Search Results - (Author, Cooperation:J. R. Heath)

2013-12-07

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Animals ; Antineoplastic Agents/*therapeutic use ; Central Nervous System Neoplasms/*drug therapy/genetics ; DNA/genetics ; Drug Resistance, Neoplasm/*genetics ; Erlotinib Hydrochloride ; Glioblastoma/*drug therapy/genetics ; Humans ; Mice ; *Molecular Targeted Therapy ; Mutation ; Neoplasm Transplantation ; Protein Kinase Inhibitors/*therapeutic use ; Quinazolines/therapeutic use ; Receptor, Epidermal Growth Factor/antagonists & inhibitors/*genetics ; Single-Cell Analysis ; Tumor Cells, Cultured ; Withholding Treatment

2016-05-27

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2018-08-24

American Society of Hematology (ASH)

0006-4971

1528-0020

Biology

Medicine

Immunobiology and Immunotherapy, Lymphoid Neoplasia, Clinical Trials and Observations

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The high resolution infrared spectrum of the C9 cluster has been measured in direct absorption by infrared diode laser spectroscopy of a pulsed supersonic carbon cluster jet. Fifty-one rovibrational transitions have been assigned to the ν6 (σu ) antisymmetric stretch fundamental of the 1Σ+9 linear ground state of C9. The measured rotational constant [429.30(50) MHz] is in good agreement with ab initio calculations and indicates an effective bond length of 1.278 68(75) A(ring), consistent with cumulenic bonding in this cluster. Several perturbations are observed in the upper state, and the upper- and lower-state centrifugal distortion constants are observed to be anomolously large, evidencing a high degree of Coriolis mixing of the normal modes.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The first characterization of the bending potential of the C7 cluster is reported via the observation of the v=11 and v=20 levels of the ν11(πu) bend as hot bands associated with the ν4(σu) antisymmetric stretch fundamental. The lower state hot band rotational constants are measured to be 1004.4(1.3) and 1123.6(9.0) MHz, constituting a 9.3% and 22% increase over the ground state rotational constant [918.89(41) MHz]. These large increases are strong evidence for extremely large amplitude, anharmonic bending modes in this cluster. In addition, quartic and sextic centrifugal distortion constants determined for the ground and ν4=1 states are found to be anomalously large and negative, evidencing strong perturbations between stretching and bending modes.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The first infrared spectrum of gas phase, jet-cooled C4 has been measured by high resolution diode laser absorption spectroscopy. Twelve rovibrational transitions are assigned to the ν3(σu) antisymmetric stretch of linear 3Σ−g C4. No evidence is observed for the bent structure of triplet C4 recently observed in a matrix study by Cheung and Graham [J. Chem. Phys. 91, 6664 (1989)]. Indeed, the measured band origin (1548.9368(21) cm−1) and effective ground state C–C bond length [1.304 31(21)A] are consistent with several ab initio predictions of a rigid, linear, cumulenic structure for this cluster radical.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A novel method is presented for growth of polycrystalline silicon films on amorphous substrates at temperatures of 540–575 °C. Grain nucleation and grain growth are performed in two steps, using Si nanocrystals as nuclei ("seeds''). The nanocrystal seeds are produced by excimer laser photolysis of disilane in a room temperature flow cell. Film (grain) growth occurs epitaxially on the seeds in a separate thermal chemical vapor deposition (CVD) step, with growth rates 10–100 times higher than similar CVD growth rates on crystal Si. Grain size and CVD growth rates are dependent on seed coverage, for seed coverage 〈0.2 monolayers.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Thin Si films have been grown isothermally on Ge(100) substrates using alternating exposures of Si2H6 and Si2Cl6, maintaining chlorine and hydrogen surface termination. At 465 °C, film growth rate is roughly 2 monolayer per cycle (one cycle equals 1 Si2H6 and 1 Si2Cl6 exposure). At 475 °C a uniform epitaxial film is obtained, while islanding is observed at higher T. This process is thermally activated and is not strictly self-limiting, but has certain desirable characteristics of atomic layer epitaxy growth.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We report here on the size-dependent kinetics of exciton recombination in a III–V quantum dot system, InP. The measurements reported include various frequency dependent quantum yields as a function of temperature, frequency dependent luminescence decay curves, and time-gated emission spectra. This data is fit to a three-state quantum model which has been previously utilized to explain photophysical phenomena in II–VI quantum dots. The initial photoexcitation is assumed to place an electron in a (delocalized) bulk conduction band state. Activation barriers for trapping and detrapping of the electron to surface states, as well as activation barriers for surface-state radiationless relaxation processes are measured as a function of particle size. The energy barrier to detrapping is found to be the major factor limiting room temperature band-edge luminescence. This barrier increases with decreasing particle size. For 30 A(ring) particles, this barrier is found to be greater than 6 kJ/mol—a barrier which is more than an order of magnitude larger than that previously found for 32 A(ring) CdS nanocrystals. © 1996 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The resonance Raman spectrum of InP