<p> We investigated the local electronic properties of the triangular-lattice materials NaxCO2 (x = 0.3, 0.72) and the superconductor Na0.3Co02-1.3H20 by 59Co and 170 Nuclear Magnetic Resonance(NMR). For Na0.72Co02 , 59Co NMR line shape shows clearly that there are two types of Co sites - Co(A) site and Co(B) site. The electronic character of Co(A) site is close to that of the less magnetic Co+3-like ion with spin rv 0, while the electronic character of Co(B) site is close to that of the strongly magnetic Co+4-like ions with spin ~ 1/2. The temperature dependence of the Knight shifts suggests that the Co(A) and Co(B) sites are electronically coupled, which is not consistent with simple phase separation. The local Co electronic environments propagate to the adjacent 0 layers through p-d hybridization. Therefore, there are two types of oxygen sites, O(A) site and O(B) site. We introduced a different route to do K vs x plot analysis for the Co sites and determined that for the Co sites, Karbitat(A) is 1.816 % and Kspin(A) is about rv 0.2 %; Karbitat(B) is 4.0255 % and Kspin(B) is at least 1.5 %. For Na0 .3Co02 , 59Co NMR line shape shows that the Co valence is averaged out in this material. There are two types of oxygen sites, 0( C) site and O(D) site, presumably because of the nearest neighbor Na+ sites. The constant behavior of Knight shifts below 100 K for both the Co and 0 sites suggests the emergence of a low temperature canonical Fermi-liquid behavior. For the superconductor Na0.3CoO2-1.3H20, both 59Co and 170 NMR line shapes show that there is only one type of Co site and oxygen site. The Knight shifts of 59Co and 170 are temperature independent below 100 K down to Tc. Combined with our spin-lattice relaxation 1/T 1 T measurements, we can rule out the possibility of ferromagnetic scenario of spin excitations above Tc. </p> / Thesis / Master of Science (MSc)

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Contemporary laser research involves the development of spectroscopic techniques to understand the microscopic structural aspects of a simple molecular system in chemical and materials to more complex biological systems such as cells. In particular, Raman spectroscopy, which provides bond specific information, has attracted considerable attention. Further with the advent of femtosecond (fs) laser, the recent trend in the field of fs chemistry is to develop nonlinear Raman techniques that allow one to acquire vibrational structural information with both fs temporal resolution as well as good spectral resolution. Among many advanced nonlinear Raman techniques, the development of fs Stimulated Raman scattering (SRS) has gathered momentum in the recent decade due to its ability to (1) provide vibrational structural information of various system including fluorescent molecules with good signal to noise ratio and (2) circumvent the limitation imposed on the spectral resolution by the necessary pulse durations according to the energy-time uncertainty principle where ‘K’ is a constant that depends on the pulse shape) unlike in the case of fs normal resonance Raman spectroscopy. We have developed a technique named “Ultrafast Raman loss spectroscopy (URLS)” that is analogues to SRS, but is more advantageous as compared to SRS and has the potential to be an alternative if not competitive tool as a vibrational structure elucidating technique. The concept and the design of this novel technique, URLS, form the core of the thesis entitled “Ultrafast Raman Loss Spectroscopy (URLS)”. Chapter 1 lays the theoretical groundwork for ultra-short pulses and nonlinear spectroscopy which forms the heart of URLS. It presents a detailed discussion on the basis behind the elementary experimental problems associated with the ultra-short laser pulses when they travel through a medium, the characterization of these ultrashort pulses as well as various non-linear phenomena induced within a medium due to the propagation of these pulses. Chapter 2 focuses on the concept of SRS which resulted into the foundation of URLS. It illustrates the theoretical as well as the experimental aspects of SRS and demonstrates the sensitivity of SRS over normal Raman spectroscopy. Chapter 3 introduces the conceptual and the technical basis which ensued into the development of URLS while Chapter 4 demonstrates its application and efficiency over its analogue SRS. URLS involves the interaction of two laser sources, viz. a picosecond (ps) pulse and a fs white light (WL), with a sample leading to the generation of loss signal on the higher energy (blue) side with respect to the wavelength of the ps pulse unlike the gain signal observed on the lower energy (red) side in SRS. These loss signals are at least 1.5 times more intense than SRS signals. Also, the very prerequisite of the experimental protocol for signal detection to be on the higher energy side by design eliminates the interference from fluorescence, which appears on the red side. Thus, the rapid data acquisition, 100% natural fluorescence rejection and experimental ease ascertain “Ultrafast Raman Loss Spectroscopy (URLS)” as a unique valuable structure determining technique. Further, the effect of resonance on the line shape of the URLS signal has been studied which forms the subject of discussion in Chapter 5. The objective of the study is to verify whether the variation of resonance Raman line shapes in URLS could provide an understanding of the mode specific response on ultrafast excitation. It is found that the URLS signal’s line shape is mode dependent and can provide information similar to Raman excitation profile (REP) in the normal Raman studies. This information can have impact on the study of