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2020


Learning to Predict Perceptual Distributions of Haptic Adjectives
Learning to Predict Perceptual Distributions of Haptic Adjectives

Richardson, B. A., Kuchenbecker, K. J.

Frontiers in Neurorobotics, 13(116):1-16, Febuary 2020 (article)

Abstract
When humans touch an object with their fingertips, they can immediately describe its tactile properties using haptic adjectives, such as hardness and roughness; however, human perception is subjective and noisy, with significant variation across individuals and interactions. Recent research has worked to provide robots with similar haptic intelligence but was focused on identifying binary haptic adjectives, ignoring both attribute intensity and perceptual variability. Combining ordinal haptic adjective labels gathered from human subjects for a set of 60 objects with features automatically extracted from raw multi-modal tactile data collected by a robot repeatedly touching the same objects, we designed a machine-learning method that incorporates partial knowledge of the distribution of object labels into training; then, from a single interaction, it predicts a probability distribution over the set of ordinal labels. In addition to analyzing the collected labels (10 basic haptic adjectives) and demonstrating the quality of our method's predictions, we hold out specific features to determine the influence of individual sensor modalities on the predictive performance for each adjective. Our results demonstrate the feasibility of modeling both the intensity and the variation of haptic perception, two crucial yet previously neglected components of human haptic perception.

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DOI [BibTex]

2020


DOI [BibTex]


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Exercising with Baxter: Preliminary Support for Assistive Social-Physical Human-Robot Interaction

Fitter, N. T., Mohan, M., Kuchenbecker, K. J., Johnson, M. J.

Journal of NeuroEngineering and Rehabilitation, 17(19), Febuary 2020 (article)

Abstract
Background: The worldwide population of older adults will soon exceed the capacity of assisted living facilities. Accordingly, we aim to understand whether appropriately designed robots could help older adults stay active at home. Methods: Building on related literature as well as guidance from experts in game design, rehabilitation, and physical and occupational therapy, we developed eight human-robot exercise games for the Baxter Research Robot, six of which involve physical human-robot contact. After extensive iteration, these games were tested in an exploratory user study including 20 younger adult and 20 older adult users. Results: Only socially and physically interactive games fell in the highest ranges for pleasantness, enjoyment, engagement, cognitive challenge, and energy level. Our games successfully spanned three different physical, cognitive, and temporal challenge levels. User trust and confidence in Baxter increased significantly between pre- and post-study assessments. Older adults experienced higher exercise, energy, and engagement levels than younger adults, and women rated the robot more highly than men on several survey questions. Conclusions: The results indicate that social-physical exercise with a robot is more pleasant, enjoyable, engaging, cognitively challenging, and energetic than similar interactions that lack physical touch. In addition to this main finding, researchers working in similar areas can build on our design practices, our open-source resources, and the age-group and gender differences that we found.

hi

DOI [BibTex]

DOI [BibTex]


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Sliding Mode Control with Gaussian Process Regression for Underwater Robots

Lima, G. S., Trimpe, S., Bessa, W. M.

Journal of Intelligent & Robotic Systems, January 2020 (article)

ics

DOI [BibTex]

DOI [BibTex]


Hierarchical Event-triggered Learning for Cyclically Excited Systems with Application to Wireless Sensor Networks
Hierarchical Event-triggered Learning for Cyclically Excited Systems with Application to Wireless Sensor Networks

Beuchert, J., Solowjow, F., Raisch, J., Trimpe, S., Seel, T.

IEEE Control Systems Letters, 4(1):103-108, January 2020 (article)

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arXiv PDF DOI Project Page [BibTex]

arXiv PDF DOI Project Page [BibTex]


Control-guided Communication: Efficient Resource Arbitration and Allocation in Multi-hop Wireless Control Systems
Control-guided Communication: Efficient Resource Arbitration and Allocation in Multi-hop Wireless Control Systems

Baumann, D., Mager, F., Zimmerling, M., Trimpe, S.

IEEE Control Systems Letters, 4(1):127-132, January 2020 (article)

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arXiv PDF DOI [BibTex]

arXiv PDF DOI [BibTex]


Self-supervised motion deblurring
Self-supervised motion deblurring

Liu, P., Janai, J., Pollefeys, M., Sattler, T., Geiger, A.

