Jet Spectroscopy and Molecular Dynamics

446 Pages·1995·8.69 MB·English

by  F. L. Bettens, A. Bauder (auth.), J. M. Hollas, D. Phillips (eds.)

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
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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