These proceedings report the lectures and seminars presented at the NATO Advanced Study Institute on "The Spectroscopy of the Excited State," held at Erice, Italy, June 9-24, 1975. This Institute was an activity of the International School of Atomic and Molecular Spectroscopy of the "Ettore Majorana" Centre for Scientific Culture. The Institute consisted of a series of lectures on the spectroscopic properties of materials in excited electronic states, that, starting at a fundamental level, finally reached the current level of research. The sequence of lectures and the organization of the material taught were in keeping with a didac­ tical presentation. In essence the course had the two-fold pur­ pose of organizing what was known on the subject, and updating the knowledge in the field. The formal lectures were complemented by seminars whose abstracts are also included in these proceedings. The proceedings report also the contributions sent by Professors R.G.W. Norrish and S. C1aesson who, unfortunately, were not able to come because of illness. A total of 62 participants and 7 lecturers came from the following countries: Belgium, Canada, Czechoslovakia, France, Germany, Israel, Italy, Japan, Netherlands, Norway, Pakistan, Poland, Sweden, Switzerland, the United Kingdom, the United States and Venezuela. The secretaries of the course were: A. La Francesca for the administrative aspects of the meeting and P.Papagiannakopou10s for the scientific aspects of the meeting.

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Interaction of Radiation with Atoms and Molecules.- Abstract.- I. Introduction.- II. Quantum Theory of Molecular Systems.- II.A. The Hamiltonian of a Molecular System.- II.B. The Adiabatic Approximation.- II.C. The Role of Symmetry.- III. Quantum Theory of the Radiative Field.- III.A. The Classical Radiative Field.- III.B. Solutions of the Field Equation.- III.C. Periodic Boundary Conditions and Density of States.- III.D. The Hamiltonian of the Radiative Field.- III.E. The Quantum Radiative Field.- III.F. The Energy Levels and the Eigenfunctions of a Radiative Field.- III.G. The Operator Vector Potential.- IV. Interaction of a Radiative Field with a Charged Particle.- IV.A. The Hamiltonian of a Charged Particle in an Electromagnetic Field.- IV.B. The Interaction of a Charged Particle with a Radiative Field.- IV.C. Radiative Processes.- IV.D. First Order Processes.- IV.E. Electric Dipole Processes.- V. Absorption and Emission of Radiation.- V.A. Transition Probabilities for Absorption and Emission.- V.B. "Upward" and "Downward" Induced Transitions.- V.C. Einstein's A and B Coefficients.- V.D. Absorption and Emission in the Electric Dipole Approximation.- V.E. Interaction of Radiation with Molecular Systems. The Franck-Condon Principle.- References.- to Molecular Spectroscopy.- Abstract.- I. Historical Introduction.- II. Theory of Electronic Spectra.- II.A. Electronic States.- 1. Classification.- 2. Electronic Selection Rules.- II.B. Vibrational Structure.- 1. Vibrational Energy Formulae.- 2. Franck-Condon Principle.- 3. Vibrational Selection Rules.- II.C. Rotational Structure.- 1. Rotational Energy Levels.- 2. Symmetry Properties.- 3. Rotational Selection Rules.- III. Molecular Orbitals and Spectra of Triatomic Molecules.- III.A. AH2 Molecules.- 1. Five Electrons: BH2, AlH2.- 2. Six Electrons: CH2, SiH2.- 3. Seven Electrons: NH2, PH2, H2O+, H2S+.- 4. Eight Electrons: H2O, H2S.- III.B. HAB Molecules.- 1. Ten Electrons: HCN, HCP.- 2. Eleven Electrons: HCO.- 3. Twelve Electrons: HNO, HPO, HCF, HCCl, HSiCl.- 4. Thirteen Electrons:HO2, HS2, HNF.- III.C. AB2 and BAC Molecules.- 1. Twelve Electrons: C3.- 2. Thirteen Electrons: CNC and CCN.- 3. Fourteen Electrons: NCN and CCO.- 4. Fifteen Electrons: CO2+, CS2+, BO2, N3, NCO, NCS, N2O+, COS+.- 5. Sixteen Electrons: CO2, CS2, COS, N2O.- 6. Seventeen Electrons: NO2.- 7. Eighteen Electrons: O3, SO2, S2O, CF2, SiF2, FNO, ClNO.- 8. Nineteen Electrons: References: ClO2, NF2.- References.