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- Atomic Spectra Atomic Structure BY Gerhard Herzberg Research Professor of Physics University of Saskatchewan TRANSLATED ‘WITH THE CO-OPERATION OF THE AUTHOR BY J. W. T. Spinks Professor of Physical Chemsitry Univeristy of Saskatchewan NEW YORK DOVER PWBLICATIONS 1944 Preface to Second Edition HE present edition of this work contains a number of cor- T rections and additions; Birge's new set of fundamental constants has been adopted throughout, and various tables, especially the table of ionization potentials, have been brought up to date. The author is indebted to Professor J. W. Ellis of the Univer- sity of California at Los Angeles for a list of errors and correc- tions; several of the author’s students have also been helpful in pointing out certain mistakes. The author is grateful to Dover Publications for their initiative and interest in making this book available again in a revised photo-offset edition in spite of war-time difficulties. G. H. COPSRIGHT, 1937, BY SASKATOON, SASK., August, 1944. PRENTICE-HALL, INC. COPYRIGHT, 1844, BY Preface . DOVER PUBLICATIONS HE present work is the translation of a volume published in T German by Theodor Steinkopff about a year ago.1 Atomic Spectra and Atomic Structure constitutes the first part of a more comprehensive course on atomic and molecular spectra which the author has prepared and given recently. FIRST EDITION, 1937 Though in the past few years several excellent accounts have SECOND EDITION, 1944 been written on the subject of atomic spectra (cf. bibliography), there is still a need for an elementary introduction that is espe- cially adapted to the beginner in this field and also to those who require a certain knowledge of the subject because of its appli- cations in other fields. For these two groups of readers the discussion of too many details and special cases does not seem desirable, since it is likely to obscure the fundamentally important points. Consequently, in thii book the main stress is laid on the basic principles of the subject. Great pains have been taken to explain them as clearly as possible. To this end numerous diagrams and spectrograms are given as illustrations. Always the experimental results serve as the starting point of the theoretical considerations. Compli- cated mathematical developments have been avoided. Instead, the results of such calculations have been accepted without proof, reference being given to sources where proof can be found. Throughout the work an effort has been made to emphasize the physical significance of the theoretical deductions. vi Preface Rather liberal use has been made of small type in the printing of certain portions of the text. These, together with the foot- notes, contain theoretical explanations and details that may very Table of Contents well be omitted in a first reading without interfering with an CBA-R PAGE understanding of the fundamental points. Throughout the book, P REFACE....................... V in making this distinction between small and ordinary type, the INTRODUCTION . 1 author has kept in mind the needs of those readers who wish to Observation of spectra. Light sources. Emission and ab- obtain a thorough knowledge of only the more important prin- sorption. Examples. Spectral analysis. Units. ciples. The part printed in ordinary type is self-sufficient and, I. THE SIMPLEST LINE SPECTRA AND THE ELEMENTS OF adequate for that purpose. ATOMIC THEORY . 11 In view of the applications, particularly to the study of molec- 1. The empirical hydrogen terms . 11 The Balmer series and the Balmer formula. Other hydrogen ular spectra and molecular structure, some points have been more series. Representation of spectral lines by terms. extensively treated than others that might appear more impor- tant from the point of view of atomic spectra alone. In general, 2. The Bohr theory of Balmer terms . 13 Basic assumptions. Electron orbits in the field of a nucleus completeness has not, been attempted except in Tables 17 and with charge Ze. Energy of Bohr’s orbits (Balmer terms). 18, which give, respectively, nuclear spin values and ionization Spectra of hydrogen-like ions. Continuuni at the series limit. potentials. In these tables, results published up to the begin- 3. Graphical representation by energy level diagrams . 23 ning of the present year have been considered. Energy level diiam and spectrum. Consideration of the quantum number k, and the fine structure of the H lines. A discussion of X-ray spectra has been omitted, as one can be Selection rule for k. found in almost any advanced physics text. Naturally in the course of the translation the author has used 4. Wave mechanics or quantum mechanics . 