摘要:

We review the history of experimental work on Rayleigh–Be´nard convection in gases, and then describe a modern apparatus that has been used in our experiments on gas convection. This system allows for the study of patterns in a cell with an aspect ratio (cell radius/fluid layer depth) as large as 100, with the cell thickness uniform to a fraction of a &mgr;m, and with the pressure controlled at the level of one part in 105. This level of control can yield a stability of the critical temperature difference for the convective onset of better than one part in 104. The convection patterns are visualized and the temperature field can be inferred using the shadowgraph technique. We describe the flow visualization and image processing necessary for this. Some interesting results obtained with the system are briefly summarized. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147511

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We describe a method of reducing the timing jitter between two passively mode‐locked femtosecond titanium:sapphire laser systems, enabling femtosecond‐resolved measurements with independently tunable pump and probe wavelengths. The scheme supplements a commercially available system (Coherent Synchrolock) which locks two Ti:sapphire lasers to a master clock. By selecting only those pulses which are temporally coincident within a user‐specified range, a timing jitter reduction between the two lasers from ∼3 ps to <200 fs FWHM can be achieved, as measured by optical cross correlation. The timing jitter between the lasers is easily varied, allowing optimization of the tradeoff between temporal resolution and throughput depending on experimental needs. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147016

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A semiconductor laser confocal microscope is developed for measuring the optical thickness of thin transparent samples at high spatial resolution. The optical sectioning capability of a confocal microscope, and the sensitivity of the laser output to optical feedback, are both utilized to detect the presence of a nonabsorbing sample in the path of the probe beam that is initially focused onto a plane mirror. The index mismatch at the sample interface defocuses the beam away from the mirror and reduces the amount of optical feedback. The optical thickness is computed from the amount of axial displacement that the mirror must be given to regain maximum feedback. The laser power output is monitored using the monitor photodiode in the laser package. When the geometrical thickness of the sample is knownapriori, the technique can be used to measure its refractive index and vice versa. The smallest and largest measurable sample thickness are determined by the sharpness of the axial intensity response of the microscope and the working distance of the focusing objective, respectively. Distortions in the central spot distributions of the response degrade the precision of the measurement technique. We demonstrate the technique for both the 830 and 780 nm laser output wavelengths, by classifying cover glasses of differing optical thickness (geometrical thickness range: 70–536 &mgr;m, refractive index=1.523), as well as to image the sudden index changes found in the phase ridge formed by two closely spaced cover glasses (surface flatness=7 &mgr;m) of different geometrical thickness. Our experiments show that at least for objectives with small numerical apertures (≤0.25), spherical aberrations are negligible and our measurements are to within the accuracy set by the glass manufacturer. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147017

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have studied the dependence of the finish of the vertex of a wedge on the diffraction pattern experimentally. It is found that the three ray interference pattern in the shadow region of the samples arises from the rounded vertex of the wedge. The method reported in this article is shown to be a valuable technique to evaluate the finish of a vertex of a small wedge with an obtuse angle. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147018

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Current instruments for the measurement of haze in aircraft windscreens generally require the removal, and possible destruction, of the windscreen to conduct the test. Most instruments available for haze measurement of samples taken from a windscreen cannot be usedinsitu. This is because the standard definition of haze requires measurement of light scattered in transmission through the sample so the instrument must have a light source and a light detector on opposite sides of the sample. One portable instrument has been reported in the literature; however, it is not readily available and, in the reported form, is subject to errors due to orientation of the instrument with respect to the sample under test, and due to background light. The orientation error arises from the typically more or less parallel orientation of scratches in the windscreen. If haze is measured in just one orientation through the sample, the haze value will depend upon whether the measurement plane is parallel to scratches or not. Background light will also affect the reading if the instrument has not been designed to eliminate most ambient light and to compensate for the remaining illumination. We report on the development of a new prototype instrument that is portable, easy to use, is relatively insensitive to ambient light, and will be less influenced by scratch orientation. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147019

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have developed a novel bistatic polarization‐modulated nephelometer that probes a medium enablinginsitumeasurement of angle‐ and polarization‐dependent light scattering through a planar interface. This is particularly important for media with a planar interface such as sea ice but also for media in which measurements at great distance or depth, e.g., the atmosphere, are desirable. The instrument sits directly and noninvasively on the planar surface of the scattering medium. Light from a laser beam is directed into the medium and the scattered light is detected by analyzer optics. The angle of both the laser and detector beams in the medium can be varied continuously. Alternating current polarimetry is used to obtain phase function and polarization information of the scattered light. Measurements from this instrument of scattering from monodisperse polystyrene (latex) spheres in the laboratory are in good qualitative agreement with the results of Mie calculations and measurements made with a conventional monostatic nephelometer. This instrument has also been used to make preliminary investigations of scattering in sea ice in Pt. Barrow, Alaska. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147020

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new micro‐Fourier transform infrared cell has been constructed and evaluated using (i) the transformation of aurichalcite crystals at 298–475 K, (ii) the adsorption/desorption of pyridine on a Cu2O/ZnO junction, and (iii) the adsorption of CO on Pt/alumina pellets; it enables spatial and time resolution of processes in such diverse samples held at known thermal and baric conditions. The potential of this cell in catalytic and analytical science is considered. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147021

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A simple surface corona discharge excitation technique has been used to generate high repetition rate, high average power rare gas halide UV lamp radiation. The lamp exhibited reliable performance with very low jitter and high shot to shot reproducibility. An average power of 9 W of UV centered at 248 nm (KrF) was obtained from a small 7.0‐cm‐diam emitting surface. XeCl lamp lifetimes of over one billion shots were obtained on a single gas fill without the need for gas circulation or cryogenic cleanup. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147022

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

B4C/W multilayer mirrors designed for an x‐ray beamline at the MEGAJOULE facility are susceptible to x‐ray damage. The OCTAL laser was implemented to generate a gold plasma with a continuous x‐ray spectrum (0.1 to 1.5 keV) used to irradiate the mirror. The fluence on the sample is 0.05 J/cm2per shot. The pulse duration is about 1 ns. The temporally integrated spectrum reflected by the mirror is analyzed by a bent crystal spectrometer and recorded on photographic film. The spectral analysis suggests that an increase of the mean period of the multilayer happens during the second shot. This expansion is estimated to be 5% with respect to the unexposed mirror. After the second shot, destruction of the multilayer coating in the central part of the sample and irreversible damages in the other part characterized by a decrease of Bragg reflectivity have been observed. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147015

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A transmission x‐ray absorption spectroscopy cell that can be used for air‐sensitive samples withinsitutreatment is described. The cell is designed with a relatively small size for use with air‐sensitive powdered catalyst samples that must be loaded in a glove box. Samples can be treatedinsituwith gas flow or vacuum and temperature control up to 500 °C. The cell is constructed of stainless steel and designed for durability as well as ease of repair. The cells are vacuum tight and equipped with beryllium windows sealed with vacuum O‐ring flanges for easy loading. Each cell, with all parts excluding the windows, costs about $2700. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147023

出版商:AIP    

年代:1996

数据来源: AIP