Molecular Physics PDF
Preview Molecular Physics
METHODS OF EXPERIMENTAL PHYSICS: L. Martori, Editor-in-Chief Claire Marton, Assistant Editor 1. Classical Methods Edited by Immanuel Estermann 2. Electronic Methods Edited by E. Bleuler and R. O. Haxby 3. Molecular Physics, Second Edition (in two parts) Edited by Dudley Williams 4. Atomic and Electron Physics—Part A: Atomic Sources and Detectors, Part B: Free Atoms Edited by Vernon W. Hughes and Howard L. Schultz 5. Nuclear Physics (in two parts) Edited by Luke C. L. Yuan and Chien-Shiung Wu 6. Solid State Physics (in two parts) Edited by K. Lark-Horovitz and Vivian A. Johnson 7. Atomic and Electron Physics—Atomic Interactions (in two parts) Edited by Benjamin Bederson and Wade L. Fite 8. Problems and Solutions for Students Edited by L. Marton and W. F. Hornyak 9. Plasma Physics (in two parts) Edited by Hans R. Griem and Ralph H. Lovberg 10. Physical Principles of Far-Infrared Radiation L. C. Robinson 11. Solid State Physics Edited by R. V. Coleman 12. Astrophysics—Part A: Optical and Infrared Edited by N. Carleton Volume 3 Molecular Physics Second Edition PART B Edited by DUDLEY WILLIAMS Department of Physics Kansas State University Manhattan, Kansas 1974 ACADEMIC PRESS · New York and London A Subsidiary of Harcourt Brace Jovanovich, Publishers COPYRIGHT © 1974, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. Ill Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NW1 Library of Congress Cataloging in Publication Data Williams, Dudley, DATE ed. Molecular physics. (Methods of experimental physics, V. 3) Includes bibliographical references. 1. Molecules. 2. Molecular theory. I. Title. II. Series. QC175.16.M6W553 539M2 73-8905 ISBN 0-12-476043-0 PRINTED IN THE UNITED STATES OF AMERICA CONTRIBUTORS TO VOLUME 3, PART B Numbers in parentheses indicate the pages on which the authors' contributions begin. THOMAS C. ENGLISH, Department of Physics and Astrophysics, Duane Physical Laboratories, University of Colorado, Boulder, Colorado (669) EDWIN N. LASSETTRE, Center for Special Studies and the Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania (868) P. S. LEUNG, Union Carbide Corporation, Sterling Forest Research Center, Tuxedo, New York (952) C. A. MCDOWELL, Department of Chemistry, University of British Co lumbia, Vancouver, British Columbia, Canada (575, 847) J. D. MEMORY, Department of Physics, North Carolina State University, Raleigh, North Carolina (465) G. W. PARKER, Department of Physics, North Carolina State University, Raleigh, North Carolina (465) AUSMA SKERBELE, Center for Special Studies and the Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania (868) G. J. SÀFFORD, Union Carbide Corporation, Sterling Forest Research Center, Tuxedo, New York (952) JENS C. ZORN, Department of Physics, University of Michigan, Ann Arbor, Michigan (669) FOREWORD Close to 12 years have elapsed since I wrote the foreword to the first edition of the volume on Molecular Physics in our series. At that time I estimated to be about halfway in our task to present a concise survey of the methods used by experimental physicists. The original concept of six volumes has since grown to ten published volumes, with several of them split into double volumes. At this time I can report on advanced plans for further additions: a volume on polymer physics, another on fluid dynamics, and a third on environmental studies. In publishing this revised edition of the volume on Molecular Physics I would like to trace briefly the reason for issuing it. My fellow editors and I were pleased with the reception of our series by our readers, as manifested by book reviews and by the distribution of the books. Several volumes were reprinted, but when it came to reprinting the molecular physics (as well as the electronics) volumes, we concluded that the time had come for major revisions. The results achieved by Pro fessor Williams are presented herewith and both he and I hope physicists will find this edition as valuable as the first version. In this new edition you will find the subjects rearranged, with a considerable amount of new material added and some of the old omitted. You will recognize also some of the authors from the first edition, with a number of new authors added. Professor Williams' introduction gives an excellent survey of the organization of the new "Molecular Physics/' It remains a pleasant duty to thank Professor Dudley Williams and all the authors for their untiring labors. The cooperation of the publishers is gratefully acknowledged, as well as all the contributions to the editorial work by Mrs. Claire Marton. L. MARTON CONTENTS OF VOLUME 3, PART A 1. Introduction by DUDLEY WILLIAMS 1.1. Introduction 1.2. Origins of the Molecular Theory 1.3. Molecular Physics 2. Molecular Spectroscopy 2.1. Microwave Spectroscopy by DAVID R. LIDE, JR. 2.2. Infrared by W. E. BLASS and A. H. NIELSEN 2.3. Electronic Spectroscopy by C. WELDON MATHEWS 2.4. Molecular Lasing Systems by GEORGE W. CHANTRY and GEOFFREY DUXBURY 3. Light Scattering by D. H. RANK and T. A. WIGGINS 3.1. Introduction 3.2. Spontaneous Rayleigh and Brillouin Scattering 3.3. Spontaneous Raman Scattering 3.4. Stimulated Scattering 3.5. Stimulated Brillouin Scattering 3.6. Stimulated Raman Scattering 3.7. Other Stimulated Effects AUTHOR INDEX—SUBJECT INDEX Xlll CONTRIBUTORS TO VOLUME 3, PART A W. E. BLASS, Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee GEORGE W. CHANTRY, Department of Trade and Industry, National Physical Laboratory, Teddington, Middlesex, England GEOFFREY DUXBURY, School of Chemistry, University of Bristol, CantocKs Close, Bristol, England DAVID R. LIDE, Jr., National Bureau of Standards, Washington, D. C. C. WELDON MATHEWS, Department of Chemistry, Ohio State University, Columbus, Ohio A. H. NIELSEN, Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee D. H. RANK, Physics Department, The Pennsylvania State University, University Park, Pennsylvania T. A. WIGGINS, Physics Department, The Pennsylvania State University, University Park, Pennsylvania DUDLEY WILLIAMS, Department of Physics, Kansas State University, Manhattan, Kansas XV 4. RESONANCE STUDIES*+ 4.1. Introduction to Magnetic Resonance Magnetic resonance occurs when transitions are induced between the Zeeman energy levels of a particle with nonzero spin, due to the inter action of the particle with electromagnetic radiation of the proper fre quency and polarization. When the particle involved is a nucleus, the process is called nuclear magnetic resonance (NMR); for electrons, the process is called electron paramagnetic resonance (EPR) or electron spin resonance (ESR).1-7 4.1.1. The Resonance Condition Two complementary descriptions of the resonance process, one classical and one quantum mechanical, will be presented. A particle of spin I has associated with it a collinear magnetic moment μ. This is, of course, reasonable on classical grounds. A spinning charged sphere may be formally resolved into current loops which behave as magnetic moments. The constant of proportionality between μ and I 1 A. Carrington and A. D. McLachlan, "Introduction to Magnetic Resonance with Applications to Chemistry and Chemical Physics." Harper, New York, 1967. 2 A. Abragam, "The Principles of Nuclear Magnetism." Oxford Univ. Press, London and New York, 1961. 8 J. W. Emsley, J. Feeney, and L. H. Sutcliffe, "High Resolution Nuclear Magnetic Resonance Spectroscopy," Pergamon, Oxford, 1965-1966. 4 D. J. E. Ingram, "Free Radicals as Studied by Electron Spin Resonance." Butter- worths, London and Washington, D. C, London, 1958. 5 C. P. Slichter, "Principles of Magnetic Resonance." Harper, New York, 1963. 6 J. A. Pople, W. G. Schneider, and H. J. Bernstein, "High Resolution Nuclear Mag netic Resonance." McGraw-Hill, 1959. 7 J. D. Memory, "Quantum Theory of Magnetic Resonance Parameters." McGraw- Hill, New York, 1968. + See also Volume 4B, Section 4.1.2.6.2, as well as Volume 6 (2nd ed.), Chapter 2.1. * Part 4 is by J. D. Memory and G. W. Parker. 465 466 4. RESONANCE STUDIES is defined by the equation μ = γΗΙ, (4.1.1) where γ is the gyromagnetic ratio of the particle, and H is Planck's constant divided by 2π. If the particle is an electron, it is more customary to write V- = gfr (4.1-2) where β is the Bohr magneton efißmc, and the ^-factor g is a dimensionless constant which is slightly over two for a free electron. 4.1.1.1. Classical Derivation of the Resonance Condition. A mag netic moment μ in a magnetic field H is subject to a torque μΧΗ, so that the equation of motion for the angular momentum 1% is έ/(Κ)/Λ = μχΗ. (4.1.3) However, since μ = γίίΐ, we have dl/dt = yKL X H = - yfiU X I. (4.1.4) The form of Eq. (4.1.4) is just that of a vector I precessing with unchanged length about an axis parallel to H with angular frequency ω = γΗ. (4.1.5) If, in addition to a strong, constant, magnetic field H, there is a much smaller magnetic field Hj rotating in the plane perpendicular to H with the same angular frequency and in the same sense as the precessing magnetic moment, then H will appear constant in time to μ, and will x provide a torque tending to tip μ away from H (see Fig. 1). This will require an absorption of energy from the source of Hj, since the energy of interaction of μ and H is E= -μ -H, (4.1.6) and the net effect of H is to increase the angle between μ and H. This x then is magnetic resonance. If the frequency of the rotating field is different from the precessional frequency, there will be no such net torque, since part of the time H will be tipping μ toward H and part of the time 1 away from H. Similarly, there will be no net effect if the precessions are in opposite senses. In a typical experiment, a linearly polarized oscillating magnetic field is used, as indicated in the block diagram of a simplified 4.1. INTRODUCTION TO MAGNETIC RESONANCE 467 FIG. 1. Precessing moment and rotating field [J. D. Memory, "Quantum Theory of Magnetic Resonance Parameters," p. 3. McGraw-Hill, New York, 1968]. NMR spectrometer (see Fig. 2). Such a linearly polarized field can, however, be resolved into two circularly polarized fields rotating in opposite senses; that component rotating in the sense opposite to that of μ may be neglected for the reasons given above. 4.1.1.2. Quantum-Mechanical Derivation of the Resonance Condition. An alternative and equally straightforward derivation of the resonance condition follows from a simple quantum-mechanical argument. The z component of spin of a particle of spin I can take on only the 21 + 1 values /, I — 1, ... , —/, so that the energy levels of a particle of spin I -Sample FIG. 2. Block diagram of a simple NMR spectrometer [J. D. Memory, "Quantum Theory of Magnetic Resonance Parameters," p. 4. McGraw-Hill, New York, 1968].
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