Reactions of Other Metastable Atoms

The third contribution is particularly devoted to the concept of so-called single source precursors (SSPs). SSPs contain all the atoms of the different elements necessary for the deposition of the desired material in one single molecule. One motivation for using this concept is to simplify the accompanying gas-phase reactions and thus reduce the process parameters to be controlled and optimised. However, SSPs may offer a unique chance of depositing metastable materials that cannot be derived by other methods. M. Veith and S. Mathur provide such an example in their paper entitled Single-Source-Precursor CVD Alkoxy and Siloxy Aluminum Hydrides . 

Instead of carboxylic acids, other carbonyl compounds can be used acid halides, esters, amides, etc. The commonly accepted general mechanism for these reactions consists of the initial nucleophilic addition of an active hydrogen compound to the electron-poor carbonyl carbon atom of the R COOI I molecule, with the formation of a metastable intermediate that can undergo a subsequent elimination reaction ... 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

Figure 4. Schematic showing a hysteresis loop for the CdSe nanociystals with the smearing of the thermodynamic transition pressure caused by the finite nature of the nanocrystal particle. The thermodynamic transition pressure is offset from the hysteresis center to emphasize that in first-order solid-solid transformations, this pressure is unlikely to be precisely centered. The lower plot shows the estimated smearing for CdSe nanocrystals as inversely proportional to the number of atoms in the crystal, at two temperatures, as discussed in the text. Note that nanocrystals are not ordinarily synAesized or studied in sizes smaller than 20 A in diameter. This figure shows that this thermal smearing is insignificant compared to the large hysteresis width in the CdSe nanociystals studied (25-130 A in diameter), such that the transition is bulk-like from this perspective. This means that observed transformations occur at pressures far from equilibrium, where there is little probability of back reaction to the metastable state once a nanociystal has transformed. In much smaller crystals or with larger temperatures, the smearing could become on the order of the hysteresis width, and the crystals would transform from one stmcture to the other at thermal equilibrium.

Chemiluminescence from CaX species (X = halogen) arises from the reaction of copper with Xj molecules. For Cu + Fj, ground-state copper atoms ( 5) react to produce A, B, and C states of CuF, with an inversion of population of the C state relative to the B state observed, whereas metastable Cu atoms ( D) are responsible for chemiluminescent reactions with the other halogens. Emission from GaF and InF ( 11) is seen in reactions of the ground-state Group Ilia atoms with F2, with population inversions formed in the vibrational levels of these excited states the products of ground-state reactions of and... 

The source of controversy over such interpretations has been the observation that other reactions of resonant and metastable noble gas atoms, for which the simple long-range model is inapplicable, occur with similar rates. For instance, the He(3 S, 3 5, and 3 P) states have no allowed radiative transitions to He(l 5) and yet these states and He(3 F) are quenched by other noble gas atoms at similar rates. Similarly, NO and H quench Ar( / ) only about twice as rapidly as they do the metastable states Ar( Fo,2) and, in fact, the rate constants for quenching the metastable states of Ar, Kr, and Xe by all except the simplest reagents are remarkably high. 

MeasarmHt Atom CoBcentiadoiis Atomic Resomncc.—Hie mediod of atomic resonance spectrometry in the vacuuin-u.v., either in absorption or in fluorescence, has become one of the most useful direct methods for the measurements of reaction rates of ground- (and metastable excited- ) state atoms. The sensitivity and scope of atomic resonance in this respect rirab, and possibly surpasses, t t of other methods sudi as e.p.r. and mass spectrometry. No recent complete review of the atomic resonance method has been given although we do not propose to give a full account here, it is fdt useful now to provide a more complete summary tban for the othm methods described in Section 2. 

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