Femtosecond Time-Resolved Four-Wave Mixing Applied to the Investigation of Excited State Dynamics by Vinu V. Namboodiri A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physics Approved by the thesis Committee: (Prof. Dr. Arnulf Materny) (Prof. Dr. Ulrich Kleinekathöfer) (PD Dr. Michael Schmitt) Date of Defence: May 18, 2010 School of Engineering and Science Femtosecond Time-Resolved Four-Wave Mixing Applied to the Investigation of Excited State Dynamics Dedicated to my family, friends and Sreeappan Abstract Laser spectroscopy in the frequency domain has, very early, developed into a pow- erful tool for the analysis of structural properties of molecules. The development of ultrafast lasers added a new dimension to conventional spectroscopy by rendering time resolved measurements possible. The high time resolution offered by picosecond and femtosecond laser pulses enabled the real time observation of extremely fast pro- cesses, such as vibrations and rotations of molecules. With pulses of duration about 100 fs, it is now possible to monitor processes such as internal conversion, vibrational relaxation, and many other processes occurring in the excited electronic states which leads to reactive or energy transfer pathways. The work presented in this thesis fo- cuses on the study of molecular dynamics in these excited electronic states using time- resolved four-wave mixing (FWM) techniques. It is demonstrated that, by combining the FWM process with an excitation pulse, it is possible to study molecular dynamics in the excited states of gaseous and condensed phase samples. The advantages of the four-wave mixing technique over the commonly used time-resolved fluorescence and pump-probe techniques are also discussed. The pump-FWM method is applied to simple molecules in the gas phase as well as to complex molecular systems where in- ternal conversion processes dominate the ultrafast dynamics. The thesis also presents preliminary studies on the effect of the surface enhancement effect of the nonlinear optical process coherent anti-Stokes Raman scattering (CARS) in presence of colloidal metal particles. vii Contents 1 Introduction 1 2 Theory 5 2.1 Femtosecond Time Resolved Spectroscopy . 5 2.2 Perturbation theory for time resolved spectroscopy . 5 2.2.1 First order polarisation . 8 2.2.2 Second order polarization . 10 2.2.3 Third order polarization . 11 2.3 Pump-Probe Spectroscopy . 13 2.4 Coherent anti-Stokes Raman Scattering (CARS) . 15 2.5 Degenerate Four Wave Mixing (DFWM) . 18 3 Experimental Setup 21 3.1 Femtosecond Laser System . 21 3.2 Optical Parametric Amplifiers . 23 3.3 Pulse Characterization . 23 3.4 Experimental Setup . 24 3.5 Phase Matching in Four-Wave Mixing . 26 4 Excited State Dynamics in Molecular Iodine 29 4.1 Molecular states of Iodine . 30 4.2 Experimental . 32 4.3 Pump-Degenerate Four-Wave Mixing in Iodine . 33 4.4 Summary . 39 5 Excited State Dynamics in b-Carotene 41 5.1 Introduction . 41 5.2 b-Carotene Photophysics . 42 5.2.1 S2 State . 42 5.2.2 S1 State . 43 ix x 5.2.3 Vibrational Relaxation . 44 5.3 pump-Four Wave Mixing (pump-FWM) on b-Carotene . 45 5.4 Experimental Setup for pump-FWM . 46 5.5 Molecular Dynamics Using pump-DFWM . 47 5.5.1 Kinetic Analysis . 51 5.5.2 Comparison between results from two-pulse pump-probe and pump-DFWM experiments . 52 5.6 Ground and Excited State Dynamics Using pump-CARS . 56 5.7 Summary . 58 6 Surface Enhanced Coherent Anti-Stokes Raman Scattering 59 6.1 Surface enhanced CARS (SE-CARS) . 61 6.1.1 Experimental . 62 6.1.2 Preparation of Silver sol . 62 6.1.3 CARS Experiment . 63 6.2 Discussion on SE-CARS . 65 6.3 Summary . 67 7 Conclusion and Outlook 69 References 73 Acknowledgment 79 List of Publications 81 Appendix 83 1 Introduction Optical spectroscopy is the most suitable experimental technique for understanding molecular structure and dynamics due to its speed and sensitivity. A careful study of the processes, such as absorption, emission, scattering, linear and circular dichroism, that occur when a light field interacts with a molecule can provide us with impor- tant information about the energy, concentration, conformation, and dynamics of the molecule. The invention of the laser, which can provide intense, coherent and tun- able radiation, revolutionized the field of spectroscopy. The increase in power of the radiation provided by lasers along with the ability to produce short pulses gave rise to a completely new nonlinear optical effects, where the absorbed radiation power depends nonlinearly on the incident power. Thereafter, nonlinear interaction of light with matter became an important field of study, which found applications in various fields including spectroscopy. Development of ultrafast laser systems brought about another breakthrough in the field of spectroscopy. Time resolved spectroscopy using picosecond (10¡12) and femtosecond (10¡15) lasers provided a new window for spectroscopists to investigate molecular dynamics, such as vibrations and rotations of molecules. Ahmed