Unit 2- Module 1-1

Unit 2- Module 1-1 - Unit 2 Module 1 Analyzing Light-Matter...

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λ * ν = c c = Speed of light = 3.00 x 10 8 m/s Unit 2 Module 1 Analyzing Light-Matter Interactions John Pollard University of Arizona Understanding the interactions between light and matter is at the core of some of the most powerful analytical methods in science. Known as spectroscopy, the study of the interaction between radiation and matter is a powerful tool in the analysis of the composition and structure of particles that make up substances. Much of what we know about the structure of atoms and molecules derives from the information gathered from spectroscopy. In most cases, spectroscopy involves either the absorption or emission of electromagnetic (EM) radiation by matter. To understand these interactions, we have to first understand the fundamental properties of light (EM radiation). There are two primary models used characterize EM radiation. The first views EM radiation as waves comprised of an electric and magnetic component. The energy of the radiation is defined by the wavelength (λ) and frequency (ν) of the radiation. Given one, the other can be calculated as both are related to the speed of light which is a constant. Observations of how light interacts with matter prompted the development of a second model that treats EM radiation as packets or “quanta” of energy called photons. The amount of energy in a photon depends on the frequency and is defined by the equation E = h ν (where h is Planck’s constant = 6.626 x 10 -34 J·s ). Even though they are quite different, the equations of both models can be combined to express the wavelength of a photon as E = hc/ λ. One of the most important properties that emerges from the “particle” model of light is that the energy of photons are quantized, that is they come in discrete energy values equal to h ν . We find that the interaction between light and matter also occurs in a quantized fashion which has large implications on our view of the structure of atoms and molecules. Absorption and Emission Absorption spectroscopy measures the energy of EM radiation absorbed by a substance as well as the probability of the absorption process. The data are represented in various ways but typically plot the absorption cross section (the probability of the absorption process) vs. the wavelength of EM radiation absorbed. Sometimes the amount of light transmitted (fraction of light that passes through a sample) is plotted against wavelength. Both create spectra that are indicative of the sample being studied and the spectra produced can be like fingerprints for the identity of a substance.
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H - Emission Spectrum ( Section in the Visible ) 410 434 486 656 nm Emission spectroscopy measures the amount of and energy of EM radiation emitted by a substance either naturally or via a stimulus like heat. All elements have signature
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This note was uploaded on 12/08/2009 for the course CHEM 151 taught by Professor Staff during the Fall '08 term at Arizona.

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Unit 2- Module 1-1 - Unit 2 Module 1 Analyzing Light-Matter...

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