A tale of coupled vibrations in solution told by coherent two-dimensional infrared spectroscopy

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2004.Vita.Includes bibliographical references.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Coherent two-dimensional infrared (2D IR) spectroscopy is used as a tool for investigating the molecular structure and dynamics of coupled vibrations in solution on a picosecond timescale. The strongly coupled asymmetric and symmetric carbonyl stretches of Rh(CO)₂C₅H₇0₂ (RDC) dissolved in hexane serve as a convenient model system. Fourier transform 2D IR spectra are obtained from heterodyne-detected third-order nonlinear signals using a sequence of broad bandwidth femtosecond IR pulses. A 2D IR correlation spectrum with absorptive lineshapes results from the addition of 2D rephasing and non-rephasing spectra, which sample conjugate frequencies in the evolution time period. The 2D IR correlation spectrum contains peaks with different positions, signs, amplitudes and lineshapes. The positions of the peaks map the transition frequencies between the ground, singly, and doubly excited states of the system, and thus describe the anharmonic vibrational potential. Peak amplitudes reflect the relative magnitudes and orientations of the transition dipole moments in the molecular frame, the electrical anharmonicity of the system, and the vibrational relaxation dynamics. The 2D line shapes are sensitive to the complicated system-bath interactions in solution. 2D IR spectra taken with varying polarization conditions and as a function of a variable waiting time can be used to isolate and quantify these spectroscopic observables. The polarization-selective 2D IR spectra of RDC in hexane are analyzed in terms of two coupled local coordinates to obtain their mutual orientation and the magnitude of the coupling between them. Evidence of vibrational coherence transfer between close-lying transition frequencies is indicated by the presence of extra induced peaks in 2D IR(cont.) rephasing spectra. The data is modeled by using Redfield theory to account for coherence transfer, vibrational dephasing and population relaxation in a multilevel vibrational system. Building on the studies of the RDC model system, 2D IR spectroscopy is used to study the thermal denaturation of RNase A by characterizing the temperature-dependent Amide I band. A nonlinear IR probe is used to study the early events in the laser temperature-jump initiated denaturation of RNase A.by Munira Khalil.Ph.D

A table-top femtosecond time-resolved soft x-ray transient absorption spectrometer

A laser-based, table-top instrument is constructed to perform femtosecond soft x-ray transient absorption spectroscopy. Ultrashort soft x-ray pulses produced via high-order harmonic generation of the amplified output of a femtosecond Ti:sapphire laser system are used to probe atomic core-level transient absorptions in atoms and molecules. The results provide chemically specific, time-resolved dynamics with sub-50-fs time resolution. In this setup, high-order harmonics generated in a Ne-filled capillary waveguide are refocused by a gold-coated toroidal mirror into the sample gas cell, where the soft x-ray light intersects with an optical pump pulse. The transmitted high-order harmonics are spectrally dispersed with a home-built soft x-ray spectrometer, which consists of a gold-coated toroidal mirror, a uniform-line spaced plane grating, and a soft x-ray CCD camera. The optical layout of the instrument, design of the soft x-ray spectrometer, and spatial and temporal characterization of the high-order harmonics are described. Examples of static and time-resolved photoabsorption spectra collected on this apparatus are presented

The impact on non-traditional security threats in Sri Lanka

The security threats exert a great challenge to national security of all states irrespective of the state’s position in the international system. In the field of security studies, with the transition of state-centric security to individual security approach, non-traditional security issues have become a novel concept of security studies. As a maritime nation Sri Lanka’s strategic location in the centre of the Indian Ocean makes it visible, perhaps vulnerable, to many external and internal threats. Hence, the government has to take immediate steps to mitigate these non-traditional security threats in order to ensure the national security and economic prosperity. This article is to examine the security environment and perspective of national security in Sri Lanka, identify the non-traditional security threats and analyse the impact on national security and identify mitigation methods to ensure national security. Methodology used were the primary and secondary data of a qualitative research method by using ethnographic study. Findings for this study is Sri Lanka must shape the security environment through an effective and realistic assessment of national interest. A great priority is to be given for the political stability while maintaining the continuous vigilance, to seek active cooperation and coordination effort within the region and beyond and engage in continuous process of confidence building within the nation and beyond. The findings of this study are the model-based approach is a better solution for Sri Lanka to overcome these security challenges, however, application of any model should be tailored to Sri Lanka’s own specifications

