摘要:

A versatile saturation recovery accessory based on a small, special‐purpose timing controller and an efficient mix of coaxial and waveguide microwave components has been added to a commercial electron paramagnetic resonance (EPR) spectrometer. The spectrometer was designed for study of the spin lattice relaxation time of transition metals and free radicals over a wide temperature range, such as are of interest in materials science and metallobiochemistry. A microprocessor‐based timing system was designed to provide a wide dynamic range with a simple user interface. Waveguide phase shifters and rotary vane attenuators were used for their precision and resetability, while coaxial components were used where their superior performance could be exploited. Sensitivity is provided by a low‐noise GaAs field‐effect transistor (FET) microwave preamplifier and a double‐balanced mixer (DBM) detector. A biphase modulator (±180° phase shifter) on the lo side of the DBM provided efficient addition/subtraction of successive recovery curves, obviating slow data transfer to a computer prior to the arithmetic operations. Because the system is designed for the study of inherently broad spectra, it does not use the magnetic field step modulation of some prior spectrometers. In addition, although most EPR spectrometers have been designed with a phase shifter in the reference arm, we have found it more convenient to keep the reference arm phase constant and adjust the phases of the pump and observe arms of this three‐arm bridge spectrometer. The saturation recovery system described here could be implemented on any commercial EPR spectrometer with only minor modifications.

ISSN:0034-6748    

DOI:10.1063/1.1143722

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The paper describes an electron paramagnetic resonance apparatus for spectroscopy and imaging at very low frequency (283 MHz). The bridge operates in a reflection cavity homodyne configuration and can be used on a very broad frequency range. The sample cavity is a one loop‐two gap resonator and accepts samples up to 5 cm in diameter and 10 cm long. These sample dimensions make the apparatus suitable for observing living samples such as rats. For the main field and gradients the apparatus uses a newly designed multipolar magnet that can provide the main field and gradients for 2D image reconstruction. The third gradient is obtained by coils mounted in the bore of the magnet. Tests on peroxilamine disulfonate samples have shown micromolar sensitivity. The minimum spatial resolution is limited by the sample linewidth and by the signal/noise. In living samples the toxicity of nitroxide radicals restricts the maximum label concentrations to values of the order of 10−4M, and the resolution seems to be limited by sensitivity to fractions of cm3.

ISSN:0034-6748    

DOI:10.1063/1.1143723

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A cw nuclear quadrupole resonance spectrometer centered on the superregenerative oscillator and working in the range 20–40 MHz is described. Both automatic gain control and sideband suppression have been incorporated by using external quench waveforms of variable mark‐to‐space ratio. Extensive use of ICs in these circuits leads to ease of construction as well as increased reliability. The S/N obtained remains unaffected by the automatic gain control while the sideband suppression does give rise to a small degradation. The spectrum of NaClO3has been recorded.

ISSN:0034-6748    

DOI:10.1063/1.1143724

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An ion trap/time‐of‐flight (TOF) mass spectrometer combination has been developed in order to combine the storage capabilities of an ion trap with the speed and resolution of a time‐of‐flight device. The ion trap is an rf quadrupole trap which operates in the total storage mode, i.e., with the dc voltage=0 on the end caps and rf voltage on the ring electrode. The trap has an ion storage time of ≥2 s at an rf potential of 310 Vpp. The stored ions are ejected into the time‐of‐flight device using a −150 V dc pulse on the exit end cap which causes the ion trajectories in the trap to become unstable. The ions are mass analyzed using either a linear or reflectron TOF. In the linear mode the resolution is 240 while in the reflectron mode a resolution of 1300 atm/z93 is achieved. The storage capabilities of this device may have important applications towards enhancing sensitivity, the study of very slow metastable decay, and photodissociation mass spectrometry and spectroscopy.

ISSN:0034-6748    

DOI:10.1063/1.1143725

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Here we describe the design, construction, and operation of an ultrahigh‐vacuum manipulator for combined molecular beam and scanning tunneling microscopy experiments. The manipulator allows (1) sample transfer, (2) liquid‐nitrogen cooling, (3) electron beam heating, (4) rotation about the vertical and horizontal axes, and (5) electrical isolation of the sample. The unique features of the design are (a) direct transfer of a 3/8‐in.‐diam sample to a load‐lock sample carrier without the use of a second transfer arm and (b) sample mounting and electron beam heating systems that permit the sample to be several hundred degrees hotter than the sample mounting plate, thus minimizing sample contamination.

