Laser and Coherence Spectroscopy

The impact which has been made on spectroscopy by lasers, and by this route on major segments of physics and chemistry, has received ample documen­ tation in the past several years. Two principal themes emerge from examina­ tion of the numerous books and monographs now available on this subject: first, an increase in spectral resolution to levels previously undreamed of; and, second, the generation of nonlinear phenomena as a result of the intense radiation fields available from laser devices. There is one additional property of laser radiation which, although used extensively in experiments, does not appear to have been as thoroughly reviewed as the foregoing aspects. This is the spatial and temporal coherence of the radiation field produced by the laser, which makes possible the coherent excitation of molecular energy states. This feature is the subject of the present volume. While the use of coherence methods in spectroscopy has been paced by lasers, it is by no me ans restricted to this technology. In the second and fourth chapters, microwave sources are discussed as generators of coherent radiation fields and are used to probe both rotational energy levels and spin states of electronically excited molecules. The phenomena discussed in this book, such as nutation, free induction decay, radiative echoes, rapid pas­ sage, and so forth, are really the same in different regions of the spectrum, and themselves echo from one chapter to the next.

1 Double-Resonance Spectroscopy.- 1.1. Introduction to Double-Resonance Methods.- 1.1.1. Introduction.- 1.1.2. Dynamics of the Interaction of Radiation and Matter.- 1.1.3. Summary of Molecular Spectroscopy.- 1.1.3.1. Rotational Energy Levels.- 1.1.3.2. Vibrational Energy Levels.- 1.1.3.3. Electronic Energy Levels.- 1.1.4. Definition of Double-Resonance Spectroscopy.- 1.1.5. Historical Survey.- 1.2. Response of a System to Pumping and Analyzing Radiation Fields.- 1.2.1. Saturation of Molecular Absorption Lines.- 1.2.2. Double Resonance in a Three-Level System.- 1.2.3. Rate-Equation Analysis of Double Resonance.- 1.3. Experimental Considerations.- 1.3.1. Radiation Sources.- 1.3.1.1. Klystrons.- 1.3.1.2. Fixed-Frequency Lasers.- 1.3.1.3. Tunable Lasers.- 1.3.2. Signal Detection and Enhancement.- 1.3.2.1. Detectors.- 1.3.2.2. Lock-In Amplifier.- 1.3.2.3. Boxcar Averager.- 1.3.2.4. Transient Recorder.- 1.3.3. Detection by Fluorescence versus Absorption Techniques.- 1.3.4. Experimental Configurations.- 1.4. Microwave-Detected Double Resonance.- 1.4.1. Microwave Pumping.- 1.4.1.1. Carbon Oxysulfide.- 1.4.1.2. Ammonia.- 1.4.1.3. Formaldehyde.- 1.4.1.4. Ethylene Oxide.- 1.4.1.5. Hydrogen Cyanide.- 1.4.1.6. Other Systems.- 1.4.2. Infrared Pumping.- 1.4.2.1. Methyl Halides.- 1.4.2.2. Ammonia.- 1.4.3. Optical Pumping.- 1.5 Infrared-Detected Double Resonance.- 1.5.1. Microwave Pumping.- 1.5.2. Infrared Pumping.- 1.5.2.1. Vibrational Energy Transfer.- 1.5.2.2. Rotational Energy Transfer.- 1.5.2.3. Dephasing, Momentum Transfer, and Molecular Alignment.- 1.5.3. Optical Pumping.- 1.6. Optically Detected Double Resonance.- 1.6.1. Microwave-Optical Double Resonance.- 1.6.1.1. Microwave-Optical Double Resonance in Atoms.- 1.6.1.2. Microwave-Optical Double Resonance in CN.