Given the importance of spectroscopy to astrophysical investigations, and certain questions and misconceptions that have been posted here at the forum, I have written an article introducing some of the basic principles of spectroscopy in the hope that such concepts would contribute positively to the understanding of, and appreciation for the significance of spectroscopy thereby enhancing the experience of those actively participating in GalaxyZoo.

This is not an introduction to astrophysical applications of spectroscopy. It is a discussion of why the energy levels of atoms are discrete and thereby lead to discrete spectral features that are highly characteristic of the atomic species, and provides a basis for understanding the internal conditions that must be satisfied for a radiative transition between energy levels to actually occur.

**Contents:**I. Introduction

II. Basic Concepts in Quantum Mechanics

III. Electronic Structure of the Hydrogen Atom

IIIa. Quantum Numbers

IIIb. The Bohr Model

IIIc. Fine Structure

IIId. Hyperfine Structure

IV. Spectroscopic Notation and Term Symbols

V. Radiative Transitions and Selection Rules

VI. Conclusions

VII. Acknowledgments

VIII. References and Notes

IX. Glossary of Symbols

X. Numerical Values of the Fundamental Physical Constants

XI. Appendix: Supplemental Material

Introduction to Electronic SpectroscopyThe document is a 446KB PDF file that you can download and study at your convenience. The file has been scanned at Jotti and VirusTotal and shown to be free of malware. File hashes follow:

MD5: 3dc0cec63a49859723f5dbc94d9fe539

SHA1: aaecbcdbcc26006fa374fd3471e0fe6e5b424a20

I would like to thank Jay Lowe for making the original manuscript into the PDF file and for hosting the document.

EDIT: Chapter 2 can be accessed via

Introduction to Spectral Line Profiles