Ultraviolet-visible absorption photonics

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. 

As we have seen, a necessary (but not sufficient) condition for a photochemical reaction is that a reactant, S, must be able to absorb light in the ultraviolet (UV)-visible range. Photon absorption produces a more reactive species, S. ... 

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. 

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). 

In the mechanism of a photochemical reaction, at least one step involves photons. The most important such step is a reaction in which the absorption of light (ultraviolet or visible) provides a reactive intermediate by activating a molecule or atom. The mechanism is usually divided into primary photochemical steps and secondary processes that are initiated by the primary steps. 

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. 

Many molecules which have absorption bands in the wavelength region of existing laser lines can be excited by absorption of laser photons into single isolated rotational-vibrational levels of the electronic ground state 1W>-103) (jn the case of infrared laser lines) or of an excited electronic state (with visible or ultraviolet lines)... 

---