Vibrationally Mediated Photodissociation

220 Pages·2009·2.82 MB·english

by  Salman Rosenwaks

Vibrationally Mediated Photodissociation

Vibrationally Mediated
Photodissociation
Salman (Zamik) Rosenwaks
Department of Physics, Ben-Gurion University of the Negev, Beer Sheva, Israel
ISBN: 978-0-85404-155-8
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rSalman(Zamik)Rosenwaks2009
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Preface
Vibrationally mediated photodissociation (VMP) deals with the influence of
vibrational excitation of the ground electronic state of a molecule on its dis-
sociation following excitation of this state to a higher electronic state. VMP
studies were initially motivated by the prospect of enhancing the photo-
dissociation cross section as a result of vibrational pre-excitation. However,
more recently the main interest in VMP has been driven by the hope that by
preparing some specific vibrational levels rather than others, the subsequent
dissociation of the electronically excited molecule could be controlled. In
other words, the goal of achieving so-called ‘‘mode-selective’’ or ‘‘bond-selec-
tive’’ dissociation has been at the heart of VMP studies. Obviously, for this
goal the molecule should have at least two vibrational modes. Therefore, the
focus of VMP studies has been on triatomic or larger molecules.
The scope of this monograph is to present the methodology of VMP via
state-of-the-art specific examples. Overviews of earlier works are included as
well,toserveasabackgroundforcurrentresearch.Thepresentationdealswith
‘‘isolated’’ neutral molecules in the gas phase; VMP in complexes, clusters or
condensed phases is not discussed. Wherever appropriate, original studies are
quoted,includingtheoriginaldrawings.Itishopedthatfromthedescriptionof
themotivation,theapproach,theexecutionoftheexperimentandtheanalysis
of the results of the specific examples, the reader will get a comprehensive
understanding of the field. In addition, a general introduction and summary
shouldgivethenecessaryorientationbeforeandafterplungingintodetails.The
limitedlengthofthemonographimposesselectionofonlyalimitednumberof
examples from the extensive literature dealing with VMP that could be dis-
cussed,butanefforthasbeenmadetogivereferencestoasmanystudiesaswe
were aware of. We apologize for any possible oversights.
The level of presentation is appropriate for senior undergraduate and
graduatestudentsofchemistryandphysics.Itishopedthatthemonographwill
serveasanintroductiontoVMPforbeginnersandasaliteratureguidetothose
acquaintedwiththesubjectbutnotnecessarilyworkingonVMP.Obviously,in
VibrationallyMediatedPhotodissociation
BySalman(Zamik)Rosenwaks
rSalman(Zamik)Rosenwaks2009
PublishedbytheRoyalSocietyofChemistry,www.rsc.org
v
vi Preface
a monograph of limited length,very detailed explanations of the experimental
and theoretical methods involved in VMP could not always be included;
however,referencestooriginalstudiesarepresentedattheendofeachchapter
and should help readers who are interested in more details.
I would like to thank the students of our laboratory and colleagues all over
the world for their learned input via very many discussions we have had in
meetings and through the mail. During the writing of the monograph I have
contactedmanyauthors ofthepaperscitedinthemonographandIapologize
for not mentioning them all, since a full list would include tens of names. I
wouldstillliketomentionthosewhosehelphasbeenparticularlyextensive,our
graduateandpostgraduatestudentsAmirGolan,ChenLevi,RanMaromand
AlexPortnovandmyfriendsandcolleaguesIlanaBar,whoalsodeservesmost
of thecredit for theVMP work conducted in our laboratory, Paul Dagdigian,
Aryeh Levin and Moshe Shapiro. Last but not least, I am greatly indebted to
themembersofmyfamily,whosenamesIamnotmentioningattheirrequest.
Without their support and patience, this monograph would not have been
written.
Salman (Zamik) Rosenwaks
Contents
Abbreviations and Acronyms xiii
Useful Physical Constants and Conversion Factors xiv
Chapter 1 Introduction 1
1.1 What is VMP and How Does it Work? 1
1.2 Why VMP? 4
1.3 Organization of the Monograph 6
References 7
Chapter 2 Theoretical Aspects 9
2.1 Photodissociation Dynamics 9
2.1.1 Potential-Energy Surfaces, the
Born–Oppenheimer Approximation and
the Franck–Condon Principle 10
2.1.2 Photodissociation Cross Sections:
Computational Approaches 13
2.1.2.1 Classical Trajectories 14
2.1.2.2 Time-Independent Approach 14
2.1.2.3 Time-Dependent Approach 15