various dynamical process involving vibrational modes like structural dynamics and coherent control. Chapter 6 demonstrates the application of URLS as a structure elucidating technique for monitoring ultrafast structural and reaction dynamics in both chemical and biological systems using α-terthiophene (3T) as the model system. The objective is to understand the mechanism of the molecular structure dependent electronic relaxation of the first singlet excited state, S1, of α-terthiophene using fs URLS. The URLS data along with the ab-initio calculations indicate that the electronic transition is associated with a structural rearrangement from a non-planar to a planar configuration in the singlet manifold along the ring deformation co-ordinate. The experimental findings suggest that the singlet state decays exponentially with a decay time constant ( 1/e) of about 145 ps and this decay could be assigned to the intersystem crossing (ISC) pathway from the relaxed S1 state to the vibrationally hot triplet state, T1*. Lastly, Chapter 7 summarizes the entire thesis and presents some possible future prospects for URLS. Considering the advantages of URLS, it is proposed that URLS can be exploited [1] to determine the structure of any fluorescent/non-florescent condensed materials and biological systems with a very good spectral resolution (10- 40 cm-1); [2] to obtain the vibrational signature of weak Raman scattering molecules and vibrational modes with relatively small Raman cross-section owing to its high detection sensitivity with good signal to noise ratio; [3] for performing fs time-resolved study by introducing an additional fs pulse for photo-excitation of the molecule and using URLS to probe the excited state dynamics with good temporal (fs) and spectral (10-40 cm-1) resolution; and lastly, [4] the high chemical selectivity of URLS and the fact that the signal is generated only within the focal volume of the lasers where all the beams overlap can be utilized for developing this method into a microscopy for labeled-free effective vibrational study of biological samples. Consequently, it is hoped that this technique, “Ultrafast Raman Loss Spectroscopy (URLS)”, would be a suitable alternative to other nonlinear Raman methods like coherent anti-Stokes Raman spectroscopy (CARS) that has made major inroads into biology, medicine and materials.

O comportamento das formas de linha dos espectros de RMN estático em função da temperatura assim como os experimentos de PURE Exchange em uma dimensão (PUREX-1D) foram utilizados para elucidar detalhes da dinâmica molecular nos PnMA. Os experimentos PUREX-1D foram realizados na faixa de temperatura de Tg-145 o C a Tg. Nesta faixa de temperatura a dinâmica molecular dos PnMA acontece com tempos de correlação entre 500 micro e 10 milissegundos. Utilizando simulação espectral foi demonstrado que os espectros PUREX-1D dependem tanto do tempo de correlação quanto da geometria das reorientações moleculares. Este fato também foi usado para avaliar as características dos movimentos moleculares responsáveis pela reaxação estrutural beta nos PnMA. Foi observado que os pequenos ângulos de reorientação da cadeia principal associados com a relaxação estrutural beta aumentam com o aumento da temperatura e que os tempos de correlação do movimento podem apresentar uma larga distribuição. Análises das formas de linha obtidas através de experimentos de polarização cruzada (CP) em função da temperatura foram realizadas para se estudar o comportamento da dinâmica molecular responsável pela relaxações estruturais alfa e beta nos PnMA. Os experimentos CP foram realizados em uma faixa de temperaturas entre Tg-50 o C e Tg.+70 o C. Nesta faixa de temperatura as figuras de linha de RMN estática são altamente dependentes da temperatura, do tamanho e do volume do ramo lateral. A dinâmica molecular dos PnMA apresentou um movimento altamente anisotrópico sendo que a isotropização da conformação do espectro induzida pelos movimentos moleculares ocorreu apenas a temperaturas muito acima da Tg, contrariando o comportamento esperado para a maioria dos polímeros amorfos. / The behavior of the static NMR line shapes as a function of the temperature as well as one-dimensional Pure Exchange NMR experiments (PUREX-1D) were used to elucidate details of the molecular dynamics in PnMAs. The 1D-PUREX experiments were carried out in the temperature range of Tg-145 oC to Tg. In this range of temperature the molecular dynamics of PnMAs was found to occur with correlation times between 500m and 10ms. Using spectral simulations it was demonstrated that the 1D-PUREX spectra depend on both, the correlation time and geometry of the molecular reorientations. Hence this feature was used to evaluate the characteristics of the molecular motions responsible by the b structural relaxation of PnMAs. It was found that the small angles reorientation of main chain associated with the b-structural relaxation increases with the temperature and the correlation times present a possible broad distribution. Analysis of the NMR line shapes obtained in Cross Polarization (CP) experiments as a function of temperature was performed to study the behavior of the molecular dynamics responsible for the b and a structural relaxation. The CP experiments were performed in the temperature range of Tg-50 oC to Tg+70 oC. In this temperature range the static NMR line shapes are highly dependent on the temperature, size and bulk of the side-group. The dynamics of the PnMA presents a highly anisotropic motion and the isotropisation of conformation induced by the motion only occurs in temperatures well above Tg, contrary to the behavior of the most fragile amorphous polymers.

