Status and Future Developments in the Study of Transport Properties

Transcription

1 Status and Future Developments in the Study of Transport Properties

2 NATO AS. Series Advanced Science Institutes Series A Series presenting the results of activities sponsored by the NA TO Science Committee, which aims at the dissemination of advanced scientific and technological knowledge, with a view to strengthening links between scientific communities. The Series is published by an international board of publishers in conjunction with the NATO Scientific Affairs Division A Life Sciences B Physics C Mathematical and Physical Sciences D Behavioural and Social Sciences E Applied Sciences F Computer and Systems Sciences G Ecological Sciences H Cell Biology I Global Environmental Change Plenum Publishing Corporation London and New York Kluwer Academic Publishers Dordrecht, Boston and London Springer-Verlag Berlin, Heidelberg, New York, London, Paris and Tokyo NATO-PCO-DATA BASE The electronic index to the NATO ASI Series provides full bibliographical references (with keywords and/or abstracts) to more than contributions from international scientists published in all sections of the NATO ASI Series. Access to the NATO-PCO-DAT A BASE is possible in two ways: - via online FILE 128 (NATO-PCG-DATA BASE) hosted by ESRIN, Via Galileo Galilei, Frascati, Italy. - via CD-ROM "NATO-PCO-DATA BASE" with user-friendly retrieval software in English, French and German ( WTV GmbH and DATAWARE Technologies Inc. 1989). The CD-ROM can be ordered through any member of the Board of Publishers or through NATO-PCO, Overijse, Belgium. Series C: Mathematical and Physical Sciences - Vol. 361

3 Status and Future Developments in the Study of Transport Properties edited by W. A. Wakeham Department of Chemical Engineering and Chemical Technology, Imperial College, London, UK A. S. Dickinson Department of Physics, University of Newcastle, Newcastle upon Tyne, U.K. F. R. W. McCourt Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada and V. Vesovic Department of Chemical Engineering and Chemical Technology, Imperial College, London, UK Springer-Science+Business Media, B.V.

4 Proceedings of the NATO Advanced Research Workshop on Status and Future Developments in the Study of Transport Properties Porto Carras, Halkidiki, Greece May 29-31, 1991 Library of Congress Cataloging-in-Publication Data ISBN ISBN (ebook) DOI / Printed on acid-free paper All Rights Reserved 1992 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1992 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

5 Table of Contents Preface... vii Overview on Intermolecular Potentials A. van der Avoird... 1 Traditional 'Transport Properties W. A. Wakeham and V. Vesovic Classical Path Methods for Lineshape Cross Sections J. M. Hutson... " Crossed Beam Studies M. Faubel Status of Kinetic Theory F. R. W. McCourt Overview on Experimental Data from Senftleben-Beenakker Effects and Depolarized Rayleigh Scattering L. J. F. Hermans Elastic and Inelastic Cross-Sections from Laser Studies of Small Molecules A. J. McCaffery Atomic Ion/Molecular Systems L. A. Viehland Classical and Semi-classical 'Treatment of Energy Transfer in Small Molecules G. D. Billing Generalized Cross-Sections for Senftleben-Beenakker Effects and Laser Studies of Molecules W.-K. Liu From Line-Broadening to Van der Waals Molecules: Complementary Ways to Probe the Anisotropic Interaction Ph. Brechignac Calculation of Pressure Broadened Spectral Line Shapes Including Collisional 'Transfer of Intensity S. Green Concluding Remarks J. J. M. Beenakker List of Participants Index

