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Jet Spectroscopy and Molecular Dynamics PDF

446 Pages·1995·8.69 MB·English
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Jet Spectroscopy and Molecular Dynamics Jet Spectroscopy aud Molecular Dyuamics Edîted by J. M. Hollas Departmcnt of Chemistry University of Reading and D. Phil1ips Dcpartment of Chemistry Imperial College University of London Springer Science+Business Media, LLC First edition 1995 © 1995 Springer Science+Business Media New York Originally published by B1ackie Academic & Professional in 1995 Softcover reprint of the hardcover Ist edition 1995 ISBN 978-94-010-4573-5 ISBN 978-94-011-1314-4 (eBook) DOI 10.1007/978-94-011-1314-4 Apart from fair deal ing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publicat ion may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK. Enquiries concern ing reproduction outside the terms stated here should be sent to the publishers at the Glasgow address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Catalog Card Number: 94-72270 ooPrinted on acid-free text paper, manufactured in accordance with ANSI/NISO Z39.48-1992 (Permanence of Paper) Preface The conditions which obtain in a supersonic jet have been referred to as those of a fourth state of matter. This may be something of an exaggeration but it does go some way towards conveying the reason for the excitement generated among those working in many branches of spectroscopy and dynamics who saw it as a means of obtaining information which could previously only have been dreamt of. In an effusive atomic or molecular beam, the species concerned could be investigated under conditions which removed pressure broadening and much of Doppler broadening from the resulting spectra. The effusive beam was in many ways the precursor of the supersonic jet but suffered by comparison in that particles in the beam have a Maxwellian velocity distribution which is the same as that of those in the reservoir of gas forming the beam. Such an effusive beam can be produced by pumping atoms or molecules through a narrow (c. 20 11m) slit or pinhole with a pressure of only a few torr on the high pressure side of the aperture. In the early 1950s it was found that, if the gas being pumped through the small aperture is atomic, typically helium or argon, and the pressure is greatly increased to a few atmospheres, the many collisions occurring in, and just downstream of, the pinhole or slit result in an extremely low translational temperature of the gas, of the order of I K, and a so-called supersonic jet results. If particularly uniform temperature and velocity are required, the outer regions of the conical jet may be removed with a skimmer to form a supersonic beam. When molecules are injected into the carrier gas, they attain a very similar translational temperature. However, because rotational energy levels are more widely spaced than translational levels, the rotational temperature is somewhat higher, typically of the order of 10K. This can be reduced further by increasing the pressure of the carrier gas. Vibrational energy levels are still more widely spaced and typical vibrational temperatures are of the order of 100 K but may vary among the vibrational modes of a polyatomic molecule. This 'fourth state of matter' consists, therefore, of molecules which are generally extremely cold, colder than could previously have been contemplated, and which have different translational, rotational and vibrational temperatures. In studies of molecular spectroscopy and dynamics these conditions have resulted in several major advantages. One of these is that very weakly bound species, such as van der Waals and hydrogen-bonded complexes and clusters, are held together at the typically low vibrational temperatures which obtain. VI PREFACE This allows the investigation of their spectroscopy and dynamics to a level of precision which was never previously approached. Another advantage is that spectra of very much larger molecules can be rotationally resolved. At room temperature the spectra of large molecules tend to be overcrowded, even to the extent of creating a pseudocontinuum, due to very closely-spaced rotational energy levels and an abundance of low-lying vibrational levels all of which are appreciably populated. In a skimmed supersonic beam, for example, individual rotational transitions in the electronic spectrum of a molecule as large as naphthalene or carbazole can be observed. This allows a detailed investigation of the molecular structure, from a rotational analysis, and of the vibrational and rotational dependence of the dynamics of far larger molecules than was previously possible. Lasers, which were developed from the 1960s onwards, have