摘要:

We have devised and implemented a novel technique that has the required sensitivity to enable measurements of the complex surface impedance (Zs) of highTcsuperconducting materials between 4.2 K and at least 200 K. The essential ideal is to employ a superconducting high‐Qcavity resonator operated at an ambient temperature of 4.2 K. The sample, mounted on a sapphire rod, is placed inside the cavity at a high magnetic field location, and is thermally insulated from the cavity walls, enabling external control of the sample temperature between 4.2 and 200 K. The cavity characteristics are dominated by the sample properties—the Pb walls maintained at 4.2 K contribute negligibly. Measurement of the cavityQand resonant frequency enables the measurement ofZsas a function of the sample temperature. The technique is applicable to both bulk and thin‐film materials. We have used this technique with success to measureZs, at 9.58 GHz, for bulk Y1Ba2Cu3Oyand La1.85Sr0.15CuO4over a temperature range from 4.2 to 100 K.

ISSN:0034-6748    

DOI:10.1063/1.1139881

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new scanning tunneling microscope (STM) system is described that has been operated in several environments for both topographic imaging and tunnel spectroscopy. This STM shows high resistance to the effects of vibration and thermal drift. The device is unique in its simplicity and has only four moving parts. In addition, the critical tip–sample approach mechanism is inherently reliable and precise. The STM system accommodates a wide range of sample geometries and requires no special sample holder.

ISSN:0034-6748    

DOI:10.1063/1.1139882

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A small scanning tunneling microscope (STM) for studying surfaces in ultrahigh vacuum is described. It has been designed to have a mechanical tip–sample gap instability of less than 0.002 Å while operating in an environment with relatively large‐amplitude, low‐frequency vibrations. Thermal fluctuations are minimized by built‐in thermal compensation. The microscope has a unique coarsezpositioner, and an open design for accommodating various types of tip and sample exchanging manipulators. A novel mechanism for fabricating tips of the same length makes it possible to approach the tip and sample without visual access. Atomic‐scale images of graphite have been obtained and are presented.

ISSN:0034-6748    

DOI:10.1063/1.1139883

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new principle for analyzing the kinetic energy as well as the angular distribution of charged particles has been discovered. A new display type analyzer was constructed according to theory. It consists of an inner hemispherical grid and an outer hemispherical electrode which has a radius two times larger than that of the inner grid. Charged particles with the same kinetic energy radiate from one point to all directions inside the inner grid, are repelled by the electric field between the two spheres, and converge at the point which is symmetric to the center of the hemispheres. When an aperture is put at the point, the angular distribution of the charged particles can be observed using a two‐dimensional detector which is positioned out of the aperture. The characteristics of the analyzer are (1) the acceptance cone is ultimately wide, for instance, 2&pgr; sr for the charged particles emitted from a flat surface, (2) the pattern obtained is not distorted, and (3) the two electrodes are very simple and easy to construct.

ISSN:0034-6748    

DOI:10.1063/1.1139884

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We are developing an apparatus to employ gas electron diffraction (GED) for the study of transient molecular species generated by a pulsed excimer/dye‐laser system, in a new type of excite‐and‐probe experiment. This paper treats problems which arise in design of a suitable pulsed electron‐beam source, to provide relatively high instantaneous current. Standard electron‐beam theory is extended to define focal conditions under which sharp patterns may be recorded. We have characterized several components of blur and distortion, particularly those due to electrostatic repulsion between the main beam and the diffracted electrons. The analysis may find use in other work, e.g., with microscopic beams.

ISSN:0034-6748    

DOI:10.1063/1.1139885

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A technique using pulsed laser desorption is described as a means of volatilizing nonvolatile and thermally labile molecules into the gas phase for entrainment in supersonic jet expansions. By carefully controlling the laser desorption power and by using a glycerol matrix to provide a thin even film of sample, repetitive desorption can be achieved over an extended period for performing jet spectroscopy. Using resonant two‐photon ionization spectroscopy while monitoring only the molecular ion in a time‐of‐flight mass spectrometer, cold sharp spectra are observed for molecules which are difficult to volatilize by conventional heating methods.

ISSN:0034-6748    

DOI:10.1063/1.1139832

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We demonstrate an optical fiber mounting device which allows microlensed fibers to be pressed repeatedly against the cleaved facets of semiconductor laser diodes without causing impact damage to the facet. The device constrains the fiber against lateral movement while using the resilience of a second fiber acting as a weak spring to allow compliance along the optical axis. Stability of the microlensed fiber is not affected.

ISSN:0034-6748    

DOI:10.1063/1.1139833

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The construction and operation of a filamentary discharge in noble gases are described. The power and spectral properties of the observed continuous vacuum–UV excimer radiation are measured and compared with values found with standard VUV sources.

ISSN:0034-6748    

DOI:10.1063/1.1139834

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Composite pulse quadrupole echo sequences significantly reduce distortions due to finite pulse‐width effects, but they also produce phase dispersion leading to distortions of the quadrupolar powder line shape. In order to correct for phase errors arising from the composite pulse, we present the composite pulse magic echo sequence. It is demonstrated both experimentally and theoretically that the composite pulse magic echo sequence can eliminate phase distortions without considerably attenuating the signal‐to‐noise ratio. The performance of the new sequence is quite good.

ISSN:0034-6748    

DOI:10.1063/1.1139835

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An atmospheric pressure ionization (API) source for a VG 7070E‐HF mass spectrometer has been built and successfully operated at an accelerating potential of 6 kV. Hydrated protons and hydrated organic ions formed in the API source can be either partially or completely disaggregated by collisionally induced dissociation before entering the mass analyzer. The protonated molecular ions of pinacolone and 2,4‐pentanedione can be fully separated (R=2800,m/z101, <1% valley) at one sixth of their maximum signal strengths, and the13C‐xylene radical ion and protonated benzaldehyde can be fully separated (R=3300,m/z107, <1% valley) at one tenth of their maximum signal strengths. With 1% transmission, the resolving power of the VG 7070E‐HF operating in the API mode is estimated to be 4800 (m/z92, 10% valley). This resolution is more than sufficient to separate two oxidized hydrocarbons whose masses differ by 0.0364 daltons, the difference between a –CH2CH2– group and a ⟩CO group, and to permit unambiguous assignment of their molecular formulas. No other form of API mass spectrometer, including the triple quadrupole version, has this capability.

ISSN:0034-6748    

DOI:10.1063/1.1139836

出版商:AIP    

年代:1988

数据来源: AIP