摘要:

A laser‐induced ionization scheme based on laser‐metal interaction is described. It is found that when a low power, pulsed UV laser beam is directed to the repeller plate of a Wiley–McLaren time‐of‐flight mass spectrometer (TOFMS), which is held at muchhigherpositive potential than the extraction grid, extensive ionization of gas phase species can take place. For monatomic species such as Ar and Xe, multiply charged ions up to Ar6+and Xe9+are obtained. For both aromatic and aliphatic organic molecules, classical electron‐impact‐type mass spectra are observed. Mass resolution is in the range of 300–400, comparable to that obtained using laser‐induced resonant two‐photon ionization (R2PI) in the same linear TOFMS. It is shown that this technique can be quite efficient in ionizing organic species. A detection limit of 5 ng and a linear dynamic range of four orders of magnitude for benzene are demonstrated. While not as sensitive as R2PI, it has a much wider applicability and more uniform ionization efficiency than R2PI. It also appears about 1000 times more sensitive than the incorporation of an electron gun in TOFMS. The technique can be interchanged with R2PI very readily by simply adjusting the position and focus of the laser beam. While the ionization mechanism involved in the technique is currently unknown, some preliminary investigation of the ionization process is reported. It is suspected that photoelectron generation from the plate surface may play some important role in producing the ions detected. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146351

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A novel Fourier transform ion cyclotron resonance ion trap consists of two conventional solid ‘‘end cap’’ electrodes and four ‘‘filar’’ electrodes, each composed of two interdigitated ‘‘combs.’’ The filar trap provides for ion dipolar excitation and ejection, dipolar detection, quadrupolar axialization, and various combinations in a single trap without additional voltage divider circuitry and signal switching between operating modes, because two different functions can be conducted simultaneously by use of two spatially multiplexed electrodes. The filar electrodes produce simulated dipolar and quadrupolar potential fields which are nearly indistinguishable (near the trap center) from those produced by solid electrodes. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146312

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A technique has been developed to determine the absolute single particle counting efficiency of a channel electron multiplier (CEM) for positively charged ions at keV energies. The calibration technique is applicable to positive ion detectors in general. Ion beam currents of C2+were created by charge transfer of C3+on H2in the thin target regime. By adjusting the H2pressure in the beam scattering chamber from 10−10to 10−6Torr, C2+currents could be created either low enough to be detected by the CEM in the particle counting mode or high enough to be measured as a current using the CEM as a Faraday cup. The CEM counting efficiency was determined by comparing the C2+count rate to the C2+current, scaling by the change in H2pressure, and also scaling by the incident C3+current for each C2+measurement. This method, which effectively uses the charge transfer cross section as a ‘‘transfer standard,’’ allows the CEM counting efficiency to be determinedinsituand to be monitored accurately over extended periods of time. The calibration requires that only the relative, not the absolute, pressure change be known. The technique does not require the value of the charge transfer cross section to be known. The accuracy of the calibration technique for the present results was 10% at a confidence level considered to be equivalent to a statistical 90% confidence level. For the present work, the peak counting efficiency of a Galileo 4039 CEM for 32.5 keV C2+ions was determined to be 96%. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146531

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A newly designed spectrometer is presented, which is eminently suited for the measurement of electron velocity distributions in the sub‐eV electron energy region. The application of this spectrometer is demonstrated in an ion‐atom collision experiment, using a time‐of‐flight technique. As an example, the low‐energy electron spectrum of 4 keV ionizing H+−H2collisions is shown. ©1995 American Institute of Physics. 

ISSN:0034-6748    

DOI:10.1063/1.1146272

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A semiempirical method of calibrating a thinned, backside illuminated charge coupled device (CCD) chip in the soft x‐ray region is presented. It is based on determining the thickness of the dead layer self‐consistently using the continuum emission from laser produced plasmas. The CCD camera system was coupled to a transmission grating spectrometer and recorded the spectrally resolved continuum emission from laser irradiated tungsten targets. The thickness of the dead layer was then determined by comparing the experimental spectra with the calculated quantum efficiency for a thinned CCD using a simplified model. In this way the CCD chip was semiempirically calibrated. The accuracy of the calibration in the soft x‐ray range was assessed by comparing the CCD recorded spectra with those recorded by a spectrometer using the absolutely calibrated Kodak 101 photographic plates and a similar transmission grating. Based on this calibration, the CCD sensitivity is deduced to be about two orders of magnitude higher than that of the Kodak plates in this wavelength range. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146228

