Experimental and theoretical investigations of the photochemistry of styrene and the creation and characterisation of shaped femtosecond ultraviolet laser pulses Abigail Nunn A thesis submitted for the degree of Doctor of Philosophy University College London March 2010 I, Abigail Nunn, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. …of making many books there is no end; and much study is a weariness of the flesh. ________________ He restoreth my soul. ________________ The flowers appear on the earth; the time of singing is come… Abstract This thesis is composed of three projects that are linked by the theme of light-molecule interactions. These are covered separately in Chapters 2, 3, and 4. In Chapter 1 some background to the thesis is described so that the various links between the science in the different chapters, and the motivation for the project as a whole are explained. Elements of photochemistry, both experimental and theoretical, are described in this chapter and some material about the most important experimental tools used in this work, ultrafast lasers, is covered, as well as the methodology of time-resolved spectroscopies, and time- resolved photoelectron spectroscopy in particular; the field of laser control of chemistry is also briefly reviewed. Chapter 2 is an account of the design and use of a UV pulse shaper and characterisation setup. This is an applied optics experiment, whose application is to the control of photochemical reactions with specifically shaped ultrafast laser light. Several demonstrations of the pulse shaping capacity of this new experiment are presented. In Chapter 3, calculation of the excited electronic states of the molecule styrene is described. This project is a computational study of the electronic spectroscopy and ionisation of the styrene molecule. In Chapter 4, the direct observation of internal conversion in styrene using time-resolved photoelectron spectroscopy is reported. This is an experimental laser spectroscopy project in which some of the results from the computations in the theory project, Chapter 3, will be used to analyse the experimental spectra. Chapter 5 summarises the conclusions drawn from Chapters 2, 3 and 4 and provides an outlook for future research based on the work in this thesis. Throughout this thesis, but more particularly in Chapters 1 and 2, there is quite a large volume of literature review and background material. This content reflects the personal perspective from which the thesis was approached. Much of the field of ultrafast optics and spectroscopy was entirely new to the writer at the outset of the PhD programme, and most of the review-based writing about these topics found here was originally written early on in the PhD project, as a means of helping to bridge the gap between work on optical experiment design and an undergraduate training in chemistry. 4 This thesis is based on the following publications Parts of Chapters 1 and 2: Femtochemistry and the control of chemical reactivity, Helen H. Fielding and Abigail D. G. Nunn, in Tutorials in molecular reaction dynamics, Edited by Mark Brouard and Claire Vallance, RSC, to be published 2010. Chapter 2: Frequency doubling and Fourier domain shaping the output of a femtosecond optical parametric amplifier: easy access to tuneable femtosecond pulse shapes in the deep ultraviolet, A. D. G. Nunn*, D. S. N. Parker*, R. S. Minns, H. H. Fielding, Applied Physics B 94, 181 (2009). Parts of Chapters 3 and 4: Ultrafast dynamics of the S2 excited state of styrene, A. D. G. Nunn, R. S. Minns, M. J. Bearpark, M. A. Robb and H. H. Fielding, Physical Chemistry Chemical Physics, in preparation. 5 Acknowledgements First of all I would like to thank my supervisor Professor Helen Fielding for allowing me to carry out a PhD in her group. I am particularly indebted to her for very thorough but very fast reading of the chapters of this thesis. I would also like to thank Dr Mike Bearpark and Professor Mike Robb at Imperial College for their supervision of my theory project; most recently, their assistance with finishing the project and writing Chapter 3 has been greatly appreciated. I owe a debt of gratitude to many friends and colleagues who have helped and encouraged me through the course of my PhD research. Thanks go to Russell for being always so patient and helpful with all my questions and over my many mishaps in the lab. Thanks also go to Dorian for sharing the many hours spent staring at spots of light in our search for t0. I would also like to thank all of the rest of the Fielding group, past and present who have been supportive colleagues and made the group such a friendly place to work: Rob, Rakhee, Eva, Corinne, Adam K, Adam