IEEE Robotics and Automation Letters, 2020 (article)

Abstract
Motion blurry images challenge many computer vision algorithms, e.g., feature detection, motion estimation, or object recognition. Deep convolutional neural networks are state-of-the-art for image deblurring. However, obtaining training data with corresponding sharp and blurry image pairs can be difficult. In this paper, we present a differentiable reblur model for self-supervised motion deblurring, which enables the network to learn from real-world blurry image sequences without relying on sharp images for supervision. Our key insight is that motion cues obtained from consecutive images yield sufficient information to inform the deblurring task. We therefore formulate deblurring as an inverse rendering problem, taking into account the physical image formation process: we first predict two deblurred images from which we estimate the corresponding optical flow. Using these predictions, we re-render the blurred images and minimize the difference with respect to the original blurry inputs. We use both synthetic and real dataset for experimental evaluations. Our experiments demonstrate that self-supervised single image deblurring is really feasible and leads to visually compelling results.

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pdf Project Page Blog [BibTex]

pdf Project Page Blog [BibTex]


Physical Variables Underlying Tactile Stickiness during Fingerpad Detachment
Physical Variables Underlying Tactile Stickiness during Fingerpad Detachment

Nam, S., Vardar, Y., Gueorguiev, D., Kuchenbecker, K. J.

Frontiers in Neuroscience, 2020 (article) Accepted

Abstract
One may notice a relatively wide range of tactile sensations even when touching the same hard, flat surface in similar ways. Little is known about the reasons for this variability, so we decided to investigate how the perceptual intensity of light stickiness relates to the physical interaction between the skin and the surface. We conducted a psychophysical experiment in which nine participants actively pressed their finger on a flat glass plate with a normal force close to 1.5 N and detached it after a few seconds. A custom-designed apparatus recorded the contact force vector and the finger contact area during each interaction as well as pre- and post-trial finger moisture. After detaching their finger, participants judged the stickiness of the glass using a nine-point scale. We explored how sixteen physical variables derived from the recorded data correlate with each other and with the stickiness judgments of each participant. These analyses indicate that stickiness perception mainly depends on the pre-detachment pressing duration, the time taken for the finger to detach, and the impulse in the normal direction after the normal force changes sign; finger-surface adhesion seems to build with pressing time, causing a larger normal impulse during detachment and thus a more intense stickiness sensation. We additionally found a strong between-subjects correlation between maximum real contact area and peak pull-off force, as well as between finger moisture and impulse.

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[BibTex]


Spatial Scheduling of Informative Meetings for Multi-Agent Persistent Coverage
Spatial Scheduling of Informative Meetings for Multi-Agent Persistent Coverage

Haksar, R. N., Trimpe, S., Schwager, M.

IEEE Robotics and Automation Letters, 2020 (article) Accepted

ics

DOI [BibTex]

DOI [BibTex]


Event-triggered Learning
Event-triggered Learning

Solowjow, F., Trimpe, S.

Automatica, 2020 (article) Accepted

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arXiv PDF Project Page [BibTex]


Safe and Fast Tracking on a Robot Manipulator: Robust MPC and Neural Network Control
Safe and Fast Tracking on a Robot Manipulator: Robust MPC and Neural Network Control

Nubert, J., Koehler, J., Berenz, V., Allgower, F., Trimpe, S.

IEEE Robotics and Automation Letters, 2020 (article) Accepted

Abstract
Fast feedback control and safety guarantees are essential in modern robotics. We present an approach that achieves both by combining novel robust model predictive control (MPC) with function approximation via (deep) neural networks (NNs). The result is a new approach for complex tasks with nonlinear, uncertain, and constrained dynamics as are common in robotics. Specifically, we leverage recent results in MPC research to propose a new robust setpoint tracking MPC algorithm, which achieves reliable and safe tracking of a dynamic setpoint while guaranteeing stability and constraint satisfaction. The presented robust MPC scheme constitutes a one-layer approach that unifies the often separated planning and control layers, by directly computing the control command based on a reference and possibly obstacle positions. As a separate contribution, we show how the computation time of the MPC can be drastically reduced by approximating the MPC law with a NN controller. The NN is trained and validated from offline samples of the MPC, yielding statistical guarantees, and used in lieu thereof at run time. Our experiments on a state-of-the-art robot manipulator are the first to show that both the proposed robust and approximate MPC schemes scale to real-world robotic systems.

