The photochemistry of dioxorhenium(V)

The excited-state properties of trans-ReO₂(py)₄⁺ (ReO₂⁺) in acetonitrile solution have been investigated. The excited-state absorption spectrum of ReO₂⁺ is dominated by bleaching of the ground state MLCT and d-d systems. The reduction potential of ReO₂^2+/+* is estimated from emission and electrochemical data to be -0.7 V (SSCE). The ReO₂⁺ excited state efficiently reduces methylviologen and other pyridinium and olefin acceptors. The resulting Re(VI) species oxidizes secondary alcohols and silanes. Acetophenone is the product of sec-phenethyl alcohol oxidation.

The emission properties of ReO₂⁺ in aqueous solutions of anionic and nonionic surfactants have been investigated. The emission and absorption maxima of ReO₂⁺ are dependent on the water content of its environment. Emission lifetimes vary over four orders of magnitude upon shifting from aqueous to nonaqueous environments. The emission lifetime has a large (8.6) isotope effect (k(H₂O)/k(D₂O)) that reflects its sensitivity towards the environment. These properties have been used to develop a model for the interactions of ReO₂⁺ with sodium dodecyl sulfate (SDS). A hydrophobic ReO₂⁺ derivative, ReO₂(3-Ph-py)₄⁺, has been used to probe micelles of nonionic surfactants, and these results are consistent with those obtained with SDS.

The emission properties of ReO₂⁺ in Nafion perfluorosulfonated membranes have been investigated. Absorption and emission spectroscopy indicate that the interior of the membrane is quite polar, similar to ethylene glycol. Two well-resolved emission components show different lifetimes and different isotope effects, indicative of varying degrees of solvent accessibility. These components are taken as evidence for chemically distinct regions in the polymer film, assigned as the interfacial region and the ion cluster region.

The unsubstituted pyridine complex shows monophasic, τ = 1.7 µs, emission decay when bound to calf thymus DNA. Switching to the 3-Ph-py complex yields a biphasic emission decay (τ₁ = 2.4 µs, τ₂ = 10 µs) indicative of an additional, solvent-inaccessible binding mode. Photoinduced electron transfer to methylviologen leads to oxidative cleavage of the DNA as detected by gel electrophoresis. Electrochemical and spectrophotometric techniques used with organic substrates also can be used to monitor the oxidation of DNA. Abstraction of the ribose 4' hydrogen by ReO₂²⁺ is a possible mechanism.

Chemistry of low-valent platinum dimers

Physical and chemical properties of low-valent platinum dimers, namely [Pt_2(P_2O_5H_2)4]^(4-) and Pt_2(µ-dppm)_2Cl_2, have been investigated using a variety of structural and spectroscopic techniques.

Platinum(II) d^8-d^8 dimers have been shown to exhibit much thermal and photochemical reactivity. Chapter 2 describes studies aimed at elucidating the excited state reduction potenetial of [Pt_2(P_2O_5H_2)4]^(4-), Pt_2, in organic media. By conducting excited state electron transfer studies using derivatized pyridiniums and benzophenones, the excited state reduction potential has been estimated to be ~2 V. The Pt_2 complex undergoes partial oxidation to form Pt(II,III) linear chains. Chapter 3 describes the structural and spectroscopic techniques used to determine the translational symmetries of these [Pt_2(P_2O_5H_2)4]^(4-) (X = Cl, Br), Pt_2X, chains. Pt_2Br has been found to be intermediate between (AAB)_n and (AABCCB)_n, while, Pt_2Cl is of (AABCCB)_n translational symmetry. Investigations into the electronic transitions of Pt_2Cl and Pt_2Br were conducted using high pressure techniques and are presented in Chapter 4. The Pt_2X electronic spectrum exhibits bands attributable to the reduced Pt2 complex and the oxidized Pt_2X_2 complex [Pt_2(P_2O_5H_2)4]^(4-) along with an intervalence charge-tranfer band characteristic of a mixed-valence solid.

