UV-Pump IR-Probe Spectroscopy of Molecules with Time-Resolution
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UV-pump IR-probe Spectroscopy of Molecules with Time-Resolution reaching the 10-fs Range Kyriaki Kosma München 2008 UV-pump IR-probe Spectroscopy of Molecules with Time-Resolution reaching the 10-fs Range Kyriaki Kosma Dissertation an der Fakultät für Physik der Ludwig-Maximilians-Universität München vorgelegt von Kyriaki Kosma aus Thessaloniki, Griechenland München, 15 Juli 2008 Erstgutachter: Ferenc Krausz Zweitgutachter: Wolfgang Domcke Tag der mündlichen Prüfung: 11-11-2008 Contents i Contents Abstract iv Introduction viii 1. Interaction of atoms with intense laser fields 1 1.1 Introduction 1 1.2 Non-linear interaction of light with matter 2 1.2.1 The optical Kerr effect 3 1.2.2 Self-phase modulation 5 1.2.3 Material dispersion and devices for pulse compression 8 1.2.4 Self-focusing 10 1.2.5 Harmonic generation 13 1.3 Temporal characterization of femtosecond pulses 14 2 Ultrashort pulse generation in the IR and UV/VUV region 16 2.1 Experimental set-up 16 2.2 Supercontinuum generation 17 2.3 Characterization of the supercontinuum radiation 19 2.3.1 Variation of the input iris 19 2.3.2 Transverse beam profile in the filament region 20 2.3.3 Influence of the interaction length 22 2.3.4 Effect of the pulse duration 23 2.3.5 Effect of the focal length 25 2.3.6 Variation of the initial chirp 26 2.3.7 Spatial variation of the supercontinuum spectrum 28 2.3.8 Argon partial pressure 29 2.3.9 Different gases 31 2.4 Pulse duration and compression in the 270 – 320 nm region of the su- percontinuum 31 2.5 Third- and fifth-harmonic generation 35 2.6 Discussion 39 2.7 Conclusions 46 3 Time-resolved spectroscopy 49 3.1 Introduction 49 3.2 Pump-probe technique 50 ii Contents 3.3 Theoretical considerations of the molecular system 51 3.3.1 The Born-Oppenheimer approximation 51 3.3.2 Beyond the Born-Oppenheimer approximation: The conical intersection 53 3.3.3 Selection rules in light absorption 56 3.4 Reaction dynamics 58 3.4.1 Deactivation processes 58 3.4.2 The pathway approach 59 3.4.3 Description of the dynamics by rate equations and periodic modulations 60 4 Dynamics of internal conversion and isomerization of ethylene and ethylene-d4 65 4.1 Experimental 65 4.2 Background 66 4.3 Results and analysis 69 4.4 Discussion 77 4.4.1 Assignments of the lifetimes and oscillations 77 4.4.1.1 The Franck-Condon region 78 4.4.1.2 Departure from S1 80 4.4.1.3 Ground-state processes 83 4.4.1.4 Results of spectroscopic relevance 86 4.4.2 Ultrafast dynamics and implication for the potentials 88 4.5 Conclusions 91 5 The ring opening of cyclohexadiene 93 5.1 Experimental 93 5.2 Background 93 5.3 Results and analysis 96 5.4 Discussion 98 5.4.1 Assignments of the lifetimes and oscillations 99 5.4.2 The reaction path 103 5.5 Conclusions 107 6 The dynamics of the dissociation of Cr(CO)6 110 6.1 Experimental 110 6.2 Background 111 Contents iii 6.3 Results and analysis 113 6.3.1 Dynamics of Cr(CO)6 excited at 270 nm 113 6.3.2 Dynamics of Cr(CO)6 excited at 318 and 345 nm 117 6.4 Discussion 118 6.4.1 Assignments of the lifetimes and oscillations 118 6.4.2 The reaction path of the dissociation 121 6.5 Conclusions 126 Appendix 129 Summary 133 References 137 Acknowledgements 151 iv Abstract Abstract This work investigates the real-time dynamics of some fundamental molecules in the gas pha- se during light-induced chemical reactions, by pushing the time resolution down to about 10 fs (30 fs initially). These reactions are the isomerization of ethylene (C2H4), the ring opening of cyclohexadiene (C6H8) and the dissociation of chromium hexacarbonyl (Cr(CO)6). These molecules were expected – and confirmed – to exhibit several consecutive very fast processes. In addition, high frequency coherent oscillations were searched and indeed re- solved; their assignment to vibrations allows conclusions on geometrical distortions of the molecule during the processes, on directions of slopes (forces) on the potential and on the path of the wave packet. The time-resolution is now better by a factor of 7 (in the near UV) to 15 (at 160 nm) than in the previous best cases. This required the development of methods to generate short UV pulses, which were not available, in contrast to the 800-nm region. The developed light sources are based on (1) supercontinuum generation and (2) third- and fifth- harmonic generation, both in argon, starting from 10-fs Ti:sapphire laser pulses. Chapters 1 and 2 report on phenomena during propagation of very short (spectrally broad) and intense femtosecond laser pulses through nonlinear media (mainly gaseous argon) and on experimental methods – exploiting self-phase modulation and harmonic generation – for pro- ducing ultrashort pulses in the region of the fundamental (800 nm) of a commercial