nanocrystals is characterized by features ascribable to both longitudinal (LO) and transverse (TO) optical modes. The intensity ratio of these modes exhibits a strong size dependence. To calculate the size dependence of the LO and TO Raman cross sections, we combine existing models of Raman scattering, the size dependence of electronic and vibrational structure, and electron vibration coupling in solids. For nanocrystals with a radius 〉10 Å, both the LO and TO coupling strengths increase with increasing radius. This, together with an experimentally observed increase in the electronic dephasing rate with decreasing size, allows us to account for the observed ratio of LO/TO Raman intensities. © 1997 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The absorption spectrum of the special C60 cluster buckminsterfullerene has been studied in a supersonic beam by laser depletion of the cold van der Waals complexes of C60 with benzene and methylene chloride. Both complexes were found to display a single, isolated absorption band in the near ultraviolet superimposed on a structureless absorption continuum. For the methylene chloride complex this feature is centered at 3860 A(ring), and is roughly 50 cm−1 wide. In the benzene van der Waals cluster, the corresponding feature is located at 3863 A(ring), and has a similar width. This spectrum is tentatively assigned to the 0–0 band of the lowest 1T1u←1Ag (LUMO+1←HOMO) transition of a truncated icosahedral carbon shell structure, broadened by coupling to the underlying quasicontinuum of ground state vibrational levels.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The laser-induced fragmentation behavior of positive carbon cluster ions has been investigated by tandem time-of-flight techniques for the jet-cooled clusters up to 80 atoms in size. Two distinct photophysical regimes were found. The first applies to clusters with 34 atoms or more, all of which dissociate to produce even numbered fragments. Large even clusters fragment by the loss of the high energy species C2, odd ones lose a C atom. The second regime applies to clusters composed of 31 or less atoms, all of which fragment by the loss of C3. These two regimes are sharply separated by C+32 which fragments to produce small cluster ions in the 10–19 atom size range. Fragmentation of the large clusters occurs on a microsecond or faster time scale only at very high levels of excitation (〉12.8 eV). These photophysical results are interpreted as consequences of the large even clusters having edgeless, spheroidal cage structures while the small ones have linear chain or ring structures.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Supersonic beams of clusters of Si and Ge atoms have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The cluster beams were characterized by F2(7.9 eV) and ArF(6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. In addition, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV) harmonics of the Nd:YAG followed by excimer laser ionization. All two-photon ionization processes were found to produce extensive fragmentation of the larger clusters. The observed fragmentation pattern for the silicon and germanium clusters were remarkably similar to each other, but drastically different from that seen for metal clusters in the same apparatus. Unlike metal clusters, which tend to lose one atom at a time, these semiconductor clusters appear to fragment by a fission process, the daughter ions falling almost exclusively in the size range from 6 to 11 atoms. Time delay studies in the two-color experiments established that clusters of both Si and Ge have excited electronic states with lifetimes of approximately 100 ns. This again is dramatically different from the behavior found with metal clusters, and indicates the feasibility of R2PI spectroscopy on these cold semiconductor particles. The existence of such long-lived excited states indicates that there is probably an energy gap between the band of electronic states being excited and the ground electronic state.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Supersonic beams of semiconductor clusters with the formula GaxAsy were generated by laser vaporization of a disc of pure GaAs mounted on the side of a pulsed supersonic nozzle. These cluster beams were characterized by laser photoionization with various fixed-frequency lasers followed by time-of-flight mass spectrometry. Mass analysis of the clusters with x+y〉10 showed all clusters in the composition range from Gax+y through GaxAsy to Asx+y to be present in roughly the amount expected from a binomial distribution. In the smaller clusters strong variations were observed from this expected binomial distirbution as a result of kinetic effects in the cluster formation process. Photoionization with an ArF excimer laser at very low pulse energy revealed a pronounced even/odd alternation in the photoionization cross section of the GaxAsy clusters, depending only on the total number of atoms in the cluster. Clusters in the 5–21 atom range with an odd number of atoms were one-photon ionized by the 6.4 eV ArF excimer laser photons. This even/odd alternation in ionization properties of the clusters supports the view that the even clusters have fully paired singlet ground states with no dangling bonds. At higher ArF excimer laser fluences, the observed mass spectrum became increasingly affected by fragmentation. As is true with bulk GaAs surfaces, these GaxAsy clusters evaporate largely by the loss of arsenic (probably As2) when heated by the laser, leaving behind clusters which are richer in gallium.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.

