1,721,017 research outputs found
Studies on the basic building blocks of light-harvesting complexes using ultrafast two-dimensional electronic spectroscopy
No full textThis dissertation describes the development of ultrafast two-dimensional electronic spectroscopy (2DES) using pump-probe geometry and its application in interrogating the underlying femtosecond to picosecond dynamics of basic building blocks in plant light- harvesting complexes (LHC). In the second chapter of this thesis, we provide the description and development of 2DES using a partially collinear geometry. The procedure for post analysis such as conversion of raw data into a purely absorptive 2D data was included in this part of the thesis. The application of 2DES is divided here into three main chapter. In chapter 3, we aims to investigate the effect of finite bandwidth of the interaction pulses in retrieval of the FFCF from 2D spectrum using three different methods (CLSω1, CLSω3, ellipticity). Although all of the methods show correct values in broad excitation bandwidth, our results show that it does not hold true when the excitation bandwidth becomes narrower than the studied absorption band. We used Chl a molecules to test and show that with the help of simulation of the 2D spectra, it is possible to recover the FFCF using any of these methods. Chl a and Chl b are major constituent pigments in LHC complexes. The primary roles of Chl molecules is to absorb light and transfer the energy on a sub-picosecond timescale to the reaction center for the light-chemical energy conversion. It is well established that proteins surrounding Chl molecules play a significant role in optimizing this process. Therefore, understanding the effect of local environment on Chls electronic transition is an important subject to study. 2DES provides a remarkably sensitive tool to study the solute-solvent interaction with high spectral and time resolution. Accompanied by the center line slope (CLS) analysis, in chapter 4, we elucidate the spectral diffusion dynamics of Chlorophyll a (Chl a) and Chlorophyll b (Chl b) in various chemical environments. 2DES was used to measure the frequency fluctuation correlation function (FFCF) of Chl a and Chl b electronic transition. Three time scales and amplitudes of the frequency fluctuations were recovered for the lowest excited state of Chl ranging from hundreds of femtoseconds to picosecond timescales and assigned as the solvation dynamics and spectral diffusion. By measuring them in various solvents, our results revealed significant differences in the extent of inhomogeneous broadening depending on the solvent used, with the biggest contribution of inhomogeneous broadening being due to the polar hydrogen bond solvent and smallest due to the nonpolar solvents. Interestingly, by comparing the results between Chl a and Chl b, our measurements indicated an effect of substituent group in porphyrin ring at position 7 on the rate of relaxation dynamics from an initially inhomogeneous broadening becoming more homogeneous at later Tw (population time). Such evolution was found to be faster for Chl a than Chl b as described in the chapter 4 of this thesis. In the last part of this study (Chapter 5), we utilized 2DES to observe the mechanism of population transfer from the Qx band (S2 state) to the Qy band (S1 state) in Chl a molecule. An ultrafast relaxation from Qx to Qy band was observed to take place in less than Tw=150 fs. Furthermore, observing the cross peak after excitation of the Qx band reveals the type of correlation between the two transition dipole moments. Our results indicate that the Qx and Qy band exhibit minimal correlation even at very short population times. We suggest a possible mechanism explaining lack of the correlation that is based on the fact that the Qx and Qy transition dipole moments are orthogonally oriented with respect to each other.Doctor of Philosoph
Ultrafast transient absorption spectroscopy : 1) Dynamics of carbonyl stretching vibration in metal carbonyl. 2) Applications to photosynthetic systems
No full textThis thesis is focused on the setup and application of ultrafast transient absorption spectroscopy. Ultrafast transient absorption spectroscopy is a four-wave-mixing technique which is also known as ultrafast pump probe spectroscopy. The information on the mechanistic and kinetic details of molecular systems can be provided through this technique.
In the first part of the thesis (Chapter 2), the dynamics of carbonyl stretching vibration in Mn2(CO)10 was studied by ultrafast infrared pump probe spectroscopy. Three vibrational frequencies at 1985 cm-1, 2016 cm-1 and 2047 cm-1 are assigned to four fundamental carbonyl stretching modes, and two of four modes are corresponding to the 2016 cm-1 frequency. The two lower vibrational frequencies 1985 cm-1 and 2016 cm-1 are due to the perpendicular carbonyl stretches while the highest frequency 2047 cm-1 is due to the coaxial carbonyl stretches. The lifetimes of these carbonyl stretching vibrations are obtained through the fit and analysis in terms of two-component exponential decays modified Gaussian function with the temporal response.