Ti:sapphire laser system, as well as outside this spectral range, in the UV (250 – 350 nm) and vacuum UV (162 nm). In a first step, 10-fs at 810-nm were achieved by self-phase modulation in argon in free propagation (with self-focusing), combined with chirped-mirror compression (see also [10] from list of publications, p. 153)1. Refocusing these pulses into argon produced a supercontinuum reaching down to 250 nm whose properties (e.g. cut-off wavelength and duration) were investigated [10, 4]. The supercontinuum allowed cutting out radiation in the UV with tunable wavelength and pulse duration of 30 fs after propagation in air [9] and 10 fs with a vacuum beam path [6]. With a much shorter argon cell, third- and fifth-harmonic pulses (270 and 162 nm) were obtained with durations (∼10 fs) slightly shorter than the 810-nm fun- damental pulses used (12 fs) [6, 3] (initially 30 fs for the TH [9]). Although further shortening 1 All the references contained in the abstract are listed on page 153. Abstract v seems feasible, these pulses were first applied to pump-probe spectroscopy before any further optimization. Chapter 3 first presents an introductory theoretical background and the framework, on which the analysis and explanation of the time-resolved findings are based. Chapters 4 to 6 then describe and discuss the results of the implementation of the ultrashort pulses in the pump-probe experiments (with 810-nm-multiphoton ionization for probing), by which the dynamical processes in the mentioned molecules are monitored in time; lifetimes as short as 12 fs could be found. With the help of the time constants, relative energies and coherent oscil- lations (vibrations), detailed conclusions on the reaction coordinates and geometrical struc- tures of important points on the potential energy surfaces could be deduced. For example in ethylene five time constants and four excited-state vibrations were found for both C2H4 and C2D4 [2]; they indicate that the initial motion involves CC twist and stretch and the conical intersection on transition to the ground state also involves H-migration. In C6H8 the initial acceleration is towards the Franck-Condon active modes (certainly a general rule), followed by an antisymmetric distortion [1]; but the ring-opening is only half completed in the lowest excited state. In Cr(CO)6 (where tunable pump radiation was used) the different initial states are depleted not via an S1 but via a common dissociative state on separate paths: the paths only merge in the primary product Cr(CO)5 in its S1 state [5]. No triplet state is involved in contrast to the previous belief. vi Zusammenfassung Zusammenfassung Diese Arbeit untersucht die Dynamik einiger fundamentaler Moleküle in der Gasphase wäh- rend lichtinduzierter chemischer Reaktionen mit verbesserter Zeitauflösung bis etwa 10 fs (anfangs 30 fs). Diese Reaktionen sind die Isomerisation von Ethylen (C2H4), die Ringöff- nung von Cyclohexadien (C6H8) und die Dissoziation von Cr(CO)6. Es wurde erwartet – und bestätigt – dass diese Moleküle mehrere aufeinanderfolgende, sehr schnelle Prozesse durchlaufen. Zusätzlich wurden hochfrequente kohärente Oszillationen gesucht und tatsächlich aufgelöst. Deren Zuordnung zu Schwingungen erlaubt Rückschlüsse auf geometrische Verformungen des Moleküls während des Prozesses, auf Richtungen von Potentialgefällen (Kräften) und auf den Weg des Wellenpakets. Die Zeitauflösung, die in die- ser Arbeit erreicht wurde, ist um einen Faktor 7 (im nahen UV) bis 15 (160 nm) besser als bisherige Bestwerte. Dazu war es notwendig, Methoden zur Erzeugung kurzer UV-Pulse zu entwickeln, welche – im Gegensatz zum 800-nm-Gebiet – nicht verfügbar waren. Die entwi- ckelten Lichtquellen basieren auf (1) Erzeugung eines Superkontinuums (SC) und (2) Er- zeugung der dritten und fünten Harmonischen, jeweils in Argon, ausgehend von einem 10-fs Ti:Saphir-Laserpuls. Kapitel 1 und 2 beschreiben die Ausbreitung von sehr kurzen, spektral breiten und inten- siven fs-Laserpulsen durch nichtlineare Medien (hauptsächlich gasförmiges Argon) und ex- perimentelle Methoden zur Erzeugung ultrakurzer Pulse im Bereich der Fundamentalen (800 nm) eines kommerziellen Ti:Saphir-Lasersystems sowie außerhalb dieses Spektralbereichs, im UV (250 – 350 nm) und Vakuum-UV (160 nm); diese Methoden basieren auf Selbst-Phasen- Modulation (SPM) und Harmonischen-Erzeugung. Im ersten Schritt wurden 10-fs-Pulse bei 810-nm mit SPM und Selbst-Fokussierung in Ar, kombiniert mit einem chirped-mirror- Kompressor, erreicht ([10] s. 153). Refokussierung dieser Pulse in Ar erzeugt ein SC- Spektrum bis 250 nm, dessen Eigenschaften (z.B. Abbruch-Wellenlänge und Dauer) unter- sucht wurden [10, 4]. Das SC ermöglichte, Strahlung im UV mit abstimmbarer Wellenlänge und Pulsdauer von 30 fs nach Ausbreitung in Luft [9] und 10 fs in Vakuum herauszuschneiden [6].