Laser near ultraviolet

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. 

Until the advent of lasers the most intense monochromatic sources available were atomic emission sources from which an intense, discrete line in the visible or near-ultraviolet region was isolated by optical filtering if necessary. The most often used source of this kind was the mercury discharge lamp operating at the vapour pressure of mercury. Three of the most intense lines are at 253.7 nm (near-ultraviolet), 404.7 nm and 435.7 nm (both in the visible region). Although the line width is typically small the narrowest has a width of about 0.2 cm, which places a limit on the resolution which can be achieved. 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

A number of investigations of the copper-group oxides and dioxygen complexes have been reported. The electronic spectra of CuO, AgO, and AuO were recorded in rare-gas matrices (9), and it was found that the three oxides could be formed effectively by cocondensation of the metal atoms with a dilute, oxygen matrix, followed by near-ultraviolet excitation. The effective wavelengths for CuO or AgO formation were X > 300 nm and for AuO was X > 200 nm. In addition, the laser fluorescence spectrum of CuO in solid Ar has been recorded (97). 

There are therefore two ways in which lasers may be used to bring about photon-assisted film formation. If the laser emits radiation in the near-ultraviolet or above, photochemical decomposition occurs in the gas phase and some unabsorbed radiation arrives at the substrate, but this latter should be a minor effect in the thin film formation. This procedure is referred to as photolysis. Alternatively, if the laser emits radiation in the infra-red, and the photons are only feebly absorbed to raise the rotational energy levels of the gaseous... 

Ruby was the first material for lasers but several other crystals are now employed. The crystals used need to contain an impurity with an energy level such that return to the ground state is only possible by a forbidden transition in the infrared, visible, or near ultraviolet. It must also be possible to populate this level via an allowed (or at least less forbidden) transition. Research has tended to concentrate on transition metal ions and lanthanide ions in various hosts since these ions have suitable transitions of the... 

Primary photochemical processes of CH3I and CF3I have been studied extensively in conjunction with the laser emission I(2/>1/2) - I(2P3/2) + hv observed at 1.315 pm in the near ultraviolet Hash photolysis. Over 90" of I atoms produced from CF3I are in the 1Pl/2 state. 

Lasers generate a high-intensity output of monochromatic photon energy, and studies of the photochemical reactions induced by this have created a virtual subdivision of photochemistry known as laser chemistry While the output of a laser can heat, anneal, burn, cut. or be usedinstrumentally as a spectral source, we are concerned here only with those chemical effects attributable to the photon output at wavelengths between near infrared and near ultraviolet, i.e.. between about 12 and 0.2 microns. 

A great deal of work has been done on the photodissociation dynamics of the halogen containing compounds. This is partially due to the production of halogen atoms and radicals that can be studied by TOF and partially due to the fact there are intense absorption bands in the ultraviolet and near ultraviolet region. These reasons, coupled with the fact that the spectroscopy of some precursor molecules is fairly well understood, have resulted in many workers looking at these compounds. Finally, some of these compounds have been studied as possible candidates for various types of lasers. 

Precision measurement of energy intervals in hydrogen and helium has been fundamental to the development of atomic theory. Relativistic and quantum-electrodynamic contributions scale with various powers of Z. Hence more information is gained by extending precise measurements to one- and two-electron ions. Laser spectroscopy is restricted to certain special transitions which fall in the infrared, visible or near-ultraviolet, and from which a useful signal can be obtained. However, where applicable, it provides precision tests of theory. The focus of this review is laser spectroscopy of the n = 2 levels of moderate-Z helium-like and hydrogen-like ions. Previous reviews may be found in [1,2,3],... 

Nonlinear techniques have been used to overeome some of the drawbacks of conventional Raman spectroscopy, particularly its low dficienev, its limitation to the visible and near-ultraviolet regions, and its susceptibility to interference from fluorescence. A major disadvantage of nonlinear methods is that they lend to be analyte specific and often require several different tunable lasers to be applicable lo diverse species. I o dale, none of the nonlinear methods has found widespread application among nonspccialisls. However, many of these methods have shown considerable promise. As less expensive and more routinely useful lasers become available, nonlinear Raman methods, particu-larlv CARS, should become more widely used. 

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