Studies of large scale unstable growth formed during GaAs(001) homoepitaxy

Atomic force and scanning tunneling microscopy studies have been performed on GaAs(001) films grown by molecular beam epitaxy. Multilayered mounds are seen to evolve when the growth conditions favor island nucleation. As the epilayer thickness is increased, these features grow in all dimensions but the angle of inclination remains approximately constant at 1°. The mounding does not occur on surfaces grown in step-flow. We propose that the multi-layered features are due to an unstable growth mode which relies on island nucleation and the presence of a step edge barrier.     

Controllability of linear systems,differential geometry curves in grassmannians and generalized grassmannians,and riccati equations

We study the linear system =Ax+Bu from a differential geometric point of view. It is well-known that controllability of the system is related to the one-parameter family of operators e^t B. We use this to give a proof of the classical controllability conditions in terms of the differential geometry of certain curves in ⁿ. We then view (t)=Im(e^t B) as a curve in appropriate Grassmannian and see that, in local coordinates, is an integral curve of the flow induced by a matrix Riccati equation. We obtain qualitative geometric conditions on that are equivalent to the controllability of the system. To get quantitiative results, we lift to a curve l' in a splitting space, a generalized Grassmannian, which has the advantage of being a reductive homogeneous space of the general linear group, GL(ⁿ). Explicit and simple expressions concerning the geometry of are computed in terms of the Lie algebra of GL(ⁿ), and these are related to the controllability of the system.James Wolper was a visiting professor in the Department of Mathematics at Texas Tech University while much of this research was conducted. He would like to express appreciation for the hospitality he received during his visit.     

Chromosomal aberrations resulting from UV exposures to G1 Xenopus cells

Abstract— The dose dependence of chromosomal aberration frequencies induced by UV light in G₁Xenopus cells has been examined. Total aberration frequencies did not exceed control levels until a dose of approximately 7.5 Jm^-2 was administered. Chromatid type aberrations were prevalent throughout the dose range 7.5–25.0 Jm^-2. Chromosome type aberration frequencies were comparatively low but exceeded control levels throughout the range 10.0–25.0 Jm^-2, Results of supporting experiments suggested that most of the chromosome type aberrations observed were in the second mitosis following exposures and were derived from aberrations which normally appear as chromatid types at the first mitosis following exposures.     

Isotope dynamic mixing and vibronic structure

Existing theories which deal with the isotope dependence of vibronic structure have neglected the isotope dependence of vibration dynamics. The latter dependence (isotope dynamic mixing) is an adiabatic correction to vibronic structure theory. A theory of vibronic structure, which includes isotope dynamic mixing, is developed. The results of model calculations are presented.     

On the dimerization of BH3 and the associated delocalization (resonance) energy

SCF calculations utilizing a basis of all s-type Gaussians have been performed to investigate the dimerization of BH₃ to B₂H₆. Localized molecular orbitals and pseudonatural orbitals are calculated. The dimerization energy obtained (11.5 kcal/mole) supports the assumption that the non-Hartree-Fock interactions (electron correlation) play a dominant role (Exp = 40–60 kcal/mole). The energy of the principal type of resonance structure based on the SCF-MO 's of the separated-but-distorted BH₃ systems is rigorously calculated and found to be 150 kcal/mole higher in energy than the SCF energy of B₂H₆. Thus, resonance stabilization (delocalization) is shown to be extremely important to the stability of the “three-center” bonds in B₂H₆, as has been suspected for many years.