Energy selection

Fig. 3.3. Experimental arrangement used by Krauss and Gruen for SSIMS [3.8] a qua-drupole mass spectrometer was used for mass analysis and a retarding-field analyzer for prior energy selection (a) ion gun (b)-(d) lenses 1-3 (e) quadrupole mass spec-...

Energy Selecting Slit (Energy Dispersive j Plane)... 

A. Energy Select acetic acid from the molecules on screen, and then select Energy from the Properties menu. The energy of acetic acid (in atomic units or au) is displayed at the bottom of the screen. Click on Done. 

One method to study energy-selected ions is threshold ionization, in which ions with precisely defined energy contents are produced. These ions can then be used to study unimolecular fragmentation, ion-molecule reactions, van der Waals clusters, and hydrogen-bonded clusters [62]. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

R.E. Alvarez and A. Macovski, Energy selective reconstructions in x-ray computed tomography, Phys. Med. Biol., 21 (1976) 733-744. 

Activation energies are energy barriers to chemical reactions. These barriers are crucial to life itself. The rate at which a molecule undergoes a particular reaction decreases as the activation barrier for that reaction increases. Without such energy barriers, complex macromolecules would revert spontaneously to much simpler molecular forms, and the complex and highly ordered structures and metabolic processes of cells could not exist. Over the course of evolution, enzymes have developed lower activation energies selectively for reactions that are needed for cell survival. 

Br- (g). The electron affinity of Br (g) is calculable by the method of lattice energies. Selecting the crystal RbBr, because Rb+ and Br have exactly the same nuclear structure, and taking the exponent of the repulsive term to be 10, we have computed, for the reaction, RbBr (c) = Rb+ (g)+Br g), Dz= —151.2 whence the electron affinity of Br (g) becomes 87.9. Using the lattice energies of the alkali bromides as calculated by Sherman,1 we have computed the values 89.6, 85.6, 84.6, 83.6, and 89.6, respectively. Butkow,1 from the spectra of gaseous TIBr, deduced the value 86.5. From data on the absorption spectra of the alkali halides, Lederle1 obtained the value 82. See also Lennard-Jones.2... 

Fig. 10.1 Schematic drawing of a fast beam apparatus. A fast atomic beam enters from the left and is excited sequentially by two different C02 lasers in electric field regions Fj and F3, respectively. F2 avoids a zero field region between them. Ions produced by highly excited atoms being ionized in the biased microwave cavity are energy selected and detected by a Johnston particle multiplier (not shown). The output signal is detected in phase with the mechanically chopped Fj laser beam (from ref. 3).

Estimates of the kinetics of methyl loss from energy-selected CztHg" species have been made by calculation.23 The hydride transfer from alkanes to carbenium ions in the gas phase is calculated to involve a species with a symmetric potential well, which is different from the situation in superacid or zeolite media.24 A correlation between the charge on a carbon and the in-plane tensor component of its 13 C chemical shift has been observed for a number of simple cationic and anionic species.25 High-level calculations... 

Now the question arises, as before, of the degree to which the rare earth s valence electrons (and in particular 5d) are populated by hybridisation with the electron-rich 7T-MO system of the C72 host molecule. Figure 9 shows resonant photoemission spectra of Ce2 C72, with photon energies selected to span the N4>5 core level excitation spectrum, as indicated in Fig. 8. 

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