In the second and major part of the thesis (Chapter 3, 4, 5, and 6), we describe the application of ultrafast transient absorption spectroscopy to the study of photosynthetic systems. Photosynthesis is the most common light reaction process in nature, which is carried out in chloroplast. Thylakoids, which are organized into sacs of green inner membranes in the chloroplast, are membranes-bound compartments and the sites of the light reaction of photosynthesis. The light reaction function mainly depends on four major protein complexes (photosystem II, photosystem I, cytochrome b6f complex and adenosine triphosphate synthase) which localize at the thylakoid membranes. Usually, the pigments for photosynthesis are bound in these components. The pigments absorb the light energy and transfer the excitation energy to the reaction centres which also contain the pigments.
Chlorophylls are the most important pigments in nature. They widely exist in the plant, bacterial and algae and absorb visible light in the red and blue wavelengths but reflect the green and near-green portions. The typical ultraviolet-visible absorption bands of chlorophylls are localized at two regions: the Soret Band and the Q bands. The Soret bands correspond to the Sn states of chlorophylls while the Q bands are corresponding to the S2 and S1 states of chlorophylls. The energy transfer dynamics between the Soret band and the Q band in chlorophyll a and b was investigated by our pump probe spectrometer. It is found that although the energy gap between the Soret band and the Q band in chlorophyll b is smaller than that in chlorophyll a, the lifetimes of the BQ energy transfer processes in chlorophyll b are longer than that in chlorophyll a.
Most of chlorophylls in nature are bound in the light-harvesting antenna pigment-protein complex (LHC) II which is a supramolecule associated with the core of photosystem II in photosynthetic systems. LHC II exists as a trimer and plays an important role in the light absorption and the efficient electron and excitation energy transfer in photosynthetic reactions. Transient absorption measurements were carried out for three types of LHC II to study the carotenoid to chlorophyll energy transfer dynamics. The excitation energy absorbed by carotenoids is transferred to both chlorophylls via the Qx bands of chlorophylls in a short time scale of few hundreds of femtoseconds in native and wild type LHC II, but in H120L mutant LHC II the excitation energy primarily is guided into chlorophyll a via the Qx bands of both chlorophylls. The energy transfer among the chlorophylls occurs in the subsequent picoseconds and tens of picoseconds.
Photosystem (PS) I complex is an important component in photosynthetic systems. In photosynthesis, PS I absorb the light energy and supply the negative redox and transfer the excitation energy as an intermediator. The energy transfer dynamics among bulk antenna pigments of PS I excited at 650 nm was measured by our transient absorption spectrometer. The excitation energy absorbed by chlorophyll b is transferred from the blue side of bulk pigments to bulk chlorophylls and red chlorophyll. The red chlorophylls absorbing the light with wavelengths at around 715 nm, 730 nm and 750 nm were observed to accept the excitation energy from the 682 nm chlorophyll pool at time scales of 1.93 ps and 10.9 ps.Doctor of Philosophy (SPMS
Applications of ultrafast multidimensional electronic spectroscopy to the study of plant light harvesting systems and colloidal quantum dots
No full textThis dissertation focuses on the development of ultrafast multidimensional electronic spectroscopy using a pump probe geometry and its application in discerning the energy transfer mechanics and band structure analysis on plant light-harvesting systems and colloidal quantum dots. In green plants, the first step of light absorbing processes is carried out and implemented by the light-harvesting complex II (LHCII). The trimeric and aggregated LHCII exhibit the unquenched and quenched excitonic states of Chlorophyll (Chl), respectively. Two-dimensional electronic spectroscopy (2DES) ,which allows direct observation of correlation of excitation and emission energy polls, enables the mapping of the pathways and dynamics from Chl b to Chl a to give insights into the mechanism of non-photochemical quenching that protects the system away from photodamage. Long-lived intermediate Chl a states are present in trimers, while in aggregates, the population decay of these excited states is significantly accelerated, suggesting that, overall, the energy transfer within the LHCII complexes is faster in the aggregated state. In addition, 2DES experiments under conditions free from singlet-singlet annihilation and anisotropic decay are done in the following study. The