RATE COEFFICIENTS IN AND SPACE SCIENCE LIBRARY

A SERIES OF BOOKS ON THE RECENT DEVELOPMENTS OF SPACE SCIENCE AND OF GENERAL GEOPHYSICS AND ASTROPHYSICS PUBLISHED IN CONNECTION WITH THE JOURNAL SPACE SCIENCE REVIEWS

Editorial Board

R.L.F. BOYD, University College, London, England W. B. BURTON, Sterrewacht, Leiden, The Netherlands C. DE JAGER, University of Utrecht, The Netherlands J. KLECZEK, Czechoslovak Academy of Sciences, Ondfejov, Czechoslovakia Z. KOPAL, University of Manchester, England R. LUST, European Space Agency, Paris, France L.1. SEDOV, Academy of Sciences of the U.S.S.R., Moscow, U.S.S.R. Z. SVESTKA, Laboratory for Space Research, Utrecht, The Netherlands

VOLUME 146 PROCEEDINGS RATE COEFFICIENTS IN ASTROCHEMISTRY

PROCEEDINGS OF A CONFERENCE HELD AT UMIST, MANCHESTER, U.K. SEPTEMBER 21-24, 1987

Edited by T. J. MILLAR and D. A. WILLIAMS Department of Mathematics. UMIST, Manchester. u.K.

KLUWER ACADEMIC PUBLISHERS DORDRECHT / BOSTON I LONDON Library of Congress Cataloging in Publication Data

Rate coefficients in astroche.istry , proceedings of a conference held In UMIST. Manchester. U.K •• September 21-24. 1987 / edited by T.J. Millar and D.A. Williams. p. em. -- (Astrophysics and space science library) Includes index. ISBN·13: 978·94-010-7851-1 1. --Congresses. 2. , Rate of- -Congresses. 1. Millar, T. J., 1952- II. Williams, D. A. (David Arnold), 1937- III. Series. QB450.R38 1988 523.02--dc19 88-12045 CIP ISBN-13: 978-94-010-7851-1 e-ISBN-I3: 978-94-009-3007-0 001: 10.1007/978-94-009-3007-0

Publisbed by Kluwer Academic Publishers, P.O. Box 17, 3300 AA Dordrecht, The Netherlands.

Kluwer Academic Publishers incorporates the publishing programmes of D. Reidel, Martinus Nijhoff, Dr W. Junk and MTP Press.

Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061, U.S.A.

In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, The Netherlands.

All Rights Reserved © 1988 by Kluwer Academic Publishers Softcover reprint of the hardcover 1st edition 1988 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. TABLE OF CONTENTS

PREFACE vii LIST OF PARTICIPANTS xi

D.C. CLARY Theory of Reactive Collisions at Low 1

D.R. BATES AND E. HERBST Radiative Association 17

D.R. BATES AND E. HERBST Dissociative Recombination: Polyatomic Positive Reactions with Electrons and Negative 41

E.F. VAN DISHOECK Photodissociation and Photoionisation Processes 49

E. ROUEFF. H. ABGRALL. J. LE BOURLOT AND Y. VIALA Radiative Pumping and Collisional Excitation of in Diffuse Interstellar Clouds 73

R. McCARROLL Charge Transfer in Astrophysical Plasmas 87

I.W.M. SMITH Experimental Measurements of the Rate Constants for Neutral-Neutral Reactions 103

D.K. BOHME Polycarbon and Hydrocarbon Ions and Molecules in Space 117

B.R. HOWE Studies of Ion- Reactions at T < 80 K 135

D. SMITH AND N.G. ADAMS Drift Tube Studies of Ion-Neutral Reactions and their Relevance to Interstellar 153

N.G. ADAMS AND D. SMITH Laboratory Studies of Dissociative Recombination and Mutual Neutralisation and their Relevance to Interstellar Chemistry 173

D.K. BOHME. S. WLODEK AND A. FOX Chemical Pathways from Atomic Silicon Ions to Silicon Carbides and Oxides 193 vi

M.J. HENCHMAN, J.F. PAULSON, D. SMITH, N.G. ADAMS AND W. LINDINGER Chemical Pathways for Deuteriun Fractionation in Interstellar Molecules 201