proved to be an extremely important tool in investigations of the spectroscopy and dynamics of molecules in supersonic jets or beams. Of particular importance are the dye lasers, for the visible and ultraviolet regions, and the diode lasers, for the near infrared. For the study of microwave spectra of jet-cooled molecules, Fourier transform techniques have proved essential. In the late 1970s the pulsed supersonic jet or beam was developed whereas earlier ones were continuous. Originally the pulsed jet was used in conjunction with a pulsed laser to conserve material and to give greater cooling of the molecules. The increased cooling was possible because of the less stringent pumping requirements allowing higher pressures to be used before the pinhole or slit. However, it was soon realised that the shortness of the laser pulses, firstly a few nanoseconds in length and then picoseconds and femtoseconds, allowed studies of the molecular dynamics to be made on extremely short timescales and at vibrational or even rotational resolution. The contributors to this volume are all international authorities on their subjects and we are extremely grateful to them for devoting a considerable amount of time in employing their expertise to make it a success. The spectroscopy of molecules, free radicals and clusters in supersonic jets and beams is covered from the microwave region, through the near infrared to the visible and ultraviolet regions. Aspects of molecular dynamics include rotational coherence phenomena, intramolecular vibrational relaxation, relaxation processes in van der Waals clusters, internal relaxation dynamics and the effects of optically dark states. The study of spectroscopy and dynamics of molecules in supersonic jets continues to develop rapidly and we hope that the present volume serves to give a general picture of the present state ofthe art and to convey much of the excitement which has been generated. J.M.H. D.P. Contributors J. Arno Chemistry Department, Texas A&M University, College Station, Texas 77843-3255, USA A. Bauder Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland F.L. Bettens Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland R.P.A. Bettens Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland J.W. Bevan Chemistry Department, Texas A&M University, College Station, Texas 77843-3255, USA T. Biirgi Institut fur Anorganische, Analytische und Physikalische Chemie, Freiestra13e 3, CH-3000 Bern 9, Switzerland P.M. Felker Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024-1569, USA J.M. Hollas Chemistry Department, University of Reading, Whiteknights, Reading, RG6 2AD, UK s. Leutwyler Institut fUr Anorganische, Analytische und Physikalische Chemie, Freiestra13e 3, CH-3000 Bern 9, Switzerland W.L. Meerts Department of Molecular and Laser Physics, University of Nijmegen, Toemooiveld, 6525 ED Nijmegen, The Netherlands T.A. Miller Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA ".J. Neusser Institut fur Physikalische und Theoretische Chemie, Technische UniversiUit Miinchen, LichtenbergstraBe 4,85748 Garching, Germany D. Phillips Chemistry Department, Imperial College, University of London, London SW7 2AY, UK D. W. Pratt Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA VIII CONTRIBUTORS L.H. Spangler Department of Chemistry, Montana State University, Bozeman, Montana 59717, USA R. Sussmann Institut fUr Physikalische und Theoretische Chemie, Technische Universitat Miinchen, Lichtenbergstraf3e 4,85748 Garching, Germany A.G. Taylor Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK M.R. Topp Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA T.G. Wright Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA Xue Qing Tan Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA A.H. Zewail Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA Contents 1 Rotational spectroscopy of weakly bound complexes I F.L. BETTENS, R.P.A. BETTENS and A. BAUDER 1.1 Introduction I 1.2 Experimental techniques 2 1.2.1 Molecular beam electric resonance 2 1.2.2 Pulsed nozzle Fourier transform microwave spectroscopy 2 1.2.3 Electric-resonance optothermal spectroscopy 5 1.3 Van der Waals complexes 5 1.3.1 Aromatic molecule...