Ultraviolet regions

Photon Chain. In the ultraviolet region quantum yields of over two (14) and as high as six (44) have been reported. Although the evidence seems to be sufficient to this author, Benson (11,13) does not believe that quantum yields of greater than two in dry ozone have been unequivocally demonstrated. He has pointed out that traces of water could greatly accelerate the rate of ozone decomposition. Nevertheless, Benson has postulated that a photon chain could yield a chain decomposition and account for quantum yields of over two, a proposal which was first made by Noyes and Leighton (64). 

The following argument was used by Benson (11) below 3100 A. (3080 A. by our calculation), sufficient energy is available for reaction (25), and at wavelengths shorter than 2680 A. (2660 A. by our calculation), reaction (26) is possible in either case a ) oxygen atom may be formed subsequently, 

Benson (13) later observes that although this reaction is about 4 kcal. endothermic, the lD oxygen atoms may have excess translational energy from 14 kcal. at 3130 A. to 46 kcal. at 2100 A. he also states that if any of this excess energy survives, it is possible to produce vibrationally excited (32M )02 which can then decompose rapidly ( 0.1 sec.) to reproduce ( D) O or else radiate to initiate a photon chain.  

Either of these processes, however, appears to be unlikely, and the first has been rejected by Benson (13). Although radiation from the 32u state is possible, it is precisely the vibrationally excited states of 32 which are known not to radiate. Emission bands in the Schumann-Runge system for v 2 have not been found (21,94,105), and the evidence is now strong that predissociation from v 3 accounts for the absence of these emission bands. Therefore, it is most probable that reac- 

Although it is possible that spin is not conserved and that 0(,D) may be formed at 3130 A., important differences were, in fact, found by Heidt (44) for the reaction at 3130 A. compared to that at 2080 A. The maximum quantum yield reported at 3130 A. was 3.5 while at 2080 A. it was 6.2, and the quantum yield at 3130 A. was independent of light intensity while at 2080 A. it was not. These differences may very well be due to the formation of 0(3P) rather than 0(10) at 3130 A. and would be particularly meaningful if the highest reported quantum yield of 3.5 was reduced to 2.0. 

---

Other atoms and molecules also show similar series of lines, often in the vacuum ultraviolet region, which fit approximately a similar fonuula ... 

Herzberg G, Lagerquist A and Malmberg C 1969 New electronic transitions of the C2 molecule absorption in the vacuum ultraviolet region Can. J. Phys. 47 2735-43... 

Ultraviolet and visible spectra arise from transitions between the electronic states in molecules. The terms electronic spectra and ultraviolet and visible spectra are synonymous and cover the range 200-800 mp.. The far-ultraviolet region 100-200 mp, only partially transmitted by quartz and appreciably absorbed by air, will not be considered. 

UV-VIS Aldehydes and ketones have two absorption bands in the ultraviolet region Both involve excitation of an electron to an antibonding tt orbital In one called a TT TT transition the electron is one of the tt electrons of the C=0 group In the other called an n ir transition it is one of the oxygen lone pair electrons Because the tt electrons are more strongly held than the lone parr electrons the transition is of... 

Emission spectroscopy is confined largely to the visible and ultraviolet regions, where spectra may be produced in an arc or discharge or by laser excitation. Absorption spectroscopy is, generally speaking, a more frequently used technique in all regions of the spectrum and it is for this reason that we shall concentrate rather more on absorption. 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

Typical recording spectrophotometers for the near-infrared, mid-infrared, visible and near-ultraviolet regions... 

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. 

An obvious difference between the emission spectra of most atoms and those we have considered so far is their complexity, the spectra showing very many lines and no obvious series. An extreme example is the spectrum of iron, which is so rich in lines that it is commonly used as a calibration spectrum throughout the visible and ultraviolet regions. 

The word laser is an acronym derived from light amplification by the stimulated emission of radiation . If the light concerned is in the microwave region then the alternative acronym maser is often used. Although the first such device to be constructed was the ammonia maser in 1954 it is the lasers made subsequently which operate in the infrared, visible or ultraviolet regions of the spectrum which have made a greater impact. 

In other regions of the spectmm, such as the infrared, visible and ultraviolet regions, the levels m and n are further apart but it turns out that the effects of saturation may be observed when N is high but considerably less than N. ... 

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. 

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. 

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

Instead of using a laser operating in the vacuum-ultraviolet region a laser operating at half the energy may be used. Then the ionization process in Figure 9.50(b) involves the... 

New to the fourth edition are the topics of laser detection and ranging (LIDAR), cavity ring-down spectroscopy, femtosecond lasers and femtosecond spectroscopy, and the use of laser-induced fluorescence excitation for stmctural investigations of much larger molecules than had been possible previously. This latter technique takes advantage of two experimental quantum leaps the development of very high resolution lasers in the visible and ultraviolet regions and of the supersonic molecular beam. 

Cerous salts in general are colorless because Ce " has no absorption bands in the visible. Trivalent cerium, however, is one of the few lanthanide ions in which parity-allowed transitions between 4f and Sd configurations can take place and as a result Ce(III) compounds absorb in the ultraviolet region just outside the visible. 

---