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At least 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Real-Time Study of Prostate Intrafraction Motion During External Beam Radiotherapy With Daily Endorectal Balloon

Purpose: To prospectively investigate intrafraction prostate motion during radiofrequency-guided prostate radiotherapy with implanted electromagnetic transponders when daily endorectal balloon (ERB) is used. Methods and Materials: Intrafraction prostate motion from 24 patients in 787 treatment sessions was evaluated based on three-dimensional (3D), lateral, cranial-caudal (CC), and anterior-posterior (AP) displacements. The mean percentage of time with 3D, lateral, CC, and AP prostate displacements >2, 3, 4, 5, 6, 7, 8, 9, and 10 mm in 1 minute intervals was calculated for up to 6 minutes of treatment time. Correlation between the mean percentage time with 3D prostate displacement >3 mm vs. treatment week was investigated. Results: The percentage of time with 3D prostate movement >2, 3, and 4 mm increased with elapsed treatment time (p < 0.05). Prostate movement >5 mm was independent of elapsed treatment time (p = 0.11). The overall mean time with prostate excursions >3 mm was 5%. Directional analysis showed negligible lateral prostate motion; AP and CC motion were comparable. The fraction of time with 3D prostate movement >3 mm did not depend on treatment week of (p > 0.05) over a 4-minute mean treatment time. Conclusions: Daily endorectal balloon consistently stabilizes the prostate, preventing clinically significant displacement (>5 mm). A 3-mm internal margin may sufficiently account for 95% of intrafraction prostate movement for up to 6 minutes of treatment time. Directional analysis suggests that the lateral internal margin could be further reduced to 2 mm.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2012

Impact of Immobilization on Intrafraction Motion for Spine Stereotactic Body Radiotherapy Using Cone Beam Computed Tomography

Purpose: Spine stereotactic body radiotherapy (SBRT) involves tight planning margins and steep dose gradients to the surrounding organs at risk (OAR). This study aimed to assess intrafraction motion using cone beam computed tomography (CBCT) for spine SBRT patients treated using three immobilization devices. Methods and Materials: Setup accuracy using CBCT was retrospectively analyzed for 102 treated spinal metastases in 84 patients. Thoracic and lumbar spine patients were immobilized with either an evacuated cushion (EC, n = 24) or a semirigid vacuum body fixation (BF, n = 60). For cases treated at cervical/upper thoracic (thoracic [T]1-T3) vertebrae, a thermoplastic S-frame (SF) mask (n = 18) was used. Patient setup was corrected by using bony anatomy image registration and couch translations only (no rotation corrections) with shifts confirmed on verification CBCTs. Repeat imaging was performed mid- and post-treatment. Patient translational and rotational positioning data were recorded to calculate means, standard deviations (SD), and corresponding margins {+-} 2 SD for residual setup errors and intrafraction motion. Results: A total of 355 localizations, 333 verifications, and 248 mid- and 280 post-treatment CBCTs were analyzed. Residual translations and rotations after couch corrections (verification scans) were similar for all immobilization systems, with SDs of 0.6 to 0.9 mm in any direction and 0.9 Degree-Sign to 1.6 Degree-Sign , respectively. Margins to encompass residual setup errors after couch corrections were within 2 mm. Including intrafraction motion, as measured on post-treatment CBCTs, SDs for total setup error in the left-right, cranial-caudal, and anterior-posterior directions were 1.3, 1.2, and 1.0 mm for EC; 0.9, 0.7, and 0.9 mm for BF; and 1.3, 0.9, and 1.1 mm for SF, respectively. The calculated margins required to encompass total setup error increased to 3 mm for EC and SF and remained within 2 mm for BF. Conclusion: Following image guidance, residual setup errors for spine SBRT were similar across three immobilization systems. The BF device resulted in the least amount of intrafraction motion, and based on this device, we justify a 2-mm margin for the planning OAR and target volume.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

Inter- and Intrafraction Target Motion in Highly Focused Single Vocal Cord Irradiation of T1a Larynx Cancer Patients