(rare gas)•• n = I. 2, complexes 5 1.3.2 Force field and derived properties of aromatic molecule.. ·rare gas complexes 9 1.3.3 Aromatic molecule diatomic molecule complexes II 1.3.4 Aromatic molecule triatomic molecule complexes 13 1.3.5 Larger complexes containing at least one aromatic molecule 15 1.4 Hydrogen bonded complexes 16 1.4.1 Complexes involving water 17 1.4.2 Complexes not involving water 19 1.5 Conclusion and outlook 22 References 24 2 Infrared spectroscopy in supersonic free jets and molecular beams 29 J. ARNO and lW. BEVAN 2.1 Introduction 29 2.2 Supersonic free jets and molecular beams 30 2.2.1 Structure and properties of continuous supersonic free jets 31 2.2.2 Cluster formation 33 2.2.3 Pulsed nozzle supersonic jets 35 2.2.4 Slit supersonic jets 36 2.2.5 Supersonic molecular beams 37 2.3 Instrumentation and techniques for infrared spectroscopy in supersonic jets and molecular beams 37 2.3.1 Fourier transform spectroscopy 38 2.3.2 Laser-based spectroscopy 42 2.4 Applications of FTIR supersonic jet spectroscopy 47 2.5 Applications of infrared laser spectrometers in supersonic jets and molecular beams 52 2.5.1 Fixed frequency lasers 52 2.5.2 Laser sideband spectrometers 52 2.5.3 Semiconductor diode lasers 56 2.5.4 Tunable lasers based on non-linear mixing techniques 60 2.5.5 F center lasers 63 2.5.6 Other laser spectrometers 65 Acknowledgements 66 ~~= ~ x CONTENTS 3 Electronic spectroscopy of free radicals in supersonic jets 74 XUE QING TAN, T.G. WRIGHT and T.A. MILLER 3.1 Introduction 74 3.2 Experimental approaches 75 3.2.1 Apparatus overview 75 3.2.2 Radical production methods 77 3.2.3 LIF experiments 81 3.2.4 REMPI and ZEKE experiments 81 3.3 Radicals studied 82 3.3.1 LIF of di- and triatomic radicals 82 3.3.2 REMPI of di- and triatomic radicals 89 3.3.3 Small hydrocarbon radicals 94 3.3.4 Alkoxy radicals and their derivatives 96 3.3.5 Aromatic radicals 100 3.3.6 Organometallic radicals 107 3.4 Conclusion 112 Acknowledgement 112 References 113 4 Structure of weakly bound complexes from electronic spectra 118 H.J. NEUSSER and R. SUSSMANN 4.1 Introduction 118 4.2 Experimental 120 4.2.1 General remarks 120 4.2.2 Mass-selective detection 121 4.2.3 Experimental set-up 122 4.3 Spectroscopy of dimers 124 4.3.1 Benzene-noble gas dimers 124 4.3.2 Dimers of fluorene and noble gas atoms 134 4.4 Spectroscopy and structure of trimers 137 4.4.1 Benzene-noble gas trimers 137 4.4.2 Carbazole-noble gas trimers 139 4.5 Benzene-molecule dimers 145 4.6 Concluding remarks 148 Acknowledgements 148 References 149 5 Jet spectra of aromatic molecules in hydrogen bonded microsolvant clusters 151 A.G. TAYLOR, T. BORGI and S. LEUTWYLER 5.1 Introduction 151 5.2 Aromatic molecule/H 20 complexes 152 5.2.1 Hydroxyaromatics 152 5.2.2 N-aromatic molecules 159 5.2.3 Benzene and toluene 161 5.2.4 Cyanobenzenes 161 5.2.5 Tautomerising molecules 163 5.3 Aromatic molecule/NH 3 complexes 164 5.3.1 Hydroxyaromatics 164 5.3.2 N-aromatic molecules 167 5.3.3 Aromatic molecules/NH 3 168 5.3.4 Molecules which undergo tautomerism 169 5.4 Comparison of experimental data and results of ab initio calculations 169 CONTENTS XI 5.4.1 Hydrogen bond energies, geometric parameters and atom charges 170 5.4.2 Vibrational frequencies 176 References 179 6 Rotational coherence phenomena 181 P.M. FELKER and A.H. ZEWAIL 6.1 Introduction 181 6.2 Alignment recurrences: the free rotational dynamics of dipole-excited species 182 6.2.1 Definitions and nomenclature 182 6.2.2 The effect of resonant, short-pulse excitation 185 6.2.3 The orientational probability density 188 6.2.4 Alignment recurrences 189 6.3 Rotational coherence phenomena: observable manifestations of free rotational dynamics 193 6.3.1 Probing of transient alignment 193 6.3.2 Rotational coherence effects in symmetric tops 198 6.3.3 Asymmetric tops 204 6.4 Rotational coherence spectroscopy 210 6.5 Results from experiment 213 6.6 Summary and conclusion 217 Acknowledgements 219 References 219 7 Ultrafast dynamics of IVR in molecules and reactions 222 P.M. FELKER and A.H. ZEWAIL 7.1 Introduction 222 7.2 Theoretical description of vibrational coherence and IVR 224 7.2.1 Two-level IVR 225 7.2.2 IVR between N levels 228 7.2.3 Types and regions of IVR 230 7.3 Applications to molecular systems: non-reactive 232 7.3.1 Anthracene 232 7.3.2 9-d t -Anthracene and dID-anthracene 242 7.3.3 trans-Stilbene 244 7.3.4 Alkylanilines: 'ring and tail' systems 248 7.3.5 p-Difluorobenzene 255 7.3.6 Techniques and other molecules 255 7.4 Effects of rotations on IVR: mismatches of rotational constants 256 7.5 IVR in reactions 261 7.5.1 Vibrational predissociation in I2 -X complexes 264 7.5.2 t-Stilbene van der Waals complexes 267 7.5.3 Hydrogen-bonded systems 270 7.5.4 Electron transfer reactions 272 7.5.5 IVR in consecutive reactions 276 7.5.6 Ground-state reactions 278 7.5.7 Isomerization reactions 278 7.6 Rotational coherence dynamics and IVR 279 7.6.1 Discussion of the phenomenon 279 7.6.2 Time-resolved fluorescence 281 7.6.3 Pump-probe fluorescence gain (PPFG) 287 7.6.4 Pump-probe fluorescence depletion (PPFD) 294 7.6.5 Pump-probe ionization gain (pPIG) 296 7.6.6 Saturation effects 301 7.6.7 Rotational coherence in reactions 302 Acknowledgements 306 References 306

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