Electronic Resource

1365-2842

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

The persistence of identification marks applied to dentures with two-spirit solvent based fibre tip pens and graphite pencil covered with an experimental and a proprietary sealant were studied under service conditions. In general, the marks applied with the pens were rapidly lost, but those covered with the sealants persisted satisfactorily. The experimental sealant performed slightly better than the proprietary product.

Electronic Resource

1365-2842

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

In order to find a denture identification method that could be rapidly applied to completed dentures, a variety of spirit pens and commercial and experimental sealants were investigated. The pens were generally rapidly removed by one or more abrasive, denture cleansing, antiseptic or mouthwash agent; the sealants exhibited greater longevity. An experimental sealant (acrylic polymer dissolved in chloroform) was found to be cheap, readily prepared, easily applied, very resistant to abrasion, unaffected by immersion denture cleansers, antiseptics and mouthwashes, to produce no more crazing on dentures than the other sealants investigated, and not to affect the transverse strength of acrylic resin.

Electronic Resource

1365-2842

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

The effect of two testing schedules on the abrasion resistance of restorative materials has been investigated.

Electronic Resource

1365-2842

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

The physical removal of plaque from dentures by brushing is important in the maintenance of oral health. However, there is a danger that brushing will abrade acrylic resin. A machine using a rotary brushing action was therefore utilized for laboratory tests of possible cleaning pastes, employing a toothbrush with good access and adaptability to denture surfaces. Crest toothpaste produced the most abrasion on acrylic resin, Dentu-Creme the least. An experimental paste produced an appreciably lower abrasion rate. Self-cured specimens wore more than heat-cured ones. The rate of abrasion decreased as the temperature was raised. Brushing with water alone, or with a 10% solution of soap, did not produce detectable wear.The wear produced by proprietary cleaners when hand brushing the lingual flanges of dentures and flat blanks of heat-cured acrylic was also measured. Boots denture cleaning paste produced the greatest abrasion, Dentu-Creme the least. The wear measured by reference to a glass plane was at least twice that recorded by the decrease in depth of a groove. The machine did not quantitatively produce the same abrasion as hand brushing, nor did it qualitatively rank the pastes in the same order of abrasiveness.The clinical relevance of these results is discussed.

Electronic Resource

1365-2842

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

A method of measuring toothbrush-dentifrice abrasion of restorative materials is described. The method also includes a quantitative assessment of surface roughness. Values of wear and surface roughness of a range of restorative materials are presented.

Electronic Resource