摘要:

Detailed analytical models of signal‐to‐noise ratios (SNR) of the conventional frequency domain (FD) and time domain (TD) photothermal measurement methodologies are developed and compared to the rate‐window photothermal method, both theoretically and experimentally. The conclusions of this study demonstrate that the lock‐in amplifier (LIA) rate‐window measurement mode in general, and the digital LIA mode, in particular, exhibits superior SNR to both the conventional frequency‐scanned LIA FD method and to the transient, time‐averaged TD method. Between the pulse‐duration‐scanned and pulse‐repetition‐period scanned rate‐window methodologies, the former clearly exhibits superior SNR. The theoretical conclusions are in agreement with experimental SNRs using the implementation of the foregoing measurement methodologies with simple infrared photothermal radiometric setups.  

ISSN:0034-6748    

DOI:10.1063/1.1144568

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A powerful infrared aerosol lidar operating at the eyesafe wavelength of 1.56 &mgr;m has been developed. More than 140 mJ per pulse are obtained from stimulated Raman backscattering narrow‐band Nd:YAG laser radiation in deuterium. The mobile lidar system was successfully applied during the VAST and MAPTIP international aerosol field campaigns in 1992 and 1993. A range between 0.3 and more than 10 km is demonstrated for slant‐path measurements in the presence of aerosols or cirrus clouds.

ISSN:0034-6748    

DOI:10.1063/1.1144569

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A picosecond time‐resolved multiplex coherent anti‐Stokes Raman scattering (CARS) spectrometer based on a new principle has been constructed using a femtosecond laser, nanosecond lasers, and a streak camera. The second harmonic of the output from the femtosecond Ti:sapphire oscillator/regenerative‐amplifier system is used for the photoexcitation. The second harmonic of a nanosecond Nd:YLF laser is used as CARS probing &ohgr;1radiation, and the output of a nanosecond broadband dye laser is used as &ohgr;2radiation. CARS signals, which are generated from the sample during the nanosecond probing pulses, are analyzed in the frequency domain by a spectrograph and are resolved in the time domain by a streak camera. This enables us multiplex CARS measurements not only in the frequency domain but also in the time domain (termed two‐dimensional multiplex CARS). Consequently, CARS spectra at various delay times are obtainable with a single measurement. Owing to the short duration and high stability of the femtosecond pumping pulses, time resolution as high as 15 ps has been achieved. Picosecond time‐resolved CARS spectra of all‐trans retinal and &bgr;‐carotene are presented to demonstrate the high capability of the constructed system. The factors which determine the time resolution of this CARS spectrometer are discussed.

ISSN:0034-6748    

DOI:10.1063/1.1145204

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Metallic toxics such as mercury, lead, cadmium, manganese, and other species are now of particular importance in relationship to coal‐based utilities, waste‐to‐energy plants, medical waste incinerators, and other industrial sources of atmospheric emissions or residues deposited in landfills. These metals are strong emitters or absorbers of middle ultraviolet light in the same general spectral region where important molecular pollutants are active. The furnace system of a commercial atomic absorption spectrometer, a continuum ultraviolet light source (a deuterium arc), and an oscillating entrance slit (adapted from second derivative spectroscopy) are applied to the measurement of toxic metals. The second‐harmonic ripple in the output of the photomultiplier tube is amplified by use of a tuned amplifier. A large signal‐to‐noise improvement is achieved when this system is calibrated and optimized with the metallic species of interest. This approximately compensates for the selectivity loss in using a continuum source in place of special lamps. Employment of this system is described in detail for the detection of mercury and the results are given for eight metals (Hg, Cd, Mn, Zn, Pb, Ni, Cu, and Cr). Data scatter when using this second‐harmonic spectrometer system is roughly 1/10 that of standard procedures.

ISSN:0034-6748    

DOI:10.1063/1.1144570

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A modified version of the classic Wiley–McLaren time‐of‐flight mass spectrometer (TOFMS) is presented here. Owing to a second‐order compensation of the initial position effect, a mass resolutionm/&dgr;mhigher than 2000 is obtained with large ion volumes, of the order of 0.2 cm3for Na+55(1265 a.m.u.). Sodium clusters Na+nwithnup to 530 (&bartil;12200 a.m.u.) are separated. The spectrometer can be used for the analysis of ionized clusters in a mass range from 1 to about 20000 a.m.u. as well as for the measurement of the kinetic energy of molecular fragments after a Coulomb explosion of a molecule. The performance of this spectrometer is demonstrated with sodium clusters and molecules.