energy transfer between the different domains within the Chl a manifold is investigated and found to proceed on time scales ranging from hundreds of femtoseconds to five picoseconds, before reaching equilibration. The bidirectional (uphill and downhill) energy transfer of the equilibration process between excited states are clearly observed in experiments. Furthermore, exciton equilibration and excitation trapping in intact Photosystem I (PSI) complexes as well as core complexes isolated from Pisum sativum are studied. Due to the flexibly tailored band structure and optical properties of semiconductor nanocrystals, colloidal semiconductor quantum dots (QDs) become the new black in modern industry. Along with the enormous application in sensing, detecting and lasing etc., understanding of the discrete-like band-edge structure caused by the quantum confined effects and eventually manipulating the corresponding optical properties become a key interest by the physicists and chemists in related field. With the above mentioned points in mind, ultrafast transient absorption (TA) measurements are performed on CdSe core type QDs and subsequently a model to explain the features of the band-edge spectra is developed and described. From our fits of the experimental TA spectra, biexcitonic binding energies for the three lowest energy transitions are obtained. Subsequently, room temperature 2DES measurements are performed on the CdSe QDs accompanying with the nodal line slope (NLS) analysis. This 2D lineshape study indicates unique determination of the contributions from different linewidth broadening sources. NLS method is much more sensitive compared with the 1D histogram TEM analysis of the size dispersion inhomogeneity. Adopting the lineshape model from TA spectroscopy and additional simulation on 2DES spectra, quantitative determination of the linewidth information for the involved the band-edge lowest three energy levels X1, X2 and X3 are available. For a pulse-shaper assisted pump-probe geometry 2D spectroscopy setup, apart from third order nonlinear optical signals, it is also possible to obtain fifth-order nonlinear optical signals. With appropriate phase-cycling schemes, the fifth-order optical signals can be measured. We report on the development of fifth-order two-quantum two-dimensional electronic spectroscopy (2Q2DES). The implementation of this 2Q2DES allows the detection of double quantum coherence, so that multi-excitonic behaviour can be probed. Chl a molecules are used to test the validity of the technique and system. Preliminary results on colloidal CdSe QDs are reported as well to gain insights into biexciton dynamics and behaviour.Doctor of Philosophy (SPMS
Ultrafast mid-infrared laser spectroscopy : applications and technique developments
No full textThis thesis discusses applications and technique development in ultrafast mid infrared nonlinear spectroscopies. After a brief introduction in Chapter 1, the vibrational dynamics of a metal carbonyl compound Os3(CO)12 are investigated with mid-infrared (MIR) pump-probe spectroscopy in Chapter 2. Specifically the vibrational relaxation dynamics of the four infrared active carbonyl stretching normal modes of Os3(CO)12 were measured using broad-band frequency resolved MIR pump-probe spectroscopy. The frequency resolved pump-probe traces measured at the fundamental absorptions exhibit marked differences: The two axial modes at frequencies of 2068 cm-1 and 2034 cm-1 yield similar bi-exponential decay traces, while the two equatorial modes at 2014 cm-1 and 2002 cm-1 show an extra rising component. The axial-equatorial combination anharmonicity constants are found to be near zero. This results in the appearance of the pump-probe signals of these combination bands at the same frequencies as the fundamental transitions, leading to interference and the resultant anomalous rising features. If unaccounted for, these interferences may lead to erroneous conclusions about the dynamics of these vibrational stretches. To avoid such pitfalls, it is therefore imperative to resolve such ambiguities. No obvious direct vibrational energy transfer between the axial and equatorial CO stretching modes was observed.
Since perturbed free induction decay was observed in the MIR studies on Os3(CO)12, this phenomena was studied in further detail for a simpler system, namely W(CO)6, which is presented in Chapter 3. Perturbation theory in the interaction picture was used to model this coherent signal for the fundamental and overtone transitions, respectively. Broadband MIR pump-probe experiments were carried out to yield transient absorption spectra and kinetic signals at negative time delays. The experimental measurements were compared and fitted with the presented modeling, showing excellent agreement.