- 1.6.1.3. Microwave-Optical Double Resonance in OH and OD.- 1.6.1.4. Microwave-Optical Double Resonance in CS.- 1.6.1.5. Microwave-Optical Double Resonance in BaO.- 1.6.1.6. Microwave-Optical Double Resonance in NO2.- 1.6.1.7. Microwave-Optical Double Resonance in NH2.- 1.6.1.8. Microwave-Optical Double Resonance in BO2.- 1.6.2. Infrared-Optical Double Resonance.- 1.6.2.1. Infrared-Optical Double Resonance in NH3.- 1.6.2.2. Infrared-Optical Double Resonance in OsO4.- 1.6.2.3. Infrared-Optical Double Resonance in Biacetyl.- 1.6.2.4. Infrared-Optical Double Resonance in F8+.- 1.6.2.5. Infrared-Optical Double Resonance in Coumarin-6.- 1.6.3. Optical-Optical Double Resonance.- 1.6.3.1. Optical-Optical Double Resonance in Atoms.- 1.6.3.2. Optical-Optical Double Resonance in Diatomic Molecules.- 1.6.3.3. Optical-Optical Double Resonance in Polyatomic Molecules.- 1.6.4. Optically Detected Double Resonance in Large Molecules.- 1.7. Molecular Information from Double-Resonance Experiments.- 1.7.1. Spectroscopic Information.- 1.7.2. Energy Transfer and Interaction Potentials.- 1.7.3. Future Directions.- References.- 2 Coherent Transient Microwave Spectroscopy and Fourier Transform Methods.- 2.1. Introduction.- 2.2. Basic Theory and Experiment.- 2.3. Transient Absorption.- 2.4. Transient Emission.- 2.5. Fast Passage.- 2.6. Fourier Transform Microwave Spectroscopy.- 2.7. Molecular Interpretation of T1 and T2.- 2.8. Conclusion.- Appendix A. Solution of the Bloch Equations.- Appendix B. Two-State Relaxation Processes.- References.- 3 Coherent Transient Infrared Spectroscopy.- 3.1. Introduction.- 3.2. Density and Population Matrices.- 3.2.1. Basic Theory.- 3.2.2. Physical Interpretation and Applicability.- 3.3. Absorption and Emission of Radiation.- 3.3.1. Polarization and Reduced Wave Equations.- 3.3.2. Steady-State Absorption: An Example.- 3.4. Solutions of the Population Matrix Equations.- 3.4.1. Introduction.- 3.4.2. Optical Bloch Equations.- 3.4.3. Matrix Solution of the Optical Bloch Equations.- 3.5. Experimental Techniques.- 3.5.1. Pulsed Laser Experiments.- 3.5.2. Stark Switching.- 3.5.3. Frequency Switching.- 3.6. Optical Nutation.- 3.7. Optical Free Induction Decay.- 3.7.1. Theory and Experiment.- 3.7.2. Superradiance.- 3.8. Photon Echo.- 3.8.1. Two-Pulse Echoes.- 3.8.2. Multiple-Pulse Echoes.- 3.9. Measurement of Level Decay Rates.- 3.9.1. Adiabatic Rapid Passage.- 3.9.2. Delayed Optical Nutation.- 3.10. Velocity-Changing Collisions.- 3.10.1. Introduction.- 3.10.2. Brownian Motion and Velocity-Changing Collisions.- 3.10.3. Photon Echoes and Velocity-Changing Collision Measurements.- Appendix A. Justification of the Reduced Wave Equation.- Appendix B. Matrix Formulation of the Bloch Equations.- References.- 4 Coherent Spectroscopy in Electronically Excited States.- 4.1. Introduction.- 4.1.1. Historical Development.- 4.1.2. Recent Advances.- 4.2. Theoretical Considerations.- 4.2.1. General Aspects of Coherence in Excited States.- 4.2.2. Equation of Motion for the Model System.- 4.2.2.1. Basic Torque Equation in the Rotating Frame.- 4.2.2.2. Addition of Feeding and Decay Terms.- 4.2.2.3. Exact Solutions, Including Feeding and Decay.- 4.2.2.4. Addition of Relaxation Terms.- 4.2.2.5. Exact Solutions, Including Feeding, Decay, and Relaxation.