Purpose: The purpose of this study was to verify clinical target volume–planning target volume (CTV-PTV) margins in single vocal cord irradiation (SVCI) of T1a larynx tumors and characterize inter- and intrafraction target motion. Methods and Materials: For 42 patients, a single vocal cord was irradiated using intensity modulated radiation therapy at a total dose of 58.1 Gy (16 fractions × 3.63 Gy). A daily cone beam computed tomography (CBCT) scan was performed to online correct the setup of the thyroid cartilage after patient positioning with in-room lasers (interfraction motion correction). To monitor intrafraction motion, CBCT scans were also acquired just after patient repositioning and after dose delivery. A mixed online-offline setup correction protocol (“O2 protocol”) was designed to compensate for both inter- and intrafraction motion. Results: Observed interfraction, systematic (Σ), and random (σ) setup errors in left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions were 0.9, 2.0, and 1.1 mm and 1.0, 1.6, and 1.0 mm, respectively. After correction of these errors, the following intrafraction movements derived from the CBCT acquired after dose delivery were: Σ = 0.4, 1.3, and 0.7 mm, and σ = 0.8, 1.4, and 0.8 mm. More than half of the patients showed a systematic non-zero intrafraction shift in target position, (ie, the mean intrafraction displacement over the treatment fractions was statistically significantly different from zero; P<.05). With the applied CTV-PTV margins (for most patients 3, 5, and 3 mm in LR, CC, and AP directions, respectively), the minimum CTV dose, estimated from the target displacements observed in the last CBCT, was at least 94% of the prescribed dose for all patients and more than 98% for most patients (37 of 42). The proposed O2 protocol could effectively reduce the systematic intrafraction errors observed after dose delivery to almost zero (Σ = 0.1, 0.2, 0.2 mm). Conclusions: With adequate image guidance and CTV-PTV margins in LR, CC, and AP directions of 3, 5, and 3 mm, respectively, excellent target coverage in SVCI could be ensured.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Limited Impact of Setup and Range Uncertainties, Breathing Motion, and Interplay Effects in Robustly Optimized Intensity Modulated Proton Therapy for Stage III Non-small Cell Lung Cancer

Purpose: To investigate the impact of setup and range uncertainties, breathing motion, and interplay effects using scanning pencil beams in robustly optimized intensity modulated proton therapy (IMPT) for stage III non-small cell lung cancer (NSCLC). Methods and Materials: Three-field IMPT plans were created using a minimax robust optimization technique for 10 NSCLC patients. The plans accounted for 5- or 7-mm setup errors with ±3% range uncertainties. The robustness of the IMPT nominal plans was evaluated considering (1) isotropic 5-mm setup errors with ±3% range uncertainties; (2) breathing motion; (3) interplay effects; and (4) a combination of items 1 and 2. The plans were calculated using 4-dimensional and average intensity projection computed tomography images. The target coverage (TC, volume receiving 95% of prescribed dose) and homogeneity index (D{sub 2} − D{sub 98}, where D{sub 2} and D{sub 98} are the least doses received by 2% and 98% of the volume) for the internal clinical target volume, and dose indexes for lung, esophagus, heart and spinal cord were compared with that of clinical volumetric modulated arc therapy plans. Results: The TC and homogeneity index for all plans were within clinical limits when considering the breathing motion and interplay effects independently. The setup and range uncertainties had a larger effect when considering their combined effect. The TC decreased to <98% (clinical threshold) in 3 of 10 patients for robust 5-mm evaluations. However, the TC remained >98% for robust 7-mm evaluations for all patients. The organ at risk dose parameters did not significantly vary between the respective robust 5-mm and robust 7-mm evaluations for the 4 error types. Compared with the volumetric modulated arc therapy plans, the IMPT plans showed better target homogeneity and mean lung and heart dose parameters reduced by about 40% and 60%, respectively. Conclusions: In robustly optimized IMPT for stage III NSCLC, the setup and range uncertainties, breathing motion, and interplay effects have limited impact on target coverage, dose homogeneity, and organ-at-risk dose parameters.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2017