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arXiv PDF DOI [BibTex]

arXiv PDF DOI [BibTex]

2015


Enzymatically active biomimetic micropropellers for the penetration of mucin gels
Enzymatically active biomimetic micropropellers for the penetration of mucin gels

Walker (Schamel), D., Käsdorf, B. T., Jeong, H. H., Lieleg, O., Fischer, P.

Science Advances, 1(11):e1500501, December 2015 (article)

Abstract
In the body, mucus provides an important defense mechanism by limiting the penetration of pathogens. It is therefore also a major obstacle for the efficient delivery of particle-based drug carriers. The acidic stomach lining in particular is difficult to overcome because mucin glycoproteins form viscoelastic gels under acidic conditions. The bacterium Helicobacter pylori has developed a strategy to overcome the mucus barrier by producing the enzyme urease, which locally raises the pH and consequently liquefies the mucus. This allows the bacteria to swim through mucus and to reach the epithelial surface. We present an artificial system of reactive magnetic micropropellers that mimic this strategy to move through gastric mucin gels by making use of surface-immobilized urease. The results demonstrate the validity of this biomimetic approach to penetrate biological gels, and show that externally propelled microstructures can actively and reversibly manipulate the physical state of their surroundings, suggesting that such particles could potentially penetrate native mucus.

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link (url) DOI [BibTex]

2015


link (url) DOI [BibTex]


The EChemPen: A Guiding Hand To Learn Electrochemical Surface Modifications
The EChemPen: A Guiding Hand To Learn Electrochemical Surface Modifications

Valetaud, M., Loget, G., Roche, J., Hueken, N., Fattah, Z., Badets, V., Fontaine, O., Zigah, D.

J. of Chem. Ed., 92(10):1700-1704, September 2015 (article)

Abstract
The Electrochemical Pen (EChemPen) was developed as an attractive tool for learning electrochemistry. The fabrication, principle, and operation of the EChemPen are simple and can be easily performed by students in practical classes. It is based on a regular fountain pen principle, where the electrolytic solution is dispensed at a tip to locally modify a conductive surface by triggering a localized electrochemical reaction. Three simple model reactions were chosen to demonstrate the versatility of the EChemPen for teaching various electrochemical processes. We describe first the reversible writing/erasing of metal letters, then the electrodeposition of a black conducting polymer "ink", and finally the colorful writings that can be generated by titanium anodization and that can be controlled by the applied potential. These entertaining and didactic experiments are adapted for teaching undergraduate students that start to study electrochemistry by means of surface modification reactions.

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DOI [BibTex]

DOI [BibTex]


Optimal Length of Low Reynolds Number Nanopropellers
Optimal Length of Low Reynolds Number Nanopropellers

Walker (Schamel), D., Kuebler, M., Morozov, K. I., Fischer, P., Leshansky, A. M.

Nano Letters, 15(7):4412-4416, June 2015 (article)

Abstract
Locomotion in fluids at the nanoscale is dominated by viscous drag. One efficient propulsion scheme is to use a weak rotating magnetic field that drives a chiral object. Froth bacterial flagella to artificial drills, the corkscrew is a universally useful chiral shape for propulsion in viscous environments. Externally powered magnetic micro- and nanomotors have been recently developed that allow for precise fuel-free propulsion in complex media. Here, we combine analytical and numerical theory with experiments on nanostructured screw-propellers to show that the optimal length is surprisingly short only about one helical turn, which is shorter than most of the structures in use to date. The results have important implications for the design of artificial actuated nano- and micropropellers and can dramatically reduce fabrication times, while ensuring optimal performance.

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DOI [BibTex]

DOI [BibTex]


A theoretical study of potentially observable chirality-sensitive NMR effects in molecules
A theoretical study of potentially observable chirality-sensitive NMR effects in molecules

Garbacz, P., Cukras, J., Jaszunski, M.

