Molecular and Laser Spectroscopy

The spectra of molecules containing more than one atom are necessarily of single atoms. They are correspondingly much more complex than those richer, not only in the number of spectral lines, but also in qualitatively different phenomena which do not have any counterpart in single atoms. Historically, molecular spectra have revealed much fundamental phy­ sics, such as the connection between nuclear spin statistics. They have pro­ vided models of physical systems which have been useful in quite different areas, such as particle physics. Most especially, molecular spectra are of fundamental importance in understanding chemical bonding. They reveal not only bond lengths but also the strength of the bonding potential between atoms. Moreover, these measurements are obtained for electronic excited states, as well as for the ground state, and for unstable short-lived molecules. In recent years, tunable lasers have provided powerful tools for the measurement and analysis of molecular spectra. Even before that, molecules were being used in lasers, most notably in the carbon dioxide laser, which finds many industrial applications.

1. Introduction.- 2. Molecular Energy States.- 2.1 The Molecular-Motion Equation, and the Hamiltonian Operator.- 2.2 Molecular Electronic States.- 2.3 Molecular Vibrational Levels.- 2.4 Molecular Rotational Levels.- 2.5 Molecular Vibration-Rotational Levels.- 2.6 Coupling of Molecular Rotation and Electronic Motion.- 2.7 Perturbations.- 2.8 Quadrupole Hyperfine Structure of Molecules.- 2.9 Magnetic Dipole Hyperfine Structure in Molecules.- 2.10 Isotopic Energy-Level Shifts.- 2.11 Molecular Rydberg States.- 3. Linear Molecular Spectroscopy.- 3.1 Infrared Pure-Rotational Spectra.- 3.2 Infrared Vibrational Spectra.- 3.3 Infrared Vibration-Rotational Spectra.- 3.4 Vibrational Band Systems of Diatomic Molecules.- 3.5 Rotational Spectra of Electronic Bands of Diatomic Molecules.- 3.6 Electric Quadrupole and Magnetic Dipole Hyperfine Spectra of Molecules.- 3.7 The Goals for Experimental Studies of Molecular Spectroscopy.- 3.8 Advances of Molecular Spectroscopy Through Linear Interaction of Molecules with Lasers.- 4. Spectral Characteristics of Molecular Two-Photon Transitions.- 4.1 Classification of Equal-Frequency Molecular Two-Photon Transitions.- 4.2 Excitation Probability of a Two-Photon Transition with One Near-Resonant Enhancing Level.- 4.3 Coarse Structure of Near-Resonantly Enhanced Molecular Two-Photon Absorption Spectra.- 4.4 Fine Structure of Near-Resonantly Enhanced Molecular Two-Photon Transitions.- 4.5 Line Shapes and Higher-Order Corrections for Near-Resonant Two-Photon Transitions in Three-Level Systems.- 4.6 Molecular Two-Photon Transitions Enhanced by Mixing Levels.- 4.7 Semiclassical Theory for a Two-Photon Transition in a Four-Level System.- 4.8 Coherent Effects on the Line Shape of a Near-Resonant Two-Photon Transition in a Four-Level System.- 5. Molecular Nonlinear Uncoupling Spectra with Doppler-Free Spectroscopy.- 5.1 Doppler-Free Saturation Spectroscopy and Its Development.- 5.1.1 Intensity Modulation Labelling.- 5.1.2 Frequency Modulation Labelling.- 5.1.3 Beam-Overlapping Modulation Labelling.- 5.2 Doppler-Free Polarization Spectroscopy and Its Development.- 5.3 Doppler-Free Two-Photon Spectroscopy and Its Development.- 5.4 Superhigh-Resolution Spectroscopy with Separated Fields.- 5.5 Applications of Nonlinear High-Resolution Laser Spectroscopy to Studies of Molecular Spectra.- 5.5.1 Accurate Measurements of the Properties of Perturbed Vibration-Rotational Bands in Diatomic Molecules.- 5.5.2 Accurate Studies of Molecular Hyperfine Spectra.- 5.5.3 Quantitative Study of Molecular Collision Mechanisms.- 5.5.4 Study of Spectroscopic Effects due to External Fields.- 5.5.5 General Features of the Applications of Doppler-Free Spectroscopy to the Study of Molecular Spectra and Molecular Structure.- 6. Molecular Nonlinear Coupling Spectral Effects.- 6.1 Background.- 6.2 Nonlinear Coupled Interaction in Three-Level Systems.- 6.3 Nonlinear Coupled Interaction in Four-Level Systems.- 6.4 Stimulated Diffuse Band Radiation via Various Excitation Processes.- 6.5 Stimulated and Coherent Radiation by Hybrid Excitation in Molecule-Atom Ensembles.- 6.5.1 Equal-Frequency Two-Step Hybrid Resonance Pumping.- 6.5.2, Unequal-Frequency Two-Step Hybrid Resonance Pumping.- 6.5.3 Two-Step Hybrid Off-Resonance Pumping.- 6.5.4 Four-Wave Mixing Following Two-Step Hybrid Excitation.- 6.5.5 Molecule-Atom Collisional Energy-Transfer Processes.- 6.6 Optically Pumped Stimulated Radiation Based on Molecular Electronic Transitions.- 6.7 Lasers Based on Molecular Photodissociation.- 6.8 Optically Pumped Far-Infrared Lasers Based on Pure Rotational Molecular Transitions.- 6.9 Optically Pumped Mid-Infrared Laser Based on Molecular Vibration-Rotational Transitions.- 6.10 Applications of Coherent Transient Spectroscopy in the Measurement of Molecular Parameters.- 6.10.1 Determination of Dipole Moments for Molecular Transitions.- 6.10.2 Lifetime of the Upper Level of a Molecular Transition.- 6.10.3 Determination of Transition Relaxation.- 6.10.4 The Fine or Hyperfine Splitting of Molecular-Spectral Lines.- 6.10.5 Study of Weak Intramolecular Coupling.- 6.10.6 Measurement of Ultrafast Inter- or Intra-Molecular Processes by Novel Transient Techniques.- 7. Simplification and Identification of Molecular Spectra.- 7.1 Laser-Induced Fluorescence.- 7.1.1 Formation and Classification of Resonant Fluorescence.- 7.1.2 Measurements and Applications.- 7.2 Population Labelling.- 7.3 Polarization Labelling.- 7.4 Two-Step Polarization Labelling.- 7.5 Modulated Polarization Two-Photon Spectroscopy.- 7.6 Molecular Energy Level Information Provided by Selective Simplified Molecular Spectra.- 7.7 Comprehensive Identification of Equal-Frequency Molecular Two-Photon Transitions.- 7.7.1 Approximate Numerical Calculations for Predicting Observable Molecular Two-Photon Absorption Frequencies.- 7.7.2 Experimental Methods.- 7.7.3 Examination of the Population Paths of a Fine Level for a Two-Photon Transition.- References.