E.F. VAN DISHOECK AND J.B. BLACK Diffuse Cloud Chemistry 209

E. HERBST Dense Chemistry 239

P.D. JB:lWN, S.B. CHARNLEY AND T.J. MILLAR Hot Molecular Cores: A Case for 263

D.R. ~, T.S. IDNTEIRO, G. PINEAU DES FOR.E:rs AND E. ROOEFF Chemistry in Shocked Interstellar Gas 271

D.A. WII.LI»f3 Dynamical Models of the Chemistry in Interstellar Clouds 281

T.J MILLAR Chemistry in Expanding Circunstellar Envelopes 287

C.M. SHARP Condensation Calculations in Circunstellar Shells for Different C/O Ratios 309

J .M.C. RAWLINGS Chemistry in Primary T Tauri Winds 315

A. DALGARNO '!be of Planetary Atmospheres 321

A. BENNETT '!be UMIST Rate File for Astrochemistry 339

NAME INDEX 347 SUBJECT INDEX 361 PREFACE

''An atteJDpt has been made to cOll1PlJte the numbers of certain JI10lecules in interstellar space , .... A search for the bands of CH, O/{, DR, en and C2 would appear to be proIDising" P Swings and L Rosenfeld Astrophysical Journal 86,483(1937) This may have been the first attempt at modelling interstellar chemistry. As with models today, the methods used lacked reliability, but the speculation was impressive! Mark Twain might well have said of this infant subject "One gets such wholesale returns of conjecture out of such a trifling investment of fact". The detection of unidentified lines around the period that Swings and Rosenfeld were writing provoked much interest, but even the most optimistic speculator could hardly have imagined developments which would occur during the next 50 years. By 1987 about 70 varieties of molecule had been identified in the interstellar and circumstellar regions, They range in complexity from simple diatomics such as H2 and CO to such species as C2HeDH, (CHs)2CO, and the largest interstellar molecule detected so far, cyano-penta­ HC11N, The study of these molecules in has developed enormously, especially over the last 20 years, and is now codified in the new subject of astrochemistry, That such a variety of should exist in tenuous regions of the Galaxy is fascinating. However, their major importance is that they allow to probe regions of the gas that are otherwise inaccessible, and to study dark, dense regions where, for example, formation may be occurring. To exploit the information contained in the observations one needs to develop models of the chemistry describing the great variety of chemical processes that can occur in interstellar and circumstellar regions, Many of these processes were correctly identified soon after the first interstellar detections, for example, in papers by Swings in 1942, by Kramers and ter Haar in 1946, and especially by Bates and Spitzer in 1951, This last paper includes almost all types of process we currently consider, including radiative and dissociative recombination, photoprocesses, ion-molecule reactions, surface reactions, and so forth. With the rapid increase in the numbers of identified types of interstellar molecules in the early 1970s, comprehensive models of interstellar chemistry were developed by a number of authors, including Klemperer, Solomon and Herbst. These models showed that much important chemistry occurs in reactions between ions and molecules, with the ionization being maintained by cosmic rays or by photoionization. Such reactions can account for the observed interstellar ions (now numbering