M, Nick, Toni, Kiera, Emma, Jadranka, Maria, Ciarán and Roman. I’d also like to thank Rosie for advice about theory, and for very welcome tea breaks on many occasions! Many other people have been helpful with practical advice and assistance during different periods of my PhD studies: Steve Firth at UCL, Matt Harvey and Simon Burbidge at the HPC service at Imperial College, Nachum Lavie at the University of Tel Aviv, members of the Walmsley Group at the University of Oxford, and other members of staff and students at UCL and Imperial. I would like to thank my cousin Meredith and her husband Bryan for arranging their wedding so that I could have a short trip to Nova Scotia to look forward to at the end of my PhD – I was very glad to be able to be there. Most of all, I would like to thank my Dad, my Mum, my brother Jamie and my sisters Ruth and Nancy. I am very grateful for all their support for my PhD studies, and for so many other things! 6 Contents Chapter 1, Introduction..................................................................................................14 1.1 Photophysics and photochemistry 15 1.2 What is an ultrafast laser pulse? 22 1.3 Time-resolved spectroscopy 30 1.4 Coherent Control 41 1.5 Summary 49 References 50 Chapter 2, Femtosecond pulse shaping and characterisation in the ultraviolet.......53 2.1 An introduction to ultrafast lasers 54 2.2 An ultrafast laser system for UV pulse shaping, spectroscopy and control 58 2.3 Pulse shaping for a coherent control experiment 64 2.4 Design of characterisation for the UV pulse shaper experiment 70 2.5 UV pulse shaping and characterisation experimental arrangement 89 2.6 Results from the UV pulse shaper 93 2.7 Further work 101 References 103 Chapter 3, Electronic structure modelling of the singlet states of styrene in the Franck-Condon region..................................................................................................106 3.1 The electronic and vibronic spectroscopy of styrene in the literature 107 3.2 Electronic structure theory methods for excited states 114 3.3 Methods of characterising the excited electronic states of styrene 122 3.4 Results and discussion – characterising the excited states of styrene 125 3.5 Results and discussion – ionisation correlations for the singlet states of styrene 130 3.6 Conclusions 134 References 135 7 Chapter 4, Time resolved photoelectron spectroscopy of styrene............................139 4.1 Excited state dynamics of styrene reported in the literature 140 4.2 Two-colour femtosecond time-resolved photoelectron imaging experiment 141 4.3 Image processing and calibration 151 4.4 Results and discussion 156 4.5 Conclusions 172 References 174 Chapter 5, Conclusion...................................................................................................177 8 List of Figures Figure 1.1 A Jablonski diagram for photophysical decay pathways from an electronically excited isolated molecule..................................................................................................16 Figure 1.2 Time and frequency domain electric-field envelope intensities of an ultrashort Gaussian laser pulse..........................................................................................................23 Figure 1.3 A plot of a laser pulse produced by a typical idealised mode-locking cavity.24 Figure 1.4 Intensity profile of a chirped, Gaussian envelope, pulse plotted as a function of time...............................................................................................................................26 Figure 1.5 Illustration of the role of phase in defining ultrafast pulse electric field structure.............................................................................................................................28 Figure 1.6 A Spectrogram showing a negatively chirped pulse.......................................30 Figure 1.7 Two-colour pump-probe experiment..............................................................32 Figure 1.8 Na2 potential energy curves and pump-probe signal......................................34 Figure 1.9 NaI potential energy curves and LIF signal....................................................36 Figure 1.10 Pump-probe excitation scheme and photoelectron kinetic energy as a function of delay for all-trans 2,4,6,8 decatetraene..........................................................38 Figure 1.11 Schematic representation of Type I and Type II Koopmans’ correlations in the time-resolved photoelectron spectroscopic observation of a non-adiabatic process..39 Figure 1.12 Schematic representation of pump-dump experiments................................43 + Figure 1.13 Potential energy curves for the Na2 and