28 Fundamental principles of wave mechanics. Mathematical every opportunity to improve the original German presentation. formulation. Physical . interpretation of the * function. The It is believed that in many instances the explanations have been Helsenberg uncertainty principle. Wave mechanics of the H atom. Momentum and angular momentum of an atom accord- clarified. Also, certain recent findings have been added. ing to wave mechanics. Transition. probabilities and selection The author is greatly indebted to Dr. J. W. T. Spinks for his rules according to wave mechanics. Quadrupole radiation and magnetic dipo e radiation. willingness to undertake the translation and for his prompt and careful work in carrying it out. He also owes many thanks to 5. Alkali spectra . 54 The principal series. Other series. Theoretical interpreta- Dr. R. N. H. Haslam, who was kind enough to read the entire tion of the alkali series. Alkali-like spark spectra. The proof and made numerous and valuable suggestions for improving Moseley lines. the presentation. Finally, the author wishes to express his appre- 6. Spectrum of helium and the alkaline earths . 64 ciation to Dr. E. U. Condon, Editor of ‘the PRENTICE-HALL Helium. Heisenberg's resonance for helium. The alkaline PHYSICS SERIES, and the staff of Prentice-Hall, Inc., for their helpful co-operation during the publication of this volume. II. MULTIPLET STRUCTURE OF LINE SPECTRA AND ELEC- G. H. TRON SPIN . 71 ’ G. Herzberg, Atomspektren und Atomstruktur (Dresden, 1936). 1. Empirical facts and their formal explanation . 71 Doublet structure of the alkali spectra. Quantum number J. Selection rule for J; compound doublets. Triplets and singlets of the alkaline earths and helium. Prohibition of interom- binations; intercombination lines. Higher multiplicities; term symbols. Alternation of multiplicities. vii . viii Contents Contents ix CHAPTER PAGE 2. Physical interpretation of the quantum numbers . 82 CHAPTER Meaning of L for several emission electrons. Physical inter- 2. Nuclear spin . 185 pretation of J: cause of multiplet splitting. Selection rule for Magnitude of the nuclear spin and its associated magnetic J. Physical interpretation of S. moment. Vector diagram allowing for nuclear spin. Selection rule for F; appearance of a hypermultiplet. Determination of 3. Space quantization: Zeeman effect and Stark effect 96 I and g from hyperfine structure. Zeeman effect of hyperfine General remarks on Zeeman effect and space quantization structure. Statistical weight. Determination of nuclear spin Normal Zeeman effect. Anomalous Zeeman effect. Pasche- by the Stem-Gerlach experiment. Results. Importance of Back effect. Stark effect. Statistical weight. nuclear spin in the theory of nuclear structure. III. THE BUILDING-UP PRINCIPLE AND THE PERIODIC SYS- VI. SOME EXPERIMENTAL RESULTS AND APPLICATIONS . 197 TEM OF THE ELEMENTS. 120 Energy level diagrams and ionization potentials . 197 1. The Pauli principle and the building-up principle . 120 Quantum numbers of the electrons in an atom. Pauli prin- Magnetic moment and magnetic susceptibility. 202 ciple. Prohibition of intercombinations. Application of the Magnetic moment of an atom. Paramagnetii. Paramag- Pauli principle. netio saturation. Diamagn etism. Paramagnetiim of ions in solutions and in solids. Magnetocalorio effect; production of 2. Determination of the term type from the electron extremely low temperatures. configuration . 128 Chemical applications . 213 Russell-Saunders coupling. Terms of non-equivalent electrons. Periodicity of chemical properties. Types of chemical binding Terms of equivalent electrons. Electron distribution with a (valence). The ionization potential. Electron affinity. Ionic number of electrons present. compounds. Atomic compounds (homopolar valence). Acti- vated states and collisions of the second kind; elementary 3. The periodic system of the elements . 138 chemical processes. H (hydrogen). He (helium). Li (lithium). Be (beryllium). B (boron). C (carbon). N (nitrogen). 0 (oxygen). F (flu-’ BIBLIOGRAPHY .................... 237 orine). Ne (neon). Succeeding periods of the periodic system. INDEX ........................ 245 IV. FINER DETAILS OF ATOMIC SPECTRA . 152 1. Intensities of spectral lines . 152 General selection rules (dipole radiation). Special selection ERRATA rules (dipole radiation). Forbidden transitions. General re- marks on the intensity ratios of allowed lines. Sum rule. Page 10. The last number in the first line of the table 2. Series limits for several outer electrons, anomalous should be: terms, and related topics . 162 1.23954 x 10-c. Series by excitation of only, one outer electron. Series by exoitation of two electrons; anomalous terms. Excitation of Page 35. The clause in the second line of the last paragraph inner electrons. Term perturbations. Pre-ionization (auto- should be in brackets. thus:
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