Spectroscopy and Dynamics of Orientationally Structured Adsorbates

The following sections are included:

• Contents

• Preface

In the last decade studies on adsorbed molecules and the monolayers formed by them have been receiving a great deal of interest due to diverse features both of applied and fundamental theoretical significance. On the one hand, adsorption and catalysis are extensively used in microelectronics, semiconductor engineering, chemical industry, and biotechnology, which calls for a deeper insight into adsorbate states. On the other hand, a system of adsorbed molecules offers a manifold of facets to be studied: it can be regarded both as an impurity subsystem with respect to a corresponding solid surface and as a quasi-two-dimensional entity in its own right, with all the particular cooperative properties arising from low dimensionality. It is also noteworthy that in most cases an adsorbed molecule is tightly bound to a surface through a single atom, whereas the other atoms (further from the surface) retain considerable mobility. Symmetric arrangement of substrate atoms often admits of several equivalent positions for the atoms of adsorbed molecules, i.e. several equivalent molecular orientations. As a result, the systems concerned are distinguished by high reorientational activity. Provided that intermolecular lateral interactions forming a molecular ensemble are strong enough to remove the orientational state degeneracy for isolated adsorbed molecules, they give rise to orientational ordering. It is evident that changes in temperature may cause phase transitions between orientationally ordered and disordered phases in such systems…

The following sections are included:

• Brief survey of experimental data and theoretical approaches

• Orientational ordering in two-dimensional dipole systems

• Lateral interactions of nonpolar molecules and expressing them in a quasi-dipole form

• Ground states and phase transitions for a system of nonpolar molecules on a square lattice

• Orientational phase transitions in planar systems of nonpolar molecules

The following sections are included:

• Complex orientational structures and corresponding Davydov-split spectral lines

• Lattice-sublattice relations and their application to orientational structures on square and honeycomb lattices

• Chain orientational structures on planar lattices

The following sections are included:

• Resonant nature of low-frequency collective excitations of adsorbed molecules

• Markov approximation for Green's functions of molecular subsystems in the condensed phase
  • The exact solution for the exchange dephasing model
  • Valence-deformation vibrations of a molecular subsystem in condensed phase
  • Perturbation theory for the transition rates in the Pauli equation

• Generalization of the exchange dephasing model to various cases of anharmonic coupling between high-frequency and low-frequency modes, and to the case of collectivized excitations in adsorbate
  • Contribution of dipolar dispersion laws to dephasing of high-frequency collective vibrations
  • A simple model for collective high-frequency and low-frequency molecular modes

The following sections are included:

• Appendix 1 Local and resonance states for a system of bound harmonic oscillators

• Appendix 2 Thermally activated reorientations and tunnel relaxation of orientational states in a phonon field

• Appendix 3 The spectral function of local vibration (low-temperature approximation involving collectivized high-frequency and low-frequency modes)

• Bibliography

• Subject index

• Author index