摘要:

In order to apply the osmotic stress method for direct measurement of forces between membranes or between macromolecules, we have designed and built an x‐ray camera which can control the sample temperature from 0 °C to 70 °C while confining the path of diffracted x rays to an evacuated space between the sample and film. The system uses a linear feedback sensor which provides ±0.1 °C accuracy and a base‐line stability of 0.02 °C over the entire operating range. The controller uses solid‐state thermoelectric modules to regulate the temperature of the sample and is capable of automatically shifting from the heating to cooling mode of operation to regulate at temperatures near room temperature. The sample solutions are mounted between two Mylar windows in a removable cell which can be cleaned and loaded outside the instrument. The film plate is mounted on a slide which can be positioned between 4 and 22 in. from the sample. A beam stop is also mounted on the film plate holder and can be adjusted 1 in. both vertically and laterally. The x‐ray entry port also has lateral and vertical adjustments for easy alignment with the collimator. The sample cell and film plate mechanism are located within an aluminum and polycarbonate chamber for operation at pressures as low as 0.001 atm.

ISSN:0034-6748    

DOI:10.1063/1.1139472

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Calibration of a Fenn‐type nozzle beam source and the limitations due to background attenuation, skimmer interference, and condensation are discussed. The nozzle flow rateNnis calculated, and the peaking factor &kgr; is determined from both radial pressure surveys and effusive‐to‐supersonic transition measurements. Stage pressure measurements verify bothNnand &kgr;. These quantities specify the ideal beam flux in the absence of attenuation, interference, or condensation. Background attentuation depends on the effective scattering cross section, which can be quite large for finely collimated beams. Serious skimmer interference occurs below a critical value of the skimmer Knudsen number and depends on individual skimmer details. Condensation is observed and found to be predictable according to the known scaling laws. A calculation of absolute beam fluxes is presented. Nonideal behavior of the speed ratio and average particle velocity are also examined. Data are given for the gases H2, He, Ne, and Ar.

ISSN:0034-6748    

DOI:10.1063/1.1139473

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Deflection of3S1and3P2metastable rare‐gas atoms in an electron‐impact‐excited thermal supersonic expansion by an inhomogeneous magnetic field produces well‐defined daughter beams for eachMstate. The deflection patterns are spatially resolved with a translatable detector and used to derive fine‐structure populations in the3P0and3P2states, and to measure absolute efficiency of state selection by optical depletion. The fine‐structure population ratio3P2/3P0is measured for electron excitation energies from 100–400 eV, showing a 20%–40% decline with increasing energy for all the rare gases. Radiative decay due to perturbation by the magnetic field appears to be negligible, but theM‐state‐resolved populations are unequal and asymmetric, perhaps due to Majorana transitions at the entrance to the field. The method may also be useful in polarized angular momentum studies of metastable atom–molecule collisions.

ISSN:0034-6748    

DOI:10.1063/1.1139474

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Design considerations and initial results are presented for a new type of time‐of‐flight photoelectron spectrometer which is particularly suited to the study of cold metal and semiconductor cluster anions prepared in a supersonic molecular beam. The desired cluster is extracted from the molecular beam, mass‐selected after an initial time‐of‐flight, and decelerated as it enters the photoelectron spectrometer. Photoelectrons ejected from the cluster by an ArF excimer laser are collected with >98% efficiency in an intense pulsed magnetic field of carefully controlled divergence. This divergent field parallelizes the photoelectron trajectories and maps smoothly onto a low, uniform magnetic field which guides the electrons along a 234‐cm flight tube leading to a microchannel‐plate detector. The strong magnetic fields and simple, open design provide excellent rejection of stray photoelectrons in a clean, ultrahigh‐vacuum environment. The UPS spectrum of Si20−is given as an example.