O comportamento das formas de linha dos espectros de RMN estático em função da temperatura assim como os experimentos de PURE Exchange em uma dimensão (PUREX-1D) foram utilizados para elucidar detalhes da dinâmica molecular nos PnMA. Os experimentos PUREX-1D foram realizados na faixa de temperatura de Tg-145 o C a Tg. Nesta faixa de temperatura a dinâmica molecular dos PnMA acontece com tempos de correlação entre 500 micro e 10 milissegundos. Utilizando simulação espectral foi demonstrado que os espectros PUREX-1D dependem tanto do tempo de correlação quanto da geometria das reorientações moleculares. Este fato também foi usado para avaliar as características dos movimentos moleculares responsáveis pela reaxação estrutural beta nos PnMA. Foi observado que os pequenos ângulos de reorientação da cadeia principal associados com a relaxação estrutural beta aumentam com o aumento da temperatura e que os tempos de correlação do movimento podem apresentar uma larga distribuição. Análises das formas de linha obtidas através de experimentos de polarização cruzada (CP) em função da temperatura foram realizadas para se estudar o comportamento da dinâmica molecular responsável pela relaxações estruturais alfa e beta nos PnMA. Os experimentos CP foram realizados em uma faixa de temperaturas entre Tg-50 o C e Tg.+70 o C. Nesta faixa de temperatura as figuras de linha de RMN estática são altamente dependentes da temperatura, do tamanho e do volume do ramo lateral. A dinâmica molecular dos PnMA apresentou um movimento altamente anisotrópico sendo que a isotropização da conformação do espectro induzida pelos movimentos moleculares ocorreu apenas a temperaturas muito acima da Tg, contrariando o comportamento esperado para a maioria dos polímeros amorfos. / The behavior of the static NMR line shapes as a function of the temperature as well as one-dimensional Pure Exchange NMR experiments (PUREX-1D) were used to elucidate details of the molecular dynamics in PnMAs. The 1D-PUREX experiments were carried out in the temperature range of Tg-145 oC to Tg. In this range of temperature the molecular dynamics of PnMAs was found to occur with correlation times between 500m and 10ms. Using spectral simulations it was demonstrated that the 1D-PUREX spectra depend on both, the correlation time and geometry of the molecular reorientations. Hence this feature was used to evaluate the characteristics of the molecular motions responsible by the b structural relaxation of PnMAs. It was found that the small angles reorientation of main chain associated with the b-structural relaxation increases with the temperature and the correlation times present a possible broad distribution. Analysis of the NMR line shapes obtained in Cross Polarization (CP) experiments as a function of temperature was performed to study the behavior of the molecular dynamics responsible for the b and a structural relaxation. The CP experiments were performed in the temperature range of Tg-50 oC to Tg+70 oC. In this temperature range the static NMR line shapes are highly dependent on the temperature, size and bulk of the side-group. The dynamics of the PnMA presents a highly anisotropic motion and the isotropisation of conformation induced by the motion only occurs in temperatures well above Tg, contrary to the behavior of the most fragile amorphous polymers.

