CHAPTER 6 - Key 6-1. (a) Coherent radiation is radiation that is made up of wave trains having identical frequencies or sets of frequencies and phase relationships that are constant with time. (b) Dispersion in a transparent medium is its variation in refractive index as a function of wavelength. (c) Anomalous dispersion is the sharp change in refractive index of a substance in a wavelength region where absorption is occurring. (d) The work function of a substance is a constant that measures the energy required to remove an electron from the surface of the substance. (e) The photoelectric effect involves the emission of electrons from the surface of a substance brought about by a beam of radiation. (f) The ground state of a molecule is its lowest energy state. (g) Electronic excitation is the process by which electrons in a substance are promoted from their ground state to higher electronic states by absorption of energy. (h) Blackbody radiation is the continuum radiation emitted by a solid when it is heated. (i) Fluorescence is a type of emission which is brought about by irradiating atoms, ions, or molecules with electromagnetic radiation. Fluorescence involves a singlet-to- singlet transition. The lifetime of the excited state in fluorescence is very short (10-5 s or less). !(j) Phosphorescence is a type of emission brought about by irradiating a molecular system with electromagnetic radiation. Phosphorescence involves a triplet-to-singlet transition and the excited state lifetime is longer than that of a fluorescing species. (k) Resonance fluorescence is a type of emission in which the radiation produced is of the same wavelength as that used to excite the fluorescence. (l) A photon is a bundle or particle of radiant energy with a magnitude of hν, where h is Planck’s constant and ν is the frequency of the radiation. (m) Absorptivity a is defined by the equation a = A/bc, where A is the absorbance of a medium contained in a cell length of b and concentration c. The path length b is expressed in cm or another specific unit of length. The concentration is expressed in units such as g/L. (n) The wavenumber of radiation is the reciprocal of the wavelength in centimeters.(o) Relaxation is a process whereby an excited species loses energy and returns to a lower energy state. (p) The Stokes shift is the difference in wavelength between the incident radiation and the wavelength of fluorescence or scattering. (p) The Stokes shift is the difference in wavelength between the incident radiation and the wavelength of fluorescence or scattering. 6-2: !!!6#5:!!!!6#6:!!!!6#10:!!!!"#$%&$'()*!+,!-%*.#/0)%.1(!2%1(3*$*4!5.6!)78! 961'.)#!5!!!:! ;<=! 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