In the later part of this thesis, we explore the technical improvements towards pulse-shaping assisted two-dimensional (2D) spectroscopies in a pump-probe geometry with a phase cycling scheme. Chapter 4 outlines the development of pulse shaping technique, which produces shaped pulse trains for 2D spectroscopies in a pump-probe geometry. The generation of amplitude, phase, and polarization controlled pulses in the MIR tunable around 3.5 µm is demonstrated. Two temporally separated sets of individually phase and amplitude shaped pulse profiles in the near-infrared are transferred into the MIR via two independent optical parametric amplification processes in two perpendicularly oriented nonlinear crystals in a common-path geometry. The resulting two shaped MIR light fields of orthogonal polarizations are temporally recombined in a birefringent material.
In Chapter 5, the necessary phase cycling schemes for 2D optical spectroscopy in a pump-probe beam geometry are presented.DOCTOR OF PHILOSOPHY (SPMS
Two-dimensional electronic spectroscopy of plant photosynthetic systems
No full textIn this project, we developed a model to simulate the dynamics of Light-harvesting Complex II
(LHCII) based on spectra from Ultrafast 2D Electronic Spectroscopy experiment. LHCII is an
important component in the photosynthesis process, which acts like an antenna to absorb light
energy and transfer to the reaction center effectively. A Hamiltonian is used to represent the
energy structure of the complex, whose energy levels are the eigenvalues and coupling strengths
are the off-diagonal elements. The population transfers between the levels represent the energy
transfer processes, which happen in femtosecond to picosecond scales. The results were compared
to previous theoretical studies and some good agreemetns can be foundBachelor of Science in Applied Physic
Technical development of coherent two-dimensional and three-dimensional optical spectroscopy
No full textThe work on this dissertation is focused on the technical development of multidimensional nonlinear optical spectroscopy. Using pulse shaping techniques, two-dimensional (2D) and three-dimensional (3D) optical spectroscopy are performed in the visible spectral region. Technical developments in generating and characterizing polarization-shaped ultraviolet (UV) pulses are explored as well, with the aim of implementing a polarization-based coherent multidimensional optical spectrometer. Polarization-shaped pulses in the ultraviolet (365 nm) are generated using two independent sum-frequency generation (SFG) processes in two orthogonally orientated β-barium borate crystals. The amplitudes, phases and the delays in the two sets of visible (650 nm) and near infrared (800 nm) pulses that contribute to the two SFG processes are controlled by a liquid crystal spatial light modulator pulse shaper (LC-SLM). The two orthogonally polarized shaped UV pulses are then temporally recombined via a birefringent crystal. The interferometric phase stability is well maintained in this common-path geometry setup. To characterize the amplitude, phase and polarization of the shaped UV pulses, we present a method referred to as “difference frequency generation cross-correlation tomographic ultrafast retrieval of transverse light E-fields” (DFG-XTURTLE) that combines the technique of tomographic ultrafast retrieval of transverse light E-fields (TURTLE) with the amplitude and phase retrieval algorithm of the cross-correlation frequency-resolved optical gating (XFROG). The handedness of the elliptically shaped pulses can also be retrieved with additional measurements with a quarter-wave retardation plate. 2D optical spectroscopy is performed in a pump-probe geometry via a pulse shaper assisted setup. An acousto-optic programmable dispersive filter (AOPDF) generates the first two interaction pump pulses with controllable delays and relative phases, while a white light continuum provides the third interaction probe pulse. Phase cycling schemes are prerequisite in order to isolate the desired signal. With additional data processing steps, we further retrieve the rephasing and non-rephasing 2D spectra with the same approach. A full discussion on the phase cycling schemes needed for coherent 3D optical spectroscopy is provided. The experimental demonstration used to measure purely absorptive fifth-order 3D electronic spectrum is based on the same experimental setup used in 2D spectroscopy. In the 3D electronic spectroscopy setup, four pulses are generated using a pulse shaper in 3D spectroscopy, and higher-order phase cycling schemes are needed to remove the unwanted signals. Through data processing, all pathways that contribute to the purely absorptive 3D spectrum are retrieved.Doctor of Philosoph
Topics in multidimensional optical spectroscopy I. Theoretical studies of finite pulse effects II. Applications to plant light-harvesting complex and nanomaterials
No full textIn Nature, there are numerous important processes happening at the picosecond, femtosecond and attosecond timescales. With the development of the ultrafast laser and techniques such as ultrafast multidimensional optical spectroscopy, scientists are able to study many of such ultrafast dynamic processes. In this thesis, we present both theoretical and experimental studies on multidimensional optical spectroscopy (MDOS) and the applications of ultrafast two-dimensional electronic spectroscopy (2DES) to the study of photosynthetic pigment-protein complexes and nanomaterials.
