scholarly journals Hyperfine anomaly in heavy atoms and its role in precision atomic searches for new physics

2021 ◽  
Vol 104 (2) ◽  
Author(s):  
B. M. Roberts ◽  
J. S. M. Ginges
Author(s):  
Kenneth H. Downing ◽  
Benjamin M. Siegel

Under the “weak phase object” approximation, the component of the electron wave scattered by an object is phase shifted by π/2 with respect to the unscattered component. This phase shift has been confirmed for thin carbon films by many experiments dealing with image contrast and the contrast transfer theory. There is also an additional phase shift which is a function of the atomic number of the scattering atom. This shift is negligible for light atoms such as carbon, but becomes significant for heavy atoms as used for stains for biological specimens. The light elements are imaged as phase objects, while those atoms scattering with a larger phase shift may be imaged as amplitude objects. There is a great deal of interest in determining the complete object wave, i.e., both the phase and amplitude components of the electron wave leaving the object.


Author(s):  
Wah Chi

Resolution and contrast are the important factors to determine the feasibility of imaging single heavy atoms on a thin substrate in an electron microscope. The present report compares the atom image characteristics in different modes of fixed beam dark field microscopy including the ideal beam stop (IBS), a wire beam stop (WBS), tilted illumination (Tl) and a displaced aperture (DA). Image contrast between one Hg and a column of linearly aligned carbon atoms (representing the substrate), are also discussed. The assumptions in the present calculations are perfectly coherent illumination, atom object is represented by spherically symmetric potential derived from Relativistic Hartree Fock Slater wave functions, phase grating approximation is used to evaluate the complex scattering amplitude, inelastic scattering is ignored, phase distortion is solely due to defocus and spherical abberation, and total elastic scattering cross section is evaluated by the Optical Theorem. The atom image intensities are presented in a Z-modulation display, and the details of calculation are described elsewhere.


Author(s):  
R. H. Morriss ◽  
J. D. C. Peng ◽  
C. D. Melvin

Although dynamical diffraction theory was modified for electrons by Bethe in 1928, relatively few calculations have been carried out because of computational difficulties. Even fewer attempts have been made to correlate experimental data with theoretical calculations. The experimental conditions are indeed stringent - not only is a knowledge of crystal perfection, morphology, and orientation necessary, but other factors such as specimen contamination are important and must be carefully controlled. The experimental method of fine-focus convergent-beam electron diffraction has been successfully applied by Goodman and Lehmpfuhl to single crystals of MgO containing light atoms and more recently by Lynch to single crystalline (111) gold films which contain heavy atoms. In both experiments intensity distributions were calculated using the multislice method of n-beam diffraction theory. In order to obtain reasonable accuracy Lynch found it necessary to include 139 beams in the calculations for gold with all but 43 corresponding to beams out of the [111] zone.


Author(s):  
J. P. Langmore ◽  
N. R. Cozzarelli ◽  
A. V. Crewe

A system has been developed to allow highly specific derivatization of the thymine bases of DNA with mercurial compounds wich should be visible in the high resolution scanning electron microscope. Three problems must be completely solved before this staining system will be useful for base sequencing by electron microscopy: 1) the staining must be shown to be highly specific for one base, 2) the stained DNA must remain intact in a high vacuum on a thin support film suitable for microscopy, 3) the arrangement of heavy atoms on the DNA must be determined by the elastic scattering of electrons in the microscope without loss or large movement of heavy atoms.


Author(s):  
J. M. Cowley ◽  
Sumio Iijima

The imaging of detailed structures of crystal lattices with 3 to 4Å resolution, given the correct conditions of microscope defocus and crystal orientation and thickness, has been used by Iijima (this conference) for the study of new types of crystal structures and the defects in known structures associated with fluctuations of stoichiometry. The image intensities may be computed using n-beam dynamical diffraction theory involving several hundred beams (Fejes, this conference). However it is still important to have a suitable approximation to provide an immediate rough estimate of contrast and an evaluation of the intuitive interpretation in terms of an amplitude object.For crystals 100 to 150Å thick containing moderately heavy atoms the phase changes of the electron wave vary by about 10 radians suggesting that the “optimum defocus” theory of amplitude contrast for thin phase objects due to Scherzer and others can not apply, although it does predict the right defocus for optimum imaging.