ISSN:0034-6748    

DOI:10.1063/1.1143726

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A new instrument bridging the gap between atomic force microscopes (AFMs) and stylus profiling instruments is described. Theconstantforceprofileris capable of subnanometer resolution over a 15‐&mgr;m vertical range with a horizontal traverse length of 50 mm. This long traverse length, coupled with the possibilities of utilizing standard radius, diamond measurement styli, make the force profiler more compatible with existing profiling instrument standards. The forces between the specimen and a diamond stylus tipped cantilever spring are sensed as displacements using a capacitance bridge. This displacement signal is then fed through a proportional plus integral controller to a high stability piezoelectric actuator to maintain a constant tip‐to‐sample force of approximately 100 nN. Much of the sensor head and traverse mechanism is made of Zerodur glass‐ceramic to provide the thermal stability needed for long travel measurements. Profiles of a 30‐nm silica step height standard and an 8.5‐&mgr;m step etched on Zerodur are presented.

ISSN:0034-6748    

DOI:10.1063/1.1143727

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The design and implementation of a flexible and modular digital control and data acquisition system for scanned probe microscopes (SPMs) is presented. The measured performance of the system shows it to be capable of 14‐bit data acquisition at a 100‐kHz rate and a full 18‐bit output resolution resulting in less than 0.02‐A˚ rms position noise while maintaining a scan range in excess of 1 &mgr;m in both theXandYdimensions. This level of performance achieves the goal of making the noise of the microscope control system an insignificant factor for most experiments. The adaptation of the system to various types of SPM experiments is discussed. Advances in audio electronics and digital signal processors have made the construction of such high performance systems possible at low cost.

ISSN:0034-6748    

DOI:10.1063/1.1143728

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have designed and constructed a high‐performance, low‐noise digital controller for ‘‘inchworm’’ piezoelectric translators. The controller features a maximum output voltage of 700 V, an rms noise level of less than 2 mV, and is capable of driving inchworms at speeds of up to 0.23 mm/s. The controller offers substantially better performance in terms of resolution, noise, speed, and cost than commercial controllers, and is suitable for atomic resolution scanning tunneling microscopy and atomic force microscopy applications.

ISSN:0034-6748    

DOI:10.1063/1.1143729

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

New ways to measure detection properties of slow‐scan charge‐coupled device (CCD) cameras suitable for electron microscopy, mainly based on the statistics of single‐electron events, are discussed. The experiments concentrate on the newly introduced Gatan 679 slow‐scan CCD camera. It has been established that for this instrument (if equipped with a thin YAG scintillator) 20% of the true intensity is recorded at the highest detectable spatial frequency (Nyquist frequency), limited mostly by the modulation transfer function of the YAG scintillator. The detection quantum efficiency is 0.45 for single 100‐kV electrons and 0.15 for single 400‐kV electrons, and is approaching unity for intensities higher than ten electrons. Furthermore, nonlinearity of the response is smaller than deviations in the image intensity due to shot noise. Examples are presented illustrating the detection properties of slow‐scan CCD cameras for electron imaging, which also include high dynamic range and negligible geometric distortion.

ISSN:0034-6748    

DOI:10.1063/1.1143730

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A number of optical streak cameras are routinely operated for experiments at the Lawrence Livermore National Laboratory Nova laser facility. We report results from a study of the accuracy and reproducibility of the sweep rate. The sweep rates, measured using a 3 GHz pulse generator, vary by ±1.3% rms for data collected routinely over a several month period. Using statistical analysis, we develop an analytic model describing the effect of noise on our measurement technique in order to estimate the actual variation in the sweep rate. Numerical simulations of the effect of noise on the uncertainty of our measurement technique show close agreement with the model. By applying this model, we estimate that the sweep rate is reproducible to at least ±1.2%, and that the measurement technique contributes ±0.5% uncertainty. Other data are presented which support this estimate.

ISSN:0034-6748    

DOI:10.1063/1.1143731

出版商:AIP    

年代:1992

数据来源: AIP