Biomedical subjects

LJ Allen

Publications and source records attributed to LJ Allen.

At least 19 recordsLinked to original sources

PubMed2000

Atomic scattering factors for K-shell and L-shell ionization by fast electrons

Atomic scattering factors have been calculated for K-shell ionization for elements in the range Z = 6 (carbon) to Z = 50 (tin) and for L-shell ionization in the range Z = 20 (calcium) to Z = 60 (neodymium). The calculations are based on relativistic Hartree-Fock wave functions for the atomic bound states and Hartree-Slater wave functions for the continuum wave functions. The results are presented in tabular form such that accurate values of the scattering factors can be obtained by cubic spline interpolation for incident electron energies between 50 and 400 keV and for scattering vectors with magnitude s = sin straight theta / lambda up to 2.5 A(-1) (2straight theta is the scattering angle and lambda the wavelength of the incident electrons). A separate parameterization of the form factors is given for 2.5 </= s </= 20 A(-1), where they are small. In addition, a simpler but less accurate parameterization of the atomic scattering factors in an exponential form has been obtained by fitting the calculated form factors in the region s </= 2.5 A(-1). The scattering factors are suitable for the calculation of ionization cross sections for use in atom location by channelling-enhanced microanalysis (ALCHEMI).

Journal Article↗

PubMed2000

Inversion of dynamical electron diffraction data including absorption

A method to invert the dynamical diffraction of high-energy electrons inside a crystal, which takes into account absorption, is discussed. It is shown that, working at fixed energy, the projected potentials associated with both the elastic and the absorptive scattering can be uniquely obtained from the scattering matrix. Inversion is possible for any principal orientation of the incident beam. Model examples are given.

Journal Article↗

PubMed1999

Retrieval of the projected potential by inversion from the scattering matrix in electron-crystal scattering.

The retrieval of a unique crystal potential from the scattering matrix S in high-energy transmission electron diffraction is discussed. It is shown that, in general, data taken at a single orientation are not sufficient to determine all the elements of S. Additional measurements with tilted incident beam are required for the determination of the whole S matrix. An algorithm for the extraction of the crystal potential from the S matrix measured at a single energy and thickness is presented. The limiting case of thin crystals is discussed. Several examples with simulated data are considered.

Journal Article↗

PubMed1999

The relationship between photosynthesis and a mastoparan-induced hypersensitive response in isolated mesophyll cells

The G-protein activator mastoparan (MP) was found to elicit the hypersensitive response (HR) in isolated Asparagus sprengeri mesophyll cells at micromolar concentrations. The HR was characterized by cell death, extracellular alkalinization, and an oxidative burst, indicated by the reduction of molecular O2 to O2. To our knowledge, this study was the first to monitor photosynthesis during the HR. MP had rapid and dramatic effects on photosynthetic electron transport and excitation energy transfer as determined by variable chlorophyll a fluorescence measurements. A large increase in nonphotochemical quenching of chlorophyll a fluorescence accompanied the initial stages of the oxidative burst. The minimal level of fluorescence was also quenched, which suggests the origin of this nonphotochemical quenching to be a decrease in the antenna size of photosystem II. In contrast, photochemical quenching of fluorescence decreased dramatically during the latter stages of the oxidative burst, indicating a somewhat slower inhibition of photosystem II electron transport. The net consumption of O2 and the initial rate of O2 uptake, elicited by MP, were higher in the light than in the dark. These data indicate that light enhances the oxidative burst and suggest a complex relationship between photosynthesis and the HR.

Journal Article↗

PubMed1996