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An atomic force microscope has been constructed which enables us to observe sample surfaces through a metallurgical optical microscope. The cantilever deflection is detected by a polarization common‐path interferometer with optical heterodyne technique. The optical system of the interferometer is incorporated in the commercial optical microscope. The common‐path interferometer is rather insensitive to the effects of environmental disturbance such as thermal drift and mechanical vibration. The sample surface and the cantilever are observed by a microscopic objective of ×20. We can adjust the interferometer easily by observing both the cantilever and laser beams. The detection system has a total rms noise of 0.35 A˚ in a frequency bandwidth of 0.5–200 Hz on a free Si3N4cantilever of 0.1 N/m spring constant. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146192

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Design details and initial results are presented for a low‐temperature scanning tunneling microscope specifically intended for measurements of ballistic‐carrier transmittance through heterostructures. The basic design is of the Besocke type, modified for ballistic electron emission microscopy and spectroscopy (BEEM). This instrument is the first to acquire BEEM spectra below 77 K. Salient features are (1) operation in a liquid‐helium storage Dewar to below 6 K, (2) a lateral positioning range of 5 mm at low temperature, and (3) lateral drift rate less than 0.2 nm/h at the lowest temperatures. For BEEM spectroscopy, the microscope’s high positional stability allows extended signal‐averaging at a single location on the sample. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146206

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

To achieve both high stability for scanning tunneling microscopy (STM) measurements and well‐defined tunnel current localization even on very rough surfaces, we have developed a new electrochemical procedure using CaCl2etching and H2SO4micro polishing technique to obtain a very reproducible tip geometry. The mean curvature radius is about 50 A˚. The contamination‐free platinum tips are usable for a long time as well in air as in ultrahigh vacuum. The tip quality has been tested by STM measurements on gold (111) surfaces and on liquid‐crystal films. Tips are stable and provide good STM image in far less time than commonly used tips. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146153

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new tip height control system for noncontact scanning force microscope (SFM) utilizing the phase‐lock technology is proposed. Its most extensive feature is that the system is unconditionally stable whereas the conventional control systems have difficulties in stability especially when the tip is at close proximity to the sample surface. In this new system, an oscillator using the cantilever as a mechanical resonator is used as a voltage controlled oscillator (VCO) in a phase‐locked loop. Its output signal phase is locked to a high precision frequency source. The force gradient detected by the cantilever equivalently alters the VCO control voltage, which is detected as a compensational signal of the loop filter output. The phase‐lock technique made it possible to reduce the tip height to an extent which was impossible with the conventional servo technology. Thus this new control technique is expected to be effective in improving the resolution of the noncontact SFM. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145274

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report on the design and testing of an ultrasensitive, electromechanical transducer for use on resonant mass gravitational wave antennae. The transducer is a superconducting, radio frequency resonant bridge circuit operating near 200 MHz. We have minimized several important sources of noise in this transducer system. The Johnson noise of the transducer circuit is reduced through using a superconducting niobium stripline circuit and low‐loss insulating materials. At a temperature of 4.2 K we have achieved unloaded electrical quality factors of 200 000. The bridge circuit is balanced by piezoelectric actuators which control the spacing between the proof mass and capacitive segments of the stripline circuit and we have achieved a residual bridge imbalance of 3×10−7. Finally, low noise cryogenically cooled field‐effect transistors are used for the first amplifier stage, enabling us to obtain an amplifier noise level which is 5400 times the quantum limit. The transducer, which has a 0.080 kg proof mass, was affixed to the end of a prototype, resonant bar, gravitational wave antenna with a mass of approximately 100 kg. The primary purpose of this small antenna was to evaluate the transducer, which is designed to be mounted on a much more massive antenna. Our theoretical analysis and measurements of the detector agree and indicate a burst noise temperature of 1.8 K using the 100 kg bar. This corresponds to a gravity wave burst sensitivity ofh=1.1×10−16, in terms of relative strain amplitude. With no other improvements, if the transducer mechanically resonant frequency were tuned to and installed on a 2000 kg antenna, the antenna would reach a noise temperature of 1.3 mK, which is equivalent to a gravitational wave burst sensitivity,h≊5.7×10−19. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145275

出版商:AIP    

年代:1995

数据来源: AIP