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Lecture10 - Absorption of energy Energy is conserved in the...

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1 Physical Chemistry Lecture 10 Photochemistry and photophysics in biological systems Light and energry Electromagnetic radiation occurs in quanta called photons The absorption of emission of electromagnetic radiation results in an exchange of energy by a molecule with the field in space Energy absorbed by system = energy lost by field Energy states of a molecule are limited to certain energies (quantum condition) Photon energy determined by its frequency Many biological processes involve exchange of energy by emission or absorption Vision Photosynthesis Vitamin D synthesis Sunburn (!!!) Planck’s relation Energy of a photon related to frequency and wavelength in a vacuum May express energy in several different units Joules Wavelength, Inverse wavelength (called wavenumber), 1/ Frequency Circular frequency, Radial frequency, Conversion between units is easy Unit commonly used depends on the region of the electromagnetic spectrum 2 2 1 h E hc E c h E photon photon photon Absorption of energy Energy is conserved in the transfer of energy from the electromagnetic field to a molecule Energy absorbed by system = energy lost by field Energy states of a molecule are limited to certain energies (quantum condition) Emission is loss of energy as the system goes to a state of lower energy Absorption and emission only indicate energy differences between states of a system Types of energy states in a molecule Electronic Vibrational Rotational Translational photon molecule E E Energy and intensity Light is described by a flow of energy The strength of the field of light is measured by the intensity Flow of energy per unit area Units of watts/m 2 When passing through a medium, energy may be absorbed and some is transmitted Absorbed light causes molecular state change dt dE A I 1 Absorption and the Bouguer- Lambert-Beer law Absorption of energy from an electromagnetic field causes Change of molecular state Decrease in intensity
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