Electromagnetic Radiation Absorption

Absorption of electromagnetic radiation is the process whereby the intensity of a beam of electromagnetic radiation is attenuated in passing through a material medium by conversion of the energy of the radiation to an equivalent amount of energy appearing within the medium; the radiant energy is converted into heat or some other form of molecular energy. A perfectly transparent medium permits the passage of a beam of radiation without any change in intensity other than that caused by the spread or convergence of the beam, and the total radiant energy emergent from such a medium equals that which entered it, whereas the emergent energy from an absorbing medium is less than that which enters, and , in the case of highly opaque media, is reduced practically to zero. No known medium is opaque to all wavelengths of the electromagnetic spectrum; similarly, no material medium is transparent to the whole electromagnetic spectrum. A medium which absorbs a relatively wide range of wavelengths is said to exhibit general absorption, while a medium which absorbs only restricted wavelength regions of no great range exhibits selective absorption for those particular spectral regions. For example, ordinary window glass is transparent to visible light, but shows general absorption for ultraviolet radiation of wavelengths below about 310 nanometers, while colored glasses show selective absorption for specific regions of the visible spectrum. The color of objects which are not self-luminous and which are seen by light reflected or transmitted by the object is usually the result of selective absorption of portions of the visible spectrum.

Lambert's law, also called Bouguer's law or the Lambert-Bouguer law, expresses the effect of the thickness of the absorbing medium on the absorption. If I is the intensity to which a monochromatic parallel beam is attenuated after traversing a thickness d of the medium, and I₀ is the intensity of the beam at the surface of incidence (corrected for loss by reflection from this surface), the variation of intensity throughout the medium is expressed by Eq. (1), in which a is a constant for the medium

I = I₀e^-αd (1)

called the absorption coefficient. This exponential relation can be expressed in an equivalent logarithmic form as in Eq. (2), where

log₁₀(I₀/I) = (α/2.303)d = kd (2)

k = α/2.303 is called the extinction coefficient for radiation of the wavelength considered. The quantity log₁₀(I₀/I) is often called the optical density, or the absorbance of the medium.

Beer's law refers to the effect of the concentration of the absorbing medium, i.e., the mass of absorbing material per unit of volume, on the absorption. This relation is of prime importance in describing the absorption of solutions of an absorbing solute, since the solute's concentration may be varied over wide limits, or the absorption of gases, the concentration of which depends on the pressure. The effects of thickness d and concentration c on absorption of monochromatic radiation can be combined in a single mathematical expression, given in Eq. (3), in which k' is a constant for a given absorbing substance

I = I₀e^-k'cd (3)

(at constant wavelength and temperature), independent of the actual concentration of solute in the solution. In logarithms, the relation becomes Eq. (4).

log₁₀(I₀/I) = (k'/2.303)cd = εcd (4)

The values of the constants k' and E in Eqs. (3) and (4) depend on the units of concentration. If the concentration of the solute is expressed in moles per liter, the constant F is called the molar extinction coefficient. Some authors employ the symbol a_M which is called the molar absorbance index, instead of ε.

If Beer's law is adhered to, the molar extinction coefficient does not depend on the concentration of the absorbing solute, but usually changes with the wavelength of the radiation, with the temperature of the solution, and with the solvent.

Absorption of radiation by matter always involves the loss of energy by the radiation and a corresponding gain in energy by the atoms or molecules of the medium. The energy absorbed from radiation appears as increased internal energy, or in increased vibrational and rotational energy of the atoms and molecules of the absorbing medium. As a general rule, translational energy is not directly increased by absorption of radiation, although it may be indirectly increased by degradation of electronic energy or by conversion of rotational or vibrational energy to that of translation by intermolecular collisions.

The energy acquired by matter by absorption of visible or ultraviolet radiation, although primarily used to excite electrons to higher energy states, usually ultimately appears as increased kinetic energy of the molecules, i.e., as heat. It may, however, under special circumstances, be reemitted as electromagnetic radiation. Fluorescence is the reemission, as radiant energy, of absorbed radiant energy, normally at wavelengths the same as or longer than those absorbed. The radiant reemission of absorbed radiant energy at wavelengths longer than those absorbed, for a readily observable interval after withdrawal of the exciting radiation, is called phosphorescence. Phosphorescence and fluorescence are special cases of luminescence, which is defined as light emission that can't be attributed merely to the temperature of the emitting body. top of page

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