Showing papers by "Bart Kahr published in 2012"

Mueller matrix polarimetry with four photoelastic modulators: theory and calibration

Abstract: A spectroscopic Mueller matrix polarimeter with four photoelastic modulators (PEMs) and no moving parts is introduced. In the 4-PEM polarimeter, all the elements of the Mueller matrix are simultaneously determined from the analysis of the frequencies of the time-dependent intensity of the light beam.

Twisted mannitol crystals establish homologous growth mechanisms for high-polymer and small-molecule ring-banded spherulites.

Abstract: D-Mannitol belongs to a large and growing family of crystals with helical morphologies (Yu, L. J. Am. Chem. Soc.2003, 125, 6380). Two polymorphs of D-mannitol, α and δ, when grown in the presence of additives such as poly(vinylpyrrolidone) (PVP) or D-sorbitol, form ring-banded spherulites composed of handed helical fibrils, where the helix axes correspond to the radial growth directions. The two polymorphs form helices with opposite senses in the presence of PVP but the same sense in the presence of D-sorbitol. The characteristic dimensions of the fibrils, including thickness, aspect ratio, and pitch, were determined by scanning probe and electron microscopies. These values must form the basis of any theory that presupposes what forces give rise to crystal twisting, a problem that has been broached but unsettled in the literature of polymer crystallization. The interdependence of the rhythmic variations of both linear and circular birefringence, as determined by Mueller matrix microscopy, informs the cooperative organization of mannitol fibers. The microstructure of mannitol ring-banded spherulites compares favorably to that of high polymers and is evaluated within the context of current theories of crystal twisting.

Mueller matrices in fluorescence scattering.

Abstract: A reappraisal of the 1929 analysis of luminescence by Soleillet reveals the form of the Mueller matrix for fluorescence scattering whose parameters are directly defined in terms of the now-familiar fluorescence anisotropy parameter. If the scattering analyte is optically active, it is further shown how fluorescence detected circular dichroism and circularly polarized luminescence can be recovered, simultaneously and free of artifacts.

Reckoning electromagnetic principles with polarimetric measurements of anisotropic optically active crystals

Abstract: A method is introduced to correlate electromagnetic theory and polarimetric experiments for anisotropic optically active crystals at oblique angles of incidence. It is based on the 4 × 4 algebraic descriptions of light propagation in layered anisotropic media, which can be simplified to 2 × 2 matrices when multiple reflections are disregarded. Spectroscopic Mueller matrix measurements in transmission and at oblique angles of incidence have been made on two uniaxial crystals: α-quartz and silver thiogallate. Their optical activity tensors have been spectroscopically determined using the method presented here.

Arago’s Best Paper

Abstract: The year 2011 is the bicentennial of Francois Arago's discovery of optical rotation. The immediate usurpation of the study of optical activity by Jean-Baptiste Biot led to the first well-known judgments of the arrangements of atoms in space. Scientists are less aware that Arago achieved something far greater than his contributions to optics, by signing the 1848 decree that abolished slavery throughout the French Empire. Opposing attitudes of Arago and Biot toward abolition, foreshadowed in their early rift over optical rotation, were surprisingly exposed in mid-century developments in chiroptics. As shown in a recent book by Levitt, Arago sought a reinvention of the whole colonial plantation system consistent with Republican principles, while Biot tried to place the cane sugar industry and slave-based economy on the quantitative foundation of saccharimetry. A reevaluation of the circumstances of abolition can celebrate both societal evolution and optical rotation on the 200th birthday of the latter. Episodes from Arago's life that arguably created his predisposition toward abolition are emphasized: He was imprisoned several times as a young man and knew the loss of liberty, his brother Jacques witnessed slavery in Brazil and advocated abolition in travel books prepared with Francois, and finally, in writing the biography of the Marquis de Condorcet, the spirit behind the first, albeit impermanent French abolition of slavery in 1794, Arago found proof of concept for his comparable challenge. Curiously, the measurement of the optical rotation of crystals and sugar, the foci of Arago and Biot, respectively, remain among the greatest challenges in polarimetry. Current developments are reviewed with respect to chiroptical anisotropy and in vivo glucose detection driven by the pandemic of diabetes, a disease diagnosed polarimetrically by Biot that claimed the life of Arago.

Prediction of Soai Reaction Enantioselectivity Induced by Crystals of N-(2-Thienylcarbonyl)glycine

Abstract: The enantioselectivity of the autocatalytic alkylation of 2-tert-butylpyrimidyl-5-carbaldehyde, initiated by chiral crystals of N-(2-thienylcarbonyl)glycine, is predicted computationally. The results are in accord with the correlation of the stereochemical outcome of the reaction and the absolute structure of the crystals determined by the anomalous dispersion of X-rays and circular dichroism spectroscopy.

Relationship between tribology and optics in thin films of mechanically oriented nanocrystals.

Abstract: Many crystalline dyes, when rubbed unidirectionally with cotton on glass slides, can be organized as thin films of highly aligned nanocrystals. Commonly, the linear birefringence and linear dichroism of these films resemble the optical properties of single crystals, indicating precisely oriented particles. Of 186 colored compounds, 122 showed sharp extinction and 50 were distinctly linearly dichroic. Of the latter 50 compounds, 88% were more optically dense when linearly polarized light was aligned with the rubbing axis. The mechanical properties of crystals that underlie the nonstatistical correlation between tribological processes and the direction of electron oscillations in absorption bands are discussed. The features that give rise to the orientation of dye crystallites naturally extend to colorless molecular crystals.

Polar alignment of dye molecules in sectors of host lattices revealed by phase sensitive second harmonic generation and scanning pyroelectric microscopy

Abstract: Phase sensitive second harmonic generation (PS-SHG) was applied to investigate the effect of polar surface recognition of dye molecules (amaranth and sunset yellow, as well as dimethylaminostyryl-pyridine) in symmetry related sectors of host lattices such as KH2PO4 and phthalic acid, respectively. PS-SHG revealed bi-polar states in all cases, meaning that sectors growing in opposite directions feature opposite absolute polarities. Scanning pyroelectric microscopy demonstrated that the dipole moments of amaranth in KH2PO4 are pointing towards the growing crystal face. The present work reveals symmetry breaking through anomalous mixed crystal growth to produce physical properties associated with polar axes.

Computational methodology for chirality determination in the Soai reaction by crystals: γ-glycine

Abstract: The autocatalytic Soai reaction gives abundant evidence of the enantioselective adsorption of organic compounds on a variety of crystals. Computational modelling can provide insight into mechanisms of enantioselectivity. Here, we use a combination of simulated annealing, forcefield, and quantum mechanical methods to examine interactions of pyrimidyl-5-carbaldehyde and 2-methylpyrimidyl-5-carbaldehyde with surfaces of γ-glycine. Using binding energy results, we predict the exposure of the pro-stereogenic S face of pyrimidyl-5-carbaldehyde (~65%) and 2-methylpyrimidyl-5-carbaldehyde (>90%) on the (1 $$ \bar{1} $$ 0) and ( $$ \bar{1} $$ 1 0) surfaces. The aim is to develop a robust computational methodology that can be applied to understanding crystal-biased asymmetric synthesis.

Histories of Crystallography by Shafranovskii and Schuh

Abstract: There are at least six book length biographies of Herman Melville (1819-1891) and ten histories of the Russian Revolution currently in print in the English language. On the other hand, if you chase after crystals not whales, or believe that the determination of the structure of matter was a historical pivot, you will be disappointed that there does not exist a single narrative history of crystallography in print in English or any other language to the best of our knowledge. By any measure, crystallography now receives scant attention by historians and scholars.