Phys. Chem. Chem. Phys., 17(35):22642-22651, May 2015 (article)

Abstract
Two recently predicted nuclear magnetic resonance effects, the chirality-induced rotating electric polarization and the oscillating magnetization, are examined for several experimentally available chiral molecules. We discuss in detail the requirements for experimental detection of chirality-sensitive NMR effects of the studied molecules. These requirements are related to two parameters: the shielding polarizability and the antisymmetric part of the nuclear magnetic shielding tensor. The dominant second contribution has been computed for small molecules at the coupled cluster and density functional theory levels. It was found that DFT calculations using the KT2 functional and the aug-cc-pCVTZ basis set adequately reproduce the CCSD(T) values obtained with the same basis set. The largest values of parameters, thus most promising from the experimental point of view, were obtained for the fluorine nuclei in 1,3-difluorocyclopropene and 1,3-diphenyl-2-fluoro-3-trifluoromethylcyclopropene.

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DOI [BibTex]

DOI [BibTex]


Dynamic Inclusion Complexes of Metal Nanoparticles Inside Nanocups
Dynamic Inclusion Complexes of Metal Nanoparticles Inside Nanocups

Alarcon-Correa, M., Lee, T. C., Fischer, P.

Angew. Chem. Int. Ed., 54(23):6730-6734, May 2015, Featured cover article. (article)

Abstract
Host-guest inclusion complexes are abundant in molecular systems and of fundamental importance in living organisms. Realizing a colloidal analogue of a molecular dynamic inclusion complex is challenging because inorganic nanoparticles (NPs) with a well-defined cavity and portal are difficult to synthesize in high yield and with good structural fidelity. Herein, a generic strategy towards the fabrication of dynamic 1: 1 inclusion complexes of metal nanoparticles inside oxide nanocups with high yield (> 70%) and regiospecificity (> 90%) by means of a reactive double Janus nanoparticle intermediate is reported. Experimental evidence confirms that the inclusion complexes are formed by a kinetically controlled mechanism involving a delicate interplay between bipolar galvanic corrosion and alloying-dealloying oxidation. Release of the NP guest from the nanocups can be efficiently triggered by an external stimulus. Featured cover article.

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DOI [BibTex]

DOI [BibTex]


Surface roughness-induced speed increase for active Janus micromotors
Surface roughness-induced speed increase for active Janus micromotors

Choudhury, U., Soler, L., Gibbs, J. G., Sanchez, S., Fischer, P.

Chem. Comm., 51(41):8660-8663, April 2015 (article)

Abstract
We demonstrate a simple physical fabrication method to control surface roughness of Janus micromotors and fabricate self-propelled active Janus microparticles with rough catalytic platinum surfaces that show a four-fold increase in their propulsion speed compared to conventional Janus particles coated with a smooth Pt layer.

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DOI [BibTex]

DOI [BibTex]


Active colloidal microdrills
Active colloidal microdrills

Gibbs, J. G., Fischer, P.

Chem. Comm., 51(20):4192-4195, Febuary 2015 (article)

Abstract
We demonstrate a chemically driven, autonomous catalytic microdrill. An asymmetric distribution of catalyst causes the helical swimmer to twist while it undergoes directed propulsion. A driving torque and hydrodynamic coupling between translation and rotation at low Reynolds number leads to drill-like swimming behaviour.

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DOI [BibTex]

DOI [BibTex]


Selectable Nanopattern Arrays for Nanolithographic Imprint and Etch-Mask Applications
Selectable Nanopattern Arrays for Nanolithographic Imprint and Etch-Mask Applications

Jeong, H. H., Mark, A. G., Lee, T., Son, K., Chen, W., Alarcon-Correa, M., Kim, I., Schütz, G., Fischer, P.

Adv. Science, 2(7):1500016, 2015, Featured cover article. (article)

Abstract
A parallel nanolithographic patterning method is presented that can be used to obtain arrays of multifunctional nanoparticles. These patterns can simply be converted into a variety of secondary nanopatterns that are useful for nanolithographic imprint, plasmonic, and etch-mask applications.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]

2013


Vision meets Robotics: The {KITTI} Dataset
Vision meets Robotics: The KITTI Dataset

Geiger, A., Lenz, P., Stiller, C., Urtasun, R.