ISSN:0034-6748    

DOI:10.1063/1.1144571

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Nuclear magnetic resonance (NMR) spectroscopy using personal computer‐based hardware has the potential of enabling the application of NMR methods to fields where conventional state of the art equipment is either impractical or too costly. With such a strategy for data acquisition and processing, disciplines including civil engineering, agriculture, geology, archaeology, and others have the possibility of utilizing magnetic resonance techniques within the laboratory or conducting applications directly in the field. Another aspect is the possibility of utilizing existing NMR magnets which may be in good condition but unused because of outdated or nonrepairable electronics. Moreover, NMR applications based on personal computer technology may open up teaching possibilities at the college or even secondary school level. The goal of developing such a personal computer (PC)‐based NMR standard is facilitated by existing technologies including logic cell arrays, direct digital frequency synthesis, use of PC‐based electrical engineering software tools to fabricate electronic circuits, and the use of permanent magnets based on neodymium‐iron‐boron alloy. Utilizing such an approach, we have been able to place essentially an entire NMR spectrometer console on two printed circuit boards, with the exception of the receiver and radio frequency power amplifier. Future upgrades to include the deuterium lock and the decoupler unit are readily envisioned. The continued development of such PC‐based NMR spectrometers is expected to benefit from the fast growing, practical, and low cost personal computer market.

ISSN:0034-6748    

DOI:10.1063/1.1144572

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A loop made from a length of transmission line with alternate electrical discontinuities (gaps) on the two conductors behaves as a resonator whose resonant frequency is a function of the transmission line parameters as well as its form and the number (n) of gaps. For a given loop diameter, one can thus design a wide range of resonant frequency by choice ofnand the characteristic impedanceZcof the line. The analysis is illustrated by a parallel‐plate transmission line resonator in the form of a bracelet. Experimental resonant frequencies obtained from resonators of various dimensions are in good agreement with calculated values. Two resonators were tested on a 3 T NMR imager: A 11 cm diameter1H surface coil, yielding results comparable to those obtained from a lumped capacitor coil and a 13 cm diameter volume coil to performinvivomuscle13C spectroscopy without1H decoupling.

ISSN:0034-6748    

DOI:10.1063/1.1144573

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A microscope using three water immersion objective lenses which realizes confocal, 4Pi‐confocal and various confocal theta microscopies in fluorescence, transmission, scattered, and reflection mode is described. An argon‐ion laser is the primary light source. A pulsed titanium‐sapphire laser allows two‐photon absorption fluorescence microscopy. The instrument has a predicted resolution of 100 nm along the illumination axis and a three‐dimensional resolution of 5×106nm3for lenses each with a numerical aperture of 0.75. This is an improvement of an order of magnitude over a confocal fluorescence microscope using the same lens. Applications of the microscope range from observation of a sample at three different angles, to confocal theta fluorescence microscopy with multiphoton absorption. Since mounting and immersion media are identical, aberrations become negligible. The large working distance of 2 mm makes the instrument ideal for the observation of biological samples of up to 1.5 mm in diameter such as drosophila embryos.

ISSN:0034-6748    

DOI:10.1063/1.1144574

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A design is presented for a cryogenic scanning tunneling microscope which has demonstrated atomic resolution at temperatures between 300 and 120 K in ultrahigh vacuum. The design features simpleinsituexchange of samples and tips, active cooling of the sample, excellent visibility of the sample‐tip region, and the use of a thermally isolated inchworm translator for coarse approach. It has demonstrated atomic resolution on SI(111)‐(7×7) and Si(001) surfaces at 120 K.

ISSN:0034-6748    

DOI:10.1063/1.1144575

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A high‐resolution noncontact scanned probe technique has been developed for sampling the internal signals of an operating integrated circuit. The signal waveform is extracted by sensing the localized electrostatic force between a small probe and point on the circuit being measured. A heterodyne approach is used to enable the sampled measurement of high‐frequency digital or analog waveforms. In conjunction with a nulling method, the technique is capable of accurate signal measurement without complex calibration or probe positioning, and can be performed over passivated structures. Measurement of digital and analog signals is demonstrated with a voltage accuracy of less than 100 mV.

ISSN:0034-6748    

DOI:10.1063/1.1144576

出版商:AIP    

年代:1994

数据来源: AIP