General Features of Spectroscopy

Case study 128 Vision Case study 128 Photosynthesis Case study 12.4 Damage of DNA by ultraviolet radiation Case study 128 Photodynamic therapy 

The same transition can be observed in absorption, when the incident radiation supplies a photon that can excite the molecule from its ground state to an excited state. 

The energy of a photon emitted or absorbed, and therefore the frequency, v (nu), of the radiation emitted or absorbed, is given by the Bohr frequency condition (Section 9.1)  

Here Ei and 2 are the energies of the two states between which the transition occurs and h is Planck s constant. This relation is often expressed in terms of the wavelength, X (lambda), of the radiation by using the relation 

The units of wavenumber are almost always chosen as reciprocal centimeters (cm ), so we can picture the wavenumber of radiation as the number of complete wavelengths per centimeter. The frequencies, wavelengths, and wavenumbers of the various regions of the electromagnetic spectrum were summarized in Fig. F.7. In this chapter we concentrate on vibrational and electronic transitions, which can be excited by the absorption of infrared and ultraviolet-visible radiation, respectively. 

Emission and absorption spectroscopy give the same information about enei level separations but practical considerations generally determine which technique is employed. Absorption of ultra violet and visible light is chiefly caused by electronic excitation, the spectrum provides limited information about the structure of the molecule. In order to obtain useful information from UV and visible range spectrum of a compound the wavelength of maximum absorption and the intensity of absorption must be measured accurately. The mechanics 

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