Fluorescence decays quickly (in microseconds or faster), and phosphorescence decays slowly (milliseconds to minutes). The decay time is the characteristic time it takes for the luminescence to disappear after the source of energy is removed or turned off. Absorption spectra can therefore be used to identify elements present in a gas or liquid. Every chemical element has absorption lines at various specific wavelengths corresponding to the differences between the energy levels of its orbitals. This light is called luminescence in general and fluorescence and phosphorescence in particular situations that are identified by the decay time of the luminescence and the nature of the excited state. An absorption spectrum is, in a sense, the opposite of an emission spectrum. Atomic absorption spectrometry (AAS) detects elements in either liquid or solid samples through the application of characteristic wavelengths of electromagnetic. In Chapter 4, we will use quantum mechanics to calculate transition moments for some molecules.Įnergy often is released from atoms, molecules, and solids as light. When an atom is excited, the valence electron moves up an energy level. Atoms can be excited when irradiated, which creates an absorption spectrum. Atoms have valence electrons, which are the outermost electrons of the atom. The absorption coefficient is of interest because it can be calculated from the transition moment, which is a quantum mechanical quantity. Figure 1.4.9: Austrian physicist Wolfgang Pauli (1900 - 1958). Also, the absorbance is proportional to a fundamental property, which is the absorption coefficient. The absorbance condenses large variations by using a logarithm so reasonably-sized graphs show both large and small variations in light intensity. Three different ways of plotting absorption spectra are used because each has particular advantages. Each of the quantities I, \(I_0\), and ε are functions of the wavelength of the light being used. The result is an absorbance spectrum that shows the intensity of emission as a function of wavelength. \) is a rearranged form of Beer’s law, as developed in a Problem at the end of this chapter.
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