Chemical Spectroscopy and Photochemistry in the Vacuum-Ultraviolet

It is probably safe to predict that the future of chemistry is linked to the excited states of molecules and to other short lived species, ions and free radicals. Molecules have only one ground state but many excited states. However large the scope of normal, ground state chemistry might be, above and beyond it lies the world of excited states, each one having its own chemis try. The electronic transitions leading to the excited states, either discrete of continuous, are examined in molecular elec tronic spectroscopy. Electronic spectroscopy is the queen of all spectroscopies: for if we have the resolution we have everything. Vnfortunately, the chemist who is interested in the structure and reactions of larger molecules must often renounce all that infor mation. The spectra are complex and often diffuse; resolution does not always help. To understand such spectra he must look at whole families of molecules; to some extent structural analogies help. Let us call this chemical spectroscopy and handle it with care. In order to understand the properties of molecules we also need theory. We know that molecular problems are, in principle, soluble by the methods of quantum mechanics. Present time quan tum chemistry is able to provide a nearly accurate description of not too large molecules in their ground states. It is probablY again safe to predict that the future of quantum chemistry is connected with molecular excited states or, generally spoken, the accurate handling of the open-shell problem.

Proceedings of the NATO Advanced Study Institute, Valmorin, Québec, Canada, August 5-17, 1973