Exploring the Margin Recipe for Online Adaptive Radiation Therapy for Intermediate-Risk Prostate Cancer: An Intrafractional Seminal Vesicles Motion Analysis

Purpose: To provide a benchmark for seminal vesicle (SV) margin selection to account for intrafractional motion and to investigate the effectiveness of 2 motion surrogates in predicting intrafractional SV coverage. Methods and Materials: Fifteen prostate patients were studied. Each patient had 5 pairs (1 patient had 4 pairs) of pretreatment and posttreatment cone beam CTs (CBCTs). Each pair of CBCTs was registered on the basis of prostate fiducial markers. All pretreatment SVs were expanded with 1-, 2-, 3-, 4-, 5-, and 8-mm isotropic margins to form a series of planning target volumes, and their intrafractional coverage to the posttreatment SV determined the “ground truth” for exact coverage. Two motion surrogates, the center of mass (COM) and the border of contour, were evaluated by the use of Pearson product-moment correlation coefficient and exponential fitting for predicting SV underdosage. Action threshold of each surrogate was calculated. The margin for each surrogate was calculated according to a traditional margin recipe. Results: Ninety-five percent posttreatment SV coverage was achieved in 9%, 53%, 73%, 86%, 95%, and 97% of fractions with 1-, 2-, 3-, 4-, 5-, and 8-mm margins, respectively. The 5-mm margins provided 95% intrafractional SV coverage in over 90% of fractions. The correlation between the COM and border was weak, moderate, and strong in the left-right (L-R), anterior-posterior (A-P), and superior-inferior (S-I) directions, respectively. Exponential fitting gave the underdosage threshold of 4.5 and 7.0 mm for the COM and border. The Van Herk margin recipe recommended 0-, 0.5-, and 0.8-mm margins in the L-R, A-P, and S-I directions based on the COM, and 1.2-, 3.9-, and 2.5-mm margins based on the border. Conclusions: Five-millimeter isotropic margins for the SV constitute the minimum required to mitigate the intrafractional motion. Both the COM and the border are acceptable predictors for SV underdosage with 4.5- and 7.0-mm action threshold. Traditional margin based on the COM or border underestimates the margin.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2011

A Pelvic Phantom for Modeling Internal Organ Motions

A pelvic phantom was developed for use in testing image-guided radiation therapy (IGRT) and adaptive applications in radiation therapy (ART) with simulating the anterior-posterior internal organ motions during prostate radiotherapy. Measurements could be done with an ionization chamber (IC) in the simulated prostate. The rectum was simulated by air-equivalent material (AEM). The volume superior to the IC placement was considered as the bladder. The extension of AEM volume could be varied. The vertical position of the IC placement could be shifted by {+-}1 cm to simulate the prostate motion parallel to the changes in bladder volume. The reality of the simulation was inspected. Three-millimeter-slice-increment computed tomography (CT) scans were taken for irradiation planning. The structure set was adapted to the phantom from a treated patient. Planning target volume was delineated according to the RTOG 0126 study. IMRT and 3D conformal radiation therapy (3D-CRT) plans were made. Prostate motion and rectum volume changes were simulated in the phantom. IC displacement was corrected by phantom shifting. The delivered dose was measured with IC in 7 cases using intensity-modulated radiation therapy (IMRT) and 3D-CRT fractions, and single square-shaped beams: anteroposterior (AP), posteroanterior (PA), and lateral (LAT). Variations from the calculated doses were slightly below 1% at IMRT and around 1% at 3D-CRT; below 4.5% at square AP beam; up to 9% at square PA beam; and around 0.5% at square LAT beam. Other authors have already shown that by using planning systems and ultrasonic and cone beam CT guidance, correction of organ motions in a real patient during prostate cancer IGRT does not have a significant dosimetric effect. The inspection of our phantom-as described here-ended with similar results. Our team suggested that our model is sufficiently realistic and can be used for IGRT and ART testing.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2017

