Fast-moving

Figure A3.9.3. Time-of-flight spectra for Ar scattered from Pt(l 11) at a surface temperature of 100 K [10], Points in the upper plot are actual experimental data. Curve tinough points is a fit to a model in which the bimodal distribution is composed of a sharp, fast moving (lienee short flight time), direct-inelastic (DI) component and a broad, slower moving, trapping-desorption (TD) component. These components are shown...

A second idea to save computational time addresses the fact that hydrogen atoms, when involved in a chemical bond, show the fastest motions in a molecule. If they have to be reproduced by the simulation, the necessary integration time step At has to be at least 1 fs or even less. This is a problem especially for calculations including explicit solvent molecules, because in the case of water they do not only increase the number of non-bonded interactions, they also increase the number of fast-moving hydrogen atoms. This particular situation is taken into account... 

A gun is used to direct a beam of fast-moving atoms or ions onto the liquid target (matrix). Figure 4.1 shows details of the operation of an atom gun. An inert gas is normally used for bombardment because it does not produce unwanted secondary species in the primary beam and avoids contaminating the gun and mass spectrometer. Helium, argon, and xenon have been used commonly, but the higher mass atoms are preferred for maximum yield of secondary ions. 

When the incident beam of fast-moving atoms or ions impinges onto the liquid target surface, major events occur within the first few nanometers, viz., momentum transfer, general degradation, and ionization. 

The momentum of a fast-moving atom or ion is di.ssipated by collision with the closely packed molecules of the liquid target. As each collision occurs, some of the initial momentum is transferred to substrate molecules, causing them in turn to move faster and strike other molecules. The result is a cascade effect that ejects some of the substrate molecules from the surface of the liquid (Figure 4.2). The process can be likened to throwing a heavy. stone into a pool of water — some... 

A typical cascade process. A fast atom or ion collides with surface molecules, sharing its momentum and causing the struck molecules to move faster. The resulting fast-moving particles then strike others, setting up a cascade of collisions until all the initial momentum has been redistributed. The dots ( ) indicate collision points, tons or atoms (o) leave the surface. 

The fast-flowing narrow liquid stream has a high relative linear velocity with respect to the slower flow of the argon gas stream. This leads to breaking up the liquid stream into fast-moving droplets, which strike the impactor bead and form much smaller droplets. 

The electric fields in such instruments are used to focus the fast-moving ion beam according to the kinetic energies of the ions contained in it. This property allows ions of individual m/z values to be focused sharply before or after deflection in the magnetic field. 

Ionizing collision. An ion/neutral reaction in which an electron or electrons are stripped from the ion and/or the neutral species in the collision. Generally, this term describes collisions of fast-moving ions or atoms with a neutral species in which the neutral species is ionized. Care should be taken to emphasize if charge stripping of the ion has taken place. 

Superelastic collision. A collision that increases the translational energy of the fast-moving collision partner. 

Fig. 4. Schematic diagrams of (a) piston-anvil quenching (39), where A is the fixed anvil B, the fast-moving piston C, copper disks and H, photo cells (b)...

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

The cyliudricaTsection provides clarification under high centrifugal gravity. In some cases, the pool should be shallow to maximize the G-force for separation. In other cases, when the cake layer is too thick inside the cylinder, the settled solids—especially the finer particles at the cake surface—entrain into the fast-moving liqmd stream above, which eventually ends up in the centrate. A slightly deeper pool... 

This material Is purified by recrystallization from ethyl acetate acetone 2 1 (v v) to give a first crop (6.8 g), and by flash chromatography of the residue from the mother liquor, using 150 g of 230-400 mesh silica gel (Merck), a 40-mm diameter column, and elution with 10 1 (v v) ethyl acetate methanol. A fast moving orange band and a slower moving lemon-yellow band can be clearly seen on the column. The lemon-yellow hand is collected from the column and evaporation gives a second crop (1.4 g) of comparably pure material. The total yield of the pale yellow isoquinoline is 8.2 g (86t), mp 135-137°C (Note 10). 

Molecular structure theory is a fast-moving subject, and a lot has happened since the First Edition was published in 1995. Chapters 3 (The Hydrogen Molecule-ion) and 4 (The Hydrogen Molecule) are pretty much as they were in the First Edition, but 1 have made changes to just about everything else in order to reflect current trends and the recent literature. I have also taken account of the many comments from friends and colleagues who read the First Edition. 

The crude product is dissolved in benzene-hexane (1 1) and applied to a column containing 125 g. of silicic acid (Note 10). Elution with the same solvent gives traces (less than 5 mg. each) of the two fast-moving components in fractions 2 and 4 (125-ml. fractions) and chromatographically pure cholane-24-aI in fractions 5-8 (Note 11). Evaporation of the pooled fractions yields 870 mg. (84%) of the pure crystalline aldehyde, m.p. 102-104°. Recrystallization from 5 ml. of acetone raises the melting point to 103-104° (Note 12). 

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