International Journal of Robotics Research, 32(11):1231 - 1237 , Sage Publishing, September 2013 (article)

Abstract
We present a novel dataset captured from a VW station wagon for use in mobile robotics and autonomous driving research. In total, we recorded 6 hours of traffic scenarios at 10-100 Hz using a variety of sensor modalities such as high-resolution color and grayscale stereo cameras, a Velodyne 3D laser scanner and a high-precision GPS/IMU inertial navigation system. The scenarios are diverse, capturing real-world traffic situations and range from freeways over rural areas to inner-city scenes with many static and dynamic objects. Our data is calibrated, synchronized and timestamped, and we provide the rectified and raw image sequences. Our dataset also contains object labels in the form of 3D tracklets and we provide online benchmarks for stereo, optical flow, object detection and other tasks. This paper describes our recording platform, the data format and the utilities that we provide.

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pdf DOI [BibTex]

2013


pdf DOI [BibTex]


Hybrid nanocolloids with programmed three-dimensional shape and material composition
Hybrid nanocolloids with programmed three-dimensional shape and material composition

Mark, A. G., Gibbs, J. G., Lee, T., Fischer, P.

NATURE MATERIALS, 12(9):802-807, 2013, Max Planck Press Release. (article)

Abstract
Tuning the optical(1,2), electromagnetic(3,4) and mechanical properties of a material requires simultaneous control over its composition and shape(5). This is particularly challenging for complex structures at the nanoscale because surface-energy minimization generally causes small structures to be highly symmetric(5). Here we combine low-temperature shadow deposition with nanoscale patterning to realize nanocolloids with anisotropic three-dimensional shapes, feature sizes down to 20 nm and a wide choice of materials. We demonstrate the versatility of the fabrication scheme by growing three-dimensional hybrid nanostructures that contain several functional materials with the lowest possible symmetry, and by fabricating hundreds of billions of plasmonic nanohelices, which we use as chiral metafluids with record circular dichroism and tunable chiroptical properties.

Max Planck Press Release.

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Video - Fabrication of Designer Nanostructures DOI [BibTex]


Chiral Colloidal Molecules And Observation of The Propeller Effect
Chiral Colloidal Molecules And Observation of The Propeller Effect

Schamel, D., Pfeifer, M., Gibbs, J. G., Miksch, B., Mark, A. G., Fischer, P.

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 135(33):12353-12359, 2013 (article)

Abstract
Chiral molecules play an important role in biological and chemical processes, but physical effects due to their symmetry-breaking are generally weak. Several physical chiral separation schemes which could potentially be useful, including the propeller effect, have therefore not yet been demonstrated at the molecular scale. However, it has been proposed that complex nonspherical colloidal particles could act as ``colloidal molecules{''} in mesoscopic model systems to permit the visualization of molecular phenomena that are otherwise difficult to observe. Unfortunately, it is difficult to synthesize such colloids because surface minimization generally favors the growth of symmetric particles. Here we demonstrate the production of large numbers of complex colloids with glancing angle physical vapor deposition. We use chiral colloids to demonstrate the Baranova and Zel'dovich (Baranova, N. B.; Zel'dovich, B. Y. Chem. Phys. Lett. 1978, 57, 435) propeller effect: the separation of a racemic mixture by application of a rotating field that couples to the dipole moment of the enantiomers and screw propels them in opposite directions. The handedness of the colloidal suspensions is monitored with circular differential light scattering. An exact solution for the colloid's propulsion is derived, and comparisons between the colloidal system and the corresponding effect at the molecular scale are made.

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Video - Nanospropellers DOI [BibTex]

Video - Nanospropellers DOI [BibTex]


Indirect absorption spectroscopy using quantum cascade lasers: mid-infrared refractometry and photothermal spectroscopy
Indirect absorption spectroscopy using quantum cascade lasers: mid-infrared refractometry and photothermal spectroscopy

Pfeifer, M., Ruf, A., Fischer, P.

OPTICS EXPRESS, 21(22):25643-25654, 2013 (article)

Abstract
We record vibrational spectra with two indirect schemes that depend on the real part of the index of refraction: mid-infrared refractometry and photothermal spectroscopy. In the former, a quantum cascade laser (QCL) spot is imaged to determine the angles of total internal reflection, which yields the absorption line via a beam profile analysis. In the photothermal measurements, a tunable QCL excites vibrational resonances of a molecular monolayer, which heats the surrounding medium and changes its refractive index. This is observed with a probe laser in the visible. Sub-monolayer sensitivities are demonstrated. (C) 2013 Optical Society of America

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DOI [BibTex]


Plasmonic nanohelix metamaterials with tailorable giant circular dichroism
Plasmonic nanohelix metamaterials with tailorable giant circular dichroism

Gibbs, J. G., Mark, A. G., Eslami, S., Fischer, P.

