Hyperthermal neutral beam etching

A pulsed beam of hyperthermal fluorine atoms with an average translational energy of 4.8 eV has been used to demonstrate anisotropic etching of Si. For 1.4 Hz operation, a room-temperature etch rate of 300 Å/min for Si(100) has been measured at a distance of 30 cm from the source. A 14% undercutting for room-temperature etching of Novolac-masked Si features was achieved under single-collision conditions, with no detectable mask erosion. Translational energy and angular distributions of scattered fluorine atoms during steady-state etching of Si by a normal-incidence, collimated beam demonstrate that unreacted F atoms can scatter inelastically, retaining a significant fraction of their initial kinetic energies. The observed undercutting can be explained by secondary impingement of these high-energy F atoms, which are more reactive upon interaction with the sidewalls than would be expected if they desorbed from the surface at thermal energies after full accommodation. Time-of-flight distributions of volatile reaction products were also collected, and they show evidence for a dominant nonthermal reaction mechanism of the incident atoms with the surface in addition to a thermal reaction channel

Gas-Surface Dynamics and Profile Evolution during Etching of Silicon

Scattering of energetic F atoms on a fluorinated Si surface is studied by molecular beam methods. The energy transfer closely follows hard-sphere collision kinematics. Energy and angular distributions of unreacted F atoms suggest significant multiple-bounce scattering in addition to single-bounce scattering and trapping desorption. An empirical model of the atom-surface interaction dynamics is used in a Monte Carlo simulation of topography evolution during neutral beam etching of Si. Model predictions of profile phenomena are validated by experiments

Sesquinary Catastrophe For Close-In Moons with Dynamically Excited Orbits

We identify a new mechanism that can lead to the destruction of small, close-in planetary satellites. If a small moon close to the planet has a sizable eccentricity and inclination, its ejecta that escape to planetocentric orbit would often re-impact with much higher velocity due to the satellite's and the fragment's orbits precessing out of alignment. If the impacts of returning ejecta result in net erosion, a runaway process can occur which may end in disruption of the satellite, and we term this process ``sesquinary catastrophe''. We expect the moon to re-accrete, but on an orbit with significantly lower eccentricity and inclination. We find that the large majority of small close-in moons in the Solar System, have orbits that are immune to sesquinary catastrophe. The exceptions include a number of resonant moonlets of Saturn for which resonances may affect the velocities of re-impact of their own debris. Additionally, we find that Neptune's moon Naiad (and to a lesser degree, Jupiter's Thebe) must have substantial internal strength, in line with prior estimates based on Roche limit stability. We also find that sesquinary instability puts important constraints on the plausible past orbits of Phobos and Deimos or their progenitors.Comment: Accepted for Ap

Polymer Translocation in Crowded Environments

We study the effect of the crowded nature of the cellular cytoplasm on the translocation of a polymer through a pore in a membrane. By systematically treating the entropic penalty due to crowding, we show that the translocation dynamics are significantly altered, leading to novel scaling behaviors of the translocation time in terms of chain length. We also observe new and qualitatively different translocation regimes depending upon the extent of crowding, transmembrane chemical potential asymmetry, and polymer length.Comment: 4 figure

The (un)availability of prognostic information in the last days of life: a prospective observational study.

OBJECTIVES: The aims of this study were (1) to document the clinical condition of patients considered to be in the last 2 weeks of life and (2) to compare patients who did or did not survive for 72 hours. DESIGN: A prospective observational study. SETTING: Two sites in London, UK (a hospice and a hospital palliative care team). PARTICIPANTS: Any inpatient, over 18 years old, English speaking, who was identified by the palliative care team as at risk of dying within the next 2 weeks was eligible. OUTCOME MEASURES: Prognostic signs and symptoms were documented at a one off assessment and patients were followed up 7 days later to determine whether or not they had died. RESULTS: Fifty participants were recruited and 24/50 (48%) died within 72 hours of assessment. The most prevalent prognostic features observed were a decrease in oral food intake (60%) and a rapid decline of the participant's global health status (56%). Participants who died within 72 hours had a lower level of consciousness and had more care needs than those who lived longer. A large portion of data was unavailable, particularly that relating to the psychological and spiritual well-being of the patient, due to the decreased consciousness of the patient. CONCLUSIONS: The prevalence of prognostic signs and symptoms in the final days of life has been documented between those predicted to die and those who did not. How doctors make decisions with missing information is an area for future research, in addition to understanding the best way to use the available information to make more accurate predictions

