N. H. Tolk

Photon emission from low-energy ion and neutral bombardment of solids

Abstract Low energy heavy particle bombardment of solid surfaces is observed to be accompanied by the emission of infrared, visible and ultraviolet radiation. Line radiation arising from transitions between discrete atomic or molecular levels may be attributed to the decay of sputtered or backscattered excited particles which have escaped the surface. Broadband continuum radiation which is also observed in low-energy heavy particle collisions with surfaces arises from the surface and appears to be a strictly solid state phenomenon. Measurement of collision induced optical radiation constitutes a powerful tool for studies of the fundamental outershelf electronic processes which result from the interaction of low-energy atomic particles with solids.

Desorption of H from Si(111) by Resonant Excitation of the Si-H Vibrational Stretch Mode

Past efforts to achieve selective bond scission by vibrational excitation have been thwarted by energy thermalization. Here we report resonant photodesorption of hydrogen from a Si(111) surface using tunable infrared radiation. The wavelength dependence of the desorption yield peaks at 0.26 electron volt: the energy of the Si-H vibrational stretch mode. The desorption yield is quadratic in the infrared intensity. A strong H/D isotope effect rules out thermal desorption mechanisms, and electronic effects are not applicable in this low-energy regime. A molecular mechanism accounting for the desorption event remains elusive.

Ultrafast carrier and phonon dynamics in Bi2Se3 crystals

in this material, 11 particularly the electron‐electron, electron‐phonon, and phonon‐phonon interactions. In this letter, we report the ultrafast time-resolved optical spectroscopy study of Bi2Se3 crystals in both the time domain and the energy domain. Our measurements reveal three underlying relaxation processes in the transient response of Bi2Se3, each associated with different physical mechanisms. It is also shown that the relative strength of these processes is sensitive to air exposure of the samples. The observed charge trapping and air doping effects are likely due to the presence of Se vacancies, a major issue material scientists working to use the properties of Bi2Se3 will face in the near term. The Bi2Se3 single crystals studied in this work were synthesized via the Bridgman method at Purdue University and Fudan University. During crystal growth, the mixture of high purity elements was first deoxidized and purified by multiple vacuum distillations, and then heated to 850‐900 °C for 15 h, followed by a slow cool down under a controlled pressure of Se to compensate for possible Se vacancies. Afterwards, the samples were zone refined at a speed of 0.5‐1.5 mm/hour with a linear temperature gradient set to 4‐5 °C /cm, until a temperature of 670 °C was reached. The as-grown Bi2Se3 crystals from both groups are naturally n-doped due to remnant Se vacancies. 4 Hall mea

Chemically Resolved Imaging of Biological Cells and Thin Films by Infrared Scanning Near-Field Optical Microscopy

The infrared (IR) absorption of a biological system can potentially report on fundamentally important microchemical properties. For example, molecular IR profiles are known to change during increases in metabolic flux, protein phosphorylation, or proteolytic cleavage. However, practical implementation of intracellular IR imaging has been problematic because the diffraction limit of conventional infrared microscopy results in low spatial resolution. We have overcome this limitation by using an IR spectroscopic version of scanning near-field optical microscopy (SNOM), in conjunction with a tunable free-electron laser source. The results presented here clearly reveal different chemical constituents in thin films and biological cells. The space distribution of specific chemical species was obtained by taking SNOM images at IR wavelengths (lambda) corresponding to stretch absorption bands of common biochemical bonds, such as the amide bond. In our SNOM implementation, this chemical sensitivity is combined with a lateral resolution of 0.1 micro m ( approximately lambda/70), well below the diffraction limit of standard infrared microscopy. The potential applications of this approach touch virtually every aspect of the life sciences and medical research, as well as problems in materials science, chemistry, physics, and environmental research.

De-excitation processes near the surface of ion bombarded SiO2 and Si

Abstract Ion bombardment induced secondary ion and optical excitation from oxide coated Si has been studied as the oxide layer is eroded away. By comparing ion and photon yields, a model for the role of resonant and Auger electron transitions in the neutralisation and de-excitation of the sputtered atoms and ions has been suggested.

Free-electron-laser near-field nanospectroscopy

First experiments at the Vanderbilt free electron lasers measured the local reflectivity of a PtSi/Si system. The reflectivity in the scanning near-field optical microscope images revealed features that were not present in the corresponding shear-force (topology) images and which were due to localized changes in the bulk properties of the sample. The size of the smallest detected features clearly demonstrated that near-field conditions were reached. The use of different photon wavelengths (0.653, 1.2, and 2.4 μm) enabled us to probe regions of different depth.

Near-monochromatic X-ray beams produced by the free electron laser and Compton backscatter

The intense photon output of a free electron laser may be made to collide with its own high energy electron beam to create nearly monochromatic x-rays using Compton backscatter techniques. These x-rays can be used for imaging and non-imaging diagnostic and therapeutic experiments. The initial configuration of the Vanderbilt Medical Free Electron Laser (Sierra Laser Systems, Sunnyvale, CA) produces intense x-rays up to 17.9 keV, although higher energies are easily attainable through the use of frequency doubling methods, alteration of the energy of the electron beam and coupling to conventional laser inputs.

Scanning near field infrared microscopy using chalcogenide fiber tips

Chalcogenide glass optical fibers were fabricated into functional apertured probes for near field scanning infrared microscopy. Probe fiber tips were chemically etched and aluminum coated for the purpose of simultaneously collecting near field shear force and optical signals. Surface topography and infrared optical reflectivity data were obtained using the tips in a scanning near field microscope while illuminating an integrated microcircuit with the output from a free electron laser operating at a λ of 4.7 μm. Approximately 25 nm topographical and 100 nm optical lateral resolution were observed.

Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation

The authors use femtosecond optical pulses to induce, control, and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ∼40K, they observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ∼TC∕2. The authors find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but independent of ambient temperature. They provide a physical interpretation of the observed light-induced collective Mn-spin precession and relaxation.

Electron and photon stimulated desorption of positive ions from alkali halide surfaces

Note: Univ wisconsin,dept phys,madison,wi 53706. Pian, tr, bell tel labs inc,murray hill,nj 07974.ISI Document Delivery No.: QV061 Reference LSE-ARTICLE-1983-013 Record created on 2006-10-03, modified on 2017-05-12