APPLIED PHYSICS LETTERS, 103(21), 2013, Featured cover article. (article)

Abstract
Plasmonic nanohelix arrays are shown to interact with electromagnetic fields in ways not typically seen with ordinary matter. Chiral metamaterials (CMMs) with feature sizes small with respect to the wavelength of visible light are a promising route to experimentally achieve such phenomena as negative refraction without the need for simultaneously negative e and mu. Here we not only show that giant circular dichroism in the visible is achievable with hexagonally arranged plasmonic nanohelix arrays, but that we can precisely tune the optical activity via morphology and lattice spacing. The discrete dipole approximation is implemented to support experimental data. (C) 2013 AIP Publishing LLC.

Featured cover article.

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DOI [BibTex]

DOI [BibTex]

2000


Phenomenological damping in optical response tensors
Phenomenological damping in optical response tensors

Buckingham, A., Fischer, P.

PHYSICAL REVIEW A, 61(3), 2000 (article)

Abstract
Although perturbation theory applied to the optical response of a molecule or material system is only strictly valid far from resonances, it is often applied to ``near-resonance{''} conditions by means of complex energies incorporating damping. Inconsistent signs of the damping in optical response tensors have appeared in the recent literature, as have errors in the treatment of the perturbation by a static held. The ``equal-sign{''} convention used in a recent publication yields an unphysical material response, and Koroteev's intimation that linear electro-optical circular dichroism may exist in an optically active liquid under resonance conditions is also flawed. We show that the isotropic part of the Pockels tensor vanishes.

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DOI [BibTex]

2000


DOI [BibTex]


Ab initio investigation of the sum-frequency hyperpolarizability of small chiral molecules
Ab initio investigation of the sum-frequency hyperpolarizability of small chiral molecules

Champagne, B., Fischer, P., Buckingham, A.

CHEMICAL PHYSICS LETTERS, 331(1):83-88, 2000 (article)

Abstract
Using a sum-over-states procedure based on configuration interaction singles /6-311++G{*}{*}, we have computed the sum-frequency hyperpolarizability beta (ijk)(-3 omega; 2 omega, omega) Of two small chiral molecules, R-monofluoro-oxirane and R-(+)-propylene oxide. Excitation energies were scaled to fit experimental UV-absorption data and checked with ab initio values from time-dependent density functional theory. The isotropic part of the computed hyperpolarizabilities, beta(-3 omega; 2 omega, omega), is much smaller than that reported previously from sum-frequency generation experiments on aqueous solutions of arabinose. Comparison is made with a single-centre chiral model. (C) 2000 Elsevier Science B.V. All rights reserved.

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DOI [BibTex]

DOI [BibTex]


Three-wave mixing in chiral liquids
Three-wave mixing in chiral liquids

Fischer, P., Wiersma, D., Righini, R., Champagne, B., Buckingham, A.

PHYSICAL REVIEW LETTERS, 85(20):4253-4256, 2000 (article)

Abstract
Second-order nonlinear optical frequency conversion in isotropic systems is only dipole allowed for sum- and difference-frequency generation in chiral media. We develop a single-center chiral model of the three-wave mixing (sum:frequency generation) nonlinearity and estimate its magnitude. We also report results from ab initio calculations and from three- and four-wave mixing experiments in support of the theoretical estimates. We show that the second-order susceptibility in chiral liquids is much smaller than previously thought.

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DOI [BibTex]

DOI [BibTex]

1998


Surface second-order nonlinear optical activity
Surface second-order nonlinear optical activity

Fischer, P., Buckingham, A.

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 15(12):2951-2957, 1998 (article)

Abstract
Following the recent observation of a large second-harmonic intensity difference from a monolayer of chiral molecules with left and right circularly polarized light, the scattering theory is generalized and extended to predict linear and circular intensity differences for the more Versatile sum-frequency spectroscopy. Estimates indicate that intensity differences should be detectable for a typical experimental arrangement. The second-order nonlinear surface susceptibility tensor is given for different surface point groups in the electric dipole approximation; it is shown that nonlinear optical activity phenomena unambiguously probe molecular chirality only for molecular monolayers that are symmetric about the normal. Other surface symmetries can give rise to intensity differences from monolayers composed of achiral molecules. A water surface is predicted to show Linear and nonlinear optical activity in the presence of an electric field parallel to the surface. (C) 1998 Optical Society of America {[}S0740-3224(98)01311-3] OCIS codes: 190.0190, 190.4350, 240.6490.

