Quenching alkali atoms

Hooymayers HP, Alkemade CTJ (1966) Quenching of excited alkali atoms and related effects in flames Part II. Measurements and discussion. J Quant Spectrosc Radiat Transfer 6 847-874... 

The latter method has the advantage of low alkali-atom densities, thus avoiding radiation trapping and chemical reactions and allows selection of the initial kinetic energy of the A atom—subject, however, to some discussion about the velocity distribution and its relaxation before quenching. The excited atoms will loose their excitation energy be either spontaneous emission... 

The problem of quenching alkali resonance radiation in E-VR energy-transfer collisions with simple molecules is important as a model case for basic processes in photochemistry and serves its own right for a variety of practical applications, such as in laser physics. It has been studied for many years in the past, but only recent progress has led to information of the final internal energy of the molecule. In particular, crossed-beam experiments with laser-excited atoms allow a detailed measurement of energy-transfer spectra. There can be no doubt that the curve-crossing... 

Precisely why the alkali atoms exhibit self quenching behavior so different from the rare gases is not fully understood, although it is no doubt due to the longer range interaction of the alkali atom with the Rydberg electron, and perhaps the Rydberg ion core as well. 

From the calculated adsorbate-substrate bond length a substantial covalent character of alkali bonding to transition metal surfaces is deduced in the low coverage limit. A spin-polarized calculation shows that the unpaired spin of the alkali atom is almost completely quenched upon chemisorption. 

A considerable amount of data has been accumulated for alkali atom-molecule collisions. The velocity dependence of the total cross section has been measured by Kramer and LeBreton (1967), Helbing and Rothe (1968) and Rothe and Helbing (1969, 1970a, 1970b). For recent results along these lines see Diiren et al. (1972) and Hermann et al. (1973). The quenching of the... 

There is increasingly good evidence that this model is applicable to the reaction of the Rj states of Ar, Kr, and Xe with the halogen molecules, X2. The ionization potentials of these states are small (Table 1) and close to those of the alkali atoms. The total quenching cross-sections for their reactions with CI2 and Brj are very close to the reaction cross-sections of K, Rb, and Cs with these halogens. For Kr( R2) + Fj, the branching ratio for the formation of KrF is approximately unity and probably similar for Xe( R2) + F , CI2, and Brj (ref. 100). For... 

A summary of Zintl phases found in alkali-tin alloys has been reported by Fassler and Hoffmann (2003) together with the description of a new Na-Sn compound. It is Na7Sn12 which was synthesized by quenching stoichiometric amounts of the elements (700°C) in a sealed Nb ampoule and further thermal treatment at 270°C for 40 days. It was described as a Zintl phase suggesting that, on the basis of its structure, its formula can be rewritten as (Na )7[(2b)Sn2 ]1[(3b)Sn ]s [(4b)Sn°]6 (where 2b, 3b, etc. denote two-three-fold bonded atoms) resulting in a polyanion i(Sn]2). 

A common result of all the experiments is that most molecules quench the alkali resonance radiation very effectively with total cross sections ranging from 10 A2 to over 200 A2. However, if the molecule BC is replaced by a rare-gas atom, the quenching cross sections become very small at thermal energies. They are probably below 10 2 A2 for quenching by helium, neon, argon, krypton, and xenon.55 The latter result is easily understood in terms of Massey s adiabatic criterion.67 If Ar is a characteristic interaction range, v the impact velocity, and AE the energy difference between initial and final electronic states E(3p) and E(3s), respectively, then we must have a Massey parameter... 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

It was recommended that alkali-metal-NH3 reduction of ketones should be conducted in the presence of NH4+ this follows from the finding that reduction of [3,3-2H2]camphor and quenching in the absence of the ion gave epimeric alcohols with one or two atoms of tracer and complex products formed by disproportionation, abstraction of H from the medium, and pinacolic coupling. In the presence of NH4C1, exclusively [3,3-2H2]-alcohols were formed with Li, Na, or K. It was concluded that a mechanism proposed by House (1972) predominated when NH4+ was the proton source.120... 

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