Raman spectroscopy involves change in the polarizability of the molecular system on excitation and is based on scattering process. Spontaneous Raman scattering is a two photon process, in which the input light initiates the excitation, which then leads to an emission of another photon due to scattering. It is extensively used to understand molecular properties. As spontaneous Raman scattering is a weak process, the detection of these weak Raman photons are rather difficult. Alternatively, resonance Raman (RR) scattering is another technique where the excitation wavelength is chosen according to the material under study. The excitation wavelength is chosen to be within the absorption spectrum of the material under study. RR spectroscopy not only provides considerable improvement in the intensity of the Raman signal, but also provides mode specific information i.e. the modes which are Franck-Condon active in that transition can be observed. There are reports on RR studies of many systems using pulsed light as an excitation source. It is necessary to use at least two pulsed laser sources for carrying out the time resolved RR spectroscopy. A single pulse source for excitation would lead to compromise either with temporal or spectral resolution which is due to the uncertainty principle. If an excitation pulse has pulse width of ~100 femtoseconds then the spectral resolution will be ~ 150 cm-1. It is clear now that for improving the temporal and spectral resolution simultaneously, usage of single pulse for Raman experiments (spontaneous scattering) is not adequate. The usage of multiple laser pulses may provide the way out to improve the resolutions. Nonlinear spectroscopy in a broad view helps in understanding the structural and dynamical properties of the molecular systems in a deeper manner. There are a number of techniques as a part of nonlinear spectroscopy that have emerged in due course to meet different requirements and to overcome some difficulties while understanding the molecular properties. Stimulated Raman (SRS) gain, coherent anti-Stokes Raman scattering (CARS) and the inverse Raman spectroscopy are a few to mention as third order nonlinear spectroscopic techniques which give the similar kind of information about the molecular systems. Stimulated Raman scattering is a more general process involved in nonlinear Raman processes. SRS involves at least two laser pulses and the difference in their frequencies should match with the vibrational frequency of the molecule. The polarization has to be matched between the Raman pump and the Raman probe pulses. We have developed a new nonlinear Raman technique in our laboratory named as ultrafast Raman loss spectroscopy (URLS) using the principles of nonlinear Raman scattering. It involves the Raman pump (~ 1 picosecond (ps) or ~ 15 cm-1spectral resolution) and Raman probe as a white light continuum (100 fs) whose frequency components ranges from 400-900 nm. The laser system consists of Tsunami which is pumped by a Millennia laser and Spitfire-Pro, a regenerative amplifier which is pumped by an Empower laser. Tsunami provides a 100 fs, 780 nm centered, 80 MHz and ~6 nJ energy laser pulses. The Tsunami output is fed into Spitfire to amplify its energy and change the repetition rate to 1 KHz. The pulse length of the input pulse is preserved in amplification. The output of amplifier is split into two equal parts; one part is used to pump the Optical Parametric Amplifier (OPA) in order to generate wavelengths in the range 480-800 nm. The output of the OPA is utilized to generate Raman pump which has to be in ps in order to get the best spectral resolution. A small portion of the other part of amplifier output is utilized to generate white light source for the Raman probe. The remaining part of the amplifier output is used to pump TOPAS to generate wavelengths in the ultraviolet region. URLS has been applied to many molecular systems which range from non-fluorescent to highly fluorescent. URLS has been demonstrated to be very sensitive and useful while dealing with highly fluorescent systems. URLS is a unique technique due to its high sensitivity and the Raman loss signal intensity is at least 1.5-2 times higher as compared to the Raman gain signal intensities. Cresyl violet perchlorate (CVP) is a highly fluorescent system. URLS has been applied to study CVP even at resonance excitation. Rhodamine B has also been studied using URLS. Spontaneous Raman scattering is very difficult to observe experimentally in such high quantum yield fluorescent systems. The variation in the lineshapes of the Raman bands for different RP excitation wavelengths in URLS spectra shows the mode dependent behavior of the absorption spectrum. The experimental observation of variation in the lineshape has been accounted using theoretical formalism. The thesis is focused on discussing the development of the new nonlinear Raman spectroscopic technique URLS in detail and its applicability to molecular systems for better understanding. A theoretical formalism for accounting the uniqueness of URLS among the other nonlinear Raman techniques is developed and discussed in various pictorial representations i.e. ladder, Feynman and closed loop diagrams. A brief overview of nonlinear spectroscopy and nonlinear Raman spectroscopy is presented for demonstrating the difference between the URLS and the other nonlinear Raman techniques.