摘要:

We present a new design for an autocorrelator to measure pulsewidths of picosecond light pulses. This device gives a real time display of the pulsewidths, enabling one to align precisely synchronously‐pumped mode‐locked dye lasers for optimum performance.

ISSN:0034-6748    

DOI:10.1063/1.1136593

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Avalanchep–nphotodiodes with uniform breakdown over the junction area are known to be capable of single‐photon detection. An experimental study has been performed on the temporal resolution of these single‐photon avalanche diodes (SPADs) in measurements of the shape of ultrashort light pulses. By using an active‐quenching circuit, pulsed operation of the device has been obtained in accurately controlled conditions, avoiding spurious effects met in previous passive‐quenching circuits. Laser pulses with durations down to 150 ps FWHM have been used; the results obtained show that the resolution is remarkably better than this value. Performances and limitations of SPADs are discussed; temporal resolutions of a few tens of picoseconds may be expected.

ISSN:0034-6748    

DOI:10.1063/1.1136594

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The operation of a proportional counter designed for resonance–electron Mo¨ssbauer spectroscopy measurement at high temperatures up to 900 °C has been investigated. In order to avoid insulation difficulties due to electric leakage on the surfaces of heated insulators, the counter is carefully constructed so that the sensitive volume of the counter can be warmed up while keeping the insulators at room temperature; all insulating materials between the anode and cathodes are located outside the electric furnace in the counter system. The counter operation is influenced by thermal electrons emitted from the cathode material; the maximum working temperature is about 800 °C when an a‐Fe foil is mounted as a cathode while operation at 900 °C is possible without the sample.57Fe Mo¨ssbauer spectra for surface layers (∼1000 A˚) have been sucessfully obtained for samples of Fe3C, Fe2O3, and a Fe. Chemical changes of the sample surfaces have been observed at temperatures greater than 600 °C by the measurements. The high‐temperature counter provides us with a new method to observe directly surface phenomena at high temperatures. Some features of the counter system are described.

ISSN:0034-6748    

DOI:10.1063/1.1136595

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The measurement of polarization of the fluorescence of solutions under high pressure is rendered uncertain by the photoelastic birefringency of the windows, and needs a correction dependent upon the fraction of the light emerging from a window with polarization normal to that of the incident light (scrambling coefficient). This correction, which increases rapidly with applied pressure, differs according to whether the method of measurement employs rotation of the excitation polarizer (T format) or the emission polarizer (L format), and the latter is shown to involve the more reliable and the smaller correction. A four window pressure bomb housing a 1 ml volume cuvette that permits absorption and fluorescence measurements up to 4 kbar is described. The polarization correction up to 2.5 kbar was determined. Its reliability was demonstrated by measurements of the fluorescence polarization spectra of fluorophores in solvents of high viscosity at all pressures and by comparison of isopiestic and isothermal measurements of a fluorophore in isobutanol, a fluid solvent with viscosity that changes rapidly with pressure or temperature.

ISSN:0034-6748    

DOI:10.1063/1.1136596

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A simple, inexpensive spectrophotometric cell was designed for use with aqueous solutions for which temperature is a maximum of 325°C and pressure, 28 MPa. The cell has an internal volume of 5 ml and a path length of 1.31 cm. Each furnace assembly is 120 mm in diameter × 150 mm high and will fit into most commercial spectrophotometers. Temperature is controlled by a standard set‐point controller and a balancing circuit that is used to maintain the temperature of the sample and reference cell within 1°C of each other at any temperature.

ISSN:0034-6748    

DOI:10.1063/1.1136597

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We describe an ozonosonde based on a homogeneous chemiluminescent reaction of olefins with ozone, allowing ozone measurements at all altitudes accessible to stratospheric balloons. The ozone–olefin reaction, which is bimolecular, gives a signal proportional to ozone mixing ratio. Typical response at 100 mb (∼16 km) with ethylene is 104counts s−1ppm−1. With 2‐methyl 2‐butene, one can extend the measurements to 1 mb. At 2 mb (43 km), we typically find 2×104counts s−1ppm−1. The time constant is 1 s, and the statistical noise of the reaction (∼1%) allows the use of the instrument to make accurate relative measurements for dynamical studies of the stratosphere. Two versions of the instrument have been flown successfully six times. We present a flight profile between 200 mb (∼12 km) and 12 mb (∼30 km), and also measurements at 2.5 mb (41 km) float level.

ISSN:0034-6748    

DOI:10.1063/1.1136598

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The molecular beam epitaxy (MBE) crystal growth technique utilizes thermal molecular beams which are stabilized by constant temperature control and controlled manually by metal shutters. Through the addition of an automated programmable shutter control system, the capabilities of the MBE system are enhanced. Two examples of automatic control of the shutters to grow epitaxial GaAs–AlGaAs layers are described. By changing the simple settings on the control units, the growth schedule can be easily altered to allow for the optimization of crystal structures used for innovative devices and basic research.

ISSN:0034-6748    

DOI:10.1063/1.1136599

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The low ultrasonic attenuation of fused quartz in the very high frequency range makes it attractive for an ultrasonic delay line. However, the temperature instability restricts its application. A method of stabilizing the delay time based on the properties of a positive temperature coefficient resistor that is used simultaneously as a temperature sensor and a regulator is discussed. This new type of temperature regulator improves the temperature coefficient of the delay time from 85 ppm/°C to 8 ppm/°C. A temperature‐stabilized wide‐band delay line has been fabricated for high definition television. A wide‐band frequency response is obtained by using LiNbO3transducers and new type bonding adhesion which is stiffened by irradiation of ultraviolet rays. The design of the delay line, which has a midband frequency of 45 MHz, a band width of 50 MHz, and a delay time of 29.55 ms, is presented and experimental results are also presented.

ISSN:0034-6748    

DOI:10.1063/1.1136600

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Design possibilities of single and double Helmholtz coils of rectangular cross section have been investigated for the purpose of generating magnetic fields which have the same or even better uniformity as compared to that achieved with the idealized coils of negligible winding cross section. It is shown that the optimum dimensions of coil configuration can be determined by means of numerical analysis, even under such practical conditions that the ratio of thickness to width of winding cross section and the turn ratio of the coil pairs should be ratios of relatively small integers. It is also suggested that the balancing method provides a possible means of extending the uniform region one and a half times as much as that achieved with the idealized coils of negligible winding cross section.

ISSN:0034-6748    

DOI:10.1063/1.1136609

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The current measured during the back step of a double potential step chronoamperometry experiment at microvoltammetric electrodes is primarily nonfaradaic and can be used for residual current correction. An instrument has been designed which provides direct readout of the corrected current. This instrument also provides differential output and adjustable delay and step times which can be important for the unusual conditions ofinvivovoltammetric experiments. All of these features are tailor made for use with microvoltammetric electrodes and are unavailable in commercial instrumentation. Experimental results indicate that potential step times as short as 100 ms can be used with back step correction with a faradaic accuracy of better than 5%. Pulse scans are compared to conventional cyclic voltammetry in the reduction of 5×10−5M potassium ferricyanide.

ISSN:0034-6748    

DOI:10.1063/1.1136610

出版商:AIP    

年代:1981

数据来源: AIP