Macroscopic topics changes

Fluctuations in the dielectric properties near the interface lead to scattering of the EW as well as changes in the intensity of the internally reflected wave. Changes in optical absorption can be detected in the internally reflected beam and lead to the well-known technique of attenuated total reflectance spectroscopy (ATR). Changes in the real part of the dielectric function lead to scattering, which is the main topic of this review. Polarization of the incident beam is important. For s polarization (electric field vector perpendicular to the plane defined by the incident and reflected beams or parallel to the interface), there is no electric held component normal to the interface, and the electric field is continuous across the interface. For p polarization (electric field vector parallel to the plane defined by the incident and reflected beams), there is a finite electric field component normal to the interface. In macroscopic electrodynamics this normal component is discontinuous across the interface, and the discontinuity is related to the induced surface charge at the interface. Such discontinuity is unphysical on the molecular scale [4], and the macroscopic formalism may have to be re-examined if it is applied to molecules within a few A of the interface. 

The topic arises from the following sequence of aspects of entropy when entropy is introduced on a thermodynamic basis the issue is the motion of heat (Jaynes, 1988), and the assessment involves calorimetry an entropy change is evaluated. When entropy is formalized with the classical view of statistical thermodynamics, the entropy is found by evaluating a configurational integral (Bennett, 1976). But a macroscopic physical system at a particular thermodynamic state has a particular entropy, a state function, and the whole description of the physical system shouldn t involve more than a mechanical trajectory for the system in a stationary, equilibrium condition. How are these different concepts compatible ... 

A related topic is the issue of time scales. Dynamic simulations of atomic behavior generally require time steps that are short enough to capture the vibrational modes of the system, whereas changes at the macroscopic scale usually occur over vastly longer time scales. Coupling between such widely varying time scales is a very important challenge, but it is not within the scope of this review. However, the problem of multiple-time-scale simulations will be discussed briefly in the discussion of dynamical methods. 

In chemistry, the study of large, or macroscopic, systems involves thermodynamics in small, or microscopic, systems, it can involve quantum mechanics. In systems that change their structures over time, the topic is kinetics. But they all have basic connections with thermodynamics. We will begin the study of physical chemistry with thermodynamics the study of heat and work in chemistry. 

The chemical change as observed in a macroscopic experiment is the result of many molecular collisions. In Section 3.1.2 we define the reaction cross-section from the macroscopic observable rate of reactive collisions. This is sufficient to discuss the dependence of the cross-section on the collision energy. Then we discuss the temperature dependence of the reaction rate as arising from this energy dependence. To understand the origin of the energy needs of chemical reactions, we provide, in Section 3.2, a microscopic interpretation of the reaction cross-section. Finally, a caveat. In this chapter we are not taking accoimt of the internal structure of the reactants. This key topic has to wait imtil Chapter 5. The only concession is that an appendix provides an extension of chemical kinetics to the important special case where we do resolve internal states. 

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