Translational and Rotational Motion of C8 Aromatics Adsorbed in Isotropic Porous Media (MOF-5): NMR Studies and MD Simulations

For this, we combined nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation to study xylene behavior in MOF-5, probing the effects of adsorbate geometry in a weakly interacting model isotropic metal organic framework (MOF) system. We employed NMR diffusometry and relaxometry techniques at low field (13 MHz) to quantify the self-diffusion coefficients (D s ) and the longitudinal relaxation times (T 1 ) of xylenes in MOF-5 as a function of temperature at the saturated loading for each xylene. These experiments reveal the translational motion activation energies to be 15.3, 19.7, and 21.2 kj mol -1 and the rotational activation energies to be 47.26, 12.88, and 11.55 for the (p-, m-, o-) xylene isomers, respectively. Paraxylene exhibits faster translational motion, yet shows four times the activation energy barrier for rotational motion vis-à-vis the other isomers. MD simulations performed on these model systems corroborate the findings for paraxylene and suggest that paraxylene has the lower free energy barrier for hopping away from its binding sites, yet has the slowest rotational motion in the plane of the xylene molecule.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Scanning Kelvin Probe Microscopy Reveals That Ion Motion Varies with Dimensionality in 2D Halide Perovskites

We study ion migration in 2D lead halide perovskites of varying dimensionality using scanning Kelvin probe microscopy (SKPM). We perform potentiometry on micrometer-scale lateral junctions in the absence of injected charge, and we compare how ion motion varies between prototypical two-dimensional n-butylammonium lead iodide perovskites (BA 2 PbI 4 , n = 1), and methylammonium-incorporated quasi-2D perovskites (BA 2 MA 3 Pb 4 I 13 , ~ < n > = 4) under the effects of illumination and temperature. We attribute the observed slow dynamics to relaxation of the bias-induced ionic charge distributions at different temperatures, and we extract the activation energies associated with the ionic motion in each case. Lastly, we propose an explanation for these phenomena by hypothesizing that ion motion in purely-2D BA 2 PbI 4 perovskite films is dominated by paired halide and halide vacancy, whereas for quasi-2D BA 2 MA 3 Pb 4 I 13 perovskites, the ion motion is a combination of both halide and methylammonium (vacancy) migration. These data show that dimensionality in these systems plays a critical role in ion dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Superresolved Motions of Single Molecular Catalysts during Polymerization Show Wide Distributions

The motion of single molecular ruthenium catalysts during and after single turnover events of ring-opening metathesis polymerization is imaged through single-molecule superresolution tracking with a positional accuracy of ±32 nm. This tracking is achieved through the real-time incorporation of spectrally tagged monomer units into active polymer chain ends during living polymerization; thus, by design, only active-catalyst motion is detected and imaged, without convolution by inactive catalysts. The catalysts show diverse individualistic diffusive behaviors with respect to time that persist for up to 20 s. Catalysts occupy three mobility populations: quasi-stationary (23%), intermediate (53%, 65 nm), and large (24%, 145 nm) step sizes. Differences in catalyst mobility populations also exist between individual aggregates (p < 0.001). Such differential motion indicates widely different local catalyst microenvironments during the catalytic turnover. These mobility differences are uniquely observable through single-catalyst microscopy and are not measurable through traditional ensemble analytical techniques for characterizing the behavior of molecular catalysts, such as nuclear magnetic resonance spectroscopy. The measured distributions of active molecular catalyst motions would not be readily predictable through modeling or first-principles, and the range likely impacts individual catalyst turnover rate and selectivity. This range plausibly contributes to property distributions observable in bulk polymers, such as molecular weight polydispersity (e.g., 1.9 in this system), leading to a revised understanding of the mechanistic, microscale origins of macroscale polymer properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Steady motion of 80-nm-size skyrmions in a 100-nm-wide track