6 PREFACE This volume contains the fourteen papers presented at the NATO-sponsored Advanced Research Workshop on the 'Status and Future Developments in the Study of Transport Properties' held in Porto Carras, Halkidiki, Greece from May 29 to May 31, The Workshop was organised to provide a forum for the discussion among practitioners of the state-of-the-art in the treatment of the macroscopic, non-equilibrium properties of gases. The macroscopic quantities considered all arise as a result of the pairwise interactions of molecules in states perturbed from an equilibrium, Maxwellian distribution. The non-equilibrium properties of gases have been studied in detail for well over a century following the formulation of the Boltzmann equation in Since then the range of phenomena amenable to experimental study has expanded greatly from the properties characteristic of a bulk, non-uniform gas, such as the viscosity and thermal conductivity, to the study of differential scattering cross-sections in molecular beams at thermal energies, to studies of spectral-line widths of individual molecules and of Van der Waals complexes and even further. The common thread linking all of these studies is found in the corresponding theory which relates them all to the potential energy function describing the interaction of pairs of molecules. Thus, accompanying the experimental development there has been a corresponding improvement in the theoretical formulation of the quantities characterising the various phenomena. In 1973 a seminar was held at Brown University, Providence, USA to mark the centenary of the publication of the Boltzmann equation. At that time, one of the principal topics of discussion was the use of the available macroscopic information to elucidate the forces between atomic systems, most especially between the noble gas atoms helium, neon, argon, krypton and xenon. The theory of the behaviour of such systems was well understood but the means of measuring accurately the traditional transport properties such as viscosity and thermal conductivity, of studying molecularbeam scattering and making spectroscopic measurements on Van der Waals' dimers were still new. However, just about ten years later, it was possible to assert that the forces of interaction among the noble gases were essentially known, although careful work continues today to obtain the most refined atomic potentials. Naturally, since that time, there has been a steadily increasing effort devoted to achieving the same success for polyatomic molecular systems. However, the topic has proved both richer and more complicated than might have been thought originally. Because polyatomic molecules possess internal energy in a variety of modes which can be exchanged upon collision and because intermolecular interaction is not spherically symmetric, a whole new set of macroscopic phenomena exist. Their effects are varied in character and can be studied by diverse techniques encompassing observations of Senftleben-Beenakker effects on the viscosity and thermal conductivity of a gas in a magnetic field, spectroscopic measurements on Van der Waals' complexes, laser spectroscopy of interacting small molecules, drift-tube mobility studies of atomicion/molecule systems, state-to-state scattering cross-sections in molecular beams and spectral line broadening. A consequence of this variety of phenomena has been a vii

7 viii divergence of direction among the workers in the field, often into different disciplines, in an attempt to circumvent the barriers to progress and, in particular, to develop new experiments to probe new phenomena. Thus, while the intermolecular pair potential has remained the common goal, it has often been, apparently, a secondary motivation. The same complicating characteristics of polyatomic molecules make the calculation of the results of their encounters very much more difficult than for atomic systems. On approximately the same timescale, developments in high-speed computers and in their appplication to the treatment of molecular collision processes have rendered possible what had hitherto been impossible. That is, it is now nearly possible to evaluate routinely, for assumed intermolecular potentials, many of the quantities that characterise the observable phenomena. This Advanced Research Workshop was therefore conceived by the organizing committee to be an ideal opportunity to bring together experts from the various disparate fields to pool their experiences and ideas with a view to re-emphasising the commonality of their endeavours. Indeed it was thought that it might even be possible to agree upon a set of representative systems to be studied by all techniques. To this end each of the papers presented was intended to provide the stimulus for lively debate - and so it proved. As the concluding remarks of Professor J.J.M. Beenakker make clear, the workshop confirmed that we are now at a turning point in the field of molecular collision processes when much of the experimentation is complete and we must await the interpretation of the results. It seems, though, that each experimental technique has its own optimum molecular systems so that there is less complementary information available than one would wish. Nevertheless, as the papers contained in this volume demonstrate, the immediate outlook for an understanding of the interaction of relatively small molecular systems is quite positive. The Advanced Research Workshop therefore accomplished its aims, and all of the participants are indebted to NATO for the financial support that made it possible. The success of the event was the result of the combined efforts of a number of groups of people. Firstly, there are the speakers, who provided thought-provoking oral presentations at the meeting and so promptly delivered written versions of their material. Secondly, there are the other participants who contributed to stimulating and valuable discussions. Finally, the management and staff of the Village Inn, Porto Carras also deserve the thanks of the participants for their warm, efficient hospitality. Special thanks are due to the members of the organising committee: Alan Dickinson, Fred McCourt, and particularly Marc Assael and Velisa Vesovic who made it all possible. Imperial College London September 1991 William A. Wakeham