vii viii

10 species) and for the enhanced abundance of deuteriUll observed in many molecules in cold interstellar clouds. As the models developed in complexity the demands on knowledge of chemical rate coefficients became severe. Even so, sophisticated models, such as that developed by Black and Dalgarno for the toward the star Zeta Qph, had some remarkable successes in description and prediction. These and later models emphasized the desperate need for accurate reaction rate data, and this perceived demand stimulated much experimental and theoretical work on appropriate atomic and molecular processes. Laboratory workers have risen magnificently to the challenge and many important rate coefficients have now been measured, and the dependence of some has been determined. These remarkable advances have occurred largely through the development, especially by Smith and Adams in Birmingham, of the Selected Ion Flow Tube apparatus. New theoretical techniques have also been developed and exploited with the greater availability of computing power. The interactioo between theoretician and experimentalist is often most fruitful, as the following pages show. The models of interstellar chemistry not only provide a stimulus to the ; in several instances the failure of theoretical models when compared to interstellar observations has led to suggestions which have resulted in new laboratory measurements and new phenomena being identified. As an example we mention the large HCS+ ICS abundance ratios observed in cold clouds. These were interpreted using a theoretical approach to ion-molecule collisions which showed that, at low temperatures, ion-dipolar molecule collisions would have rate coefficients much greater than that predicted by the ADO theory or measured at room temperature, a result confirmed by subsequent experiments and new theoretical developments. Because of the central importance of reaction rate data to models developed at UHIST and elsewhere we thought it appropriate to hold a conference to review recent progress in theoretical and experimental work in this area, and to set it in context with the astrochemical modelling. The aim was specifically to identify those reactions and processes, crucial in the modelling, that are uncertain and in need of study. UHIST seemed to us an appropriate venue for such a meeting, not only because it is a centre for astrochemical modelling, but also because it is an institution long devoted to scientific research, an institution with which Dalton - the founder of atomic theory - and his pupil Joule - a founder of thermodynamics - were connected in byegone years. The interdisciplinary of our subject would, we feel, have appealed to them. Some 50 years after the orlglnS of astrochemistry, therefore, the conference was held at UHIST (September 1987) with the title "Reactive rate coefficients in Astrophysics". Particular prominence was given to the 17 major invited reviews which were intended to cover the main areas of laboratory and and the astrochemical modelling. Papers based on these reviews for~ the bulk of this book. A few contributed papers were also accepted and are included. We believe that taken together these papers form a useful and coherent account of today's experimental and theoretical techniques, give some indication of the present activity in astrochemical modelling, and contain many suggestions for a lifetime of future work . We are especially grateful to the Science and Engineering Research Council for financial support through the Collaborative Computational Projects No. 6 (Heavy Particle Dynamics) and No. 7 (Analysis of Astronomical Spectra), to Drs N G Adams, D C Clary, D R Flower and E Herbst for advice on the scientific content of the meeting and to Dr A E Lynas~ray, then the CCP7 Secretary, for administrative support.

T J Millar and D A Williams Mathematics Department UMIST March 1988 LIST OF PARTICIPANTS

N.G. ADAMS, Dept. of Space Research, University of Birmingham, PO Box 363, Birmingham B15 ZTT, UK R.J. ALLAN, Daresbury Laboratory, Warrington WA4 4AD, UK G.G. BALINT-KURTI, School of Chemistry, Univ. of Bristol, Bristol BS8 1TS UK D.R. BATES, Dept. of Applied Mathematics and Theoretical Physics, The Queen"s University, Belfast BT7 1NN, UK D.L. BAULCH, Dept. of , Univ. of Leeds, Leeds LS2 9JT UK A. BENNETT, Dept. of Mathematics, UMIST, PO Box 88, Manchester M60 1QD UK

J.H. BLACK, Steward Observatory, Univ. of Arizona, Tuscon, AZ 85721, USA D.K. BOHHE, Dept. of Chemistry, York University, 4700 Keele Street Downsview, Ontario, CANADA M3J IP3 P.D. BROWN, Dept. of Mathematics, UMIST, PO Box 88, Manchester M60 1QD UK S.B. CHARNLEY, Inst. fur extraterrestriche Physik, HPI fur Physik und Astrophysik, D-8046 Garching bei Munchen, WEST GERMANY D.C. CLARY, University Chemical Laboratory, Lensfield Road, Cambridge CB2 lEW, UK J.N.L. CONNOR, Dept. of Chemistry, Univ. of Manchester, Manchester M13 9PL, UK C. COURBIN, Daresbury Laboratory, Warrington WA4 4AD, UK A. DALGARNO, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA C.E. DATED, University Chemical Laboratory, Lensfield Road, Cambridge CB2 lEW, UK J. DAVIDSSON, Dept. of Physical Chemistry GU, Chalmers University of Technology, S-412 96 Goteborg, SWEDEN D.J. DEFREES, Molecular Research Institute, 701 Welch Rd, Suite 213, Palo Alto, CA 94305, USA xi xii

A.S. DICKINSON, Dept. of Physics, Univ. of Newcastle, Newcastle-upon-Tyne NE1 7RU, UK

C.H. DEPUY, Dept. of Chemistry, Univ. of Colorado, Campus Box 215, Boulder, CO 80309, USA

