摘要:

The line shape problem for the Balle–Flygare‐type microwave spectrometer is reexamined experimentally using the method of sine and cosine Fourier transforms with a phase correction. Two previous studies [E. J. Campbell, L. W. Buxton, T. J. Balle, M. R. Keenan, and W. H. Flygare, J. Chem. Phys.74, 829 (1981), and F. J. Lovas and R. D. Suenram, J. Chem. Phys.87, 2010 (1987)] are reviewed. Those results are combined with new experimental data to obtain a general description of line shapes obtained from this device. Our emphasis here will be on using specific examples to clarify the relationship between the frequency domain representations of experimentally measured time‐domain signals and the originating active species distributions.

ISSN:0034-6748    

DOI:10.1063/1.1143956

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The most important factors contributing to line broadening and background intensity in electron spectra are identified and new design principles for high quality measurements are presented. It is shown that time‐dependent potential gradients in the gas cell are responsible for a large part of the ‘‘normal’’ line broadening, while scattering processes inside the gas cell are responsible for a large part of the background intensity. By designing the experiment according to these results, gas‐phase electron spectra can be recorded routinely at an instrument resolution level of better than 5 meV even at comparatively high gas cell pressures necessary to give high intensity for weak lines, and in principle unlimited counting times, at a much improved signal‐to‐background level. The resulting improvements in the spectral quality are demonstrated by spectra of Ar and HBr.

ISSN:0034-6748    

DOI:10.1063/1.1143957

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electrostatic analyzers with different geometries have been used extensively in space plasma investigations. When the energy resolution requirements are such that &Dgr;E/E≊10% the instrument is rather simple to design and to manufacture. However, when an energy resolution of ≊1% is required, the manufacturing costs increase rapidly. For analyzers with an energy resolution of ≊10% the calibration can also be used to correct for manufacturing errors when they appear. In this paper, it has been shown that to reach an energy resolution &Dgr;E/E=(1.6±0.2)% using a hemispherical electrostatic analyzer with outer and inner radii 101.25 and 98.75 mm respectively and plate thickness 0.4 mm, an accuracy of 25 &mgr;m in the alignment of one hemisphere with respect to the other is sufficient. It is also shown that the effect on the energy resolution of an indentation of the surface of one of the conductors can be neglected as long as its depth is less than ≊100 &mgr;m. Larger indentations can easily be detected in the manufacturing process. For each case considered, the maximum permitted loss of transmitted particles with respect to the transmission of an ideal instrument has been defined as 10%. Similarly, it has been specified that the deviations in the distributions of entrance angles of transmitted particles shall be less than 0.1°.  

ISSN:0034-6748    

DOI:10.1063/1.1143958

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Advantages of wavelength‐dispersive spectrometers over energy‐dispersive spectrometers for parallel detection of x rays are discussed and factors involved in the design of wavelength‐dispersive spectrometers are reviewed. The dispersion and collection solid angle of fixed‐position Bragg diffractors used for parallel detection of x rays are calculated and the properties of four such spectrometers based on singly and doubly curved diffractors are compared with the properties of scanning monochromators. It is concluded that the previously studied parallel‐detection spectrometers based on Bragg diffractors are less advantageous than scanning monochromators but they may be preferred when it is necessary to detect many characteristic x‐ray lines simultaneously or to avoid the use of moving parts.

ISSN:0034-6748    

DOI:10.1063/1.1143959

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this article we describe a new type of powder diffractometer that gives a significant improvement in count rate over existing methods. The key to the new technique is the use of a flat, diffracted beam monochromator in combination with a position sensitive detector. The incident x rays fall on a flat plate or capillary sample and multiple diffraction lines are reflected by the monochromator out of the equatorial plane onto a position sensitive detector. Since the monochromator is tuned for the incident beam energy, background from fluorescence or other scattering is eliminated and data can be recorded over a range of several degrees thereby providing a large improvement in counting efficiency over conventional diffractometers. This new method is also compared to a previous article which employed a focusing analyzer crystal and position sensitive detector.

ISSN:0034-6748    

DOI:10.1063/1.1143960

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The double‐crystal x‐ray monochromator (spectrometer) in (+,+) setting is high‐resolution equipment. Focusing and a considerable increase of the x‐ray monochromatic flux without any lowering of resolution may be reached if one of the crystals is properly bent into conical shape. The parameters of the bending are dependent on the wavelength. This ideal conical bending may be replaced by an easier‐realizable cylindrical bending where the virtual source and the focus are located on the axis of the cylinder. In this case the higher monochromatic flux, as compared with the flat crystals, may be reached only for Bragg angles from about 35° to about 60°. The highest flux is for the Bragg angle &thgr;=45° and its nearest neighborhood.

ISSN:0034-6748    

DOI:10.1063/1.1143961

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A simple and efficient electron‐impact ionization method for time‐of‐flight mass spectrometers is described. This method utilizes the photoelectrons emitted from the accelerating electrode surfaces upon UV laser irradiation as the electron source. Since no modification of the spectrometer is required, it provides a convenient way to perform electron‐impact ionization in a time‐of‐flight mass spectrometer originally designed for laser ionization. The detection sensitivity (∼109/cm3) and mass resolution (∼150) achieved by this method in our apparatus are sufficient for diagnostic purposes for cluster beam experiments. The observed ion intensities suggest that the photoelectron current density produced is comparable to those generated by conventional thermionic emission electron sources. This technique is generally applicable as an ionization method in time‐of‐flight mass spectrometers and is particularly useful when the available lasers are not suitable for photoionization detection due to wavelength or intensity limitations.

ISSN:0034-6748    

DOI:10.1063/1.1143962

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new reflectron time‐of‐flight mass spectrometer using a pulsed CO2laser for desorption of neutral molecules, followed by a pulsed, tunable UV laser for resonant multiphoton postionization is described. The innovative features of this instrument are the following: spatially resolved desorption in the &mgr;m range, a simple and efficient ion extraction geometry, and several improvements in sample loading, positioning, and observation. Excellent sensitivity in the attomole range and a mass resolution ofM/&Dgr;M≊2000 are demonstrated. Primary applications include spatially resolved organic trace analysis and the detection of selected chemical compounds in complex mixtures.

ISSN:0034-6748    

DOI:10.1063/1.1143963

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We designed and constructed an ultrahigh vacuum scanning tunneling microscope (UHV‐STM) which is available for the measurement of tunneling spectra at low temperature. The sample holder is cooled by thermal conduction to the stage that thermally connects with the refrigerator head. We cleaved Bi2Sr2CaCu2Oy(Bi2212 highTcsuperconductor) single crystals under ultrahigh vacuum conditions, and measured tunneling spectra of the Bi–O surface at 35 K. The spectra were smeared due to the fact that the STM tip was at high temperature (≳100 K).

ISSN:0034-6748    

DOI:10.1063/1.1143964

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A two‐dimensional micropositioning device for scanning tunneling microscope (STM) probes has been developed. This device uses the principle of piezoelectric inertial translation to produce a controlled stepping motion of the probe along vertical and horizontal axes over distances of several mm. The tip micropositioner is controlled by the same electrical signals that drive the scanning piezoelectric element, thus alleviating the need for additional electronic control elements. This device has been tested on STMs operating in air and in ultrahigh vacuum environments.

ISSN:0034-6748    

DOI:10.1063/1.1143965

出版商:AIP    

年代:1993

数据来源: AIP