Absorption cross section, chemical

Zirconium is used as a containment material for the uranium oxide fuel pellets in nuclear power reactors (see Nuclearreactors). Zirconium is particularly usehil for this appHcation because of its ready availabiUty, good ductiUty, resistance to radiation damage, low thermal-neutron absorption cross section 18 x 10 ° ra (0.18 bams), and excellent corrosion resistance in pressurized hot water up to 350°C. Zirconium is used as an alloy strengthening agent in aluminum and magnesium, and as the burning component in flash bulbs. It is employed as a corrosion-resistant metal in the chemical process industry, and as pressure-vessel material of constmction in the ASME Boiler and Pressure Vessel Codes. 

In a Mdssbauer transmission experiment, the absorber containing the stable Mdssbauer isotope is placed between the source and the detector (cf. Fig. 2.6). For the absorber, we assume the same mean energy q between nuclear excited and ground states as for the source, but with an additional intrinsic shift A due to chemical influence. The absorption Une, or resonant absorption cross-section cr( ), has the same Lorentzian shape as the emission line and if we assume also the same half width , cr( ) can be expressed as ([1] in Chap. 1)... 

The mechanisms for the NMHCs (except DMS) required to fully characterise OH chemistry were extracted from a recently updated version of the Master Chemical Mechanism (MCM 3.0, available at http //mcm.leeds.ac.uk/MCM/). The MCM treats the degradation of 125 volatile organic compounds (VOCs) and considers oxidation by OH, NO3, and O3, as well as the chemistry of the subsequent oxidation products. These steps continue until CO2 and H2O are formed as final products of the oxidation. The MCM has been constructed using chemical kinetics data (rate coefficients, branching ratios, reaction products, absorption cross sections and quantum yields) taken from several recent evaluations and reviews or estimated according to the MCM protocol (Jenkin et al., 1997, 2003 Saunders et al., 2003). The MCM is an explicit mechanism and, as such, does not suffer from the limitations of a lumped scheme or one containing surrogate species to represent the chemistry of many species. 

Mossbauer spectroscopy involves the measurement of minute frequency shifts in the resonant gamma-ray absorption cross-section of a target nucleus (most commonly Fe occasionally Sn, Au, and a few others) embedded in a solid material. Because Mossbauer spectroscopy directly probes the chemical properties of the target nucleus, it is ideally suited to studies of complex materials and Fe-poor solid solutions. Mossbauer studies are commonly used to infer properties like oxidation states and coordination number at the site occupied by the target atom (Flawthome 1988). Mossbauer-based fractionation models are based on an extension of Equations (4) and (5) (Bigeleisen and Mayer 1947), which relate a to either sums of squares of vibrational frequencies or a sum of force constants. In the Polyakov (1997)... 

The incorporation of Cr" + ions in crystals is presently an active research subject, due to the possibility of realizing new broadly tunable solid state lasers in the infrared, which will operate at room temperature. Moreover, the spectroscopic properties of this ion are particularly useful in the development of saturable absorbers for Q-switching passive devices. At the present time, Cr + YAG is the most common material employed as a passive Q-switch in Nd YAG lasers. This is because the ions provide an adequate absorption cross section at the Nd + laser wavelength (1.06 /um), together with the good chemical, thermal, and mechanical properties of YAG crystals, which are required for stable operation. 

A quantitative determination of two-photon absorption cross-sections from direct measurements of chemical yield was performed by Speiser and Kimel i ) who studied the two-photon-induced photolysis of iodoform with a Q-switched ruby laser. 

Again the radiative association kinetics described above allow a direct comparison for some realistic values of k and k. For most chemically activated systems at the threshold for unimolecular dissociation, the observed radiative rate constants are of the order of 10-100 s and hence are much below the values expected for k of about 10 s . Therefore, the first limit is most likely to be valid, with the interesting conclusion that the observed unimolecular dissociation rate constant will depend only on the photon density and the absorption cross section (rate constant) at a given wavelength. 

The most important applications of zirconium involve its alloys, Zircaloy. The aUoy offers excellent mechanical and heat-transfer properties and great resistance to corrosion and chemical attack. This, in conjunction with the fact that zirconium has a low neutron absorption cross section, makes this ahoy a suitable choice as a construction material for thermal nuclear reactors and nuclear power plants. Other uses are as an ingredient of explosive mixtures, as getter in vacuum tubes, and in making flash bulb, flash powder (historical), and lamp filaments, in rayon spinnerets, and in surgical appliances. 