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DOI [BibTex]

1998


DOI [BibTex]


Linear electro-optic effect in optically active liquids
Linear electro-optic effect in optically active liquids

Buckingham, A., Fischer, P.

CHEMICAL PHYSICS LETTERS, 297(3-4):239-246, 1998 (article)

Abstract
A linear effect of an electrostatic field F on the intensity of sum- and difference-frequency generation in a chiral liquid is predicted. It arises in the electric dipole approximation. The effect changes sign with the enantiomer and on reversing the direction of the electrostatic field. The sum-frequency generator chi(alpha beta gamma)((2)) (-omega(3);omega(1),omega(2)), where omega(3) = omega(1) + omega(2), and the electric field-induced sum-frequency generator chi(alpha beta gamma delta)((3))(-omega(3);omega(1),omega(2),0)F-delta interfere and their contributions to the scattering power can be distinguished. Encouraging predictions are given for a typical experimental arrangement. (C) 1998 Elsevier Science B.V. All rights reserved.

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DOI [BibTex]

DOI [BibTex]


Monolayers of hexadecyltrimethylammonium p-tosylate at the air-water interface. 1. Sum-frequency spectroscopy
Monolayers of hexadecyltrimethylammonium p-tosylate at the air-water interface. 1. Sum-frequency spectroscopy

Bell, G., Li, Z., Bain, C., Fischer, P., Duffy, D.

JOURNAL OF PHYSICAL CHEMISTRY B, 102(47):9461-9472, 1998 (article)

Abstract
Sum-frequency vibrational spectroscopy has been used to determine the structure of monolayers of the cationic surfactant, hexadecyltrimethylammonium p-tosylate (C(16)TA(+)Ts(-)), at the surface of water. Selective deuteration of the cation or the anion allowed the separate detection of sum-frequency spectra of the surfactant and of counterions that are bound to the monolayer. The p-tosylate ions an oriented with their methyl groups pointing away from the aqueous subphase and with the C-2 axis tilted, on average, by 30-40 degrees from the surface normal. The vibrational spectra of C(16)TA(+) indicate that the number of gauche defects in the monolayer does not change dramatically when bromide counterions are replaced by p-tosylate. The ends of the hydrocarbon chains of C16TA+ are, however, tilted much further from the surface normal in the presence of p-tosylate than in the presence of bromide. A quantitative analysis of the sum-frequency spectra requires a knowledge of the molecular hyperpolarizability tensor: the role of ab initio calculations and Raman spectroscopy in determining the components of this tensor is discussed.

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DOI [BibTex]

DOI [BibTex]


Ultraviolet resonance Raman study of drug binding in dihydrofolate reductase, gyrase, and catechol O-methyltransferase
Ultraviolet resonance Raman study of drug binding in dihydrofolate reductase, gyrase, and catechol O-methyltransferase

Couling, V., Fischer, P., Klenerman, D., Huber, W.

BIOPHYSICAL JOURNAL, 75(2):1097-1106, 1998 (article)

Abstract
This paper presents a study of the use of ultraviolet resonance Raman (UVRR) spectroscopic methods as a means of elucidating aspects of drug-protein interactions. Some of the RR vibrational bands of the aromatic amino acids tyrosine and tryptophan are sensitive to the microenvironment, and the use of UV excitation radiation allows selective enhancement of the spectral features of the aromatic amino acids, enabling observation specifically of their change in microenvironment upon drug binding. The three drug-protein systems investigated in this study are dihydrofolate reductase with its inhibitor trimethoprim, gyrase with novobiocin, and catechol O-methyltransferase with dinitrocatechol. It is demonstrated that UVRR spectroscopy has adequate sensitivity to be a useful means of detecting drug-protein interactions in those systems for which the electronic absorption of the aromatic amino acids changes because of hydrogen bonding and/or possible dipole-dipole and dipole-polarizability interactions with the ligand.

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DOI [BibTex]

DOI [BibTex]