ISSN:0034-6748    

DOI:10.1063/1.1139475

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The pseudorandom method for time‐of‐flight analysis has the advantage over the conventional method of a greatly improved efficiency, but, according to previous theory, the disadvantage that the time resolution is worse. Here we present an improved theory of pseudorandom modulation. We show that the resolution of this method and the conventional method are identical. This conclusion is in agreement with experimental results. The difference between the previously given and present treatment of the pseudorandom method for time‐of‐flight analysis is discussed. We show that the treatment given previously does not apply to a possible, real experiment, but only to the nonphysical case of an infinitely good time resolution. Second, we show that the additional broadening which was found previously for the pseudorandom method is not inherent to the method, but is due to a nonoptimal choice of the deconvolution function.

ISSN:0034-6748    

DOI:10.1063/1.1139476

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

This paper describes the design of the first calorimeter capable of measuring heats of adsorption of gases on single‐crystal surfaces in ultrahigh vacuum. The calorimeter is of the quasi‐adiabatic type, and measures the temperature rise as gas adsorbs. The calorimeter has been used to measure the heat of adsorption of CO and C2H4on Pt(111). The calorimeter seemed to perform quite well in these experiments, and yielded results that are reasonable. Thus, it is suggested that a calorimeter would be a useful addition to a surface scientist’s arsenal.

ISSN:0034-6748    

DOI:10.1063/1.1139477

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The design, construction, and performance of a new high‐sensitivity dc SQUID magnetometer, covering a circular area of 93‐mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first‐order gradiometers, located on a spherical surface of 125‐mm radius and with the symmetry axis tilted 30° with respect to the vertical. The pickup coil diameter is 20 mm, and the channels are separated by 36.5 mm from each other in a hexagonal array. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 fT/(Hz)1/2, mainly limited by the thermal noise in the radiation shields of the Dewar. The optimization of the coil configuration and the measurement system is discussed in detail, and a system to determine automatically the position and orientation of the Dewar with respect to certain fixed points on the subject’s head is described. Finally, some examples of measurements carried out with the new device are given.

ISSN:0034-6748    

DOI:10.1063/1.1139478

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A new method is reported for calibrating bremsstrahlung spectra in the 200–1000‐keV range with x‐ray activation techniques. This approach is particularly significant for pulsed sources that cannot be characterized with conventional spectroscopy methods.

ISSN:0034-6748    

DOI:10.1063/1.1139661

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A fiber heterodyne interferometer probe free from external disturbance, which can measure small displacement of an object as a pressure transducer or a vibrational object, is described. The phase fluctuation in the fiber due to external disturbance is excluded using a reference beat signal of light beams from a reflection mirror on a graded index rod lens. A graded index‐type multimode fiber connected to one end of a single‐mode fiber by arc fusion splicing is used to improve acceptance of the light. The characteristics of the fiber probe are also described.

ISSN:0034-6748    

DOI:10.1063/1.1139479

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The detector of the IMST/SA‐CNRS improved Lyman‐alpha hygrometer is a solar‐blind photomultiplier tube that ensures large signal‐to‐noise ratio and allows a miniaturization of the sampling volume. This PM tube has a relatively large spectral response and can detect, in addition to the Lyman‐alpha line, hydrogen molecular transitions emitted by the discharge source between 120 and 180 nm. In order to avoid a spurious sensitivity to oxygen density fluctuations, an active filtering cell is added to absorb the undesired lines. The instrument sensitivity is calculated with the help of a two‐gas, three‐spectral‐line UV‐beam absorption model, fitted to calibration measurements. The technique of the active filtering associated with the three‐line absorption model produces the following advantages: (1) reduced sensitivity to oxygen fluctuations, (2) a clear physical meaning to the model terms allowing the sensitivity computation in most conditions, (3) increased signal‐to‐noise ratio, and (4) in most conditions, improved sensitivity to water vapor compared to the monochromatic Lyman‐alpha line.

ISSN:0034-6748    

DOI:10.1063/1.1139480

出版商:AIP    

年代:1987

数据来源: AIP