Cross-section, absolute

Irvin J A and Dagdigian P J 1980 Chemiluminescence from the Ca(4s3d D) + O2 reaction absolute cross sections, photon yield, and CaO dissociation energy J. Chem. Rhys. 73 176-82... 

Faubel M, Martinez-Haya R, Rusin L Y, Tappe U and Toennies J P 1997 Experimental absolute cross sections for the reaction F + D2 at collision energies 90-240 meV J. Phys. Chem. A 101 6415-28... 

Collisional ionization can play an important role in plasmas, flames and atmospheric and interstellar physics and chemistry. Models of these phenomena depend critically on the accurate detennination of absolute cross sections and rate coefficients. The rate coefficient is the quantity closest to what an experiment actually measures and can be regarded as the cross section averaged over the collision velocity distribution. 

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

The lab angular distributions shown in Fig. 12 contain information about the cross-section for each reaction. In practice, extraction of an absolute cross-section is difficult because of uncertainties in the number densities of the reactant beams and the ionization efficiencies of the products.130,135 However, in the determination of the product branching ratio, many of... 

Below the ODT such a label highlights the polymer-polymer interface. A main peak around Q" =0.02 A" corresponding to a lamellar periodocity 2 n/diain with di j =3l5 A is observed. Its visibility results from the asymmetric nature of the diblock. We note the existence of a second order peak, which is well visible at Todt=433 K. At large Q>Q the scattering is dominated by the form factor of the PEP-label in the environment of the deuterated monomers at the interface. This form factor may be described by a Debye function A)ebye( ) (Eq. 3.23). The absolute cross-section for these labels is given by ... 

The Intensity ratio between these satellite lines" and the main fluorescence line yields the absolute cross sections for the different collision-induced rotational transitions, provided the He pressure and the lifetime of the excited Na state are known. Both parameters have been measured, giving cross sections between = 1.5 A for A/ = 1 transitions and 6 = 0.3 A for A/ = 5... 

In most cases, except those in earlier comparative studies between the real-photon method and the dipole-simulation method, the absolute cross-section values obtained by both methods agree with each other [27]. Comparison of obtained cross-section values between the two methods were discussed in detail [27, 2, and references therein] and summarized in conclusion [5]. It should be noted, at least briefly, that it is essentially difficult to accurately obtain the absolute values of photoabsorption cross sections (u) in the dipole-simulation experiments, and it is necessary to use indirect ways in obtaining those values as the application of the TKR sum rule, Eq. (3), to the relative values of the cross sections obtained partly with theoretical assumptions. Moreover, in some cases, in relatively earlier dipole-simulation experiments, particularly of corrosive molecules upon their electron optics with poorer energy resolutions, serious discrepancy from the real-photon experiments was clearly pointed out in the obtained absolute values of photoabsorption cross sections [5,20,25-28]. 

As for the absolute values of photoionization quantum yields ( ],), a situation of the dipole-simulation experiments in comparison with the real-photon experiments is much more serious and controversial because their absolute scales are determined by the assumption that the photoionization quantum yields should be unity around 20 eV photon energy in addition to the above-mentioned difficulty in obtaining absolute cross-section values in the dipole-simulation experiments. 

Because of the uncertainties in the absolute cross sections for HOBr, both the DeMore et al. (1997) recommendation and the more recent values of Ingham et al. (1998) are given in Table 4.34. 

In summary, it may be concluded that the uncertainty in calculating absolute cross-sectional areas, the variation in cross-sectional areas with the BET C value and the fact that on porous surfaces less area is available for larger adsorbate molecules all point to the need for a universal, although possibly arbitrary, standard adsorbate. The unique properties of nitrogen have led to its acceptance in this role with an assigned cross-sectional area of 16.2 usually at its boiling point of —195.6 °C. 

Figure 39. Absolute cross section for excitation of NO[A (o = 0)]— NO(/4) transitions in H+-NO collisions as function of kinetic energy.288...

In order to determine absolute cross sections the ion yield /m+, defined... 

Schneider, H., Tobehn, I., Ebel, F. and Hippier, R. (1993). Absolute cross sections for inner shell ionization by lepton impact. Phys. Rev. Lett. 71 2707-2709. 

The calculation of absolute cross sections requires knowledge of the transition dipole function which, unfortunately, is rarely known. Therefore, all examples which we will discuss in this monograph are relative cross sections and the constant C will be mostly ignored in what follows. As stressed in Section 1.4, the total photodissociation cross section is the... 

Dub6, L. and Herzenberg, A. (1979). Absolute cross sections from the boomerang model for resonant electron-molecule scattering, Phys. Rev. A 20, 194-213. 

Bergmann, K. and Demtroder, W. (1972). Inelastic cross section of excited molecules. III. Absolute cross section for rotational and vibrational transitions in the Na2(B1IIu) state, J. Phys. B At. Mol. Phys., 5, 2098-2106. 

Consider a polymer solution consisting of protonated and deuterated polymers (concentrations CH and CD respectively) that have the same degree of polymerization N. The scattered intensity is proportional to the structure factors for the polymer-polymer correlations SHH(Q), SDD(Q), SHIj(Q), polymer-solvent correlations SpS(Q), and solvent-solvent correlations Sss(Q). The protonated, deuterated and solvent molecule scattering lengths are called aH, aD and as and ap is the average polymer scattering length. The scattered intensity can be expressed in terms of an absolute cross section as ... 

Frommhold L, Hong-Hong K, Proffitt MH (1978) Absolute cross sections for collision-induced depolarized scattering of light in krypton and xenon. Mol Phys 35 665-700... 

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