Vacuum ultraviolet region

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... 

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... 

Usually, the ultraviolet and visible regions of the spectrum are recorded. Many of the most intense emission lines lie between 200 nm and 400 nm. Some elements (the halogens, B, C, P, S, Se, As, Sn, N, and O) emit strong lines in the vacuum ultraviolet region (170-200 nm), requiring vacuum or purged spectrometers for optimum detection. 

When the wavelengths exceed the values for which crystals are available, it is necessary to go to gratings, preferably in vacuum. The problems then begin to resemble those encountered in the vacuum ultraviolet region. 

The photodimerization of simple isolated olefinic bonds is rarely observed because of the absorption of these compounds in the high-energy or vacuum-ultraviolet region. One case reported is that of the photo-dimerization of 2-butene.<2) Irradiation of liquid cw-2-butene with light from a cadmium (A = 229, 227, 214 nm) or zinc (A = 214 nm) lamp was reported to lead to dimers (1) and (2) ... 

E. Hayon and J. McGarvey, Flash photolysis in the vacuum ultraviolet region of sulfate, carbonate, and hydroxyl ions in aqueous solutions. J. Phys. Chem. 71, 1472-1477 (1967). 

Nonmetals cannot generally be determined by direct measurement of atomic absorption in a flame because their absorption lines occur in the vacuum ultraviolet region where gases of the flame and atmosphere absorb strongly. Some can be determined by absorption of metastable lines. For example, phosphorous can be determined by the atomic absorption of a metastable line at 2135 A. A number of indirect methods for the determination of nonmetals have been described 19). 

Aromatic compounds have very high molar absorptivities that usually lie in the vacuum ultraviolet region and are not useful for routine analysis. Modest absorption peaks are found between 200 and 300 nm. Substituted benxene compounds show dramatic effects from electron-withdrawing substituents. These substituents are known as auxo-chromes since they do not absorb electromagnetic radiation but they have a significant effect on the main chromophore. For example, phenol and aniline have molar absorptivities that are six times the molar absorptivity of benzene or toluene at similar wavelengths. 

Perhaps all the elements present in the periodic table might be excited to yield respective emission spectra by employing a huge energetic source. However, it has a serious drawback because most of the spectral lines invariably fall within the vacuum-ultraviolet region thereby rendering their critical studies rather difficult. Hence, the emission spectroscopy is exclusively limited to metals and metalloids. The non-metals, for instance Phosphorus, Sulphur, Carbon etc. are not limited to these studies. 

Because of such difficulties as the featureless absorption and emission spectra in the vacuum ultraviolet region, very weak and energy-dependent fluorescence intensity, short excited-state lifetime, etc. the photophysics and photochemistry of alkanes is much less known than those of other organic molecules, for instance, aromatic hydrocarbons. In this chapter, the present status was reviewed. 

EFFECT ON WATER MOLECULE 5.1. Vacuum Ultraviolet Region... 

Though most of the oxygen in the atmosphere has been formed by photosynthesis in plants, some is produced by photolysis of water vapour in the vacuum ultraviolet region A <200 nm. Photolysis of N2, NO, N02, NHa, CO, 002 and small aliphatic hydrocarbons (alkanes) set up complex reactions in the upper atmosphere. 

CIO and BrO abundances are detected simultaneously and continuously as the airstream passes through the instrument. They are not detected directly but are chemically converted to Cl and Br atoms by reaction with reagent nitric oxide gas that is added to the airstream inside the instrument. The Cl and Br atoms are then detected directly with resonance fluorescence in the 2D5/2 -> 2P3/2 transitions in the vacuum ultraviolet region of the spectrum. In resonance fluorescence, the emissions from the light sources are resonantly scattered off of the Cl and Br atoms in the airstream and are detected by a photomultiplier tube set at right angles to both the light source and the flow tube. The chemical conversion reactions... 

Fig. VI —I. Absorption coefficients of water in the vacuum ultraviolet region. k is given in units of atm-.1 cm"base e, 0°C. From Watanabe and Zelikotf (1016), reprinted by permission. Copyright 1953 by the American Institute of Physics.

Since the photochemical reaction is initiated by absorption of light in the visible, ultraviolet, and vacuum ultraviolet regions, an understanding of atomic and molecular spectroscopy is required. Chapter I gives a brief introduction to the electronic stales and transitions in atoms and simple molecules. 

The absorption in the ultraviolet and visible regions consists of the Hartley bands (2000 to 3200 A), the Huggins bands (3000 to 3600 A), and the Chappuis bands (4400 to 8500 A). The absorption coefficients of these bands arc given in Figs. VI I2 andVI 12b. Figure VI 12c shows the absorption coefficients in the vacuum ultraviolet region. 

Predissociation is apparent below 3750 A. The absorption coefficients in the vacuum ultraviolet region 1300 to 1840 A have recently been measured by Basco and Morse (70). This is shown in Fig. VI 17. 

The absorption coefficients in the near and vacuum ultraviolet regions have been measured by Moule and Foo (721) and by Okabe ct al. (766) and are shown in Fig. VII 9. 

The ultraviolet region is important in spectroscopic studies since it energetically corresponds to electronic transitions in monatomic, polyatomic and free radical species. The vacuum ultraviolet region (1000—2000A), while significant for many laboratory spectroscopic applications, is relatively unimportant in relation to field applications of expls since atmospheric... 

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