Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.     

A non-empirical LCAO MO SCF investigation of the ground and core-hole states of the cyclopentyl cation

ΔSCF computations have been performed on both classical and non-classical structures for the cyclopentyl cation. The C 1s core-level data confirm that the reported experimental shift data are consistent with a classical structure.     

The role of band structure effects on electron capture and real space transfer in semiconductor superlattices

We report pseudopotential calculations of the electron transfer times from wells to barriers in GaAs---AlAs superlattices—as a function of well and barrier widths—with a view to establishing the role of level crossing and valley mixing. Such momentum mixing effects have recently been invoked with some success to make contact with pressure induced cross-interface luminescence spectra in which the observable in question is the luminescence lifetime^1,2. Our studies of Stark spectra in these structures have shown similiar level crossing and valley mixing³ effects. However this is the first systematic study of the transfer times versus structure. It transpires that details of the microscopic geometry play a significant part in determining the degree of momentum mixing and the corresponding rates of charge transfer.     

Avoiding conflicts between the natural behavior of the animal and the demands of discrimination experiments.

Auditory discrimination experiments are traditionally designed without regard for ethological or ecological concerns, yet land dwelling mammals may have specialized behavior with respect to sound sources. Auditory discriminations occur under field conditions, and there is some fit or matching of the animal's behavior to the acoustic environment. Understanding this fit requires a knowledge of specializations. Understanding the specializations may also guide the design of discrimination experiments. This paper reviews a number of auditory discrimination experiments that were designed to reveal some of the specialized behaviors. These experiments showed the following: (i) The position of a sound source is the dominant sensory dimension, over riding the quality of the sound; (ii) the effect of reinforcing a response in the presence of a sound is to strengthen the response of approaching the source. This effect is ubiquitous in discrimination tasks; (iii) sounds that are novel at the start of discrimination training more rapidly gain control of responding than sound to which the animal has been pre-exposed; (iv) novel low-intensity sounds elicit approach and exploration of the source. These behaviors rapidly adapt. These four behavioral attributes are considered in terms of their impact upon behavior in the field, and of the requirements they impose on the design of experimental discriminations.     

Incompatible blends: Thermal effects in a model system

There has been a growing interest in compatible blends in which one of the polymers is crystallizable. At appropriate compositions some of this polymer will crystallize, the remainder forms a compatible blend with the other polymer component; attempts have been made to relate melting point depression of the crystalline component to diluent effects, i.e., polymer/polymer interactions. Melting point is also a function of lamella thickness, the determination of which is very difficult in such systems. It is assumed in this type of study that the melting point of a crystal in an incompatible matrix will be unchanged. In an attempt to test this assumption we have determined the melting behavior of isothermally grown, single crystals of polyethylen (from the same preparation) embedded in a variety of incompatible matrices. The matrices used were polybutadiene, polystyrene, and polyphenylene oxide. This work shows that polymer single crystals embedded in an incompatible polymer matrix can show apparent melting point depressions of up to 5°. This size of depression represents a significant part of that reported for compatible systems.     

The interaction of divalent tin compounds with transition metal carbonyls

The reaction of tin(II) halides, cyclopentadienyls, and β-diketonates with Co₂(CO)₈, Fe₂(CO)₉, and Group VI metal carbonyls is described.     

Single mode multiline emission from a hybrid CO2 laser

A simple technique of obtaining single mode multiline emission from a hybrid CO₂ laser is described. This involves operation of its low pressure section below lasing threshold and a judicious positioning of the axial modes with respect to the corresponding rotational line centres. Emission with comparable intensities on up to four rotational lines in the P-branch of the 10 μm band has been achieved.     

Entropy rigidity for finite volume strictly convex projective manifolds

Geometriae Dedicata - We prove entropy rigidity for finite volume strictly convex projective manifolds in dimensions $$\ge 3$$ , generalizing the work of [1] to the finite volume setting. The...