Analytical Chem istry - DePauw University

548 Analytical Chemistry 2.0Table 10.1 Examples of Spectroscopic Techniques Involving an Exchange of EnergyBetween a Photon and the SampleRegion ofType of Energy Transfer Electromagnetic Spectrum Spectroscopic Technique aabsorption g-ray Mossbauer spectroscopyX-rayX-ray absorption spectroscopyUV/VisUV/Vis spectroscopyatomic absorption spectroscopyIRinfrared spectroscopyraman spectroscopyMicrowavemicrowave spectroscopyRadio waveelectron spin resonance spectroscopynuclear magnetic resonance spectroscopyemission (thermal excitation) UV/Visatomic emission spectroscopyphotoluminescence X-ray X-ray fluorescenceUV/Visfluorescence spectroscopyphosphorescence spectroscopyatomic fluorescence spectroscopychemiluminescence UV/Vis chemiluminescence spectroscopya Techniques discussed in this text are shown in italics.We can divide spectroscopy into two broad classes of techniques. In oneclass of techniques there is a transfer of energy between the photon and thesample. Table 10.1 provides a list of several representative examples.In absorption spectroscopy a photon is absorbed by an atom or molecule,which undergoes a transition from a lower-energy state to a higherenergy,or excited state (Figure 10.4). The type of transition depends on thephoton’s energy. The electromagnetic spectrum in Figure 10.3, for example,shows that absorbing a photon of visible light promotes one of the atom’sor molecule’s valence electrons to a higher-energy level. When an moleculeabsorbs infrared radiation, on the other hand, one of its chemical bondsexperiences a change in vibrational energy.Figure 10.4 Simplified energy diagram showing theabsorption and emission of a photon by an atom ora molecule. When a photon of energy hn strikes theatom or molecule, absorption may occur if the differencein energy, DE, between the ground state and theexcited state is equal to the photon’s energy. An atomor molecule in an excited state may emit a photonand return to the ground state. The photon’s energy,hn, equals the difference in energy, DE, between thetwo states.Energyabsorptionemissionexicted stateshν ΔE = hν ΔE = hνground statehν