CALTECH APH 9A - Einstein coefficients (12 pages)

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Einstein coefficients



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Einstein coefficients

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Lecture Notes


Pages:
12
School:
California Institute of Technology
Course:
Aph 9a - Solid-State Electronics for Integrated Circuits
Solid-State Electronics for Integrated Circuits Documents

Unformatted text preview:

Einstein coefficients cross sections f values dipole moments and all that Robert C Hilborn Department of Physics Amherst College Anherst MA 01002 The relationships among various parameters describing the strength of optical transitions in atoms and molecules are reviewed The application of these parameters to the description of the interaction between nearly monochromatic directional light beams and atoms and molecules is given careful attention Common pitfalls in relating these parameters are pointed out This is a revised February 2002 version of a paper that originally appeared in Am J Phys 50 982 986 1982 I INTRODUCTION Several parameters are commonly used to describe the strength of atomic and molecular optical transitions The Einstein A and B coefficients f values also called oscillator strengths and transition dipole moments are all atomic and molecular parameters related to the strength of the transition In many practical situations on the other hand it is useful to define an absorption coefficient or for lasers a gain coefficient to describe the absorption or amplification of a beam of light passing through a medium consisting of the atoms or molecules of interest From a kinetics point of view the absorption or scattering of radiation is described as a reaction or scattering process and the probability of absorption or scattering is given in terms of a cross section It is the purpose of this paper to review the relationships among these descriptions of the light matter interaction and to point out common pitfalls in using these relationships An examination of books1 7 dealing with light matter interaction shows a wide variety of expressions relating these parameters Differences among these expressions involving factors of 2 o etc of course can be traced to differing units used in the definitions of the parameters However these differences are often exacerbated because many different and often not clearly defined measures of light intensity are used in the



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