Electrostatics play a large role in the spectroscopy of condensed phases. In this section we review some of the basic definitions.
Polarizability
Molecular polarizability is a crucially important aspect of polarization. For example, solvent water has a relatively large dipole moment (1.86 Debye in the gas phase). However, water is polarizable as well, which means that it has an even large dipole moment in solution. The mutual interaction of water molecules has a profound effect on the properties of water. This type of effect is important for dielectric polarization discussed in the next section since the polarizability is related to the dielectric constant. In this section we discuss the experimental origin of that relationship in the Clausius-Mosotti and Debye equations.
Dielectric Polarization
Dielectric polarization gives rise to numerous important effects in spectroscopy. The bathochromic and hypsochromic shifts studied in Organic Chemistry coures have their origins in dielectric polarization. There are two major effects that need to be considered. The local field contains contributions from an internal field and a reaction field. The internal field, which is usally treated using the spherical cavity approximation to explain how the alignment of solvent dipoles in an applied electric field actally increases the field felt by the molecule. The reaction field, on the other hand, is present even when there is no applied field. It is an effect of the ordering of solvent dipoles around a solute that has a dipole moment.
Solvatochromism and Electrochromism
Solvatochromism is the tendency of a solvent to shift the absorption maximum of molecules that have a ground state dipole moment. The ground state dipole moment tends to polarize the solvent due to the reaction field, which in turn causes the energetic relationship of the molecule to change. Specifically, the difference dipole moment (Delta_mu = mu_excited - mu_ground) of the molecule determines whether the absorption spectrum shifts to the red (Delta_ma < 0) or the blue (Delta_mu > 0). Thus, the transition energy is affected. Electrochromism is the experimental approach to measurement of difference dipole moments and difference polarizabilities using an externally applied electric field.
