TEXT BOOK MOLECULAR SPECTRA and MOLECULAR STRUCTURE I. SPECTRA OF DIATOMIC MOLECULES by GERHARD HERZBERG. Originally published in 1939. PREFACE: Eleven years ago I published a volume entitled Molecular Spectra and Molecular Structure I. Diatomic Molecules which was followed in 1945 by a second volume Infrared and Raman Spectra of Polyatomic Molecules. The first volume has been out of print for a number of years but the demand for it seemed to justify a new edition. Although the book has been completely revised and brought up to date, its general plan has remained substantially unchanged. Concerning this plan it seems therefore appropriate to quote from the preface of the first edition I have endeavored to give a presentation which is readable by the beginner in the field and also will be useful to those who do or want to do research work in this field. In order to assist the former, I have frequently made use of small type for those sections that are not necessary for an understanding of the fundamentals. For the benefit of those working in the field, numerous references to original papers have been included. A satisfactory presentation of molecular spectra and molecular structure is nowadays not possible without treating thoroughly, apart from the empirical results, the theoretical background also. Therefore I have included as much of the theory of molecular spectra as is possible without going into the more difficult mathematical details. A large number of diagrams, graphical repre sentations of eigenfunctions and potential curves, as well as energy level dia grams, serve to illustrate and to explain the theory. On the other hand, I have added numerous carefully selected spectrograms of bands and band systems some of which have been taken specially for this purpose in order to give an accurate idea of tho experimental material that forms the basis of the developments. While of course most of the material presented is not new, it seems that the actual procedure followed in analyzing a band spectrum has not previously been given as specifically in a book of this kind. The same holds for the ap plications of band spectra to other parts of physics, to chemistry, and to astro physics given in the last chapter. I hope that both these features will be found useful. In the eleven years since the publication of the first edition the subject Spectra of Diatomic Molecules has developed vigorously even though not as rapidly as in the preceding two decades. Most of the progress made has been consolidation and slow evolution rather than revolution. Exceptions to this statement are the amazing advances made by applying the new tools of molecular beams and microwaves to diatomic molecular problems. Naturally I have incorporated these advances of recent years in the present new edition...