摘要:

Recent advances in sources, optics, and detectors for soft x rays have led to rapid developments in applications to microscopy. The history of the field and the advantages of soft x rays for the examination of biological specimens are briefly described. Limitations in resolution and radiation induced damage are mentioned before embarking on a survey of the various approaches in use. Contact microscopy is the highest resolution technique available today. It is particularly powerful when used with flash sources for stop‐action imaging. Imaging with Fresnel zone plates provides direct magnification, high resolution, and short exposure times. Scanning microscopy leads naturally to digitally recorded images, with minimal radiation exposure to the specimen. The focused beam for scanning can be generated with multilayer mirrors, or with zone plates. This last approach is treated in greater detail. Extensions of soft x‐ray imaging to three dimensions, and to elemental mapping, are considered as important areas under active development.

ISSN:0034-6748    

DOI:10.1063/1.1138464

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A rigorous and convenient method of correcting for the wavelength variation of the instrument response function in time correlated photon counting fluorescence decay measurements is described. The method involves convolution of a modified functional formF˜sof the physical model with a reference data set measured under identical conditions as the measurement of the sample. The method is completely general in that an appropriate functional form may be found for any physical model of the excited state decay process. The modified function includes a term which is a Dirac delta function and terms which give the correct decay times and preexponential values in which one is interested. None of the data is altered in any way, permitting correct statistical analysis of the fitting. The method is readily adaptable to standard deconvolution procedures. The paper describes the theory and application of the method together with fluorescence decay results obtained from measurements of a number of different samples including diphenylhexatriene, myoglobin, hemoglobin, 4’, 6‐diamidine–2‐phenylindole (DAPI), and lysine–trytophan–lysine.

ISSN:0034-6748    

DOI:10.1063/1.1138457

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The fluorescence spectra of excited free radicals have been recorded using a pulsed laser to generate these reaction intermediates in the modified cell compartment of a commercial gated spectrofluorimeter. The technique, illustrated with data for the diphenylmethyl and diphenylhydroxymethyl radicals can also be applied to the study of the kinetics of free radical reactions.

ISSN:0034-6748    

DOI:10.1063/1.1138465

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The construction of a versatile, single‐beam spectrophotometer expanded from a previously reported circular dichrometer utilizing an elasto‐optic modulator (EOM) for producing circular polarization, is described. The new spectrophotometer is capable of measuring circular dichroism as well as linear dichroism and light‐induced changes in absorbance and fluorescence yield, the latter being two of the most commonly measured properties in photosynthesis research. The use of an EOM, along with a phase‐sensitive detector, affords high sensitivity and at the same time, in conjuction with a mechanical phosphoroscopic device, a high degree of immunity from artifacts in studies employing actinic light. A description is given of the generation and use of timing signals essential for control and signal processing. Its performance is illustrated by examples drawn from light‐induced absorbance and fluorescence‐yield changes associated with photosynthetic electron transport and changes in redox state of electron carriers. This photometer, with very little modification, may also be used to perform a host of other types of measurements.

ISSN:0034-6748    

DOI:10.1063/1.1138466

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The Mueller matrix approach is used to analyze the problems associated with the calibration of a modern CD spectropolarimeter constructed on the basis of a modulation technique in polarization spectroscopy. As a result, the performance of the CD spectropolarimeter is found to be determined by the characteristics of the photomultiplier as a partial polarizer, the residual static strain birefringence of the modulator, and the first harmonic response of a lock‐in amplifier. Furthermore, the CD signal is greatly affected by the circular birefringence of a sample in addition to the above three factors. Thus, the necessity of checking these factors before the calibration is carried out using a standard sample is pointed out.

ISSN:0034-6748    

DOI:10.1063/1.1138467

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Calorimeters have been constructed for measuring heat changes accompanying biochemical reactions in samples of small volume. They can measure heat changes of 0.05 mJ in reaction volume of 1 or 2 ml with a time resolution better than 1 s. The instruments have proven useful in studies of some biochemically interesting reactions, where high sensitivity and good time resolution are required.

ISSN:0034-6748    

DOI:10.1063/1.1138468

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A new high‐energy spectrometer for the coincident detection of both outgoing electrons in an electron impact ionization, or (e, 2e), experiment is described. The absolute triple differential cross section can be measured with this instrument to a precision of 10%. Selected results on rare gases are presented, demonstrating the ability of the spectrometer to either probe the target structure in impulsive or binary (e,2e) experiments, or to test the scattering mechanism in nonimpulsive conditions, both for valence or inner shells.

ISSN:0034-6748    

DOI:10.1063/1.1138469

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A new type of detector for the determination of absolute molecular‐beam intensities is described. The intensity is obtained from the work done on a cylindrical rotating body by the torque exerted by the impinging beam particles. The detector takes advantage of the very low and constant bearing friction of magnetically suspended rotors and of the complete particle momentum accommodation on uncleaned surfaces. The detection limit of the present design isImin0=1.2×1015/Mv˜, whereMis the atom mass andv˜ the average velocity of the beam particles in ms−1. The molecular‐beam detector turns out to be an absolute instrument. Principle, experimental performance, and measurements with helium nozzle beams are reported.

ISSN:0034-6748    

DOI:10.1063/1.1138470

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An analysis of the molecular effusive flow pattern through a narrow rectangular slit orifice is given in terms of the arrival flux on a parallel detector plane. It is shown that the advantage of having an orifice in the form of a long narrow slit in terms of a pronounced peaked incident beam at the detector plane is only achieved at detector plane separations less than the length of the orifice slit. At separations of the detector plane larger than twice the slit length the arrival flux distribution asymptotically approaches that of an infinitesimal orifice source of arbitrary shape. This is in contrast to recent calculations of flux density versus off‐axis detector position reported by Perram and Dorko in Rev. Sci. Instrum.54,275 (1983).

ISSN:0034-6748    

DOI:10.1063/1.1138474

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Two simple methods are described for an accurate perpendicular alignment of a laser beam and an atomic beam, which is required for a velocity independent, i.e., Doppler‐free interaction, of the laser beam with the atomic beam. With the first method a free running single‐mode cw dye laser beam and an atomic beam are aligned perpendicular with an accuracy of 0.25 mrad. This alignment takes typically 2 h. The second method needs a laser beam that is absolutely stabilized to the investigated atomic transition. With this method a 0.2‐mrad accurate perpendicular alignment is obtained within only 5 min.

ISSN:0034-6748    

DOI:10.1063/1.1138475

出版商:AIP    

年代:1985

数据来源: AIP