The Absorption Of Light By Molecules

A. The Interaction of Light with Molecular Electronic Structure

A light wave is an electromagnetic disturbance traveling in a straight line with an in-vacuo speed (c) of 3.0 X 1010 cm/s. Perpendicular to the direction of travel of the wave there is an alternating electric field, and perpendicular to the direction of travel of the wave and to the plane of oscillation of the electric field vector there is an alternating magnetic field. The frequency of oscillation of the electric and magnetic field vectors is called the frequency of the light, v. The distance traveled by the wave during the period of one complete cycle of the electric vector is called the wavelength of the light, X. The speed, frequency, and wavelength of the light wave are related by the equation c = Xv (1)

Because of the electric field associated with light, an electron placed in the path of a light wave will experience a force and is capable of absorbing energy from the electric field of the light wave. If an electron belonging to a molecule in its ground electronic state absorbs energy from the electric field of a light wave, the electron will be promoted to an unoccupied orbital and will be transported from one site to another in the molecule. The net result will be that the molecule will have absorbed energy from the light and will be raised from the ground state to an electronically excited state. However, not all frequencies of light can be ab sorbed by a given molecule. Quantum theory tells us that the energy associated with one wavelength of light of frequency v is hc

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