Zewail was awarded Nobel prize for his pioneering work in femtochemistry, which demon- strated the use of femtosecond pulses for real time probing of chemical reaction dy- namics. The observation of coherent dynamics resulting from the excitation of molecu- lar systems by femtosecond lasers is the heart of femtochemistry. The new insight into the dynamical aspects of molecules in short time scales brought great advances in the field of molecular physics. Several nonlinear optical techniques were then used along with ultra short pulses to probe different aspects of molecular dynamics occurring in pico and femtosecond time scales. The high peak intensities of ultrafast lasers often lead to different optical nonlin- earities. Among different nonlinear optical processes, third order nonlinear optical effects, due to the many degrees of freedom they offer, have proven to be ideal candi- 1 2 Introduction dates for time resolved spectroscopy to address different aspects of dynamics occur- ing in molecules [1, 2]. Most commonly used third order nonlinear optical processes are the pump-probe and four wave mixing (FWM). The pump-probe technique was used extensively by Zewail for his works on the ultrafast dynamics of the chemical bond [3]. Their works showed that the key to obtaining spectral and structural infor- mation in ultrafast time resolved measurements is based on the generation and detec- tion of wavepackets [3,4]. Transient absorption techniques was the primary technique used to gain information about the ultrafast primary processes in photosynthesis and vision [5–10]. In recent years four wave mixing spectroscopy has rapidly been devel- oped to study the dynamics of molecules in condensed, liquid and vapor phases. Four wave mixing technique using ultrashort pulses can be used in a variety of different forms such as coherent anti-Stokes Raman scattering (CARS), degenerate four wave mixing (DFWM), transient grating (TG), photon echo (PE) etc. to ad- dress different aspects of molecular dynamics. Coherent anti-Stokes Raman scatter- ing (CARS) was first used by the groups of Lauberau, Zinth and Kaiser to moni- tor vibrational dynamics of molecules occurring in a femtosecond time scale [11–13]. Materny and co-workers time resolved femtosecond CARS and DFWM processes to observe wavepacket dynamics occuring on the ground and excited states of iodine vapour [14, 15]. By varying the time delay of one of the incident pulses and keeping the other two overlapped in time, they were able to observe wavepacket dynamics in both the ground and excited electronic states. Zewail and co-workers used pump- degenerate four wave mixing (DFWM) to investigate molecular dynamics. Here the pump pulse initiates the molecular dynamics and the DFWM process is used as a probe [16]. Dietzek et. al. used femtosecond time resolved four wave mixing for studying excited state dynamics in different systems [17, 18]. Brown et. al. used off resonant transient grating (TG) technique for the study of molecular dynamics in gas- phase systems ranging from single atoms to large polyatomic molecules [19]. Ultrafast energy transfer and solvation dynamics has been extensively investigated by Fleming et. al. using photon echo (PE) techniques [6,20,21]. Femtosecond nonlinear techniques such as PE and TG have also been used to investigate primary processes in photosyn- thesis [22–24]. Femtosecond stimulated Raman technique using a narrow band pump laser and a broad band probe laser has gained much interest in recent years. It can give vibrational structural information with high temporal and spectral resolution [25–28]. The work presented in this thesis is divided into two parts. One part of the work uses time resolved four wave mixing techniques using femtosecond laser pulses to in- vestigate molecular dynamics taking place in the excited states of simple and complex 3 molecules. The degenerate four wave mixing technique with an additional pump pulse is applied to investigate the vibrational dynamics occurring in the excited states of the diatomic system iodine. Iodine is a much investigated molecule whose excited state properties are well characterized. Using iodine as a model system, the applicability of such time resolved nonlinear techniques in investigating higher lying excited states can be verified. The technique will then be extended to a more complex molecule. The photosynthetic pigment molecule b-carotene was chosen as the other system for investigation. Studies on the excited states of this molecule are important due to the important role it plays in plant photosynthesis [29]. Time resolved third order nonlinear optical processes are used in order to understand population and vibrational dynamics occurring in the excited states of b-carotene. The population dynamics af- ter photo excitation is probed using a pump-DFWM scheme, where the pump pulse promotes population into the excited state and the subsequent flow of population is monitored using a DFWM process as probe.