摘要:

The motion of ions evaporating in the subnanosecond period before a pulse has matured is invoked in the interpretation of resolution limiting energy deficits in the TOF atom‐probe. The second order, nonlinear differential equation of motion in the time and space dependent field is numerically solved by computer. Two shapes of the pulse front, in which the final voltage level is approached either from below or from an overshoot are considered. The latter case accounts for the actual observations by giving the proper magnitude and mass dependence of the energy deficits.

ISSN:0034-6748    

DOI:10.1063/1.1686807

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Energy deficits of field evaporated ions which limit the mass resolution of the straight TOF atom‐probe are measured by incorporating a 90° deflecting energy discriminator. The observed dependence on the mass‐to‐charge ratio shows premature evaporation during the less than perfect subnanosecond pulse front to be the cause of the energy spread. For narrow beam apertures the mass resolution may be improved by tilting the detector plane to provide a compensating shorter path for the slower ions. For wider apertures an energy focusing combination of straight path sections and a 163° toroidal deflector, as conceived by Poschenrieder, has been adapted to our atom‐probe. Offering a mass resolution of better than 1/1000 and complete rejection of artifacts, the compensated atom‐probe FIM is now a microanalytical tool of ultimate sensitivity and high reliability.

ISSN:0034-6748    

DOI:10.1063/1.1686808

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The measurement of the phase fluctuations caused by spontaneous emission in the oscillation of a He&sngbnd;Ne laser operating at 1.15 &mgr; is reported. The frequency fluctuations in the signal resulting from heterodyning two independent modes in the laser were measured asf2(&ngr;) =(9.6±2)×10−2Hz2/Hz for 1.65×107stored photons per mode andf2(&ngr;) =(2.7±0.3)×10−1Hz2/Hz for 6.2×106stored photons per mode. These values are to be compared with the theoretical minima for these cases, which are calculated to bef2(&ngr;) =5.1×10−2Hz2/Hz andf2(&ngr;) =1.4×10−1Hz2/Hz, respectively.

ISSN:0034-6748    

DOI:10.1063/1.1686809

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A procedure for obtaining Fourier transform (FT) NMR spectra on a Perkin‐Elmer model R ‐20B spectrometer is described. The procedure requires no changes in the basic spectrometer, so that changing between continuous wave (cw) and FT modes can be performed simply. A stable radio‐frequency source, a power amplifier, a pulse gate, and an FT data system are required.

ISSN:0034-6748    

DOI:10.1063/1.1686810

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A three‐element radio interferometer can measure the complex ratio of spatial‐frequency components in the presence of atmospheric phase fluctuations, even when the interference fringes are badly distorted by the fluctuations. The measurement is based on the principle that the phase‐modulated fringes of two interferometers will have essentially identical envelopes if the interferometer baselines are not greatly different. A least‐squares fitting process determines the amplitude ratio and phase difference with variances only slightly higher than would be obtained in the absence of phase fluctuations. Three applications of the basic technique to measuring a spatial‐frequency spectrum are discussed.

ISSN:0034-6748    

DOI:10.1063/1.1686811

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The unique features of pulse counting coupled with anodic current measurement by an electrometer are used to determine photomultiplier gain accurately under actual experimental conditions.

ISSN:0034-6748    

DOI:10.1063/1.1686812

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

An electrometer is employed to investigate dead time effects in scaling and multiscaling. The electrometer method as presented gives accurate values for the dead time over a wide range of counting rates. An expression is also developed for the ``between channel'' dead time of a multiscaler, and a simple method of determining this time is also given. The technique as outlined allows the exact determination of the nonlinearity caused by dead time losses.

ISSN:0034-6748    

DOI:10.1063/1.1686813

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

An on‐line system to measure correlation functions in a plasma is described. By combining a fast buffer memory and a digital computer, the system can analyze signals with up to 4 MHz bandwidth and has direct applications to pulsed plasmas. Hardware and software for the system are described together with a few applications.

ISSN:0034-6748    

DOI:10.1063/1.1686814

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

We have constructed a temperature controller utilizing a thermistor in a high impedance ac Wheatstone bridge. It maintains the temperature of a 1.3 liter volume constant to within ±15 &mgr;°C, over a 40 h period. The controller employs a ratio transformer, a simple homemade preamplifier, a commercial lock‐in amplifier, a small bipolar power amplifier, and a thermoelectric device. Access to the controlled region can be obtained in a few seconds.

ISSN:0034-6748    

DOI:10.1063/1.1686815

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

An instrument employing several novel features has been assembled for the measurement of magnetic and natural circular dichroism (MCD and CD) in the visible, ultraviolet and near infrared spectral regions. The instrument is fitted with two magnets: an electromagnet for routine use and a superconducting magnet for use when higher performance is required. The contributions of CD and MCD are separated by measuring the net spectrum twice; first with the magnetic field parallel to the Poynting vector of the optical beam and then with it antiparallel. The value of the net optical activity for each wavelength increment is converted to digital form and the manipulations required to separate CD and MCD and signal averaging are performed by a computer. The data can be plotted in a variety of formats on a Cal Comp plotter. The instrument is equipped with a Cary model 14 prism‐grating monochromator which, unlike double quartz prism monochromators, has good wavelength dispersion from the uv to the near ir spectral regions. Performance in the ultraviolet has been considerably improved by replacing the previously used electro‐optical modulator (Pockels cell) with a quartz strain‐birefringence modulator. The capabilities of this instrument are illustrated by spectra for the natural CD of ribulose‐1,5‐diphosphate carboxylase at 22° from 195 to 245 nm, the MCD of metmyoglobin cyanide at −196° and 22° from 340 to 470 nm, and the MCD of oxyhemoglobin at 4° from 700 to 1100 nm.

ISSN:0034-6748    

DOI:10.1063/1.1686816

出版商:AIP    

年代:1974

数据来源: AIP