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Preface; Contents; Acronyms; 1 A Quick Look at the Diffuse Interstellar Medium; 1.1 Introduction; 1.2 Overview of the ISM and Its Role in Spiral Galaxies; 1.2.1 A Remark on Physical Processes in the ISM; 1.3 How Does the ISM Manifest Itself in a Galaxy Like the Milky Way?; 1.4 Why Does It Break Up into Phases?; 1.5 The Phases of the ISM: Observational Signatures; 1.5.1 The Hot Ionized Medium; 1.5.2 The Warm Ionized Medium; 1.5.3 The Warm Neutral Medium; 1.5.4 The Cold Neutral Medium; 1.6 Molecular Gas: Why Some of It Is a Phase and Some Isn't; 1.7 The Transition from Atomic to Molecular Gas
1.7.1 Photodissociation Regions: PDRs1.7.2 Diffuse vs. Dark Clouds; 1.8 The Role of Dust; 1.9 Cosmic Rays; 1.10 The Concept of a ``Molecular Cloud''; References; 2 Radiative Transfer Considerations; 2.1 Introduction; 2.2 The Transfer of Radiation Through the Interstellar Medium; 2.2.1 Statistical Balance; 2.2.2 Radiative Processes in the Rayleigh-Jeans Limit; 2.2.3 The Line Profile Function; 2.2.4 Obtaining Column Densities from Absorption Lines: Equivalent Width and Curveof Growth Methods; 2.2.5 Obtaining Column Densities from Emission Lines; 2.2.6 An Example: HI-The 21 Centimeter Line
2.2.6.1 HI Emission2.2.6.2 HI Absorption; 2.2.6.3 HI Absorption Continuum Measurements; 2.2.7 Line Pumping by the Cosmic Background Radiation: The Optical CN Transitions; 2.2.8 Collisional Excitation in a Nearly CollisionlessMedium; 2.2.9 Dynamical and Nonlocal Complications to Radiative Transfer; 2.2.9.1 Escape Probability and Photon Trapping; 2.2.9.2 Velocity Gradients and Line Transfer; 2.2.10 Elemental Abundances and Depletion in the Diffuse Medium; 2.2.11 H2 and Fine Structure Transitions
2.2.12 Diffuse Interstellar Features in Emission and Absorption: Radiative Excitationsand Fluorescence2.3 Radio Continuum Measurements of the Diffuse Medium; References; 3 The Diffuse ISM from the Ground: Chemistry and Tracers; 3.1 Introduction; 3.2 The First Detections of Interstellar Molecules; 3.3 Astrochemistry; 3.3.1 General Considerations; 3.3.2 The Oxygen Network; 3.3.3 The Carbon Network; 3.3.4 The Nitrogen Network; 3.3.5 Cosmic Rays as Agents of Astrochemical Processing; 3.4 Molecular Tracers of the Diffuse ISM in the Radio Regime; 3.4.1 The CO Rotational Transitions
3.4.2 The OH 18 cm Lines3.4.3 The CH 3.3GHz Lines; 3.4.4 The H2CO 4.8 GHz Line; 3.4.5 The HCO+(1-0) Transition; 3.4.6 C3H2, C2H, and C3H+; 3.4.7 HF, a Tracer of Molecular Hydrogen; 3.5 Optical Manifestations of Diffuse Molecular Gas and Dust; 3.5.1 CH, CH+, CN; 3.5.2 C2; 3.5.3 The Diffuse Interstellar Bands; 3.5.4 Red Luminescence; 3.6 The Molecular Medium in a Cosmological Line of Sight: PKS 1830-211; References; 4 Observing the Diffuse ISM: Making Sense of the RadioObservations; 4.1 Introduction; 4.2 Radio Observations: Practical Considerations; 4.2.1 The Radiometer Equation
1.7.1 Photodissociation Regions: PDRs1.7.2 Diffuse vs. Dark Clouds; 1.8 The Role of Dust; 1.9 Cosmic Rays; 1.10 The Concept of a ``Molecular Cloud''; References; 2 Radiative Transfer Considerations; 2.1 Introduction; 2.2 The Transfer of Radiation Through the Interstellar Medium; 2.2.1 Statistical Balance; 2.2.2 Radiative Processes in the Rayleigh-Jeans Limit; 2.2.3 The Line Profile Function; 2.2.4 Obtaining Column Densities from Absorption Lines: Equivalent Width and Curveof Growth Methods; 2.2.5 Obtaining Column Densities from Emission Lines; 2.2.6 An Example: HI-The 21 Centimeter Line
2.2.6.1 HI Emission2.2.6.2 HI Absorption; 2.2.6.3 HI Absorption Continuum Measurements; 2.2.7 Line Pumping by the Cosmic Background Radiation: The Optical CN Transitions; 2.2.8 Collisional Excitation in a Nearly CollisionlessMedium; 2.2.9 Dynamical and Nonlocal Complications to Radiative Transfer; 2.2.9.1 Escape Probability and Photon Trapping; 2.2.9.2 Velocity Gradients and Line Transfer; 2.2.10 Elemental Abundances and Depletion in the Diffuse Medium; 2.2.11 H2 and Fine Structure Transitions
2.2.12 Diffuse Interstellar Features in Emission and Absorption: Radiative Excitationsand Fluorescence2.3 Radio Continuum Measurements of the Diffuse Medium; References; 3 The Diffuse ISM from the Ground: Chemistry and Tracers; 3.1 Introduction; 3.2 The First Detections of Interstellar Molecules; 3.3 Astrochemistry; 3.3.1 General Considerations; 3.3.2 The Oxygen Network; 3.3.3 The Carbon Network; 3.3.4 The Nitrogen Network; 3.3.5 Cosmic Rays as Agents of Astrochemical Processing; 3.4 Molecular Tracers of the Diffuse ISM in the Radio Regime; 3.4.1 The CO Rotational Transitions
3.4.2 The OH 18 cm Lines3.4.3 The CH 3.3GHz Lines; 3.4.4 The H2CO 4.8 GHz Line; 3.4.5 The HCO+(1-0) Transition; 3.4.6 C3H2, C2H, and C3H+; 3.4.7 HF, a Tracer of Molecular Hydrogen; 3.5 Optical Manifestations of Diffuse Molecular Gas and Dust; 3.5.1 CH, CH+, CN; 3.5.2 C2; 3.5.3 The Diffuse Interstellar Bands; 3.5.4 Red Luminescence; 3.6 The Molecular Medium in a Cosmological Line of Sight: PKS 1830-211; References; 4 Observing the Diffuse ISM: Making Sense of the RadioObservations; 4.1 Introduction; 4.2 Radio Observations: Practical Considerations; 4.2.1 The Radiometer Equation