The current-driven movement of magnetic skyrmions along a nanostripe is essential for the advancement and functionality of a new category of spintronic devices resembling racetracks. Despite extensive research into skyrmion dynamics, experimental verification of current-induced motion of ultra-small skyrmions within an ultrathin nanostripe is still pending. Here, we unveil the motion of individual 80 nm-size skyrmions in an FeGe track with an ultrathin width of 100 nm. The skyrmions can move steadily along the track over a broad range of current densities by using controlled pulse durations of as low as 2 ns. The potential landscape, arising from the magnetic edge twists in such a geometrically confined system, introduces skyrmion inertia and ensures efficient motion with a vanishing skyrmion Hall angle. Our results showcase the steady motion of skyrmions in an ultrathin track, offering a practical pathway for implementing skyrmion-based spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Quantum effects on dislocation motion from ring-polymer molecular dynamics

Quantum motion of atoms known as zero-point vibration was recently proposed to explain a long-standing discrepancy between theoretically computed and experimentally measured low-temperature plastic strength of iron and possibly other metals with high atomic masses. This finding challenges the traditional notion that quantum motion of atoms is relatively unimportant in solids comprised of heavy atoms. Here we report quantum dynamic simulations of quantum effects on dislocation motion within the exact formalism of Ring-Polymer Molecular Dynamics (RPMD). To extend the reach of quantum atomistic simulations to length and time scales relevant for extended defects in materials, we implemented RPMD in the open-source code LAMMPS thus making the RPMD method widely available to the community. We use our RPMD/LAMMPS approach for direct calculations of dislocation mobility and its effects on the yield strength of α-iron. Our simulation results establish that quantum effects are noticeable at temperatures below 50 K but account for only a modest (≈13% at T = 0 K) overall reduction in the Peierls barrier, at variance with the factor of two reduction predicted earlier based on the more approximate framework of harmonic transition state theory. Our results confirm that zero-point vibrations provide ample additional agitation for atomic motion that increases with decreasing temperature, however its enhancing effect on dislocation mobility is largely offset by an increase in the effective atom size, an effect known as quantum dispersion that has not been accounted for in the previous calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Complex motion of steerable vesicular robots filled with active colloidal rods

Abstract While the collective motion of active particles has been studied extensively, effective strategies to navigate particle swarms without external guidance remain elusive. We introduce a method to control the trajectories of two-dimensional swarms of active rod-like particles by confining the particles to rigid bounding membranes (vesicles) with non-uniform curvature. We show that the propelling agents spontaneously form clusters at the membrane wall and collectively propel the vesicle, turning it into an active superstructure. To further guide the motion of the superstructure, we add discontinuous features to the rigid membrane boundary in the form of a kinked tip, which acts as a steering component to direct the motion of the vesicle. We report that the system’s geometrical and material properties, such as the aspect ratio and Péclet number of the active rods as well as the kink angle and flexibility of the membrane, determine the stacking of active particles close to the kinked confinement and induce a diverse set of dynamical behaviors of the superstructure, including linear and circular motion both in the direction of, and opposite to, the kink. From a systematic study of these various behaviors, we design vesicles with switchable and reversible locomotions by tuning the confinement parameters. The observed phenomena suggest a promising mechanism for particle transportation and could be used as a basic element to navigate active matter through complex and tortuous environments.

42 ENGINEERING↗
DOE OSTI2024

Quantifying motion blur by imaging shock front propagation with broadband and narrowband X-ray sources

