A branch of chemistry in which reactions are induced or altered by laser light. The initial part of any photochemical reaction involves an optical transition to some excited state of molecule. These excited states could involve electronic, vibrational, and rotational excitation. The particular photochemical product that results from the absorption of light depends on the specific excited state species created during the irradiation. Thus the properties of the light source often determine the photochemical product. Lasers have had an immense impact on the field of photochemistry by providing scientists with an intense, polarized, and nearly monochromatic source of light. There are lasers that extend from wavelengths of less than 110 nanometers (vacuum ultraviolet) to more than 100,000 nm (far-infrared); for comparison, the entire visible spectrum extends from only 400 nm (violet) to 700 nm (red).