Photophysical investigations of a new luminescent chromophore, Pt_2(µ-dppm)_2Cl_2, a d^9-d^9 dimer, and its analogs are described in Chapter 5. The absorption band directly responsible for the observed emission is believed to be very weak and, as of yet, unobserved. Attempts to determine the spin multiplicty and approximate energy of this unobserved transition are described in Chapter 6. Excited-state energy transfer studies indicate that this absorption band is a triplet transition at -13,000 cm^(-1). Although, the Pt_2(µ-dppm)_2Cl_2 excited state is non-luminescent in fluid solution, it has been shown to undergo thermal electron transfer to tetracyanoethylene and photoinduced electron transfer to methylviologen. These experiments are presented in Chapter 7. Preliminary studies, described in Chapter 8, of non-bridged d^9-d^9 platinum(I) dimers have shown that [Pt_2(CNCH_3)_6]^(2+) serves as a versatile precursor in the synthesis of new d^8-d^8 A-frame complexes.

2,4-dimethylene-1,3-cyclobutanediyl and 2,4-dimethylenebicyclobutane. Synthesis, spectroscopy, and reactivity of a triplet biradical and its covalent isomer

The preparation and direct observation of triplet 2,4-dimethylene-1,3- cyclobutanediyl (1), the non-Kekule isomer of benzene, is described. The biradical was generated by photolysis of 5,6-dimethylene-2,3- diazabicyclo[2.1.1]hex-2-ene (2) (which was synthesized in several steps from benzvalene) under cryogenic, matrix-isolation conditions. Biradical 1 was characterized by EPR spectroscopy (‌‌‌‌‌│D/hc│ =0.0204 cm^(-1), │E/hc│ =0.0028 cm^(-1)) and found to have a triplet ground state. The Δm_s= 2 transition displays hyperfine splitting attributed to a 7.3-G coupling to the ring methine and a 5.9-G coupling to the exocyclic methylene protons. Several experiments, including application of the magnetophotoselection (mps) technique in the generation of biradical 1, have allowed a determination of the zero-field triplet sublevels as x = -0.0040, y = +0.0136, and z = -0.0096 cm^(-1), where x and y are respectively the long and short in-plane axes and z the out-of-plane axis of 1.

Triplet 1 is yellow-orange and displays highly structured absorption (λ_(max)= 506 nm) and fluorescence (λ_(max) = 510 nm) spectra, with vibronic spacings of 1520 and 620 cm^(-1) for absorption and 1570 and 620 cm^(-1) for emission. The spectra were unequivocally assigned to triplet 1 by the use of a novel technique that takes advantage of the biradical's photolability. The absorption є = 7200 M^(-1) cm^(-1) and f = 0.022, establishing that the transition is spin-allowed. Further use of the mps technique has demonstrated that the transition is x-polarized, and the excited state 1s therefore of B_(1g) symmetry, in accord with theoretical predictions.

Thermolysis or direct photolysis of diazene 2 in fluid solution produces 2,4- dimethylenebicyclo[l.l.0]butane (3), whose ^(l)H NMR spectrum (-80°C, CD_(2)Cl_(2)) consists of singlets at δ 4.22 and 3.18 in a 2:1 ratio. Compound 3 is thermally unstable and dimerizes with second-order kinetics between -80 and -25°C (∆H^(‡) = 6.8 kcal mol^(-1), (∆s^(‡) = -28 eu) by a mechanism involving direct combination of two molecules of 3 in the rate-determining step. This singlet-manifold reaction ultimately produces a mixture of two dimers, 3,8,9- trimethylenetricyclo[5.1.1.0^(2,5)]non-4-ene (75) and trans-3,10-dimethylenetricyclo[6.2.0.0^(2,5)]deca-4,8-diene (76t), with the former predominating. In contrast, triplet-sensitized photolysis of 2, which leads to triplet 1, provides, in addition to 75 and 76t, a substantial amount of trans-5,10- dimethylenetricyclo[6.2.0.0^(3,6)]deca-3,8-diene (77t) and small amounts of two unidentified dimers.

In addition, triplet biradical 1 ring-closes to 3 in rigid media both thermally (77-140 K) and photochemically. In solution 3 forms triplet 1 upon energy transfer from sensitizers having relatively low triplet energies. The implications of the thermal chemistry for the energy surfaces of the system are discussed.

Part 1: Longitudinal static and dynamic effects in semiconductor lasers. Part II: Spectral characteristics of passively mode-locked quantum well lasers

In the first part of this thesis a study of the effect of the longitudinal distribution of optical intensity and electron density on the static and dynamic behavior of semiconductor lasers is performed. A static model for above threshold operation of a single mode laser, consisting of multiple active and passive sections, is developed by calculating the longitudinal optical intensity distribution and electron density distribution in a self-consistent manner. Feedback from an index and gain Bragg grating is included, as well as feedback from discrete reflections at interfaces and facets. Longitudinal spatial holeburning is analyzed by including the dependence of the gain and the refractive index on the electron density. The mechanisms of spatial holeburning in quarter wave shifted DFB lasers are analyzed. A new laser structure with a uniform optical intensity distribution is introduced and an implementation is simulated, resulting in a large reduction of the longitudinal spatial holeburning effect.

A dynamic small-signal model is then developed by including the optical intensity and electron density distribution, as well as the dependence of the grating coupling coefficients on the electron density. Expressions are derived for the intensity and frequency noise spectrum, the spontaneous emission rate into the lasing mode, the linewidth enhancement factor, and the AM and FM modulation response. Different chirp components are identified in the FM response, and a new adiabatic chirp component is discovered. This new adiabatic chirp component is caused by the nonuniform longitudinal distributions, and is found to dominate at low frequencies. Distributed feedback lasers with partial gain coupling are analyzed, and it is shown how the dependence of the grating coupling coefficients on the electron density can result in an enhancement of the differential gain with an associated enhancement in modulation bandwidth and a reduction in chirp.

In the second part, spectral characteristics of passively mode-locked two-section multiple quantum well laser coupled to an external cavity are studied. Broad-band wavelength tuning using an external grating is demonstrated for the first time in passively mode-locked semiconductor lasers. A record tuning range of 26 nm is measured, with pulse widths of typically a few picosecond and time-bandwidth products of more than 10 times the transform limit. It is then demonstrated that these large time-bandwidth products are due to a strong linear upchirp, by performing pulse compression by a factor of 15 to a record pulse widths as low 320 fs.

A model for pulse propagation through a saturable medium with self-phase-modulation, due to the a-parameter, is developed for quantum well material, including the frequency dependence of the gain medium. This model is used to simulate two-section devices coupled to an external cavity. When no self-phase-modulation is present, it is found that the pulses are asymmetric with a sharper rising edge, that the pulse tails have an exponential behavior, and that the transform limit is 0.3. Inclusion of self-phase-modulation results in a linear upchirp imprinted on the pulse after each round-trip. This linear upchirp is due to a combination of self-phase-modulation in a gain section and absorption of the leading edge of the pulse in the saturable absorber.

Identification of thermally-tagged coherent structures in the zero pressure gradient turbulent boundary layer

A zero pressure gradient boundary layer over a flat plate is subjected to step changes in thermal condition at the wall, causing the formation of internal, heated layers. The resulting temperature fluctuations and their corresponding density variations are associated with turbulent coherent structures. Aero-optical distortion occurs when light passes through the boundary layer, encountering the changing index of refraction resulting from the density variations. Instantaneous measurements of streamwise velocity, temperature and the optical deflection angle experienced by a laser traversing the boundary layer are made using hot and cold wires and a Malley probe, respectively. Correlations of the deflection angle with the temperature and velocity records suggest that the dominant contribution to the deflection angle comes from thermally-tagged structures in the outer boundary layer with a convective velocity of approximately 0.8U∞. An examination of instantaneous temperature and velocity and their temporal gradients conditionally averaged around significant optical deflections shows behavior consistent with the passage of a heated vortex. Strong deflections are associated with strong negative temperature gradients, and strong positive velocity gradients where the sign of the streamwise velocity fluctuation changes. The power density spectrum of the optical deflections reveals associated structure size to be on the order of the boundary layer thickness. A comparison to the temperature and velocity spectra suggests that the responsible structures are smaller vortices in the outer boundary layer as opposed to larger scale motions. Notable differences between the power density spectra of the optical deflections and the temperature remain unresolved due to the low frequency response of the cold wire.

Synthesis and characterization of 1,1-di-tert-butyldiazene

The synthesis and direct observation of 1,1-di-tert-butyldiazene (16) at -127°C is described. The absorption spectrum of a red solution of 1,1-diazene 16 reveals a structured absorption band with λ max at 506 run (Me_2O, -125°C). The vibrational spacing in S_1 is about 1200 cm^(-1). The excited state of 16 emits weakly with a single maximum at 715 run observed in the fluorescence spectrum (Me_2O:CD_2Cl_2, -196°C). The proton NMR spectrum of 16 occurs as a singlet at 1.41 ppm. Monitoring this NMR absorption at -94^0 ± 2°C shows that 1,1-diazene 16 decomposes with a first-order rate of 1.8 x 10^(-3) sec(-1) to form isobutane, isobutylene and hexarnethylethane. This rate is 10^8 and 10^(34) times faster than the thermal decomposition of the corresponding cis and trans 1,2-di-tert-butyldiazene isomers. The free energy of activation for decomposition of 1,1-diazene 16 is found to be 12.5 ± 0.2 kcal/mol at -94°C which is much lower than the values of 19.1 and 19.4 kcal/lmole calculated at -94°C for N-(2,2,6,6- tetramethylpiperidyl)nitrene (3) and N-(2,2,5,5- tetrarnethylpyrrolidyl)nitrene (4), respectively. This difference between 16 and the cyclic-1,1-diazenes 3 and 4 can be attributed to a large steric interaction between the tert-butyl groups in 1,1-diazene 16.

In order to investigate the nature of the singlet-triplet gap in 1,1-diazenes, 2,5-di-tert-butyl-N-pyrrolynitrene (22) was generated but was found to be too reactive towards dimerization to be persistent. In the presence of dimethylsulfoxide, however, N-pyrrolynitrene (22) can be trapped as N-(2,5-di-tert-butyl- N'-pyrrolyl)dimethylsulfoxirnine (38). N-(2,5-di-tert-butyl-N'-pyrrolyl)dimethylsulfoximine (38-d^6) exchanges with free dimethylsulfoxide at 50°C in solution, presumably by generation and retrapping of pyrrolynitrene 22.

Physical characterization of the rack effect and hydrogen bond networks in blue copper proteins

A summary of previous research is presented that indicates that the purpose of a blue copper protein's fold and hydrogen bond network, aka, the rack effect, enforce a copper(II) geometry around the copper(I) ion in the metal site. In several blue copper proteins, the C-terminal histidine ligand becomes protonated and detaches from the copper in the reduced forms. Mutants of amicyanin from Paracoccus denitrificans were made to alter the hydrogen bond network and quantify the rack effect by pK_a shifts.

The pK_a's of mutant amicyanins have been measured by pH-dependent electrochemistry. P94F and P94A mutations loosen the Northern loop, allowing the reduced copper to adopt a relaxed conformation: the ability to relax drives the reduction potentials up. The measured potentials are 265 (wild type), 380 (P94A), and 415 (P94F) mV vs. NHE. The measured pK_a's are 7.0 (wild type), 6.3 (P94A), and 5.0 (P94F). The additional hydrogen bond to the thiolate in the mutants is indicated by a red-shift in the blue copper absorption and an increase in the parallel hyperfine splitting in the EPR spectrum. This hydrogen bond is invoked as the cause for the increased stability of the C-terminal imidazole.

Melting curves give a measure of the thermal stability of the protein. A thermodynamic intermediate with pH-dependent reversibility is revealed. Comparisons with the electrochemistry and apoamicyanin suggest that the intermediate involves the region of the protein near the metal site. This region is destabilized in the P94F mutant; coupled with the evidence that the imidazole is stabilized under the same conditions confirms an original concept of the rack effect: a high energy configuration is stabilized at a cost to the rest of the protein.

Spectroscopic studies of gas-phase molecular clusters

Spectroscopic investigations of hydrogen-bonding and van der Waals' interactions m molecular clusters were studied by the techniques of infrared predissociation and resonance-enhanced multiphoton ionization spectroscopies (REMPI). Ab initio calculations were applied in conjunction for data interpretation.

The infrared predissociation spectroscopy of CN^-•(H_2O)_n (n = 2 - 6) clusters was reported in the region of 2950 - 3850 cm^(-1). The hydrogen bondings for the C-site and N-site binding, and among the water molecules were identified for n = 2 to 4. A spectral transition was observed for n = 5 and 6, implying that the anion was surface-bound onto the water aggregates in larger clusters.

The infrared predissociation spectroscopy of Br^-•(NH_3) and I^-•(NH_3)_n (n =1-3) clusters was reported in the region of 3050-3450 cm^(-1). For the Br^-•(NH_3) complex, a dominating ionic NH stretch appeared at 3175 cm^(-1), and the weaker free NH stretch appeared at 3348 cm^(-1). The observed spectrum was consistent to the structure in which there was one nearly linear hydrogen bond between Br^- and the NH_3 moiety. For the I^- •(NH_3) complex, five distinct IR absorption bands were observed in the spectrum. The spectrum was not consistent with basic frequency patterns of three geometries considered in the ab initio calculations - complex with one, two and three hydrogen bondings between I^- and the NH_3 moiety. Substantial inhomogenous broadening were displayed in the spectra for I^-•(NH_3)_n (n =2-3), suggesting the presence of multiple isomers.

The REMPI spectroscopy of the bound 4p ^2П 1/2 and ^2П 3/2 states, and the dissociative 3d ^2Σ^+ 1/2 state in the Al•Ar complex was reported. The dissociative spectrum at Al^+ channel suggested the coupling of the 4p ^2П 1/2,3/2 states to the repulsive 3d ^2Σ^+ 1/2 state. The spin-electronic coupling was further manifested in the dissociative Al^+ spectrum of the 3d ^2Σ^+ 1/2 state. Using the potential energy curves obtained from ab initio calculations, a bound → continuum Franck-Condon-intensity simulation was performed and compared with the one-photon 3d ^2Σ^+ 1/2 profile. The agreement provided evidence for the petturbation above the Al(3d)Ar dissociation limit, and the repulsive character of the 3d ^2Σ^+ 1/2 state.

Studies in nuclear reactor dynamics : I. The accuracy of point kinetics. II. The effect of delayed neutrons on the spectrum of the group-diffusion operator

This thesis is a theoretical work on the space-time dynamic behavior of a nuclear reactor without feedback. Diffusion theory with G-energy groups is used.

In the first part the accuracy of the point kinetics (lumped-parameter description) model is examined. The fundamental approximation of this model is the splitting of the neutron density into a product of a known function of space and an unknown function of time; then the properties of the system can be averaged in space through the use of appropriate weighting functions; as a result a set of ordinary differential equations is obtained for the description of time behavior. It is clear that changes of the shape of the neutron-density distribution due to space-dependent perturbations are neglected. This results to an error in the eigenvalues and it is to this error that bounds are derived. This is done by using the method of weighted residuals to reduce the original eigenvalue problem to that of a real asymmetric matrix. Then Gershgorin-type theorems .are used to find discs in the complex plane in which the eigenvalues are contained. The radii of the discs depend on the perturbation in a simple manner.

In the second part the effect of delayed neutrons on the eigenvalues of the group-diffusion operator is examined. The delayed neutrons cause a shifting of the prompt-neutron eigenvalue s and the appearance of the delayed eigenvalues. Using a simple perturbation method this shifting is calculated and the delayed eigenvalues are predicted with good accuracy.

Investigation of bulk indium antimonide as heterodyne detector for the submillimeter wavelength region

Bulk n-lnSb is investigated at a heterodyne detector for the submillimeter wavelength region. Two modes or operation are investigated: (1) the Rollin or hot electron bolometer mode (zero magnetic field), and (2) the Putley mode (quantizing magnetic field). The highlight of the thesis work is the pioneering demonstration or the Putley mode mixer at several frequencies. For example, a double-sideband system noise temperature of about 510K was obtained using a 812 GHz methanol laser for the local oscillator. This performance is at least a factor or 10 more sensitive than any other performance reported to date at the same frequency. In addition, the Putley mode mixer achieved system noise temperatures of 250K at 492 GHz and 350K at 625 GHz. The 492 GHz performance is about 50% better and the 625 GHz is about 100% better than previous best performances established by the Rollin-mode mixer. To achieve these results, it was necessary to design a totally new ultra-low noise, room-temperature preamp to handle the higher source impedance imposed by the Putley mode operation. This preamp has considerably less input capacitance than comparably noisy, ambient designs.

In addition to advancing receiver technology, this thesis also presents several novel results regarding the physics of n-lnSb at low temperatures. A Fourier transform spectrometer was constructed and used to measure the submillimeter wave absorption coefficient of relatively pure material at liquid helium temperatures and in zero magnetic field. Below 4.2K, the absorption coefficient was found to decrease with frequency much faster than predicted by Drudian theory. Much better agreement with experiment was obtained using a quantum theory based on inverse-Bremmstrahlung in a solid. Also the noise of the Rollin-mode detector at 4.2K was accurately measured and compared with theory. The power spectrum is found to be well fit by a recent theory of non- equilibrium noise due to Mather. Surprisingly, when biased for optimum detector performance, high purity lnSb cooled to liquid helium temperatures generates less noise than that predicted by simple non-equilibrium Johnson noise theory alone. This explains in part the excellent performance of the Rollin-mode detector in the millimeter wavelength region.

Again using the Fourier transform spectrometer, spectra are obtained of the responsivity and direct detection NEP as a function of magnetic field in the range 20-110 cm^-1. The results show a discernable peak in the detector response at the conduction electron cyclotron resonance frequency tor magnetic fields as low as 3 KG at bath temperatures of 2.0K. The spectra also display the well-known peak due to the cyclotron resonance of electrons bound to impurity states. The magnitude of responsivity at both peaks is roughly constant with magnet1c field and is comparable to the low frequency Rollin-mode response. The NEP at the peaks is found to be much better than previous values at the same frequency and comparable to the best long wavelength results previously reported. For example, a value NEP=4.5x10^-13/Hz^1/2 is measured at 4.2K, 6 KG and 40 cm^-1. Study of the responsivity under conditions of impact ionization showed a dramatic disappearance of the impurity electron resonance while the conduction electron resonance remained constant. This observation offers the first concrete evidence that the mobility of an electron in the N=0 and N=1 Landau levels is different. Finally, these direct detection experiments indicate that the excellent heterodyne performance achieved at 812 GHz should be attainable up to frequencies of at least 1200 GHz.

Microwave absorption and emission from magnetized afterglow plasmas

The microwave scattering properties of an axially magnetized afterglow plasma column in an S-band waveguide have been investigated experimentally. The column axis is perpendicular to the electric field and the direction of wave propagation in the H_(10)-mode waveguide. Strong absorption is found in the range of upper hybrid frequencies, ω_c ≤ ω ≤ [ω^2_c + ω^2_p(r,t)]^(1/2) where ω_c is the electron cyclotron frequency and ω_p is the locally and temporally varying electron plasma frequency. With the high absorption the noise emission approaches the blackbody limit. A microwave radiometer has been used to measure the noise power and with a comparison and null-technique the electron temperature. As emission and absorption are largely confined to a resonant layer, spatially resolved temperature data are obtained. Time resolution is obtained by gating the radiometer. The peak electron density is derived from the emission or absorption onset at the maximum upper hybrid frequency and confirmed by independent measurements. With this diagnostic technique the electron density and temperature decay has been studied under a variety of experimental conditions. Ambipolar diffusion and collisional cooling essentially account for the plasma decay, but impurities and metastable ions play an important role. The diagnostic method is successfully applied in a microwave heating experiment. The existence of absorbing resonant layers is shown by a peak in the radial temperature profile where the local upper hybrid frequency equals the heating frequency. The knowledge of the plasma parameters is important in the study of hot plasma effects. Buchsbaum-Hasegawa modes are investigated in a wide range of magnetic fields (.5 < ω_c/ω < .985).

Experimental and theoretical developments in extended x-ray absorption fine structure (EXAFS) spectroscopy

To obtain accurate information from a structural tool it is necessary to have an understanding of the physical principles which govern the interaction between the probe and the sample under investigation. In this thesis a detailed study of the physical basis for Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is presented. A single scattering formalism of EXAFS is introduced which allows a rigorous treatment of the central atom potential. A final state interaction formalism of EXAFS is also discussed. Multiple scattering processes are shown to be significant for systems of certain geometries. The standard single scattering EXAFS analysis produces erroneous results if the data contain a large multiple scattering contribution. The effect of thermal vibrations on such multiple scattering paths is also discussed. From symmetry considerations it is shown that only certain normal modes contribute to the Debye-Waller factor for a particular scattering path. Furthermore, changes in the scattering angles induced by thermal vibrations produces additional EXAFS components called modification factors. These factors are shown to be small for most systems.

A study of the physical basis for the determination of structural information from EXAFS data is also presented. An objective method of determining the background absorption and the threshold energy is discussed and involves Gaussian functions. In addition, a scheme to determine the nature of the scattering atom in EXAFS experiments is introduced. This scheme is based on the fact that the phase intercept is a measure of the type of scattering atom. A method to determine bond distances is also discussed and does not require the use of model compounds or calculated phase shifts. The physical basis for this method is the absence of a linear term in the scattering phases. Therefore, it is possible to separate these phases from the linear term containing the distance information in the total phase.

Electron tunneling in proteins and water

The subject of this thesis is electronic coupling in donor-bridge-acceptor systems. In Chapter 2, ET properties of cyanide-bridged dinuclear ruthenium complexes were investigated. The strong interaction between the mixed-valent ruthenium centers leads to intense metal-to-metal charge transfer bands (MMCT). Hush analysis of the MMCT absorption bands yields the electronic-coupling strength between the metal centers (H_(AB)) and the total reorganization energy (λ). Comparison of ET kinetics to calculated rates shows that classical ET models fail to account for the observed kinetics and nuclear tunneling must be considered.

In Chapter 3, ET rates were measured in four ruthenium-modified highpotential iron-sulfur proteins (HiPIP), which were modified at position His50, His81, His42 and His18, respectively. ET kinetics for the His50 and His81 mutants are a factor of 300 different, while the donor-acceptor separation is nearly identical. PATHWAY calculations corroborate these measurements and highlight the importance of structural detail of the intervening protein matrix.

In Chapter 4, the distance dependence of ET through water bridges was measured. Photoinduced ET measurements in aqueous glasses at 77 K show that water is a poor medium for ET. Luminescence decay and quantum yield data were analyzed in the context of a quenching model that accounts for the exponential distance dependence of ET, the distance distribution of donors and acceptors embedded in the glass and the excluded volumes generated by the finite sizes of the donors and acceptors.

In Chapter 5, the pH-dependent excited state dynamics of ruthenium-modified amino acids were measured. The [Ru(bpy)_(3)] ^(2+) chromophore was linked to amino acids via an amide linkage. Protonation of the amide oxygen effectively quenches the excited state. In addition. time-resolved and steady-state luminescence data reveal that nonradiative rates are very sensitive to the protonation state and the structure of the amino acid moiety.

Upper hybrid resonance absorption and scattering from a plasma column

The electromagnetic scattering and absorption properties of small (kr~1/2) inhomogeneous magnetoplasma columns are calculated via the full set of Maxwell's equations with tensor dielectric constitutive relation. The cold plasma model with collisional damping is used to describe the column. The equations are solved numerically, subject to boundary conditions appropriate to an infinite parallel strip line and to an incident plane wave. The results are similar for several density profiles and exhibit semiquantitative agreement with measurements in waveguide. The absorption is spatially limited, especially for small collision frequency, to a narrow hybrid resonant layer and is essentially zero when there is no hybrid layer in the column. The reflection is also enhanced when the hybrid layer is present, but the value of the reflection coefficient is strongly modified by the presence of the glass tube. The nature of the solutions and an extensive discussion of the conditions under which the cold collisional model should yield valid results is presented.

Analyses of planetary atmospheres across the spectrum : from Titan to exoplanets

Planetary atmospheres exist in a seemingly endless variety of physical and chemical environments. There are an equally diverse number of methods by which we can study and characterize atmospheric composition. In order to better understand the fundamental chemistry and physical processes underlying all planetary atmospheres, my research of the past four years has focused on two distinct topics. First, I focused on the data analysis and spectral retrieval of observations obtained by the Ultraviolet Imaging Spectrograph (UVIS) instrument onboard the Cassini spacecraft while in orbit around Saturn. These observations consisted of stellar occultation measurements of Titan's upper atmosphere, probing the chemical composition in the region 300 to 1500 km above Titan's surface. I examined the relative abundances of Titan's two most prevalent chemical species, nitrogen and methane. I also focused on the aerosols that are formed through chemistry involving these two major species, and determined the vertical profiles of aerosol particles as a function of time and latitude. Moving beyond our own solar system, my second topic of investigation involved analysis of infra-red light curves from the Spitzer space telescope, obtained as it measured the light from stars hosting planets of their own. I focused on both transit and eclipse modeling during Spitzer data reduction and analysis. In my initial work, I utilized the data to search for transits of planets a few Earth masses in size. In more recent research, I analyzed secondary eclipses of three exoplanets and constrained the range of possible temperatures and compositions of their atmospheres.