摘要:

A single‐rod, high pulse‐energy, Nd:YAG laser is presented that is actively mode‐locked simultaneously at the third and fourth harmonics of the round‐trip frequency andQ‐switched after a prelasing period. The output consists of highly stable pulses, separated by the round‐trip time, of about 90 ps duration and 1 mJ energy. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147277

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An investigation is performed on the characteristics of the self‐referencing resonant fiber optic intensity based sensor supported by a Mach–Zehnder topology. Both transmissive and reflective configurations are analyzed. Via the definition of the measurement parameter (Rparameter) the linearity and sensitivity of the sensor are addressed. Theoretical and experimental results are compared. The problem of sensor design is under consideration. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147278

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have characterized the output of inexpensive, commercially available single quantum well (SQW) blue and green light emitting diodes (LEDs). The SQW LEDs were amplitude modulated with the output from a frequency generator while biased through a bias tee with 5 mA of current. The blue SQW LED produced 800 &mgr;W of light centered at 466 nm, with a −3 dB bandwidth of 58 MHz. The green SQW LED produced 543 &mgr;W of light centered at 522 nm, with a −3 dB bandwidth of 26 MHz. Modulated light was available to approximately 100 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetime of a standard fluorophore (fluorescein) was measured in the frequency domain using the phase‐modulation technique, and gave results similar to those obtained with a 488 nm argon ion laser modulated with a Pockels cell. To demonstrate the usefulness of the SQW LED source, we also performed measurements with the fluorescentpH indicator SNAFL‐2. Again, these results compared favorably with those obtained with the laser. When compared to a laser modulated with a Pockels cell, the SQW LEDs were smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the SQW LEDs at high frequencies, along with the blue or green outputs, allow their use as inexpensive light sources in fluorescence lifetime optical sensors and even fluorometers. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147279

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Solid state Ti:sapphire lasers were constructed and combined with nonlinear optics to produce a 100 fs resolution absorption spectrometer with tunability from 50 000 to 800 cm−1and <1 m o.d./s sensitivity. Tunable light generation and array detection technology are presented, including a discussion of the suitability of different infrared nonlinear optical materials. A systematic approach to stable infrared pulse generation is provided. Critical parameters of the pump/probe beam geometry are enumerated, revealing detrimental nonlinear contributions to the observed pump‐induced change in probe absorbance. These effects include pump‐induced probe phase modulation and pump‐induced refractive index lenses. Measurements and simulations of the unwanted features in various system geometries reveal their systematic control and elimination. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147280

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An unconventional electrodynamic suspension system with transparent planar electrodes is described which stably levitates charged solid particles or liquid droplets without the need for feedback control. The system has been used with particles ranging from about 1 to 100 &mgr;m diam, under vacuum and within stationary and flowing gases. Operation within low conductivity liquids is possible in principle. The suspension system consists of six transparent conducting electrodes arranged as faces of a hollow cube. Four of these electrodes are driven by a variable‐frequency two‐phase ac source operating in the low audio frequency range. Advantages of this type of trap for aerosol studies include relatively wide‐angle optical access and a geometry naturally suited to the use of three‐axis dc crossfields for particle manipulation. Conditions for stable levitation are reviewed as well as methods for determining the radius, mass, charge, and density of a spherical levitated object. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147281

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The applications of liquid crystal variable retarders in the production of spin‐polarized electrons and in the determination of the polarization of optical radiation from atoms excited by polarized electrons are discussed. The advantages of using liquid crystal variable retarders in the measurement of Stokes parameters are insensitivity to the incident photon direction, large transmission diameter, variable retardation over large wavelength range without mechanical movement, andinsitucalibration. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147282

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Using x‐ray spectroscopic techniques, we have investigated the properties of an electron beam ion trap (EBIT) after the electron beam is switched off. In the absence of the electron beam, bare, and hydrogenlike Kr35+and Kr36+ions remain trapped due to externally applied magnetic and electric fields for at least 5 s; xenon ions with an openLshell, i.e., Xe45+–Xe52+, remain trapped at least as long as 20 s. The ion storage time in this ‘‘magnetic trapping mode’’ depends on the pressure of background atoms as well as on the value of the externally applied trapping potential, and even longer ion storage times appear possible. The magnetic trapping mode enables a variety of new opportunities for atomic physics research involving highly charged ions, which include the study of charge transfer reactions, Doppler‐shift‐free measurements of the Lamb shift, measurements of radiative lifetimes of long‐lived metastable levels, or ion‐ion collision studies, by x‐ray or laser spectroscopy, and mass spectrometry. Because the trap is filledinsituduring the electron trapping phase, transfer losses associated with filling the trap from an external source are avoided. We present spectra of theK‐shell emission from heliumlike and hydrogenlike Kr34+and Kr35+as well as Xe52+and Xe53+that are produced by charge transfer reactions in collisions between ions and neutral atoms. Marked differences withK‐shell spectra produced by electron‐impact excitation are evident. We use the measurements to infer the Lamb shift contribution to the energy of the 1s1/2–2p3/2transition in hydrogenlike Xe53+and determine it to be 31 276(12) eV. The measurement technique can be applied to any ion produced in an EBIT so that Doppler‐shift‐free Lamb shift measurements of hydrogenlike U91+are within reach. We also illustrate the utility of the magnetic mode for lifetime determinations by measuring the 3.92(13) ms radiative decay of the 1s2s 3S1level in heliumlike N5+. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147276

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have investigated by probes and by photodetachment the plasma properties of two negative ion sources of different volume. We compared the data relevant to these two sources and identify the effects induced by the change in volume and the isotope effects. The observed scaling law for the electron temperature is in each caseTe∼I0.27d. Positive ions are predominantly lost to the walls, but due to the increasing influence of their loss by dissociative recombination when the source volume is increased, the scaling laws arene∼I0.7d, for the small source, andne∼I0.59d, for the large source. Atoms limit the production of negative ions and destroy them. Thus we see an isotope effect, with higher negative ion density in hydrogen than in deuterium, and the effect of the source volume, with the negative ion density larger in the small source than in the large one. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147283

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this article, we describe the performance characteristics for a selected number of target ion sources that will be employed for initial use at the Holifield Radioactive Ion Beam Facility as well as prototype ion sources that show promise for future use for radioactive ion beam applications. A brief review of present efforts to select target materials and to design composite target matrix/heat‐sink systems that simultaneously incorporate the short diffusion lengths, high permeabilities, and controllable temperatures required to effect fast and efficient diffusion release of the short‐lived species is also given. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147284

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Neutron crystal diffraction experiments are complementary to x‐ray diffraction studies of macromolecules because neutrons are strongly scattered by hydrogen and/or deuterium and can therefore be used to locate these atoms directly in a macromolecule. Since hydrogen atoms constitute about half of the total number of atoms in a macromolecule, and are functionally and structurally important in determining the biological properties of the system, a detailed knowledge of their location can often provide unique information about the system under study. However, because of the relatively low flux (on the order of 106neutrons/s/cm2) of neutrons available at the highest flux steady‐state reactors, high resolution diffraction experiments require large crystals and long data collection times. Typical crystal volumes exceed a cubic millimeter and data collection times approach three to four months to collect complete data to atomic resolution. Because of these limitations, relatively few high resolution neutron structures have been completed to date. To improve data collection efficiency and throughput, we have redesigned the macromolecular neutron crystallography beam line (H3A) at Brookhaven National Laboratory’s high flux beam reactor. In this report, the new facility is described. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147285

出版商:AIP    

年代:1996

数据来源: AIP