2.1.2.4 ComparisonoftheTime-Independent
and Time-Dependent Approaches 18
2.1.3 Radiationless Transitions between PESs, the
Role of Conical Intersections 18
2.1.4 Vector Correlations in VMP 20
2.2 Intramolecular Vibrational Dynamics: Normal
and Local Modes and the Role of Intramolecular
Vibrational Redistribution 21
VibrationallyMediatedPhotodissociation
BySalman(Zamik)Rosenwaks
rSalman(Zamik)Rosenwaks2009
PublishedbytheRoyalSocietyofChemistry,www.rsc.org
vii
viii Contents
2.2.1 Normal Modes and Local Modes 22
2.2.2 Intramolecular Vibrational Redistribution 23
References 27
Chapter 3 Experimental Methods 29
3.1 Preparation and Detection of Vibrational States 32
3.1.1 Nonspecific Vibrational Excitation 32
3.1.2 One-Photon, State-Selected Vibrational
Excitation 32
3.1.3 Two-Photon, State-Selected Vibrational
Excitation 33
3.1.3.1 Overtone–Overtone Double
Resonance 33
3.1.3.2 Preparation by Stimulated Raman
Excitation and Detection by
Coherent Anti-Stokes Raman
Spectroscopy and Photoacoustic
Raman Spectroscopy 34
3.1.3.3 Preparation by Stimulated Emission
Pumping 35
3.1.3.4 Preparation by Stimulated Raman
Adiabatic Passage 35
3.1.4 Monitoring Vibrational Excitation 36
3.2 Excitation and Detection of Electronic States 36
3.3 Detection and Characterization of
Photofragments 37
3.3.1 Spontaneous Emission and Laser-Induced
Fluorescence 37
3.3.2 Resonantly Enhanced Multiphoton Ionization 37
3.3.3 Doppler Profiles 38
3.3.4 High-n Rydberg Time-of-Flight 38
3.3.5 Photofragment Translational Spectroscopy 39
3.3.6 Velocity-Map Imaging 39
References 40
Chapter 4 VMP of Diatomic Molecules and Radicals 43
4.1 HF 44
4.2 HCl 44
4.3 HBr 45
4.4 HI 46
4.5 OH, OD, SH and SD Radicals 47
4.6 O 48
2
References 48
Contents ix
Chapter 5 VMP of Triatomic Molecules Excluding Water 51
5.1 VMP of O 51
3
5.1.1 Spectral Features of O Relevant to VMP 51
3
5.1.2 VMP of O as a Source of O(1D) 53
3
5.1.3 Vector Correlations in the VMP of O 54
3
5.2 VMP of OCS 55
5.2.1 Spectral Features of OCS Relevant
to VMP 55
5.2.2 VMP of OCS in the Second and Higher
Absorption Bands 55
5.2.3 VMP of OCS in the First Absorption Band 57
5.2.4 Vector Correlations in the VMP of OCS 59
5.3 OCSe 60
5.4 N O 60
2
5.5 CS 61
2
5.6 ICN 63
5.7 HCN, DCN, HOCl, DOCl and HOBr 63
References 64
Chapter 6 VMP of Water Isotopologues 68
6.1 Spectral and Dynamical Features of Water Relevant
to VMP 68
6.2 VMP of H O and HOD in the Second Absorption
2
Band 70
6.3 VMP of H O in the First Absorption Band 71
2
6.3.1 Vibrational-State Dependence of H O
2
Absorption Cross Section 71
6.3.2 Vibrational Distributions of the OH
Photofragments 73
6.3.3 Rotational, Spin-Orbit and L-Doublet State
Distributions of the OH Photofragments 75
6.3.3.1 Rotational Distributions 77
6.3.3.2 Spin-Orbit and L-Doublet
Distributions 78
6.3.4 Vector Correlations in the VMP of H O 79
2
6.3.5 VMP of H O as a Spectroscopic Tool 79
2
6.4 VMP of HOD 81
6.4.1 VMP of HOD Pre-Excited to Fundamental
OH or OD Vibrations 82
6.4.2 VMP of HOD Pre-Excited to OH Overtones 84
6.4.3 VMP of HOD Pre-Excited to OD Overtones 88
6.5 VMP of D O 90
2
References 90
x Contents
Chapter 7 VMP of Tetratomic Molecules 94
7.1 VMP of Acetylene Isotopologues 94
7.1.1 SpectralandDynamicalFeaturesofAcetylene
Relevant to VMP 94
7.1.2 VMP of C H 96
2 2
7.1.2.1 Adiabatic VMP of C H 97
2 2
(a) The 4n Region 97
CH
(b) Comparison of VMP for Different
Vibrational Levels in the 5n Region 97
CH
(c) Rotational Effects in the 5v
CH
Region 101
(d) Comparison of VMP for Different
Vibrational Levels in the 4n Region 103
CH
(e) The 1n Region 104
CH
7.1.2.2 Nonadiabatic VMP of C H 104
2 2
7.1.3 VMP of C HD 107
2
7.1.3.1 Adiabatic VMP of C HD – Evidence
2
for Rotational Dependence of H/D
Branching Ratio 107
7.1.3.2 Nonadiabatic VMP of C HD 110
2
7.2 VMP of Ammonia Isotopologues 110
7.2.1 Spectral Features of Ammonia Relevant
to VMP 110
7.2.2 VMP of NH 111
3
7.2.2.1 VMP of NH Prepared at Low
3
Vibrational Energies 111
7.2.2.2 VMP of NH Prepared at High
3
Vibrational Energies 116
7.2.2.3 VMP of NH as a Spectroscopic
3
Tool 117
7.2.3 VMP of NHD and NH D 118
2 2
7.3 VMP of HNCO 120
7.3.1 Spectral Features of HNCO Relevant
to VMP 120
7.3.2 A Brief Survey of the VMP of HNCO 121
7.3.3 Comparison of VMP of HNCO 3n to
1
One-Photon Dissociation of Thermal
Molecules 123
7.3.4 Application of VMP to HNCO
Spectroscopy 128
7.4 VMP of Other Tetratomic Molecules 129
7.4.1 Hydrogen Peroxide Isotopologues 131
7.4.2 Nitrous Acid 134
7.4.3 Hydrazoic Acid 135
References 136