Photochemistry can be defined as chemical reactions that are initiated by light. In order to photoreact, a molecule must absorb light energy. The molecules made by Professor McMahon and his group absorb light in the visible and ultraviolet region. This light energy causes them to photoreact to form other molecules while isolated in the argon matrix. In order to determine where a molecule absorbs light, an absorption spectrum can be taken. Peaks in the spectrum correspond to light energy that excites the molecule and causes it to react. Below is an absorption spectrum of a molecule the McMahon group studies. It has a peak with a maximum at 494 nanometers. This wavelength of light is in the visible region. If you shine light of this wavelength on the molecule, it will react to form another molecule!

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