Most commercial spectrometers report absorbance, as defined in Eq. (Q), versus wavelength. This is very important to recognize, since as we will see later, calculations of the rate of light absorption in the atmosphere require the use of absorption coefficients to the base e rather than to the base 10. While the recent atmospheric chemistry literature reports absorption cross sections to the base e, most measurements of absorption coefficients reported in the general chemical literature are to the base 10. If these are to be used in calculating photolysis rates in the atmosphere, the factor of 2.303 must be taken into account. 

Different types of chemical reactions involve different types of vibrational modes, e.g. dissociation reactions may be controlled by stretching vibrations, isomerizations by skeletal modes, and so on. The argument that infrared quanta are relatively energy-poor and infrared transitions generally have low absorption cross sections, especially if multiphoton excitation is required, limits the choice of suitable molecular transitions. With respect to these constraints the type of reaction chosen and described below was dissociation, involving molecules with maximal transition dipole moments, comparatively weak bonds to be broken, and vibrational excitation in the mid-infrared spectral range. 

Fig. Vl-2. (a) Absorption coefficients of H,S in the region 1800 to 3000 A. k in units of mm 1 cm 1, base 10, room temperature. 10 3 (mni 1 cm- ), base 10 corresponds to 1.91 (atm 1 cm 1), base e. From Goodeve and Stein (410), reprinted by permission of The Chemical Society. (A) Absorption cross sections of U2S in the region 1200 to 2000 A. a is given in units of 10 111 cm1, base e, room temperature. From Watanabe and Jursa (1018), reprinted by permission. Copyright 1964 by the American Institute of Physics.

Fig. VII-24. Absorption cross sections of chlorine nitrate in the near ultraviolet a is in units of cm2 molec"1, base e, room temperature. Reprinted with permission from F. S. Rowland, J. E. Spencer, and M. J. Molina, J. Phys. Chem. 80, 2711 (1976). Copyright by the American Chemical Society.

The alkylnitrites have large UV absorption cross-sections, comparable to that of HONO [31]. These compounds can be prepared in pure form and are reasonably stable both in the vapor and liquid phases when kept in the dark. As will be illustrated later, the selection of a specific RONO for a given experiment is based largely on the consideration of potential chemical and/or analytical interference problems arising from the accompanying carbonyl products. 

If, in centrosymmetric molecules, states to which a transition is forbidden in the normal absorption spectrum can make important contributions to x(3)> this suggests a strategy for enhancing the figure of merit, x(3)/a> of such a nonlinear material. Chemically introducing low lying states with gerade symmetry and thus small or zero absorption cross sections has the potential to enhance the x(3) but not increase the absorption probability, a. 

However, what unite all applications of NIRS for PAC are the unique features of the NIR spectrum. The NIR is in effect the chemical spectroscopy of the hydrogen atom in its various molecular manifestations. The frequency range of the NIR from about 4000 cm-1 up to 12 500 cm-1 (800-2500 nm) covers mainly overtones and combinations of the lower-energy fundamental molecular vibrations that include at least one X—H bond vibration. These are characteristically significantly weaker in absorption cross-section, compared with the fundamental vibrational bands from which they originate. They are faint echoes of these mid-IR absorptions. Thus, for example, NIR absorption bands formed as combinations of mid-IR fundamental frequencies (for example v + u2), typically have intensities ten times weaker than the weaker of the two original mid-IR bands. For NIR overtone absorptions (for example 2v, 2v2) the decrease in intensity can be 20-100 times that of the original band. 

For most small systems the chemical and electronic channels are rather obvious and therefore the labels 7 and e will be omitted hereafter. Normally, one measures integral absorption cross sections,... 

The quantum chemical calculations on Fe(CO)5 indicate that the lowest-energy accessible excited state is MC in character. The oscillator strength for the ground state to MC transition is small and most ultrafast experiments use either single or multiphoton excitation to MLCT states. The difference in product distribution depending on the excitation pulse duration points to an enhanced absorption cross section for the Fe(CO)5 excited state over the ground-state species to both pump and probe pulses. This tends to complicate the apparent photochemistry. Consequently the use of short-pulse single-photon excitation provides a better picture of the excited-state dynamics. 

Photons are emitted by the sun. When a molecule absorbs a photon it may dissociate breaking one or more chemical bonds. The rate of a reaction in the atmosphere, e.g. ABC + hv A + BC, depends on the absorption cross section of the molecule ABC, the photolysis quantum yield and the actinic solar flux, shown in Figure 7.1, all of which are wavelength dependent. 

We have investigated the atmospheric implications ot our newly calculated absorption cross sections with the Garcia-Solomon 2D dynamical/chemical model [85,86], to which we have added sultur chemistry and aerosol microphysics [87]. The model spans 56 pressure levels trom 2 to 112 km above sea level, and 36 latitudes trom 89.5°S to 89.5°N. Further details ot the sultur chemistry and... 

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