摘要:

The quantum dynamics of cryochemical solid-state reactions in the region of the low-temperature limit for the rate constant is considered in the framework of multidimensional nuclear tunnelling when low-frequency displacements of heavy particles lead to the formation of reaction complexes with high barrier transparency.

ISSN:0144-235X    

DOI:10.1080/01442359209353265

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

An overview of the studies of vibronic transitions in the rare earth ions is given with special attention to the intraconfigurational transitions. First the general theory is shortly reviewed, later that of the rare-earth vibronic transitions. The vibronic spectroscopy of Gd3+is extensively dealt with, because it is an ideal ion for this type of study due to its non-degenerate ground state. Vibronic coupling with several vibrational modes has been observed, also with modes in which the central ion is not directly involved. Attention has also been given to Eu3+and Pr3+and some other specific cases of interest. The use of Gd3+as a probe ion is pointed out. A comparison with relating physical phenomena is made. The experimental results are compared with the theoretical expressions derived up till now. The agreement is satisfactory, although of a qualitative nature. The concentration dependence of the vibronic intensity is not well understood. The role of opposite-parity configurations is probably of more importance than generally accepted and the admixture into the 4fnconfiguration not equal for all its levels.

ISSN:0144-235X    

DOI:10.1080/01442359209353266

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The dynamics of adsorption for molecular hydrogen on different metal surfaces (Ni, Cu, A1) is discussed in this review. Surface parameters like geometric and electronic structure, surface defects and impurities, surface coverage and surface temperature as well as gas parameters like translational, vibrational and rotational energy and angle of incidence influence the sticking probability of H2. On transition metal surfaces the sticking coefficient is rather high, whereas on noble and simple metals the sticking coefficient is exceedingly small. Only translationally and/or vibrationally hot molecules are able to adsorb on the latter surfaces. These features are discussed in the context of appropriate potential energy surfaces which govern the adsorption process. Experimental methods to determine absolute sticking coefficients as function of different gas parameters are also described.

ISSN:0144-235X    

DOI:10.1080/01442359209353267

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The effects of time lags on the kinetic behaviour of chemical systems are considered. Such lags can result from spatial separation of components of a system, from the introduction of a delayed feedback, or as a way of representing the effects of intermediate steps in a series of reactions. The mathematical apparatus for treating systems with delay is introduced. Then, a number of examples, both experimental and theoretical, are discussed. It is seen that time lags can produce major, qualitative changes in the behaviour of a system, and that it is sometimes possible to reduce significantly the number of variables needed to describe a system if one utilizes differential delay rather than ordinary differential equations.

ISSN:0144-235X    

DOI:10.1080/01442359209353268

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The paper considers the current status of the study of the transport and related effective cross-sections, for dilute molecular gases, that emerge from the kinetic theory as a unifying formulation of a number of diverse phenomena. The discussion concentrates upon the effective cross-sections that occur in the description of transport and relaxation phenomena in dilute gases and gas mixtures both in the absence of magnetic or electric fields and in their presence. The relationships of these phenomena to others such as light scattering, nuclear magnetic resonance relaxation and spectroscopic phenomena through effective cross-sections is illustrated. It is argued that, in general, experimental information on the effective cross-sections is either abundant or could be made so with little technological development and an investment only in time and effort. Equally the kinetic theory of dilute polyatomic gases is now both developed and secure. Thus the stage has been set for the application of the development of new methods of evaluating the effective cross-sections from prescribed intermolecular potentials that has characterised the last decade. These particular developments are reviewed in some detail to establish a set of guidelines for their application. While the guidelines are based on a limited set of studies they indicate the most fruitful paths for further progress towards the goal of understanding the role of particular molecular collisions in determination of specific observables which is otherwise concealed by the complicated averaging processes of the kinetic theory of gases.

ISSN:0144-235X    

DOI:10.1080/01442359209353269

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor