The steady-state response of multidegree-of-freedom systems with a spatially localized nonlinearity

This thesis is concerned with the dynamic response of a General multidegree-of-freedom linear system with a one dimensional nonlinear constraint attached between two points. The nonlinear constraint is assumed to consist of rate-independent conservative and hysteretic nonlinearities and may contain a viscous dissipation element. The dynamic equations for general spatial and temporal load distributions are derived for both continuous and discrete systems. The method of equivalent linearization is used to develop equations which govern the approximate steady-state response to generally distributed loads with harmonic time dependence.

The qualitative response behavior of a class of undamped chainlike structures with a nonlinear terminal constraint is investigated. It is shown that the hardening or softening behavior of every resonance curve is similar and is determined by the properties of the constraint. Also examined are the number and location of resonance curves, the boundedness of the forced response, the loci of response extrema, and other characteristics of the response. Particular consideration is given to the dependence of the response characteristics on the properties of the linear system, the nonlinear constraint, and the load distribution.

Numerical examples of the approximate steady-state response of three structural systems are presented. These examples illustrate the application of the formulation and qualitative theory. It is shown that disconnected response curves and response curves which cross are obtained for base excitation of a uniform shear beam with a cubic spring foundation. Disconnected response curves are also obtained for the steady-state response to a concentrated load of a chainlike structure with a hardening hysteretic constraint. The accuracy of the approximate response curves is investigated.

Sputtering by multiply-charged ions, and preferential sputtering of isotopic mixtures

Collector-type experiments have been conducted to investigate two different aspects of sputtering induced by keV ions. The first study looked for possible ejection mechanisms related to the primary charge state of the projectile. Targets of CsI and LiNbO_3 were bombarded with 48 keV Ar^(q+), and a Au target was bombarded with 60 keV Ar^(q+), for q = 4, 8, and 11. The collectors were analyzed using heavy-ion Rutherford backscattering spectroscopy to determine the differential angular sputtering yields; these and the corresponding total yields were examined for variations as a function of projectile charge state. For the Au target, no significant changes were seen, but for the insulating targets slight (~10%) enhancements were observed in the total yields as the projectile charge state was increased from 4+ to 11+.

In the second investigation, artificial ^(92)Mo/^(100)Mo targets were bombarded with 5 and 10 keV beams of Ar^+ and Xe^+ to study the isotopic fractionation of sputtered neutrals as a function of emission angle and projectile fluence. Using secondary ion mass spectroscopy to measure the isotope ratio on the collectors, material ejected into normal directions at low bombarding fluences (~ 10^(15) ions cm^(-2)) was found to be enriched in the light isotope by as much as ~70‰ compared to steady state. Similar results were found for secondary Mo ions sputtered by 14.5 keV O^-. For low-fluence 5 keV Xe^+ bombardment, the light-isotope enrichment at oblique angles was ~20‰ less than the corresponding enrichment in the normal direction. No angular dependence could be resolved for 5 keV Ar^+ projectiles at the lowest fluence. The above fractionation decreased to steady-state values after bombarding fluences of a few times 10^(16) ions cm^(-2) , with the angular dependence becoming more pronounced. The fractionation and total sputtering yield were found to be strongly correlated, indicating that the above effects may have been related to the presence of a modified target surface layer. The observed effects are consistent with other secondary ion measurements and multiple-interaction computer simulations, and are considerably larger than predicted by existing analytic theory.

Periodic solutions of integro-differential equations which arise in population dynamics

The problem of the existence and stability of periodic solutions of infinite-lag integra-differential equations is considered. Specifically, the integrals involved are of the convolution type with the dependent variable being integrated over the range (- ∞,t), as occur in models of population growth. It is shown that Hopf bifurcation of periodic solutions from a steady state can occur, when a pair of eigenvalues crosses the imaginary axis. Also considered is the existence of traveling wave solutions of a model population equation allowing spatial diffusion in addition to the usual temporal variation. Lastly, the stability of the periodic solutions resulting from Hopf bifurcation is determined with aid of a Floquet theory.

The first chapter is devoted to linear integro-differential equations with constant coefficients utilizing the method of semi-groups of operators. The second chapter analyzes the Hopf bifurcation providing an existence theorem. Also, the two-timing perturbation procedure is applied to construct the periodic solutions. The third chapter uses two-timing to obtain traveling wave solutions of the diffusive model, as well as providing an existence theorem. The fourth chapter develops a Floquet theory for linear integro-differential equations with periodic coefficients again using the semi-group approach. The fifth chapter gives sufficient conditions for the stability or instability of a periodic solution in terms of the linearization of the equations. These results are then applied to the Hopf bifurcation problem and to a certain population equation modeling periodically fluctuating environments to deduce the stability of the corresponding periodic solutions.

Some problems in nonlinear elasticity

Two separate problems are discussed: axisymmetric equilibrium configurations of a circular membrane under pressure and subject to thrust along its edge, and the buckling of a circular cylindrical shell.

An ordinary differential equation governing the circular membrane is imbedded in a family of n-dimensional nonlinear equations. Phase plane methods are used to examine the number of solutions corresponding to a parameter which generalizes the thrust, as well as other parameters determining the shape of the nonlinearity and the undeformed shape of the membrane. It is found that in any number of dimensions there exists a value of the generalized thrust for which a countable infinity of solutions exist if some of the remaining parameters are made sufficiently large. Criteria describing the number of solutions in other cases are also given.

Donnell-type equations are used to model a circular cylindrical shell. The static problem of bifurcation of buckled modes from Poisson expansion is analyzed using an iteration scheme and pertubation methods. Analysis shows that although buckling loads are usually simple eigenvalues, they may have arbitrarily large but finite multiplicity when the ratio of the shell's length and circumference is rational. A numerical study of the critical buckling load for simple eigenvalues indicates that the number of waves along the axis of the deformed shell is roughly proportional to the length of the shell, suggesting the possibility of a "characteristic length." Further numerical work indicates that initial post-buckling curves are typically steep, although the load may increase or decrease. It is shown that either a sheet of solutions or two distinct branches bifurcate from a double eigenvalue. Furthermore, a shell may be subject to a uniform torque, even though one is not prescribed at the ends of the shell, through the interaction of two modes with the same number of circumferential waves. Finally, multiple time scale techniques are used to study the dynamic buckling of a rectangular plate as well as a circular cylindrical shell; transition to a new steady state amplitude determined by the nonlinearity is shown. The importance of damping in determining equilibrium configurations independent of initial conditions is illustrated.

Anatomical and developmental patterns of interleukin-2 gene expression in the mouse: analysis of IL-2 expressing cells and partial characterization of interactions involved in mediating IL-2 gene expression in vivo

Interleukin-2 (IL-2) is an important mediator in the vertebrate immune system. IL-2 is a potent growth factor that mature T lymphocytes use as a proliferation signal and the production of IL-2 is crucial for the clonal expansion of antigen-specific T cells in the primary immune response. IL-2 driven proliferation is dependent on the interaction of the lymphokine with its cognate multichain receptor. IL-2 expression is induced only upon stimulation and transcriptional activation of the IL-2 gene relies extensively on the coordinate interaction of numerous inducible and constitutive trans-acting factors. Over the past several years, thousands of papers have been published regarding molecular and cellular aspects of IL-2 gene expression and IL-2 function. The vast majority of these reports describe work that has been carried out in vitro. However, considerably less is known about control of IL-2 gene expression and IL-2 function in vivo.

To gain new insight into the regulation of IL-2 gene expression in vivo, anatomical and developmental patterns of IL-2 gene expression in the mouse were established by employing in situ hybridization and immunohistochemical staining methodologies to tissue sections generated from normal mice and mutant animals in which T -cell development was perturbed. Results from these studies revealed several interesting aspects of IL-2 gene expression, such as (1) induction of IL-2 gene expression and protein synthesis in the thymus, the primary site of T-cell development in the body, (2) cell-type specificity of IL-2 gene expression in vivo, (3) participation of IL-2 in the extrathymic expansion of mature T cells in particular tissues, independent of an acute immune response to foreign antigen, (4) involvement of IL-2 in maintaining immunologic balance in the mucosal immune system, and (5) potential function of IL-2 in early events associated with hematopoiesis.

Extensive analysis of IL-2 mRNA accumulation and protein production in the murine thymus at various stages of development established the existence of two classes of intrathymic IL-2 producing cells. One class of intrathymic IL-2 producers was found exclusively in the fetal thymus. Cells belonging to this subset were restricted to the outermost region of the thymus. IL-2 expression in the fetal thymus was highly transient; a dramatic peak ofiL-2 mRNA accumulation was identified at day 14.5 of gestation and maximal IL-2 protein production was observed 12 hours later, after which both IL-2 mRNA and protein levels rapidly decreased. Significantly, the presence of IL-2 expressing cells in the day 14-15 fetal thymus was not contingent on the generation of T-cell receptor (TcR) positive cells. The second class of IL-2 producing cells was also detectable in the fetal thymus (cells found in this class represented a minority subset of IL-2 producers in the fetal thymus) but persist in the thymus during later stages of development and after birth. Intrathymic IL-2 producers in postnatal animals were located in the subcapsular region and cortex, indicating that these cells reside in the same areas where immature T cells are consigned. The frequency of IL-2 expressing cells in the postnatal thymus was extremely low, indicating that induction of IL-2 expression and protein synthesis are indicative of a rare activation event. Unlike the fetal class of intrathymic IL-2 producers, the presence of IL-2 producing cells in the postnatal thymus was dependent on to the generation of TcR+ cells. Subsequent examination of intrathymic IL-2 production in mutant postnatal mice unable to produce either αβ or γδ T cells showed that postnatal IL-2 producers in the thymus belong to both αβ and γδ lineages. Additionally, further studies indicated that IL-2 synthesis by immature αβ -T cells depends on the expression of bonafide TcR αβ-heterodimers. Taken altogether, IL-2 production in the postnatal thymus relies on the generation of αβ or γδ-TcR^+ cells and induction of IL-2 protein synthesis can be linked to an activation event mediated via the TcR.

With regard to tissue specificity of IL-2 gene expression in vivo, analysis of whole body sections obtained from normal neonatal mouse pups by in situ hybridization demonstrated that IL-2 mRNA^+ cells were found in both lymphoid and nonlymphoid tissues with which T cells are associated, such as the thymus (as described above), dermis and gut. Tissues devoid of IL-2 mRNA^+ cells included brain, heart, lung, liver, stomach, spine, spinal cord, kidney, and bladder. Additional analysis of isolated tissues taken from older animals revealed that IL-2 expression was undetectable in bone marrow and in nonactivated spleen and lymph nodes. Thus, it appears that extrathymic IL-2 expressing cells in nonimmunologically challenged animals are relegated to particular epidermal and epithelial tissues in which characterized subsets of T cells reside and thatinduction of IL-2 gene expression associated with these tissues may be a result of T-cell activation therein.

Based on the neonatal in situ hybridization results, a detailed investigation into possible induction of IL-2 expression resulting in IL-2 protein synthesis in the skin and gut revealed that IL-2 expression is induced in the epidermis and intestine and IL-2 protein is available to drive cell proliferation of resident cells and/or participate in immune function in these tissues. Pertaining to IL-2 expression in the skin, maximal IL-2 mRNA accumulation and protein production were observed when resident Vγ_3^+ T-cell populations were expanding. At this age, both IL-2 mRNA^+ cells and IL-2 protein production were intimately associated with hair follicles. Likewise, at this age a significant number of CD3ε^+ cells were also found in association with follicles. The colocalization of IL-2 expression and CD3ε^+ cells suggests that IL-2 expression is induced when T cells are in contact with hair follicles. In contrast, neither IL-2 mRNA nor IL-2 protein were readily detected once T-cell density in the skin reached steady-state proportions. At this point, T cells were no longer found associated with hair follicles but were evenly distributed throughout the epidermis. In addition, IL-2 expression in the skin was contingent upon the presence of mature T cells therein and induction of IL-2 protein synthesis in the skin did not depend on the expression of a specific TcR on resident T cells. These newly disclosed properties of IL-2 expression in the skin indicate that IL-2 may play an additional role in controlling mature T-cell proliferation by participating in the extrathymic expansion of T cells, particularly those associated with the epidermis.

Finally, regarding IL-2 expression and protein synthesis in the gut, IL-2 producing cells were found associated with the lamina propria of neonatal animals and gut-associated IL-2 production persisted throughout life. In older animals, the frequency of IL-2 producing cells in the small intestine was not identical to that in the large intestine and this difference may reflect regional specialization of the mucosal immune system in response to enteric antigen. Similar to other instances of IL-2 gene expression in vivo, a failure to generate mature T cells also led to an abrogation of IL-2 protein production in the gut. The presence of IL-2 producing cells in the neonatal gut suggested that these cells may be generated during fetal development. Examination of the fetal gut to determine the distribution of IL-2 producing cells therein indicated that there was a tenfold increase in the number of gut-associated IL-2 producers at day 20 of gestation compared to that observed four days earlier and there was little difference between the frequency of IL-2 producing cells in prenatal versus neonatal gut. The origin of these fetally-derived IL-2 producing cells is unclear. Prior to the immigration of IL-2 inducible cells to the fetal gut and/or induction of IL-2 expression therein, IL-2 protein was observed in the fetal liver and fetal omentum, as well as the fetal thymus. Considering that induction of IL-2 protein synthesis may be an indication of future functional capability, detection of IL-2 producing cells in the fetal liver and fetal omentum raises the possibility that IL-2 producing cells in the fetal gut may be extrathymic in origin and IL-2 producing cells in these fetal tissues may not belong solely to the T lineage. Overall, these results provide increased understanding of the nature of IL-2 producing cells in the gut and how the absence of IL-2 production therein and in fetal hematopoietic tissues can result in the acute pathology observed in IL-2 deficient animals.

Flight dynamics in Drosophila through a dynamically-scaled robotic approach

Flies are particularly adept at balancing the competing demands of delay tolerance, performance, and robustness during flight, which invites thoughtful examination of their multimodal feedback architecture. This dissertation examines stabilization requirements for inner-loop feedback strategies in the flapping flight of Drosophila, the fruit fly, against the backdrop of sensorimotor transformations present in the animal. Flies have evolved multiple specializations to reduce sensorimotor latency, but sensory delay during flight is still significant on the timescale of body dynamics. I explored the effect of sensor delay on flight stability and performance for yaw turns using a dynamically-scaled robot equipped with a real-time feedback system that performed active turns in response to measured yaw torque. The results show a fundamental tradeoff between sensor delay and permissible feedback gain, and suggest that fast mechanosensory feedback provides a source of active damping that compliments that contributed by passive effects. Presented in the context of these findings, a control architecture whereby a haltere-mediated inner-loop proportional controller provides damping for slower visually-mediated feedback is consistent with tethered-flight measurements, free-flight observations, and engineering design principles. Additionally, I investigated how flies adjust stroke features to regulate and stabilize level forward flight. The results suggest that few changes to hovering kinematics are actually required to meet steady-state lift and thrust requirements at different flight speeds, and the primary driver of equilibrium velocity is the aerodynamic pitch moment. This finding is consistent with prior hypotheses and observations regarding the relationship between body pitch and flight speed in fruit flies. The results also show that the dynamics may be stabilized with additional pitch damping, but the magnitude of required damping increases with flight speed. I posit that differences in stroke deviation between the upstroke and downstroke might play a critical role in this stabilization. Fast mechanosensory feedback of the pitch rate could enable active damping, which would inherently exhibit gain scheduling with flight speed if pitch torque is regulated by adjusting stroke deviation. Such a control scheme would provide an elegant solution for flight stabilization across a wide range of flight speeds.

Modeling and analysis of hysteretic structural behavior

For damaging response, the force-displacement relationship of a structure is highly nonlinear and history-dependent. For satisfactory analysis of such behavior, it is important to be able to characterize and to model the phenomenon of hysteresis accurately. A number of models have been proposed for response studies of hysteretic structures, some of which are examined in detail in this thesis. There are two popular classes of models used in the analysis of curvilinear hysteretic systems. The first is of the distributed element or assemblage type, which models the physical behavior of the system by using well-known building blocks. The second class of models is of the differential equation type, which is based on the introduction of an extra variable to describe the history dependence of the system.

Owing to their mathematical simplicity, the latter models have been used extensively for various applications in structural dynamics, most notably in the estimation of the response statistics of hysteretic systems subjected to stochastic excitation. But the fundamental characteristics of these models are still not clearly understood. A response analysis of systems using both the Distributed Element model and the differential equation model when subjected to a variety of quasi-static and dynamic loading conditions leads to the following conclusion: Caution must be exercised when employing the models belonging to the second class in structural response studies as they can produce misleading results.

The Massing's hypothesis, originally proposed for steady-state loading, can be extended to general transient loading as well, leading to considerable simplification in the analysis of the Distributed Element models. A simple, nonparametric identification technique is also outlined, by means of which an optimal model representation involving one additional state variable is determined for hysteretic systems.

Interaction of aqueous inorganic orthophosphate and phosphate rock

An experimental study was made of the interaction of phosphate rock and aqueous inorganic orthophosphate, calcium, and hydroxyl ions. A model of the reaction was developed by observing electron diffraction patterns in conjunction with concentration changes of chemical components. The model was applied in explaining the performance of batch suspensions of powdered phosphate rock and packed columns of granular phosphate rock. In both cases the reaction consisted initially of a rapid nucleation phase that occurred in a time period of minutes. In the batch system the calcium phosphate nuclei then ripened into larger micro-crystals of hydroxyapatite, which eventually became indistinguishable from the original phosphate rock surface. During column operation the high supersaturation ratio that existed after the rapid nucleation phase resulted in a layer of small nuclei that covered a slowly growing hydroxyapatite crystal.

The column steady-state rate constant was found to increase with increasing temperature, pH, and fluoride concentration, and to decrease with increasing concentrations of magnesium sulfate, ammonium chloride, and bicarbonate ion.

An engineering feasibility study indicated that, based on economic considerations, nucleation of apatite on phosphate rock ore has a potential use as a wastewater phosphate removal treatment process.

Formal methods for control synthesis in partially observed environments : application to autonomous robotic manipulation

Modern robots are increasingly expected to function in uncertain and dynamically challenging environments, often in proximity with humans. In addition, wide scale adoption of robots requires on-the-fly adaptability of software for diverse application. These requirements strongly suggest the need to adopt formal representations of high level goals and safety specifications, especially as temporal logic formulas. This approach allows for the use of formal verification techniques for controller synthesis that can give guarantees for safety and performance. Robots operating in unstructured environments also face limited sensing capability. Correctly inferring a robot's progress toward high level goal can be challenging.

This thesis develops new algorithms for synthesizing discrete controllers in partially known environments under specifications represented as linear temporal logic (LTL) formulas. It is inspired by recent developments in finite abstraction techniques for hybrid systems and motion planning problems. The robot and its environment is assumed to have a finite abstraction as a Partially Observable Markov Decision Process (POMDP), which is a powerful model class capable of representing a wide variety of problems. However, synthesizing controllers that satisfy LTL goals over POMDPs is a challenging problem which has received only limited attention.

This thesis proposes tractable, approximate algorithms for the control synthesis problem using Finite State Controllers (FSCs). The use of FSCs to control finite POMDPs allows for the closed system to be analyzed as finite global Markov chain. The thesis explicitly shows how transient and steady state behavior of the global Markov chains can be related to two different criteria with respect to satisfaction of LTL formulas. First, the maximization of the probability of LTL satisfaction is related to an optimization problem over a parametrization of the FSC. Analytic computation of gradients are derived which allows the use of first order optimization techniques.

The second criterion encourages rapid and frequent visits to a restricted set of states over infinite executions. It is formulated as a constrained optimization problem with a discounted long term reward objective by the novel utilization of a fundamental equation for Markov chains - the Poisson equation. A new constrained policy iteration technique is proposed to solve the resulting dynamic program, which also provides a way to escape local maxima.

The algorithms proposed in the thesis are applied to the task planning and execution challenges faced during the DARPA Autonomous Robotic Manipulation - Software challenge.

Neutron slowing down with inelastic scattering

The emphasis in reactor physics research has shifted toward investigations of fast reactors. The effects of high energy neutron processes have thus become fundamental to our understanding, and one of the most important of these processes is nuclear inelastic scattering. In this research we include inelastic scattering as a primary energy transfer mechanism, and study the resultant neutron energy spectrum in an infinite medium. We assume that the moderator material has a high mass number, so that in a laboratory coordinate system the energy loss of an inelastically scattered neutron may be taken as discrete. It is then consistent to treat elastic scattering with an age theory expansion. Mathematically these assumptions lead to balance equations of the differential-difference type.

The steady state problem is explored first by way of Laplace transformation of the energy variable. We then develop another steady state technique, valid for multiple inelastic level excitations, which depends on the level structure satisfying a physically reasonable constraint. In all cases the solutions we generate are compared with results obtained by modeling inelastic scattering with a separable, evaporative kernel.

The time dependent problem presents some new difficulties. By modeling the elastic scattering cross section in a particular way, we generate solutions to this more interesting problem. We conjecture the method of characteristics may be useful in analyzing time dependent problems with general cross sections. These ideas are briefly explored.

Conduction through thin titanium dioxide films

Conduction through TiO₂ films of thickness 100 to 450 Å have been investigated. The samples were prepared by either anodization of Ti evaporation of TiO₂, with Au or Al evaporated for contacts. The anodized samples exhibited considerable hysteresis due to electrical forming, however it was possible to avoid this problem with the evaporated samples from which complete sets of experimental results were obtained and used in the analysis. Electrical measurements included: the dependence of current and capacitance on dc voltage and temperature; the dependence of capacitance and conductance on frequency and temperature; and transient measurements of current and capacitance. A thick (3000 Å) evaporated TiO₂ film was used for measuring the dielectric constant (27.5) and the optical dispersion, the latter being similar to that for rutile. An electron transmission diffraction pattern of a evaporated film indicated an essentially amorphous structure with a short range order that could be related to rutile. Photoresponse measurements indicated the same band gap of about 3 ev for anodized and evaporated films and reduced rutile crystals and gave the barrier energies at the contacts.

The results are interpreted in a self consistent manner by considering the effect of a large impurity concentration in the films and a correspondingly large ionic space charge. The resulting potential profile in the oxide film leads to a thermally assisted tunneling process between the contacts and the interior of the oxide. A general relation is derived for the steady state current through structures of this kind. This in turn is expressed quantitatively for each of two possible limiting types of impurity distributions, where one type gives barriers of an exponential shape and leads to quantitative predictions in c lose agreement with the experimental results. For films somewhat greater than 100 Å, the theory is formulated essentially in terms of only the independently measured barrier energies and a characteristic parameter of the oxide that depends primarily on the maximum impurity concentration at the contacts. A single value of this parameter gives consistent agreement with the experimentally observed dependence of both current and capacitance on dc voltage and temperature, with the maximum impurity concentration found to be approximately the saturation concentration quoted for rutile. This explains the relative insensitivity of the electrical properties of the films on the exact conditions of formation.

Isotopic fractionation in sputtering

Isotopic fractionation due to sputtering has been investigated via a collector type experiment in which targets of known isotopic composition have been bombarded with several keV Ar⁺ and Xe⁺ ions with fluences down to 3.0x10¹⁴ ions/cm² , believed to be the lowest fluences for which such detailed measurements have ever been made. The isotopes were sputtered onto carbon collectors and analyzed with Secondary Ion Mass Spectroscopy (SIMS.) There is clear indication of preferential effects several times that predicted by the dominant analytical theory. Results also show a fairly strong angular variation in the fractionation. The maximum effect is usually seen in the near normal direction, measured from the target surface, falling continuously, by a few percent in some cases, to a minimum in the oblique direction. Measurements have been made using Mo isotopes: ¹⁰⁰Mo and ⁹²Mo and a liquid metal system of In:Ga eutectic. The light isotope of Mo is found to suffer a 53 ± 5‰ (note: 1.0‰ ≡ 0.1%) enrichment in the sputtered flux in the near normal direction, compared to the steady state near normal sputtered composition, under 5.0 keV Xe⁺ bombardment of 3.0 x 10¹⁴ ions/cm². In the liquid metal study only the angular dependence of the fractionation could be measured due to the lack of a well defined reference and the nature of the liquid surface, which is able to 'repair' itself during the course of a bombardment. The results show that ¹¹³In is preferentially sputtered over ¹¹⁵In in the near normal direction by about 8.7 ± 2.7‰ compared to the oblique direction. ⁶⁹Ga, on the other hand, is sputtered preferentially over ⁷¹Ga in the oblique direction by about 13 ± 4.4‰ with respect to the near normal direction.

Kinetic investigations of heterogeneously catalyzed reactions on the Ru(001) and Pt(110)-(1x2) surfaces

The initial probabilities of activated, dissociative chemisorption of methane and ethane on Pt(110)-(1 x 2) have been measured. The surface temperature was varied from 450 to 900 K with the reactant gas temperature constant at 300 K. Under these conditions, we probe the kinetics of dissociation via trapping-mediated (as opposed to 'direct') mechanism. It was found that the probabilities of dissociation of both methane and ethane were strong functions of the surface temperature with an apparent activation energies of 14.4 kcal/mol for methane and 2.8 kcal/mol for ethane, which implys that the methane and ethane molecules have fully accommodated to the surface temperature. Kinetic isotope effects were observed for both reactions, indicating that the C-H bond cleavage was involved in the rate-limiting step. A mechanistic model based on the trapping-mediated mechanism is used to explain the observed kinetic behavior. The activation energies for C-H bond dissociation of the thermally accommodated methane and ethane on the surface extracted from the model are 18.4 and 10.3 kcal/mol, respectively.

The studies of the catalytic decomposition of formic acid on the Ru(001) surface with thermal desorption mass spectrometry following the adsorption of DCOOH and HCOOH on the surface at 130 and 310 K are described. Formic acid (DCOOH) chemisorbs dissociatively on the surface via both the cleavage of its O-H bond to form a formate and a hydrogen adatom, and the cleavage of its C-O bond to form a carbon monoxide, a deuterium adatom and an hydroxyl (OH). The former is the predominant reaction. The rate of desorption of carbon dioxide is a direct measure of the kinetics of decomposition of the surface formate. It is characterized by a kinetic isotope effect, an increasingly narrow FWHM, and an upward shift in peak temperature with Ɵ_T, the coverage of the dissociatively adsorbed formic acid. The FWHM and the peak temperature change from 18 K and 326 K at Ɵ_T = 0.04 to 8 K and 395 K at Ɵ_T = 0.89. The increase in the apparent activation energy of the C-D bond cleavage is largely a result of self-poisoning by the formate, the presence of which on the surface alters the electronic properties of the surface such that the activation energy of the decomposition of formate is increased. The variation of the activation energy for carbon dioxide formation with Ɵ_T accounts for the observed sharp carbon dioxide peak. The coverage of surface formate can be adjusted over a relatively wide range so that the activation energy for C-D bond cleavage in the case of DCOOH can be adjusted to be below, approximately equal to, or well above the activation energy for the recombinative desorption of the deuterium adatoms. Accordingly, the desorption of deuterium was observed to be governed completely by the desorption kinetics of the deuterium adatoms at low Ɵ_T, jointly by the kinetics of deuterium desorption and C-D bond cleavage at intermediate Ɵ_T, and solely by the kinetics of C-D bond cleavage at high Ɵ_T. The overall branching ratio of the formate to carbon dioxide and carbon monoxide is approximately unity, regardless the initial coverage Ɵ_T, even though the activation energy for the production of carbon dioxide varies with Ɵ_T. The desorption of water, which implies C-O bond cleavage of the formate, appears at approximately the same temperature as that of carbon dioxide. These observations suggest that the cleavage of the C-D bond and that of the C-O bond of two surface formates are coupled, possibly via the formation of a short-lived surface complex that is the precursor to to the decomposition.

The measurement of steady-state rate is demonstrated here to be valuable in determining kinetics associated with short-lived, molecularly adsorbed precursor to further reactions on the surface, by determining the kinetic parameters of the molecular precursor of formaldehyde to its dissociation on the Pt(110)-(1 x 2) surface.

Overlayers of nitrogen adatoms on Ru(001) have been characterized both by thermal desorption mass spectrometry and low-energy electron diffraction, as well as chemically via the postadsorption and desorption of ammonia and carbon monoxide.

The nitrogen-adatom overlayer was prepared by decomposing ammonia thermally on the surface at a pressure of 2.8 x 10^(-6) Torr and a temperature of 480 K. The saturated overlayer prepared under these conditions has associated with it a (√247/10 x √247/10)R22.7° LEED pattern, has two peaks in its thermal desorption spectrum, and has a fractional surface coverage of 0.40. Annealing the overlayer to approximately 535 K results in a rather sharp (√3 x √3)R30° LEED pattern with an associated fractional surface coverage of one-third. Annealing the overlayer further to 620 K results in the disappearance of the low-temperature thermal desorption peak and the appearance of a rather fuzzy p(2x2) LEED pattern with an associated fractional surface coverage of approximately one-fourth. In the low coverage limit, the presence of the (√3 x √3)R30° N overlayer alters the surface in such a way that the binding energy of ammonia is increased by 20% relative to the clean surface, whereas that of carbon monoxide is reduced by 15%.

A general methodology for the indirect relative determination of the absolute fractional surface coverages has been developed and was utilized to determine the saturation fractional coverage of hydrogen on Ru(001). Formaldehyde was employed as a bridge to lead us from the known reference point of the saturation fractional coverage of carbon monoxide to unknown reference point of the fractional coverage of hydrogen on Ru(001), which is then used to determine accurately the saturation fractional coverage of hydrogen. We find that ƟSAT/H = 1.02 (±0.05), i.e., the surface stoichiometry is Ru : H = 1 : 1. The relative nature of the method, which cancels systematic errors, together with the utilization of a glass envelope around the mass spectrometer, which reduces spurious contributions in the thermal desorption spectra, results in high accuracy in the determination of absolute fractional coverages.

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.

Generalizations and extensions of the Fokker-Planck-Kolmogorov equations

The problem of determining probability density functions of general transformations of random processes is considered in this thesis. A method of solution is developed in which partial differential equations satisfied by the unknown density function are derived. These partial differential equations are interpreted as generalized forms of the classical Fokker-Planck-Kolmogorov equations and are shown to imply the classical equations for certain classes of Markov processes. Extensions of the generalized equations which overcome degeneracy occurring in the steady-state case are also obtained.

The equations of Darling and Siegert are derived as special cases of the generalized equations thereby providing unity to two previously existing theories. A technique for treating non-Markov processes by studying closely related Markov processes is proposed and is seen to yield the Darling and Siegert equations directly from the classical Fokker-Planck-Kolmogorov equations.

As illustrations of their applicability, the generalized Fokker-Planck-Kolmogorov equations are presented for certain joint probability density functions associated with the linear filter. These equations are solved for the density of the output of an arbitrary linear filter excited by Markov Gaussian noise and for the density of the output of an RC filter excited by the Poisson square wave. This latter density is also found by using the extensions of the generalized equations mentioned above. Finally, some new approaches for finding the output probability density function of an RC filter-limiter-RC filter system driven by white Gaussian noise are included. The results in this case exhibit the data required for complete solution and clearly illustrate some of the mathematical difficulties inherent to the use of the generalized equations.