Subject rare gases

The spectroscopy of elementaiy systems such as atoms / molecules trapped in the volume or at the surface of small rare-gas clusters is a subject of continuous... 

Symmetry. For the dissimilar rare gas pairs, the subject matter of this Section, exchange symmetry of the atoms does not exist. We will return to a discussion of symmetry in the next Chapter, when discussing binary induced spectra of like molecular pairs. 

Among van der Waals complexes, the complexes of simple molecules with rare gas atoms has received a great deal of attention because the PES of these species can be reconstructed from spectroscopic data. Studies of isolated complexes in supersonic jets provide a unique opportunity to determine experimentally the parameters of complex multidimensional PES, which may have several minima, with accuracy approaching 0.01-0.001 cm-1 in some cases. This feat seemed to be a formidable challenge only a few years ago On the other hand, in view of their relative simplicity these complexes are reasonable subjects for quantum chemical calculations, so reliable comparisons of theory with experiment are possible. 

At this point we note that the overall form of the absorption, fluorescence emission and excitation profiles for Agx, Ag + and AgP+ for AgxNaX and AgxNaY is superficially reminiscent of those observed for Ag°, Ag2 0, and Ag3 ° entrapped in rare gas solids (4-10). However, a number of important differences are also apparent. These details are discussed for each silver guest as a necessary prelude to the subject of metal-support interactions. 

The reason we employ two rather distinct methods of inquiry is that neither, by itself, is free of open methodological issues. The method of molecular dynamics has been extensively applied, inter alia, to cluster impact. However, there are two problems. One is that the results are only as reliable as the potential energy function that is used as input. For a problem containing many open shell reactive atoms, one does not have well tested semiempirical approximations for the potential. We used the many body potential which we used for the reactive system in our earlier studies on rare gas clusters containing several N2/O2 molecules (see Sec. 3.4). The other limitation of the MD simulation is that it fails to incorporate the possibility of electronic excitation. This will be discussed fmther below. The second method that we used is, in many ways, complementary to MD. It does not require the potential as an input and it can readily allow for electronically excited as well as for charged products. It seeks to compute that distribution of products which is of maximal entropy subject to the constraints on the system (conservation of chemical elements, charge and... 

The latter systems have only two internal degrees of freedom, however, (in the rigid molecule approximation) and the rare gas dimers have just a single one, of course. Some ab initio studies have been made of molecular Van der Waals (or hydrogen bonded) systems with more internal coordinates but mostly they concern only specific points or one-dimensional cuts (e.g. distance curves for fixed molecular orientations) of the potential (hyper) surface. One exception is the case of the simplest molecular dimer (112)2, which has been studied in detail, both ab and experimentally Another exception form the two Van der Waals molecules, (C2H and ( 2)2, of which the complete potential surfaces have been obtained in our institute via ab initio calculations. The N2—N2 potential, in particular, has been the subject of much previous (s ni-) empirical work . The dimers (N2)2 and (C2HJ2 have been investi-... 

There is a trend away from the study of particular molecules, such as benzene, which have been regarded as being examples of specific chromophore properties which are essential for a complete understanding of behaviour in more complex situations. Simple molecules are however still important subjects for research. The lifetimes of AgO molecules in rare gas matrices have been... 

The powerful technique of two-photon absorption, which permits limitations of Doppler broadening to be overcome (see Chapter 1), has been used in a study on sodium atoms in which the Stark effect in the 5s 2Si and 4d D and D levels was observed.188 The radiative lifetimes of the S and D Rydberg levels of Na,189 the use of laser-induced resonance fluorescence for the measurement of small concentrations of Na vapour,170 the quenching of Na(32P) and K(42P) by N2, 02, H2, and HaO,171 the chemi-ionization reactions of photoexcited atoms,172 and excitation of the K(42P ) level in collisions with rare-gas atoms173 have been the subjects of recent reports. 

The electronic spectra of rare gas dimers have been a subject of interest for many years, mainly because these dimers are model systems for studying van der Waals interactions, and because of their potential as media for VUV and XUV lasers. Yet very little is known about the excited states of these dimers. Two experimental techniques were combined in our laboratory for this investigation four-wave summixing (4-WSM) and a pulsed supersonic jet to produce rotationally and vibrationally cold dimer molecules. In this way it was possible to resolve rovibronic structures in several isotopic band systems of Xe2, Kr2 and Ar2, in the region 150 to 104 nm, to determine the relevant molecular constants, and to calculate potential energy curves for the ground states and the three lowest (stable) excited states, for all three dimers [41,42,43]. 

Results of experimental studies of solids under pressure can be described by the equations of state (EOS). So many equations of state involving the temperature, volume and pressure of a condensed phase have been proposed and used that a full review of them is out of question. Besides books dealing with the subject in general, many papers are more concerned with some specific element such as carbon, or aluminium, or the rare-gas solids, or the behavior of solids under extreme pressures, etc. Useful compilations of EOSs are available [267-269]. The most popular among physicists are the EOSs of Murnaghan [270], Birch [271], Vinet et al. [272,273] and Holzapfel et al. [274-277]. The latter two EOSs have similar forms ... 

The review is not meant to be exhaustive. Rather, theory and experiment as they relate to a number of important systems are treated. The lattice dynamics of the rare gas crystals are discussed only as they relate to quantum lattice dynamics. For the interested reader, good and comprehensive reviews on this subject are available (Horton, 1968). 

It might be mentioned that there is quite a large empty space in the interior of the fullerene molecule. It has proven possible to trap various rare-gas atoms and small molecules within the fullerene interior, and the subject has been studied in some detail. Helium and argon, for example, can be inserted into the fullerene cage at high temperatures and pressures. The resulting structures are referred to as endohedral complexes, and they can be identified by mass spectrometry. ... 

The preceding are examples of some of the rich chemistry and kinetics that are still unknown about the rare gas halide lasers. Many other examples abound, but they are either not yet the subject of current research or there is still a significant amount of additional research needed. These include the kinetics of specific (v, J) states in XeF (a bound-bound laser transition), any quantitative understanding of ArF (one of the more potentially useful lasers), measurements of halogen bumup phenomena in KrF lasers, electron temperatures or distributions, the energy to produce an ion pair in actual laser mixtures, and a host of other challenging puzzles. On top of these "chemical physics" challenges there still remain numerous... 

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... 

The pivot of this presentation will be the interrelation of the electronic properties and their evolution with increasing density. Of central importance for this subject is the variation of the electron mobility with changing pressure and temperature. This is extensively dealt with in G.R. Freeman s lecture in this Proceedings and is therefore only briefly referred to here. Moreover, because of the limited scope, a wide and extensively studied field involving doped rare gas liquids and solids will also be only briefly mentioned a review of this field is included in the book by Schwentner et al., 1985. For the same reason. 

To show the main aspects of the phase diagram for rare gases adsorbed on graphite. The richness of the phases that are found and the differences between the phase diagrams for each rare gas have been the subject of many experimental, theoretical, and computer simulation studies. 

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