Nature ◽  
2011 ◽  
Author(s):  
Eugenie Samuel Reich
Keyword(s):  

2019 ◽  
Author(s):  
Drew P. Harding ◽  
Laura J. Kingsley ◽  
Glen Spraggon ◽  
Steven Wheeler

The intrinsic (gas-phase) stacking energies of natural and artificial nucleobases were explored using density functional theory (DFT) and correlated ab initio methods. Ranking the stacking strength of natural nucleobase dimers revealed a preference in binding partner similar to that seen from experiments, namely G > C > A > T > U. Decomposition of these interaction energies using symmetry-adapted perturbation theory (SAPT) showed that these dispersion dominated interactions are modulated by electrostatics. Artificial nucleobases showed a similar stacking preference for natural nucleobases and were also modulated by electrostatic interactions. A robust predictive multivariate model was developed that quantitively predicts the maximum stacking interaction between natural and a wide range of artificial nucleobases using molecular descriptors based on computed electrostatic potentials (ESPs) and the number of heavy atoms. This model should find utility in designing artificial nucleobase analogs that exhibit stacking interactions comparable to those of natural nucleobases. Further analysis of the descriptors in this model unveil the origin of superior stacking abilities of certain nucleobases, including cytosine and guanine.


Edupedia ◽  
2018 ◽  
Vol 3 (1) ◽  
pp. 77-88
Author(s):  
Ali Fatoni

The integration of science is discussed today. The figures in this issue appear in the world. Mentioned among them Naquib al-Attas,and in Indonesia who keen to speak scientific integration is Amin Abdullah.This speech led to the birth of the 2013Curriculum in Indonesia with the demands of all subjects must contain a spiritual attitude (KI-1). This creates difficulties for teachers. Training and education program for teacher in applying The 2013 Curriculum is not technically in touch with their difficulties.Training and education program for teachermostly touchonly on aspects of teaching skills. This research is present to fill the gap that has not been filled by thattraining and education program. The results of this study is a simple description of the process of developing a physics textbook that begins from the study of old books and relevant theories for thisnew developmenttextbook to compiled new physics textbookincluding the content of Islamic values.


1993 ◽  
Vol 10 (2) ◽  
pp. 153-164
Author(s):  
Mahmoud Dhaouadi

There is no question that contemporary western civilization has beendominant in the field of science since the Renaissance. Western scientificsuperiority is not limited to specific scientific disciplines, but is rather anovetall scientific domination covering both the so-called exact and thehuman-social sciences. Western science is the primary reference for specialistsin such ateas as physics, chemistry, biology, medicine, economics,psychology, and sociology. It is in this sense that Third World underdevelopmentis not only economic, social, and industrial; it also suffersfrom scientific-cultutal underdevelopment, or what we call "The OtherUnderdevelopment" (Dhaouadi 1988).The imptessive progress of western science since Newton and Descartesdoes not meari, however, that it has everything tight or perfect. Infact, its flaws ate becoming mote visible. In the last few decades, westernscience has begun to experience a shift from what is called classical scienceto new science. Classical science was associated with the celestialmechanics of Copernicus, Kepler, Newton, the new physics of Galileo,and the philosophy of Descartes. Descartes introduced a radical divisionbetween mind and matter, while Newton and his fellows presented a newscience that looked at the world as a kind of giant clock The laws of thisworld were time-reversible, for it was held that there was no differencebetween past and future. As the laws were deterministic, both the pastand the future could be predicted once the present was known.The vision of the emerging new science tends to heal the division betweenmatter and spirit and to do away with the mechanical dimension ...


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