MDOS spectra are affected by the use of finite bandwidth excitation pulses due to the convolution properties of the multi-pulse interactions. Disentangling the effects of finite bandwidth is not trivial. In Chapters 2 and 3 of this thesis, we present theoretical studies on how the finite bandwidth and the duration of excitation pulses alter the MDOS spectra. We derive an approximate and simplified expression that can be used to calculate the MDOS spectra that includes finite bandwidth effects. We found that for third-order two-dimensional optical spectra, in certain conditions, the obtained expressions are exact and contain only simple multiplications of the pulse spectra and the system response functions. We also obtained expressions for fifth-order optical spectra, such as two-quantum two-dimensional and three dimensional optical spectra. In these cases, due to the higher complexity caused by a higher number of pulse interactions, the obtained formulae are only approximate. However, by comparing the simulations using the exact formalism and our approximated expressions, the approximated formula can reproduce most of the main spectral features very well. These results will be beneficial to the MDOS community by providing simpler and more resource-efficient ways of incorporating the finite bandwidth effects of excitation pulses into the simulation of MDOS spectra. It will also facilitate the extraction from experimental MDOS spectra, of relevant information contained in the systems’ response functions.
Chapters 4 and 5 of this thesis present the application of 2DES on studying the ultrafast excitation energy transfer (EET) processes in Light-harvesting complex II (LHCII) the most abundant light-harvesting antenna on Earth. LHCII plays an important role in photosynthesis due to its main functions of harvesting sunlight energy and funnelling it to the reaction center to regulate photosynthesis. As the first pigment protein complex in eukaryotic plants having its crystal structure solved, LHCII has been intensively studied during the past decades. Many experimental and theoretical studies have been proposed to try to solve the EET scheme. However, due to the highly congested spectral features and ultrafast dynamics in the femtosecond timescale, there is no consensus on the model for LHCII EET dynamics. In our studies, we implement 2DES with the capability of correlating the excitation and emission frequencies to track the ultrafast EET dynamics of LHCII. 2DES measurements of LHCII at various temperatures elucidate the temperature-dependent excitation energy transfer dynamics of the Chl a manifold of LHCII. Global analysis results reveal three main timescales for the energy equilibration dynamics of LHCII. A clear indication of the temperature dependence of uphill/downhill energy transfer processes was also discerned, which follows the detailed-balance condition. In Chapter 5, we take a closer look at the cryogenic 77 K 2DES measurement. We construct a phenomenological model to fit our 2DES data and extract the spectral information of the excitonic states, as well as the kinetic parameters. Due to the restriction of the model and complexity of the system, the EET network cannot be constructed uniquely. However, based on the agreement of our fitting results with the previously proposed structure-based calculations, we have proposed two EET schemes with tentative pigment assignments. In addition, the existence of a ‘bottleneck’ state in the intermediate wavelength region (660-670 nm) is supported by our results. Our model also determines three terminal excitons having the lowest energy levels, and the excitation can always be equilibrated among these three states without any significant EET directly connecting them. These equilibration processes happen via the uphill energy flow to the higher excitonic levels bridging the three lowest termini and this equilibration mechanism accelerates the EET at higher temperatures and results in the temperature dependence of EET processes in LHCII.
