It is a great challenge in chemistry to clarify every detail of reaction processes. In older days chemists mixed starting materials in a flask and took the resul- tants out of it after a while, leaving all the intermediate steps uncleared as a sort of black box. One had to be content with only changing temperature and pressure to accelerate or decelerate chemical reactions, and there was almost no hope of initiating new reactions. However, a number of new techniques and new methods have been introduced and have provided us with a clue to the examination of the black box of chemical reaction. Flash photolysis, which was invented in the 1950s, is such an example; this method has been combined with high-resolution electronic spectroscopy with photographic recording of the spectra to provide a large amount of precise and detailed data on transient molecules which occur as intermediates during the course of chemical reac- tions. In 1960 a fundamentally new light source was devised, i. e. , the laser. When the present author and coworkers started high-resolution spectroscopic stud- ies of transient molecules at a new research institute, the Institute for Molecu- lar Science in Okazaki in 1975, the time was right to exploit this new light source and its microwave precursor in order to shed light on the black box.

Table of Contents:
1. Introduction.- 1.1 Historical Background of Free Radical Studies by High-Resolution Spectroscopy.- 1.2 Significance of High-Resolution Spectra of Transient Molecules.- 2. Theoretical Aspects of High-Resolution Molecular Spectra.- 2.1 Molecular Rotation.- 2.1.1 Angular Momentum.- 2.1.2 Rotational Energy of a Rigid Body.- 2.2 Vibration-Rotation Interaction.- 2.3 Fine and Hyperfine Structures.- 2.3.1 Electron Spin and Electron Orbital Angular Momenta.- 2.3.2 Fine-Structure Hamiltonian.- 2.3.3 Hamiltonian Matrix Elements Appropriate for Hund's Case (a).- 2.3.4 ?-Type Doubling in 2? States.- 2.3.5 Hamiltonian Matrix Elements Appropriate for Hund's Case (b).- 2.3.6 Rotational Structure of a 2E Molecule.- 2.3.7 Hyperfine Interaction.- 2.3.8 Matrix Elements of the Hyperfine Interaction Appropriate for Case (a)?.- 2.3.9 Matrix Elements of the Hyperfine Interaction Appropriate for Case (b)?J.- 2.3.10 Hyperfine Interactions Caused by More than One Nucleus.- 2.3.11 Hyperfine Interactions in a C3v Symmetric Top Molecule.- 2.4 Vibronic Interaction Including the Renner-Teller Effect.- 2.4.1 Born-Oppenheimer Approximation and Vibronic Interaction.- 2.4.2 Renner Effect on a Linear Polyatomic Molecule in a ? Electronic State.- 2.4.3 Rotational Energy Levels of a Linear Triatomic Molecule Affected by Vibronic Interactions.- 2.4.4 Hyperfine Structure in a 2? Polyatomic Molecule Affected by Rovibronic Interactions.- 2.5 Zeeman and Stark Effects.- 2.5.1 Zeeman Effect of a Molecule Without Orbital Angular Momentum.- 2.5.2 Zeeman Effect of a Molecule with Electron Orbital Angular Momentum.- 2.5.3 Stark Effect.- 3. Experimental Details.- 3.1 Microwave Spectrometer.- 3.1.1 Requirements to be Met by a Microwave Spectrometer for the Study of Transient Molecules.- 3.1.2 Historical Survey of Microwave Studies of Transient Molecules.- 3.1.3 High Sensitivity Millimeter-Wave Spectrometer to Study Transient Molecules.- 3.2 Infrared Laser Spectrometers.- 3.2.1 Diode Laser Spectrometer.- 3.2.2 Infrared Laser Magnetic Resonance Spectrometer.- 3.2.3 Far-Infrared Laser Magnetic Resonance Spectrometer.- 3.2.4 Difference Frequency Laser Spectrometer.- 3.3 Dye Laser Spectroscopic System.- 3.3.1 CW Dye Laser.- 3.3.2 Doppler-Limited Excitation Spectroscopy Using a CW Dye Laser as a Source.- 3.3.3 Intermodulated Fluorescence Spectroscopy.- 3.4 Double Resonance Spectroscopy.- 3.4.1 Microwave or Radio Frequency Optical Double Resonance.- 3.4.2 An MODR Spectrometer to Study Transient Molecules.- 3.4.3 Infrared-Optical Double Resonance.- 3.5 Generation of Transient Molecules.- 4. Individual Molecules.- 4.1 Diatomic Free Radicals.- 4.1.1 2? and 3? Diatomic Molecules.- 4.1.2 2? Diatomic Molecules.- 4.1.3 Molecules in ? States.- 4.2 Linear Polyatomic Molecules.- 4.2.1 Molecules in ? Electronic States.- 4.2.2 Molecules in ? Electronic States.- 4.3 Nonlinear XY2- and XYZ-Type Triatomic Free Radicals.- 4.3.1 Nonlinear XYZ-Type Free Radicals of Cs Symmetry.- 4.3.2 Nonlinear XY2 -Type Free Radicals of C2v Symmetry.- 4.3.3 Molecular Structure and Anharmonic Potential Constants of Nonlinear XYZ-Type Molecules of Cs Symmetry.- 4.4 Symmetric Top and Other Polyatomic Free Radicals.- 4.4.1 The Methyl Radical CH3.- 4.4.2 The Trifluoromethyl Radical CF3.- 4.4.3 Structures and Internal Motions of the Methyl Radical and Its Derivatives.- 4.4.4 The NO3 Radical.- 4.4.5 The Methoxy Radical CH3O.- 4.5 Fine and Hyperfme Interactions in Free Radicals.- 4.5.1 Hyperfme Interaction Constants of Diatomic Molecules.- 4.5.2 Spin-Rotation Interaction in Nonlinear Polyatomic Molecules.- 4.5.3 Hyperfme Interaction in Nonlinear Polyatomic Molecules.- 4.6 Molecules in Metastable Electronic States.- 4.6.1 LEF and MODR Study of H2CS in the a3A2 State.- 4.6.2 Perturbations in the Singlet-Singlet Transition of a Few Simple Carbenes.- 5. Applications and Future Prospects.- 5.1 Applications to Chemical Reactions.- 5.2 Applications to Atmospheric Chemistry.- 5.3 Applications to Astronomy.- 5.3.1 HCCN.- 5.3.2 Phosphorus-Containing Compounds.- 5.4 Future Developments.- 5.4.1 Possible Improvements of Spectroscopic Techniques.- 5.4.2 Future Trends in High-Resolution Spectroscopic Studies of Transient Molecules.- References.