Rare Gas Activities

Methods which have been used to count rare gas activities may be conveniently divided into several categories. The choice of method will be determined by the usual considerations of kind and energy of radiation, level of activity, half-life of activity, and type of information desired. 

1) Incorporation of the rare gas to be counted in the filling of a Geiger-Muller or proportional counter The use of this technique involves one in all the problems of counter construction and filling. The experimenter faced for the first time with these problems in detail must be referred to standard texts on the subject. Only a few comments will be made here. This is of 

If absolute counting must be attempted, corrections are necessary for end effects (electric field distortion at the tube extremities) and for wal) effects depending on the specific ionization of the radiation in the counter filling and the minimum number of ion pairs required for detection. Counters may be 

Counter fillings may be made using any of the rare gases and a suitable 

When low levels of Kr are to be counted it should be borne in mind 

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Dissociation of halomethanes, particularly CF4, by collisions with rare gas active species has also been studied under various experimental conditions. The cross sections of each product ion in the M+/CF4 (M = Ar, Ne, He) reaction system from thermal to 50 eV have been determined using guided ion-beam techniques72. It was found that the energy dependence of the cross sections can be understood by considering the energies needed to access specific electronic states of CF4+ 72. 

In a rare gas FA, metastable atoms, atomic ions, and dimer ions are present as rare gas active species [e.g. He(2 S), FIe (2 Si/2), and Hef in a He flow and Ar( Po,2), Ar" ( Pi/2,3/2), and ArJ in an Ar flow]. The contribution of active ionic species to the observed emissions was examined by applying an electrostatic potential (—50 to 50 V) to a pair of ion-collector grids placed between the discharge region and the reaction zone. The overlap of emissions due to ionic reactions with those due to neutral reactions has often made detailed analysis of each reaction difficult. This problem was overcome using an ion modulation technique, by which emissions from ionic reactions can be detected exclusively. " ... 

At Lawrence Radiation Laboratory these same tubes are used for counting and following the decay of a variety of rare gas activities. They are suit-... 

Indirect study of rare gas activities through their daughters ... 

The feasibility in specific instances of counting rare gas activities... 

L.Yu. Kupriyanov, Vzaimodeistvie Metastabilnykh Atomov Inert-nykh Gazov s Chstoy i Aktivirovannoy Metallami Povrkhostjru Okislov (Interaction of Rare Gas Metastable Atoms with Pure and Metal-Activated Surface of Oxides) Dissertation, Cand. Sci. (Phys.-Math.)Moscow, 1985, 140 p. 

The glass photolysis cell has a radius of. 75 cm and an active length of 10 cm. The CaF2 windows of the cell are protected from photoproducts by a curtain of rare gas which flows over the windows and out the exhaust ports without mixing with the sample gases. 

The reaction rate constant increases with increasing temperature (except for rare gas-phase reactions with negative activation energies), and may either... 

Several types of collision-induced light scattering spectra are known. We have already mentioned the depolarized translational spectra of rare gas pairs and bigger complexes which arise from the anisotropy of the diatom polarizability. Contrary to the infrared inactivity of like pairs, e.g., Ar-Ar like pairs are Raman active. Furthermore, polarized translational spectra... 

These lasers are also called—incorrectly— excimer lasers. It will be clear that they could be called exciplex lasers. The active material is a gas mixture which contains a halogen (F2 or Cl2 in most cases) and a rare gas such as Kr, Ar or Xe. These cannot form any stable compounds in their ground states, but excited state species do exist and can fluoresce. These excited state species e.g. KrF) are formed through the recombination of ions, for instance... 

In association reactions of this type, where a new bond is formed, the intermediate has excess vibrational energy equal to the bond energy of the newly formed bond and is thus unstable with respect to dissociation back to reactants unless stabilized by collision. The situation is very similar to that prevailing in neutral systems for atom-atom or radical-radical recombinations, as such larger systems are analogous to those studied by Rabinovitch and co-workers241-243 by chemical-activation methods. Colli-sional stabilization or deactivation may result from V-T transfer if the third body, Mit is monoatomic (a rare-gas atom) or from V-V transfer if it is polyatomic. 

A number of recent investigations have been concerned with the mobility of heavy atoms in rare gas matrices. Although not directly related to tunneling processes, they are concerned with important fundamental dynamics of atoms and small molecules in low-temperature solids, so we shall briefly review selected examples here. A typical experiment of this type includes the photolytic formation of atoms (see the review by Perutz [1985]) with subsequent detection of the decrease in atom concentrations due to bimolecular recombination. In most cases the rates are diffusion limited, and the temperature dependences are characteristic of thermally activated transfer. 

From the rare gas isolation studies of trisilver (8,9), it is thought that the Ag,0 structural isomer has an equilateral triangular form (trapped in a 60° trisubstitutional vacancy site) with an E electronic ground state. Its spectroscopy is best described in terms of a vibronically active, pseudo rotating triatomic, as calculated for Li, (23) and observed for K, (24). 

H-exchange reaction between CH3 and CH4 have been compared in the gas-phase and in rare gas solution (compressed Ar and Xe as solvents) over a wide range of internal pressures. Depending on the solvent and its internal pressure, relatively large rate enhancements have been calculated for this methyl/methane H-abstraction reaction. The major reason for this static solvent enhancement of rate in rare gas solutions seems to be the compact (tight) character of the activated complex, [CH . .. H ... 

Speaking of these isotope effects, the amazing observations that the presence of methane practically eliminates any activation of D2 by Cu and that the use of a molecular nitrogen matrix makes the CuD and D subproducts become almost negligible (using rare gas matrices these subproducts were almost as abundant as CuH and H) is one of the most attractive open questions in these experiments (Ozin, G.A. Gracie, C. and Mattar, S.M., unpublished data, 1984). 

An important implication is that collisional activation within the cluster can be described as a sequence of binary events. In other words, it is typically one rare gas atom at a time that undergoes a close in collision with a reactant, as illustrated in Fig. 5. 

The high relative velocities following impact of a cluster on a surface suggests that such dissociation processes can readily take place when a diatomic molecule embedded inside the cluster is activated by a collision. Molecular dynamics simulations show that beyond a threshold, the yield of dissociation of halogen molecules solvated in a rare gas cluster is a rapidly increasing function of the collision velocity and can reach 100%, see Fig. 6. This, unlike the surface impact induced dissociation of unclustered, cold, halogen molecules where the yield reaches a plateau of below 40%. ° ... 

Due to their long range attraction the reactants cluster together and the rare gas atoms surround the reactive molecules. The three roles of the cluster are as in the previous cases, to make sure that the reactions occur through the bulk of the cluster and not only in the layer nearest to the surface, to activate the reactants and to stabilize the products. 

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