18_Spectrophotometry - Energy of photon = h h Plank’s...

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Energy of photon = h ! h - Plank’s constant ! - frequency (sec -1 ) "! = c c - speed of light As " increases, the frequency and energy of the photon decreases. Common " units μ m - 10 -6 m nm - 10 -9 m A - 10 -10 m Type wavelength Interaction # < 10 nm nuclear emission X-ray < 10 nm atomic ionization UV 10-380 nm electronic transitions Vis 380-800 nm electronic transitions IR 800nm-100 μ m bond interaction Radio meters nuclear absorption Light is absorbed by a substance only when the energy corresponds to some energy need or transition of a substance. Changes in our substance can be Electronic Vibrational Rotational The last two are only seen for molecules.
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Absorption of light is a complicated process. Each electronic state is subdivided into a number of vibrational sub-levels. In turn, each vibrational sub-level is further divided into rotational sublevels. Electronic levels (UV interactions) Vibrational sublevels (IR interactions) Rotational sublevels (IR interactions) With atoms, our simplest case, we still have a relative complex absorption process. Even for hydrogen, we end up with a complex line spectrum due to the major electronic transitions and the sublevels -s, p, d, f. With molecules, we not only have electronic but vibrational and rotational sub-levels. Interactions with other molecules and a solvent will also have an effect. These result in ‘band’ spectra.
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