Hard Spheres in Vesicles: Curvature-Induced Forces and Particle-Induced Curvature

We explore the interplay of membrane curvature and nonspecific binding due to excluded-volume effects among colloidal particles inside lipid bilayer vesicles. We trapped submicron spheres of two different sizes inside a pear-shaped, multilamellar vesicle and found the larger spheres to be pinned to the vesicle's surface and pushed in the direction of increasing curvature. A simple model predicts that hard spheres can induce shape changes in flexible vesicles. The results demonstrate an important relationship between the shape of a vesicle or pore and the arrangement of particles within it.Comment: LaTeX with epsfig; ps available at http://dept.physics.upenn.edu/~nelson/index.shtml Phys Rev Lett in press (1997

Depletion forces near curved surfaces

Based on density functional theory the influence of curvature on the depletion potential of a single big hard sphere immersed in a fluid of small hard spheres with packing fraction \eta_s either inside or outside of a hard spherical cavity of radius R_c is calculated. The relevant features of this potential are analyzed as function of \eta_s and R_c. There is a very slow convergence towards the flat wall limit R_c \to \infty. Our results allow us to discuss the strength of depletion forces acting near membranes both in normal and lateral directions and to make contact with recent experimental results

How do palliative care doctors recognise imminently dying patients? A judgement analysis.

OBJECTIVES: To identify a group of palliative care doctors who perform well on a prognostic test and to understand how they make their survival predictions. DESIGN: Prospective observational study and two cross-sectional online studies. SETTING: Phase I: an online prognostic test, developed from a prospective observational study of patients referred to palliative care. Phase II: an online judgement task consisting of 50 hypothetical vignettes. PARTICIPANTS: All members of the Association of Palliative Medicine (APM) were eligible (n=~1100). 99 doctors completed the prognostic test and were included in the phase I analysis. The top 20% were invited to participate in phase II; 14/19 doctors completed the judgement task and were included in the phase II analysis. MEASURES: Phase I: participants were asked to give a probability of death within 72 hours (0%-100%) for all 20 cases. Accuracy on the prognostic test was measured with the Brier score which was used to identify the 'expert' group (scale range: 0 (expert)-1 (non-expert)). Phase II: participants gave a probability of death within 72 hours (0%-100%). A mixed model regression analysis was completed using the percentage estimate as the outcome and the patient information included in the vignettes as the predictors. RESULTS: The mean Brier score of all participants was 0.237 (95% CI 0.235 to 0.239). The mean Brier score of the 'experts' was 0.184 (95% CI 0.176 to 0.192). Six of the seven prognostic variables included in the hypothetical vignettes were significantly associated with clinician predictions of death. The Palliative Performance Score was identified as being the most influential in the doctors' prognostic decision making (β=0.48, p<0.001). CONCLUSIONS: This study identified six clinical signs and symptoms which influenced the judgement policies of palliative care doctors. These results may be used to teach novice doctors how to improve their prognostic skills

A coil-globule transition of a semiflexible polymer driven by the addition of spherical particles

The phase behaviour of a single large semiflexible polymer immersed in a suspension of spherical particles is studied. All interactions are simple excluded volume interactions and the diameter of the spherical particles is an order of magnitude larger than the diameter of the polymer. The spherical particles induce a quite long ranged depletion attraction between the segments of the polymer and this induces a continuous coil-globule transition in the polymer. This behaviour gives an indication of the condensing effect of macromolecular crowding on DNA.Comment: 12 pages, 4 figure