- Laser Excitation of Optical Spectra.- Abstract.- I. Characteristics and Types of Lasers.- I.A. CW Lasers.- I.B. Pulsed Lasers.- II. Excitation of Molecular Spectra.- II.A. Band Spectra.- II.B. Resonance Fluorescence.- III. Saturation Spectroscopy.- IV. Two-Photon Spectroscopy.- IV.A. Detection of Two-Photon Processes.- IV.B. High Resolution Spectra without Doppler Broadening.- References.- Techniques of Flash Photolysis.- Abstract.- I. Introduction.- II. Discharge Circuits.- II.A. Series Resistors for Flash Lamps.- II.B. Flash Lamp Design.- III. Examples of Flash Photolysis Apparatuses.- IV. Kinetic Studies by Means of Flash Photolysis.- V. Calculation of Rate Processes Which Are as Fast as the Flash Decay.- VI. Numerical Analysis of Experimental Data References.- Some Fast Reactions in Gases Studied by Flash Photolysis and Kinetic Spectroscopy.- I. Introduction.- II. Flash Photolysis and Kinetic Spectroscopy.- III. Vibrational Excitation by Primary Reaction. The Flash Photolysis of Nitrosyl Halides -- Vibrational Relaxation.- IV. Vibrational Excitation by Secondary Reactions.- IV.A. The Reactions of Oxygen Atoms.- IV.B. The Photolysis of Ozone.- V. Application of the Adiabatic Method. The Study of Explosive Processes Exemplified by the Oxidation of Hydrides.- VI. A General Mechanism for the Combustion of Hydrides.- References.- Electronic and Vibrational Energy Transfer.- Abstract.- I. Introduction.- II. Vibration - Translation Transfer.- III. Vibration - Vibration Transfer.- IV. Vibrational Relaxation in Excited Electronic States.- V. Electronic to Vibrational Energy Transfer.- VI. Electronic - Electronic Transfer.- References.- The Chemical Production of Excited Species.- Abstract.- I. Introduction.- II. The Formation of Excited Species in Combination Processes.- II.A. Introduction.- II.B. Direct Two-Body Combination.- II.C. Two-Body Combination with Curve-Crossing (Preassociation).- II.D. Three-Body Combination.- III. Chemiluminescence in Atom Transfer Processes.- III.A. Adiabatic Atom Transfer Reactions.- III.B. Non-Adiabatic Atom Transfer Reactions.- IV. Chemiluminescence in Other Chemical Systems.- References.- Two-Photon Spectroscopy in the Gas Phase.- Abstract.- I. Introduction.- II. Theory.- III. Methods of Assignment in Two-Photon Spectroscopy.- IV. Lifetimes.- References.- Lifetime Spectroscopy.- Abstract.- I. Introduction.- II. New Techniques.- III. Conclusion and Future.- References.- The Study of Electronic Spectra in Crystalline Solid Solutions.- Abstract.- I. Introduction.- II. Naphthalene in Durene.- III. Orientation of Molecules in Solid Solution.- IV. Spectrum Analysis.- IV.A. Pyrazine and Pyridine.- IV.B. Trans-Stilbene.- IV.C. Benzoic Acid Dimer.- IV.D. Vibronic Interference.- V. Conclusions.- References.- Core Excitation and Electron Correlation in Crystals.- Abstract.- I. Fundamentals of Core-Exciton Theory.- I.A. Representations by Bloch and Wannier Functions.- 1. A Ground State of a Model Insulator.- 2. States of One-Electron Excitation.- I.B. Exciton Wavefunctions.- 1. Frenkel Excitons.- 2. Wannier Excitons.- I.C. Core-Excitons.- 1. Energy Matrix in the Wannier-Function Representation.- 2. Approximations in Core-Excitons.- I.D. Hole-Electron Interactions.- 1. The Excited-Electron Orbital.- 2. Complementary States.- 3. Multiplets.- I.E. Spin-Orbit Interactions.- II. Vacuum-Ultraviolet Absorption of Alkali Halides.- II.A. Application of the Core-Exciton Theory.- 1. The Starting Model.- 2. Calculation of Exciton Multiplets.- 3. Transition Dipole Moments.- 4. Excitation Transfer.- II.B. Interpretation of the Spectra.- 1. The Energy Level Diagram.- 2. Comparison with the Experiments.- II.C. Effects of Electron Correlation.- 1. Comparison with the Wannier Exciton Theory.- 2. Importance of the Configuration Mixing.- 3. Relaxation of the Excited-Electron Orbitals.- III. X-Ray Spectroscopy of Transition-Metal Compounds.- III.A. Covalency in the Absence of a Hole.- 1. A Covalency Parameter.- 2. Hartree-Fock Equation.- 3. Calculati…