Manipulating valence and core electronic excitations of a transition-metal complex using UV/Vis and X-ray cavities

We demonstrate how optical cavities can be exploited to control both valence- and core-excitations in a prototypical model transition metal complex, ferricyanide ([Fe(iii)(CN)6]3−), in an aqueous environment. The spectroscopic signatures of hybrid light-matter polariton states are revealed in UV/Vis and X-ray absorption, and stimulated X-ray Raman signals. In an UV/Vis cavity, the absorption spectrum exhibits the single-polariton states arising from the cavity photon mode coupling to both resonant and off-resonant valence-excited states. We further show that nonlinear stimulated X-ray Raman signals can selectively probe the bipolariton statesviacavity-modified Fe core-excited states. This unveils the correlation between valence polaritons and dressed core-excitations. In an X-ray cavity, core-polaritons are generated and their correlations with the bare valence-excitations appear in the linear and nonlinear X-ray spectra

Resonant Inelastic X-ray Scattering Calculations of Transition Metal Complexes within a Simplified Time-Dependent Density Functional Theory Framework

We present a time-dependent density functional theory (TDDFT) approach to compute the light-matter couplings between two different manifolds of excited states relative to a common ground state in the context of 4d transition metal systems. These quantities are the necessary ingredients to solve the Kramers-Heisenberg (KH) equation for resonant inelastic X-ray scattering (RIXS) and several other types of two-photon spectroscopies. The procedure is based on the pseudo-wavefunction approach, where the solutions of a TDDFT calculation can be used to construct excited-state wavefunctions, and on the restricted energy window approach, where a manifold of excited states can be rigorously defined based on the energies of the occupied molecular orbitals involved in the excitation process. Thus, the present approach bypasses the need to solve the costly TDDFT quadratic-response equations. We illustrate the applicability of the method to 4d transition metal molecular complexes by calculating the 2p4d RIXS maps of three representative ruthenium complexes and comparing them to experimental results. The method can capture all the experimental features in all three complexes to allow the assignment of the experimental peaks, with relative energies correct to within ∼0.6 eV at the cost of two independent TDDFT calculations

Sticky Ends in a Self-Assembling ABA Triblock Copolymer: The Role of Ureas in Stimuli-Responsive Hydrogels

Directing polymer self-assembly through noncovalent interactions is a powerful way to control the structure and function of nanoengineered materials. Dynamic hydrogen bonds are particularly useful for materials with structures that change over time or in response to specific stimuli. In the present work, we use the supramolecular association of urea moieties to manipulate the morphology, thermal response, and mechanical properties of soft polymeric hydrogels. Urea-terminated poly(isopropyl glycidyl ether)-b-poly(ethylene oxide)-b-poly(isopropyl glycidyl ether) ABA triblock copolymers were synthesized using controlled, anionic ring-opening polymerization and subsequent chain-end functionalization. Triblock copolymers with hydroxy end-groups were incapable of hydrogelation, while polymers terminated with meta-bis-urea motifs formed robust gels at room temperature. Rheometric analysis of the bulk gels, variable-temperature infrared spectroscopy (VT-IR), differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) confirmed the formation of structured hydrogels via association of the meta-bis-urea end-groups. Monourea end-groups did not result in the same regular structure as the meta-bis-urea. In future, the reported hydrogels could be useful for elastomeric, shape-morphing 3D-printed constructs, or as biomimetic scaffolds with precisely tailored porosity and mechanical properties