K. ERIKSSON, Astronomiska Observatoriet, Box 515, S-751 20 Uppsala SWEDEN

D.R. FlDWER, Dept. of Physics, The University, Durham DH1 3LE, UK

K. GILES, Dept. of Space Research, University of Birmingham, PO Box 363 Birmingham B15 2TT, UK

T. HASEGAWA, Canadian Institute for Theoretical Astrophysics, Univ. of Toronto, CANADA M5S 1A1

M.J. HENCHMAN. Dept. of Chemistry, Brandeis Univ., Waltham, MA 02254 USA

E. HERBST, Dept. of Physics, Duke Univ., Durham, NC 27706, USA

C.R. HERD, Dept. of Space Research, University of Birmingham, PO Box 363 Birmingham B15 2TT, UK

B. JANSEN OP DE HAAR, University Chemical Laboratory, Lensfield Road Cambridge CB2 lEW, UK

A.P. JONES, Dept. of Mathematics, UMIST, PO Box 88, Manchester M60 100 UK

P.G. JUDGE, Dept. of Theoretical Physics, Univ. of Oxford, 1 Keble Road, Oxford OX1 3NP, UK

J. LE BOURLO'I', DAMAp, Observatoire de Meudon, Place Jules Janssen F-92195 Meudon Principal Cedex, FRANCE

A.E. LYNAS-GRAY, Dept. of Physics and Astronomy, University College, Gower Street, London WC1 BBT, UK

R. McCARROLL, Univ. P. et M. Curie, Lab. de Physique et Optique Corpusculaires, Tour 12-E5, F-75252 Paris Cedex, FRANCE B.J. McINTOSH, Dept. of Space Research, University of Birmingham. PO Box 363, Birmingham B15 2TT, UK

J.B. MARQUETTE, Laboratoire d'Aerothermique, 4ter Routes des Gardes F-92190 Meudon, FRANCE

G. MAUCLAIRE, LPCR - Bat. 350, Univ. de Paris-Sud, F-91405 Orsay Cedex, FRANCE xiii

T.J. MILLAR, Dept. of Mathematics, UMIST, PO Bax 88, Manchester MOO 100 UK S. MILLER, Dept. of Physics and Astronomy, University College, Gower Street, London WCl 6BT, UK

T.S. MONTEIRO, Dept. of Physics, The University, Durham DHl 3tH, UK

L.A.M. NEJAD, Insitute of AstronolllY, Madingley Road, Cambridge CB3 OHA UK

G. PINEAU DES FORETS, DAPHE, Observatoire de Paris, F-92195 Heudon Principal Cedex, FRANCE J.H.C. RAWLINGS, Dept. of Mathematics, UMIST, PO Box 88, Manchester M60 100, UK

E. ROUEFF, DAMAp, Observatoire de Meudon, Place Jules Janssen, F-92195 Meudon Principal Cedex, FRANCE

B. R. ROWE, Laboratoire d' Aerothrmique, 4ter Routes des Gardes, F-92190 Meudon, FRANCE

C.M. SHARP, Institut fur Astrophysik, Karl-Schwarzschild-Str. 1, D-8046 Garching bei Hunchen, WEST GERMANY

D. SMITH, Dept. of Space Research, University of Birmingham, PO Box 363, Birmingham B15 2Tl', UK

I.W.H. SMITH, Dept. of Chemistry, University of Birmingham, PO Bax 363, Birmingham B15 2Tl', UK

N.D. TWIDDY, Dept. of Physics, University College, Penglais, Aberystwyth SY23 3BZ, UK

P. VALl RON , Groupe d' Astrophysique, CERHO, B. P. 68, F-38402 St. Martin d'Heres Cedex, FRANCE

E.F. VAN DISHOECK, Insitute for Advanced Study, School of Natural Sciences, Princeton, NJ 08450, USA

Y. -P. VI ALA , DEMIRH, Observatoire de Meudon, F-92195 Meudon Principal Cedex, FRANCE

J.C. WHITEHEAD, Dept. of Chemistry, Univ. of Manchester, Manchester M13 SPL, UK D.C.B. WHITTET, School of Physics and Astronomy, Lancashire Polytechnic, Preston PR1 ZDQ, UK xiv

U. WILHELHSSON, Dept. of Physical Chemistry GU, Chalmers University of Technology, S-412 96 Goteborg, SWEDEN D.A. WILLIAMS, Dept.of Mathematics, UMIST, PO Box 88, Manchester M60 1QD UK