摘要:

Instrumental aspects of low‐energy electron microscopy are reviewed with a view toward the future evolution of this reemergent technology. Both elastically scattered and inelastically excited electrons in the 0–1000‐eV range may be used to form direct rather than scanned images of surfaces in the 3–10‐nm resolution range. Different instrumental setups may be used to form images that selectively contain information about the topography, crystalline structure, chemical composition, or magnetic orientation of the first few monolayers. Frequently, parallel imaging allows observation of dynamic processes occurring during observation. Key electron optical elements and their systemic relationships are described in the context of a still hypothetical generalized instrument that would allow complementary exploitation of many contrast modes. Image quality issues such as resolution, sensitivity, statistics, and contrast selectivity are considered with a view toward their optimization, in many cases by drawing ideas and technologies from other fields of microscopy.

ISSN:0034-6748    

DOI:10.1063/1.1143377

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A novel experimental technique to observe the Bragg scattering of evanescent neutron waves is presented. The so‐called EVA diffractometer, installed at the high‐flux reactor of the Institut Laue‐Langevin, allows the identification of neutron scattering from single crystal surfaces. We discuss the experimental setup, intensity, and resolution considerations and some first Bragg scattering signals from evanescent neutron waves excited at CaF2, InP, and MnF2single crystal surfaces. The experimental results are discussed within the framework of the so‐called distorted wave Born approximation and within a dynamical scattering theory. The limitations of both theoretical approaches to describe the Bragg scattering of evanescent neutron waves are indicated.  

ISSN:0034-6748    

DOI:10.1063/1.1143841

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Long pulse operation of a multijoule transverse‐electric‐CO2laser using a magnetic‐spiker sustainer excitation in conjunction with a pulse forming network is described. The laser has produced 4.5 J in a 5 &mgr;s long pulse at a high efficiency of ∼10% from an active discharge volume of 416 cm3. The overall system is simple and reliable.

ISSN:0034-6748    

DOI:10.1063/1.1143378

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A high‐resolution photoacoustic Ti:sapphire/dye ring laser spectrometer with a high level of automation is described. It produces high quality Doppler‐limited overtone spectra. The performance of the spectrometer has been demonstrated by measuring both low‐ and high‐resolution spectra of acetylene in the wave number regions 10850–12900 cm−1and 11500–11650 cm−1, respectively. The spectra show large signal‐to‐noise ratios (≳7000) for strong absorption peaks. Some strong absorption lines have been compared with published FTIR spectral data.

ISSN:0034-6748    

DOI:10.1063/1.1143379

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In order to make a microscopic observation of the surface elastic wave on inhomogeneous opaque materials, an apparatus for Brillouin scattering spectroscopy was constructed utilizing a commercial microscope. An optical system which serves both as an inlet of a laser beam and as an outlet of the scattered light was made and incorporated in the microscope. The apparatus was proved to be particularly useful to make microscopic‐scale observations of surface elastic waves on metal films.

ISSN:0034-6748    

DOI:10.1063/1.1143380

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A broadband coherent anti‐Stokes Raman spectroscopy (CARS) system optimized for single shot measurements in turbulent flames is described. The CARS spectrometer incorporates an intensified photodiode array (IPDA) with a P46‐phosphor based intensifier that is shown to have an order of magnitude less image persistence than P20‐phosphor based IPDAs, and thus largely eliminates the temperature errors that can result from this image persistence. The low and high signal nonlinearity of IPDAs incorporating both P20 and P46 phosphor based intensifiers is described. The CARS signal is dispersed with a single element concave holographic diffraction grating, which is fiber‐optically coupled. To provide the necessary dynamic range (approximately 1000:1) to handle CARS spectra whose temperatures can vary from 300 K to adiabatic flame temperatures a fiber‐optic splitter was employed and the nonlinearity of the IPDA detectors was characterized. A method of determining the nonlinearity was developed that was convenient and the fiber‐optic link was shown to provide a stable calibration of this nonlinearity. The problem of dye laser spectral drift, which is particularly severe in a long duration experiment, was addressed by constructing a compact high resolution spectrometer to continuously monitor the dye laser. The dye laser center frequency was maintained constant by manually adjusting an intracavity tuning filter. The accuracy of this technique and its effect on CARS derived temperatures was evaluated. The standard method of determining the instrument function by fitting to 295 K air spectra was compared to other methods and its use was shown to result in an uncertainty of less than 0.5% in CARS derived temperatures. The operation of this CARS instrument was demonstrated in a turbulent diffusion flame. The varying composition in these flames was accounted for by fittingC, the ratio of the nitrogen mole fraction to the total third order nonlinear susceptibility.

ISSN:0034-6748    

DOI:10.1063/1.1143381

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The design of this linear ultraviolet (UV) flashlamp ensures an equilibrium between the mercury cathode erosion and the return of mercury atoms from the discharge, making the cathode self‐replenishing. The lamp, which requires a simmer current of only 0.3 A, can operate in the region of high pulse peak currents and current densities where a conventional xenon flashlamp experiences a rapid deterioration. The lamp has a minimal length of 7.5 cm and can be loaded up to 100 W/cm. It has demonstrated about 30% output in the UV region and has proven efficient for dissociation of organic compounds and for disinfection, and has promise for other photochemical applications.

ISSN:0034-6748    

DOI:10.1063/1.1143382

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have developed a new method of signal detection that permits the direct measurement of the wavelength derivative transmittance or reflectance spectra of any sample that can be studied by optical spectroscopy. This method uses multiple lock‐in amplifiers where the output of one lock‐in is, in turn, measured by another. In doing this we can achieve both high signal‐to‐noise ratios and eliminate variations in the background response of the spectrometer to approximately one part in 104, while keeping the spectrometer simple in design.

ISSN:0034-6748    

DOI:10.1063/1.1143383

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A shielded cylindrical space‐charge‐limited diode ionization detector with high sensitivity and high resolution has been constructed to detect ionization in metal vapors, e.g., multiphoton ionization in the sodium atom and collisional associative ionization in the Rydberg states of the sodium dimer. The diode design and construction and measurements of its characteristics are described. Using this diode detector, a wide variety of high resolution spectroscopy can be obtained using techniques such as two‐photon excitation, optical–optical double resonance, perturbation‐facilitated optical–optical double resonance, and all‐optical triple resonance.

ISSN:0034-6748    

DOI:10.1063/1.1143384

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The sensitivity of an optical field sensor with a Pockels crystal of LiNbO3for measuring a field distribution in a long‐gap discharge has been much improved by using an intensity difference method with an intensity‐modulated laser beam. The signal‐to‐noise ratio (S/N) is also improved by 37 dB from that without the above‐mentioned method. The resultant sensitivity of the new field measuring system is 100 times higher than that of the previous one. Finally, it is possible to measure an electric field down to 7.5 V/cm in the long‐gap discharge with high accuracy corresponding to theS/Nratio of 20 dB and 1.5 V/cm with the accuracy of 6.0 dB.

ISSN:0034-6748    

DOI:10.1063/1.1143385

出版商:AIP    

年代:1992

数据来源: AIP