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Unformatted text preview: 44 Chem 24 Thurs Jan 13 Classical view of Spectrosopy A light field can interact with a dipole or it can induce a dipole Induced dipoles are most important Understanding the frequency and phase dependence of how light interacts with a dipole or induced dipole is the central picture you should get from this lecture 45 + + E =0 in no field Induced Dipoles and Electric Fields. The electric field can induce a dipole moment in a molecule the magnitude of in is proportional to the polarizeability of the molecule This can be a changing dipole moment that accompanies a molecular vibration It can be the reorganization of electronic structure when an electron moves from one orbital to another 46 + + E =0 in no field Induced Dipoles and Electric Fields. in is the same units as . An induced dipole responds essentially the same, at the instant in time that it is induced, as does the permanent dipole Thus, we can refer to and in similarly, from the point of view of how a dipole responds to an applied E field 47 Molecular Dipoles, Electric Fields, and Dielectric Constants. Debye showed that for molecules in a gas or in a dilute solution, and are related to the dielectric constant of the substance T ek N M B A 2 3 4 2 1 k B = Boltzmann Constant thus, at high T, dominates 3 4 2 1 A hF hF N M at high frequencies ( hF ), the permanent dipoles can no longer follow the field, and so the dielectric constant is again dominated by . hF = n 2 = square of the refractive index M = molecular weight = density see E&C ch 11 Molecular Dipoles, Electric Fields, and Dielectric Constants. T ek N M B A 2 3 4 2 1 k B = Boltzmann Constant thus, at high T, dominates M = molecular weight = density this is the static, or lowfrequency, dielectric constant 48 Molecular Dipoles, Electric Fields, and Dielectric Constants. T ek N M B A 2 3 4 2 1 k B = Boltzmann Constant M = molecular weight = density Note that if we measure as a function of T, then we can extract both and Measurements of the dielectric constant of a substance are important is typically measured through the use of a parallel plate capacitor 49 When the applied field has a low frequency, the oscillator follows the field exactly Low frequency means that the applied frequency << resonance frequency of oscillator At resonance, the oscillator lags 90 behind the applied field called 90 out of phase At high frequency, the oscillator lags 180 behind the applied field, and the response is very weak High frequency means that the applied frequency >> resonance frequecy of oscillator hanging meter stick demo General Response of an Oscillator to an applied AC field 50 Hb = hemoglobin 51...
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