- 4.2.2.6. Discussion.- 4.2.2.7. Inhomogeneous Relaxation and Expected Line Shapes.- 4.2.3. Relationship Between the Geometrical Model and Double-Resonance Observables.- 4.2.3.1. Density Matrix and Dipole Emission.- 4.2.3.2. Probe Pulse Method.- 4.2.4. Experiments Utilizing Optically Detected Coherence.- 4.2.4.1. Introduction.- 4.2.4.2. Transient Nutation.- 4.2.4.3. Free Induction Decay and Spin Echo.- 4.2.4.4. Echo Trains and Coherent Averaging.- 4.2.4.5. Spin Locking and Coherent Averaging.- 4.2.4.6. Rotary Echoes and Driving-Field Inhomogeneities.- 4.2.4.7. Adiabatic Demagnetization and Rapid Passage.- 4.3. Experimental Methods.- 4.3.1. Excited Triplet States and Phosphorescence Spectroscopy.- 4.3.2. Conventional Techniques: Optically Detected Magnetic Resonance.- 4.3.3. Pulse Techniques in Optically Detected Magnetic Resonance.- 4.3.3.1. Transient Nutation and Pulse Timing.- 4.3.3.2. Short Coherence Sequences.- 4.3.3.3. Long Coherence Sequences.- 4.3.3.4. Triplet-State Multiplets and Orientation Factors.- 4.4. Applications.- 4.4.1. Preliminaries.- 4.4.2. Addition of Energy Exchange to the Equations of Motion.- 4.4.2.1. Loss of Spin Memory in the Slow Exchange Limit.- 4.4.2.2. Retention of Spin Memory in Scattering in the Fast Exchange Limit.- 4.4.3. Energy Transfer Studies Using Coherent Spectroscopy Techniques.- 4.4.4. Vibrational Relaxation Studies Using Coherence Techniques.- 4.4.5. Energy Transfer Studies Using an Ordered State.- References.- 5 Resonant Scattering of Light by Molecules: Time-Dependent and Coherent Effects.- 5.1. Elementary Time-Dependent Theory Related to Luminescence.- 5.1.1. Introduction.- 5.1.2. Scattering Theory.- 5.1.3. Approximate Model for the Photon States.- 5.1.4. Molecular States.- 5.1.5. Matrix Elements of G(?).- 5.1.6. Excitation of an Isolated Resonant State: A Two-Level System.- 5.1.7. Semiclassical Analogy.- 5.2. Applications of Scattering Theory to Model Systems.- 5.2.1. Three-Level System.- 5.2.2. Scattering of an Exponentially Decaying Pulse.- 5.2.3. Semiclassical Treatment of the Three-Level System.- 5.2.4. Resonance and Near-Resonance Raman Scattering.- 5.3. Nature of the Electromagnetic Field.- 5.3.1. Definition of the Field Variables.- 5.3.2. Radiation-Matter Interaction.- 5.3.3. States of the Radiation Field.- 5.3.4. Measurables of the Field and Photon Experiments.- 5.4. Theory of Light Scattering with Well-Defined Light Sources.- 5.4.1. More General Approach to Light Scattering.- 5.4.2. Spectral Content of a Scattered Coherent Pulse.- 5.4.3. Scattering from a Gaussian Pulse.- 5.4.4. Scattering from a Weak Stationary Light Beam.- 5.5. Effects of Intermolecular Interactions on Luminescence.- 5.5.1. Resonance Scattering (Raman Fluorescence) in the Presence of Fluctuations.- 5.5.2. Random Modulation in Resonance Raman Scattering.- 5.5.3. Classical Character of Fluorescence.- 5.5.4. Absorption and Scattering.- 5.5.5. Spectroscopic Selection Rules for Resonance Raman, Fluorescence, and Phosphorescence.- 5.6. Two-Photon Induced Light Scattering.- 5.6.1. Two-Photon Processes.- 5.6.2. Scattering Induced by Two-Photon Excitation: Hyper Raman Scattering.- 5.7. Recent Resonance Fluorescence Concepts and Experiments.- Appendix. Contour Integration.- References.- Author Index.