inst section occur in the wave mechanical treatment of the harmonic oscillator1). Although this is a very simple mechanical system the analysis of its properties is of great importance because of its application to the quantum theory of radiation. 

Perhaps best known of Perrin s work is his spirited defense of kinetic theory and physical atomism entitled Les atomes (1913), in which he made use of his own work on Brownian motion, in combination with studies of cathode rays and x-rays, ionization, radioactivity, radiation, and quantum theory.72 About the time of the 1911 Solvay physics conference, Perrin shifted from Brownian motion to work on thin films, fluorescence, and photochemistry, partly to test the early quantum theory and especially to study individual atom-based fluctuations. 

R. E. Moss, Advanced molecular quantum mechanics. An Introduction to Relativistic Quantum Mechanics and the Quantum Theory of Radiation, Chapman Hall, London, 1973. 

Radiation is fundamentally different from conduction as it describes the transfer of heat between two substances that are not in contact with each other. Like conduction, radiation is an independent form of heat transfer. Ignoring the conflicts of wave and quantum theory, radiation, refers to the transmission of electromagnetic energy through space. 

We now consider the effect of exposing a system to electromagnetic radiation. Our treatment will involve approximations beyond that of replacing (3.13) with (3.16). A proper treatment of the interaction of radiation with matter must treat both the atom and the radiation field quantum-mechanically this gives what is called quantum field theory (or quantum electrodynamics). However, the quantum theory of radiation is beyond the scope of this book. We will treat the atom quantum-mechanically, but will treat the radiation field as a classical wave, ignoring its photon aspect. Thus our treatment is semiclassical. 

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