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Glossary Quantum jumps: When many absorbers are present, one may define the transition rate from one energy level to another. However, when only one absorber is present, discrete jumps can be observed as the absorber jumps from one quantum state to another. These quantum jumps occur stochastically, with a rate that agrees with the normal transition rate only on average. Site: The specific local environment around an absorbing impurity center. Phonon: Lattice vibration of a solid host matrix. Zero-phonon transitions: Optical or vibrational transitions of an impurity center in which no phonons of the host matrix are created or destroyed. Such transitions are only appreciable when the electron-phonon (or vibration-phonon) coupling is not too large.
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Polymeric Materials for Electronics Packaging
W
hile there has been a number of books addressing packaging, and to some extent polymers, virtually none has emphasized the synthetic and physical chemistry of these systems. This new volume fills that gap, addressing the many aspects relating to the development of novel polymeric materials and processes. The emphasis is on chemistry and materials science, rather than circuitry, its electrical capabilities, or its design characteristics. Covering a broad spectrum of subjects, this 38-chapter book focuses on four general areas: • physical chemistry of materials • properties and applications of encapsulants • properties and applications of gels • printed circuit board substrates and materials for circuit board substrates. Also included is a review of the marketing trends which drive packaging technology. This unique volume will be helpful to scientists engaged in materials development for electronic packaging, electronic packaging engineers, and to technologists who monitor developments in the electronics industry and related fields. John H. Lupinski, Editor, General Electric Company Robert S. Moore, Editor, Eastman Kodak Company Developed from a symposium sponsored by the Division of Polymeric Materials: Science and Engineering and of Polymer Chemistry of the American Chemical Society ACS Symposium Series No. 407 512 pages (1989) Clothbound ISBN 0-8412-1679-9 LC 89-37412 US & Canada $99.95 Export $119.95
American Chemical Society Distribution Office, Dept. 45 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE
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Homogeneous absorption line: The absorption profile of a single impurity center. In most cases, the homogeneous lines of all of the impurity centers in the solid are identical in width; hence, this width is called the homogeneous width. Inhomogeneous absorption line: The type of absorption line that occurs for zero-phonon transitions of impurity centers in solids at low temperatures. The inhomogeneous line is almost always much wider than the homogeneous width because of the spread of center frequencies for the various impurities. Number fluctuations: For an additive physical quantity that is measured for many independent members of a random ensemble, the value of the quantity fluctuates with a relative size equal to 1 over the square root of N, the number of members sampled. Coherent transient techniques: Methods for measuring the homogeneous width using pulses shorter than all the excited-state lifetimes, such as photon echoes, stimulated photon echoes, and free-induction decays. Quantum limit: In optical spectroscopy, the signal-to-noise regime where the noise is dominated by Poisson shot noise fluctuations of the light beam itself. Quadratic Stark effect: The shifting of the resonance frequency of a center in an external electric field in which the shift is proportional to the square of the electric field. Laser FM spectroscopy: An optical detection technique in which a sample with narrow absorption features is probed with a frequency-modulated (or phase-modulated) light beam. The signal derives from the conversion of FM into AM by the sample absorption.
el system composed of pentacene substitutional impurities in p-terphenyl crystals at 1.5 Κ (Figure 1). A penta cene molecule can substitute for any one of the four p-terphenyl molecules in the low-temperature unit cell (7), giving rise to four Si *- So 0-0 optical absorption origins near 593 nm: Οι, Ο2, O3, and O4. We will focus on the origins Oi and 02, because the homogeneous linewidths are smaller for these than for the other origins. (See Reference 8 for details of the low-temperature crys tal structure.) In our ^ΙΟΟ-μπι-thick samples at impurity concentrations of 10~7 mol/ mol, therefore, the background "hay stack" is composed of 1013 host mole cules plus 106 additional pentacene molecules. Our method of selecting a single resonant impurity for optical probing relies on the phenomenon of inhomogeneous broadening that occurs for all zero-phonon transitions at low temperatures. Zero-phonon transitions can become very narrow as the tem perature is lowered, because broaden ing processes in which a host phonon scatters off the electronic excited state are quenched. In essence, the absorp
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tion lines for the various guests become so narrow that the normally hidden distribution of center frequencies be comes apparent (Figure 2). The lowtemperature inhomogeneous profile (for a particular origin) is composed of many narrow homogeneous (usually Lorentzian) absorption lines with a dis tribution of center frequencies caused by dislocations, point defects, or ran dom internal electric and strain fields and field gradients in the host material. Clearly, one absorber may be selected for spectroscopy by proceeding out into the wings of the inhomogeneous line, as shown on the right side of Figure 2. It would also be possible to drastically lower the impurity concentration and work near the center of the inhomogen eous line. We chose the approach de scribed herein because by tuning to the center of the inhomogeneous line where Ν is large, we can use a conve niently large alignment signal to opti mize the detection system. Then, by tuning out into the wings, single-mole cule absorptions can be observed. Because of the randomness associat ed with imperfections in the host mate rial, inhomogeneous absorption lines