Time-integrated radiography using MeV Bremsstrahlung X-ray sources is the norm for imaging during system-level testing of components and structures under dynamic condition. One source of error in the analysis of the time-integrated radiography data sets stems from motion blur which smears out sharp interfaces to a greater degree with longer exposure times, which become necessary to provide sufficient signal-to-noise with low X-ray penetration of objects of interest. To quantify motion blur, a 1D shock wave through PMMA was investigated experimentally at The Dynamic Compression Sector at The Advanced Photon Source (DCS@APS) with tapered broadband and 25.46 ± 1.06 keV narrowband X-rays. Four cameras with different exposure times were used for each experiment to compare the effect that exposure time has on motion blur. In addition, our methodology to accurately simulate motion blur in terms of transmission and shape is presented and compared to our experimental results and quantified. There is a high level of agreement between the experimental and simulation results across the range of data sets investigated in this study with a percent difference range of 0.29–1.31% for the four shots. The methodology of this work serves as a steppingstone towards a physically validated model that could be used in conjunction with experimental results to deconvolve physical parameters, densities, and interfaces of interest in a way that would not be possible with experimental results alone.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Designing Dual-functionalized Gels for Self-reconfiguration and Autonomous Motion

Human motion is enabled by the concerted expansion and contraction of interconnected muscles that are powered by inherent biochemical reactions. One of the challenges in the field of biomimicry is eliciting this form of motion from purely synthetic materials, which typically do not generate internalized reactions to drive mechanical action. Moreover, for practical applications, this bio-inspired motion must be readily controllable. Herein, we develop a computational model to design a new class of polymer gels where structural reconfigurations and internalized reactions are intimately linked to produce autonomous motion, which can be directed with light. These gels contain both spirobenzopyran (SP) chromophores and the ruthenium catalysts that drive the oscillatory Belousov-Zhabotinsky (BZ) reaction. Importantly, both the SP moieties and the BZ reaction are photosensitive. When these dual-functionalized gels are exposed to non-uniform illumination, the localized contraction of the gel (due to the SP moieties) in the presence of traveling chemical waves (due to the BZ reaction) leads to new forms of spontaneous, self-sustained movement, which cannot be achieved by either of the mono-functionalized networks.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque

Spin-polarized electrons can move a ferromagnetic domain wall through the transfer of spin angular momentum when current flows in a magnetic nanowire. Such current induced control of a domain wall is of significant interest due to its potential application for low power ultra high-density data storage. In previous reports, it has been observed that the motion of the domain wall always happens parallel to the current flow – either in the same or opposite direction depending on the specific nature of the interaction. In contrast, here we demonstrate deterministic control of a ferromagnetic domain wall orthogonal to current flow by exploiting the spin orbit torque in a perpendicularly polarized Ta/CoFeB/MgO heterostructure in presence of an in-plane magnetic field. Reversing the polarity of either the current flow or the in-plane field is found to reverse the direction of the domain wall motion. Notably, such orthogonal motion with respect to current flow is not possible from traditional spin transfer torque driven domain wall propagation even in presence of an external magnetic field. Therefore the domain wall motion happens purely due to spin orbit torque. These results represent a completely new degree of freedom in current induced control of a ferromagnetic domain wall.

Science & Technology - Other Topics↗
DOE OSTI2011

Designing mechano-responsive microcapsules that undergo self-propelled motion.

Biological cells are capable of sensing mechanical cues and responding to these signals by undergoing morphological changes and directed motion. A significant challenge is creating cell-like objects that can translate mechanical stimuli into analogous behavior. Herein, we use computational modeling to design a simple mechanosensitive 'cell' that responds to mechanical deformation through a shape change that allows it to undergo self-sustained, directed movement. Our cellular object is formed from a nanoparticle-filled microcapsule that is located on an adhesive substrate in solution. In response to a locally applied force, the deformed capsule releases nanoparticles that bind to the surface and dynamically create adhesion gradients. Due to the self-generated gradients, the capsule moves autonomously from regions of less adhesion to greater adhesion. During the capsule's motion, new nanoparticles are released that both sustain and propagate the adhesion gradients and thus, the capsule sustains autonomous movement along its path (until it is depleted of nanoparticles). The self-sustained motion occurs only if the permeability of the capsule's shell depends on mechanical deformation. We isolate critical parameters that control the dynamic behavior of this mechano-responsive capsule. Our findings can facilitate the fabrication of devices that are powered by the autonomous movement of microscopic synthetic cells. Additionally, the capsules could serve as sensors for mechanical strain, indicating the presence of strain fields by their spontaneous motion and release of nanoparticles; the latter behavior could be exploited in the fabrication of self-healing materials.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt($\mathrm{II}$) complex

