Photon ultraviolet-visible absorption

All photochemical and photophysical processes are initiated by the absorption of a photon of visible or ultraviolet radiation leading to the formation of an electronically-excited state. 

Photochemistry is the branch of chemistry which relates to the interactions between matter and photons of visible or ultraviolet light and the subsequent physical and chemical processes which occur from the electronically excited state formed by photon absorption. 

The selective absorption of ultraviolet, visible and infrared radiation by molecules is explained in a descriptive manner that stresses how the noncontinuous energy requirements of chemical substances can only be satisfied by photons that have energy values equivalent to that of the differences in energy levels of the molecule in question. The meaning and quantitative significance of Beer s Law is briefly discussed. The components of a simple spectrophotometer are illustrated, accompanied by a demonstration of the operation of a spectrophotometer in the laboratory. Actual applications of the techniques of spectrophotometry are described during the presentation of relevent topics, for example, in drug identification. 

By far the most widely used method of generating electronically excited molecules is through the absorption of one photon of visible or ultraviolet light. The excitation can be performed directly or via a sensitizer, the latter often allows the controlled formation of excited states at a long wavelength. 

Luminance was measured with a luminance meter Topcon BM 8 at room temperature. EL spectra were taken with an optical multichannel analyzer (Hamamatsu Photonics PMA 10). Electronic absorption spectra were taken with a Shimadzu 2200A ultraviolet-visible spectrophotometer. 

The processes of excitation of a solute molecule A by absorption of a photon of ultraviolet light, and of fluorescence by emission of a photon of visible light, may be represented as... 

Three-photon absorption has also been observed by multiphoton ionization, giving Rydberg states of atoms or molecules [36]. Such states usually require vacuum ultraviolet teclmiques for one-photon spectra, but can be done with a visible or near-ultraviolet laser by tluee-photon absorption. 

Colorimetry, in which a sample absorbs visible light, is one example of a spectroscopic method of analysis. At the end of the nineteenth century, spectroscopy was limited to the absorption, emission, and scattering of visible, ultraviolet, and infrared electromagnetic radiation. During the twentieth century, spectroscopy has been extended to include other forms of electromagnetic radiation (photon spectroscopy), such as X-rays, microwaves, and radio waves, as well as energetic particles (particle spectroscopy), such as electrons and ions. ... 

Two-photon absorption has been observed in the microwave region with an intense klystron source but in the infrared, visible and ultraviolet regions laser sources are necessary. 

From comparison of the data presented in Table 2.2 [8], it is obvious that the energy of the microwave photon at a frequency of 2.45 GHz (0.0016 eV) is too low to cleave molecular bonds and is also lower than Brownian motion. It is therefore clear that microwaves cannot induce chemical reactions by direct absorption of electromagnetic energy, as opposed to ultraviolet and visible radiation (photochemistry). 

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