Chapter 6 presents the application of 2DES on studying the photophysics of nanomaterials. Due to intrinsic limitations in synthesis procedure, the nanomaterials, like quantum dots (QDs) and nanoplatelets (NPLs) always exhibit heterogeneity in size, shape, and spectroscopic properties. We perform 2DES measurements on zero-dimensional CdSe QDs and two-dimensional CdSe NPLs. Employing the nodal line slope and central line slope analysis, we can extract information about the systems’ homogeneity, inhomogeneity and spectral diffusion dynamics from our measured 2DES data. The results show that no spectral diffusion dynamics occur for the CdSe QDs. On the other hand, spectral diffusion was observed in the 5 mono-layers CdSe NPLs’ heavyhole transition. The normalized frequency fluctuation correlation function of the CdSe NPLs’ heavy-hole transition was measured to have a major fast decay component at 80 fs, followed by a slower diffusion component of 2.5 ps.Doctor of Philosoph
Generation of mid-infrared laser using solid state laser and nonlinear optical frequency conversion
No full textCoherent MIR radiation has several applications such as remote sensing, laser surgery, nonlinear spectroscopy, countermeasures, etc. Generation of coherent MIR radiation is achieved by solid state laser and quantum cascade laser with direct MIR lasing, or nonlinear frequency conversion from laser with shorter wavelength. In this thesis, we developed several different laser configurations to access the MIR wavelengths.
Firstly, a high repetition rate high power Ho:YAG laser was developed to generate high average power MIR ns pulse in an intra-cavity ZGP OPO configuration. A high energy Ho:YAG MOPA laser pumped ZGP crystal in an extra-cavity OPO configuration was demonstrated. A novel uncoated wedge ZGP OPO configuration was studied, addressing the AR-coating limitation encountered in conventional ZGP OPO.
Secondly, Coherent Polarization Locking configuration was implemented in Ho:YAG oscillator. Studies show the Coherent Polarization Locking configuration improves the beam quality in CW operation and reduces the risk of intra-cavity optical damage in pulse operation. Improvement in beam quality is due to the reduction of thermal related effects in the oscillator. Splitting the intra-cavity intensity in the Coherent Polarization Locking cavity reduces the possibility of intra-cavity optical damage.
Lastly, a high energy Yb:CaF2 regenerative amplifier was developed as the pump source for a multi-stage ZGP OPA setup to generate fs MIR laser pulses. To increase the efficiency of fs MIR generation, a high energy Ho:YAG regenerative amplifier was explored as the direct pump source for ZGP OPA system.DOCTOR OF PHILOSOPHY (SPMS
Solvent effects on the spectral diffusion dynamics of chlorophylls
No full textChlorophyll a (Chl a) and Chlorophyll b (Chl a) are pigment molecules that are vital to the process of photosynthesis in green plants. These molecules bind to the protein matrix through ligation with side groups in the protein scaffold. The manner in which protein side groups bind to the Mg2+ ions play an important role in the function of Chls, tuning the energy based on the nature of Chl-ligand interactions which aids in the acceptance of light energy for photosynthesis. In this thesis, we aim to understand the relaxation dynamics of Chl a and Chl b monomers in solvents to provide a greater insight of its behaviour in the protein matrix. We use ultrafast two-dimensional electronic spectroscopy (2DES) to determine the frequency fluctuation correlation function (FFCF) in a range of solvents, measuring the spectral diffusion process of the Qy transition using the Centre Line Slope (CLS) method.
In this thesis, we experiment with how the interactions between the surrounding solvent molecules and Chl affect the FFCF in Chl a and Chl b. In chapter 2, the factor discussed is the polarity and hydrogen bonding ability of the surrounding solvent. We measure the FFCF decay for Chl a and Chl b in a protic solvent, methanol and aprotic solvents, tetrahydrofuran and diethyl ether, all of which have varying polarity. We find that the polarity of the solvent does not affect the FFCF decay but a significant ~ 40 ps component is only present in the protic solvent, methanol. We also observe that Chl b exhibits a higher level of inhomogeneous broadening as compared to Chl a despite their similarities in molecular structure.
Next, in chapter 3 we discuss the effect of Lewis basicity of the surrounding solvent. We perform 2DES experiments on Chls in solvents of various Lewis basicity, which is measured by the pKa and BF3 affinity. From the decay of the FFCF, we find that certain timescales of the spectral diffusion dynamics are correlated to the Lewis basicity of the solvent. Additionally, control experiments and theoretical calculations are used to try and better understand the molecular mechanism causing this correlation. The sensitivity of Chls to the Lewis basicity of their surrounding solvent is due to the dynamics of the dative bond between the Mg2+ of the Chl and the heteroatom of the solvent molecule. We are able to show that the 2DES method can act as a sensitive probe to study the molecular mechanism associated with the Lewis basicity of a solvent.Master of Scienc
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