Inhalt
Vacuum Ultraviolet and Photoelectron Spectroscopy Historical Background and Survey.- V.U.V. Spectra of Polyatomic Molecules.- Photoionization.- Photoelectron Spectroscopy.- References.- Aspects of Molecular Rydberg States.- References.- Photoelectron Spectroscopy.- - Electrons and Photons.- Some Experimental Aspects of Electron Energy Analysis.- Light Sources.- Photoelectron Bands Designated According to the Ionic States.- Vibrational Fine Structure.- Vibrational Fine Structure in the Third Band of Formaldehyde.- Vibrational Fine Structure in the Ketene Photoelectron Spectrum.- Vertical Ionization.- Interpretative Methods Involving Molecular Orbital Calculations.- Correlations between P.E. Spectral Data and Other Physical Parameters.- Photoelectron Spectra of Transient Species.- Photoelectron Spectra from Solids.- References.- Absolute Intensities and Classification of Transitions in Electron Impact Spectroscopy.- Generalized Oscillator Strengths.- Determination of Oscillator Strengths.- Oscillator Strengths Obtained by Extrapolation.- Optical Oscillator Strengths from Forward Scattering.- Classification of Transitions. Dipole-Quadrupole Behavior.- Franck-Condon Principle. Invariance in Relative Intensities.- Singlet-Triplet Transitions.- Strongly Forbidden Transitions at Low Kinetic Energies.- Identification of Triplets by Positive Ion Scattering.- Recent Results.- Deviations from Born Approximation at Small Scattering Angles. Selection Rules.- Singlet-Triplet Excitation at High Kinetic Energy. 11S?23S Transition in Helium.- Theoretical Study of Singlet-Triplet Selection Rules.- Other Singlet-Triplet Transitions.- Calculation of Singlet-Triplet Energy Differences.- Excited Triplet of H2O.- Other Recent Researches.- Addendum on Benzene.- References.- Photoionization of Free Radicals.- Methods of Production.- Available Data.- Hydrocarbon and Fluorocarbon Radicals.- Atmospheric Radicals.- O(3P).- N(4S).- OH.- O3 and HNO3.- References.- Photoelectron Spectroscopy of Molecular Beams.- and Chemical Studies.- Physical Studies.- Comparison of Photoelectron Intensities and Franck-Condon Factors in the Photoionization of H2, HD and D2.- Rotational Band Shapes in the Photoelectron Spectra of HF and DF.- Spin-Orbit Coupling and the Ratio of Photoionization Cross Sections in Some Atoms.- Summary and Conclusions.- References.- High Resolution Vacuum Ultraviolet Spectroscopy of Diatomic Molecules.- Instrumentation.- Spectrographs.- Sources.- Spectra.- Spectra of Valence States.- Spectra of Rydberg States.- BH.- Nitric Oxide.- Nitrogen.- Fluorine.- Hydrogen Fluoride.- Conclusions.- References.- Optical Properties of Liquids in the Vacuum UV.- Experimental Background.- Theory.- Carbon Ring Liquids.- Water.- Other Liquids.- Discussion.- References.- The Far Ultraviolet Absorption Spectra of Organic Molecules: Lone Pairs and Double Bonds.- Halogen Derivatives of Methane.- Halogen Derivatives of Ethane.- Ethylene Derivatives.- Benzene Derivatives.- Molecules Containing Divalent Sulfur.- References.- The Ethane Problem.- The Ground State.- The Ion.- The Ultraviolet Spectra.- Possible Weak Transitions.- The Primary Steps.- References.- A Pot-Pourri of Ultraviolet and Photoelectron Spectra of Iodides.- PE Spectrum of Vinyl Iodide.- UV Spectra of Alkyl Iodides.- PE Spectra of Fluoroiodoethanes.- References.- Beyond the Orbital Approximation.- The Need for a Language Adapted to Very Elaborate Wave-Functions.- The Loge Theory.- Change in the Localizability of Electrons during Ionization.- The Localizability of the Electron as a Starting Point to Calculate an Elaborate Wave-Function.- Loge Theory and Exciton Localizability.- References.- Natural and Magnetic Circular Dichroism Spectroscopy in the Vacuum Ultraviolet.- General.- Natural Circular Dichroism.- Magnetic Circular Dichroism.- Instrumentation.- The Natural CD Spectra of Benzene Chromophores.- The Natural CD Spectra of Ethylene Chromophores.- MCD Spectra of Benzene and Toluene.- References.- (H3C)2C=C(CH3)2, (H3C)2BN(CH3)2, (H3C)BF and (H3C)3B. Further Evidence for a ? ? ?* Assignment of the OLEFIN UV Mystery Band.- Helium (I) PE Spectra.- Far UV Spectra.- Concluding Remarks.- References.- Photoelectron Spectra of Moderate Sized Molecules.- The Interpretation of the PE Spectra of Structurally Similar Molecules.- A Comparison of the Bonding of First and Second Row p-Block Elements.- Photoelectron Studies of the Bonding of Particular Elements.- The Investigation of Through Space and Through Bond Interactions.- The Correlation of PES with UV Spectra and Other Techniques.- Theoretical Studies.- References.- HAM a Semi-Empirical MO Theory.- The HAM Method.- Shielding (preliminary form).- Electrostatic Interaction (preliminary form).- Results.- References.- Ab Initio Calculations for Excited States of Molecules.- Geometrical Aspects..- Use of Ab Initio Walsh-Type Diagrams.- Some Examples.- Calculation of Vertical Transition Energies.- Consideration of the Vibrational Structure of Electronic Transitions.- References and Footnotes.- Threshold Electron-Impact Spectroscopy.- Interaction of Electrons and Molecules.- Experimental Techniques.- Applications of Threshold Methods.- Saturated Hydrocarbons.- Olefins.- Other Unsaturated Hydrocarbons.- Molecules Containing a Carbonyl Group.- Molecules Containing a Nitrogen Atom.- Other Molecules.- Conclusions.- References.- Spectra of Free Radicals and Molecular Ions Produced by Vacuum Ultraviolet Photolysis in Low-Temperature Matrices.- General Principles.- Stabilization and Spectra of Free Radicals.- Stabilization and Spectra of Molecular Ions.- References.- Rydberg States of Diatomic and Polyatomic Molecules.- Atomic Calibration.- Results.- Conclusions.- References.- Theory of Intravalency and Rydberg Transitions in Molecules.- Questions.- Theory of Excited States, Non-Closed Shell States.- Semi-Internal Orbitals.- The Charge Wave Function, ?c of a Non-Closed Shell State.- Energy of a Non-Closed Shell State.- Relation of the Intravalency and Rydberg States of Spectra.- Classification of Transitions into Intravalency, Pre-Rydberg and Rydberg Types.- Spectral Form of the Charge Wave Function of an Intravalency State.- The Charge Wave Function of a Pre-Rydberg State.- Electric Dipole Oscillator Strengths of Intravalency$$ \left( {{\rm{\bar V}}} \right) $$ and Pre-Rydberg(pR) Transition…