Ultrafast excited-state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. Here, in this work, we investigated a cyclometalated platinum(II) dimer known to undergo a Pt-Pt bond contraction in metal-metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we are able to correlate coherent vibrational wavepacket (CVWP) evolution with stimulated emission (SE) dynamics of the 1 MMLCT state. The results demonstrated that the 145 cm -1 CVWP motions with the damping times of ~ 0.9 ps and ~ 2 ps originate from coherent Pt-Pt stretching vibrations in the singlet and triplet MMLCT states, respectively. On the basis of excited-state potential energy surface calculations in our previous work, we rationalized that the CVWP transfer from Franck-Condon (FC) state to the 3 MMLCT state was mediated by a triplet ligand-centered ( 3 LC) intermediate state through two step intersystem crossing (ISC) on a time scale shorter than a period of the Pt-Pt stretching wavepacket motions. Moreover, it was found that the CVWP motion with 110 cm-1 frequency decays with the damping time of ~ 0.2 ps, well matching the time constant of 0.253 ps corresponding to a redshift in the SE feature at early time. This observation indicates that the Pt-Pt bond contraction changes the stretching frequency from 110 to 145 cm -1 and stabilize the 1 MMLCT state relative to the 3 LC state with the ~ 0.2 ps time scale. Thus, the ultrafast ISC from the 1 MMLCT to the 3 LC states occurs before the Pt-Pt bond shortening. The findings herein provide insight into understanding the impact of Pt-Pt bond contraction on ultrafast branching of the 1 MMLCT population into the direct ( 1 MMLCT → 3 MMLCT) and indirect ISC pathways ( 1 MMLCT → 3 LC → 3 MMLCT) in the Pt(II) dimer. These results revealed intricate excited state electronic and nuclear motions that could steer the reaction pathways with levels of details that have not achieved before

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Direct numerical simulation of electrokinetic instability and transition to chaotic motion

A new type of instability—electrokinetic instability—and an unusual transition to chaotic motion near a charge-selective surface (semiselective electric membrane, electrode, or system of micro-/nanochannels) was studied by the numerical integration of the Nernst-Planck-Poisson-Stokes system and a weakly nonlinear analysis near the threshold of instability. A special finite-difference method was used for the space discretization along with a semi-implicit 31/3 -step Runge-Kutta scheme for the integration in time. Two kinds of initial conditions were considered: (a) white-noise initial conditions to mimic “room disturbances” and subsequent natural evolution of the solution, and (b) an artificial monochromatic ion distribution with a fixed wave number to simulate regular wave patterns. The results were studied from the viewpoint of hydrodynamic stability and bifurcation theory. The threshold of electroconvective movement was found by the linear spectral stability theory, the results of which were confirmed by numerical simulation of the entire system. Our weakly nonlinear analysis and numerical integration of the entire system predict possibility of both kinds of bifurcations at the critical point, supercritical and subcritical, depending on the system parameters. The following regimes, which replace each other as the potential drop between the selective surfaces increases, were obtained: one-dimensional steady solution, two-dimensional steady electroconvective vortices (stationary point in a proper phase space), unsteady vortices aperiodically changing their parameters (homoclinic contour), periodic motion (limit cycle), and chaotic motion. The transition to chaotic motion does not include Hopf bifurcation. The numerical resolution of the thin concentration polarization layer showed spike-like charge profiles along the surface, which could be, depending on the regime, either steady or aperiodically coalescent. The numerical investigation confirmed the experimentally observed absence of regular (near-sinusoidal) oscillations for the overlimiting regimes. There is a qualitative agreement of the experimental and the theoretical values of the threshold of instability, the dominant size of the observed coherent structures, and the experimental and theoretical volt–current characteristics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