摘要:

Significant improvements in the performances of the Super‐ACO storage ring free‐electron laser (FEL) at 800 MeV have been obtained recently: enhancement of the output power in the ultraviolet, laser duration of 10 h for the same injection of positrons, long‐term stability with a micropulse of 60 ps FWHM. A first series of experiments using this FEL has then been successfully performed. Taking advantage of the time structure, the polarization and the high power of the source at 350 nm, the polarized fluorescence decays of the reduced nicotinamide adenine dinucleotide coenzyme (NADH) were studied in aqueous solution, using the single‐photon counting (SPC) technique. The experimental setup is described, including the Super‐ACO FEL characteristics and diagnostics. The FEL working point has been first optimized by monitoring the SPC apparatus function. A complete fluorescence experiment required about 30 min of data acquisition, during which the best integrated instrumental response had a FWHM of 110 ps. Measurements performed in such a way lead to the unambiguous separation of two close lifetime components of 0.28 and 0.62 ns in the fluorescence decays of NADH at 20 °C, in good agreement with previous works. The thermodynamic parameters obtained from temperature studies show that the NADH fluorescence heterogeneity is consistent with the ground‐state folding equilibrium of the coenzyme, as characterized by many other spectroscopic techniques. From the fluorescence anisotropy decays, an apparent hydrodynamic radius of about 6 A˚ is determined, while on the other hand, a large initial depolarization of the fluorescence indicates a fast independent motion of the nicotinamide ring. The quality of the collected data fully meets the requirements for the study of more complex systems such as fluorescent compounds bound to proteins or membranes. Thus, the feasibility of use of a storage ring UV FEL for this type of time‐resolved experiments on the subnanosecond time scale has been demonstrated.

ISSN:0034-6748    

DOI:10.1063/1.1144880

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A Pockels cell with an operating range of 2 to 10 &mgr;m has been designed, constructed, and tested for use with the Vanderbilt free electron laser. The Pockels cell can be continuously adjusted to switch out from 80 ns to the full 6 &mgr;s duration of the FEL macropulse. The extinction ratio is better than 180:1 and the optical pulse is square with rise and fall times under 10 ns. The high voltage drivers are based on semiconductor switches that are compact and mounted directly to the Pockels cell; this design avoids problems associated with fast, high voltage pulses traveling in cables. In addition, designs for two alternative drivers to generate optical pulses with durations ranging from a few nanoseconds down to the subnanosecond regime are presented here. Prototypes using single high voltage transitions have been constructed and tested electrically.  

ISSN:0034-6748    

DOI:10.1063/1.1144881

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We describe a complete diode laser system stabilized at 852 nm on theD2line of Cs with a linewidth narrower than 100 kHz. This system is intended to be employed for optical pumping, cooling, and detection of Cs atoms in atomic frequency standards. The square root of the Allan variance, evaluated by measuring the beat‐note of two similar systems, is better than 1×10−12for &tgr; up to 1000 s. A month of continuous locked operation has been observed.

ISSN:0034-6748    

DOI:10.1063/1.1144882

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We describe a single crystal microspectrophotometer for optical monitoring of protein crystals during time‐resolved x‐ray diffraction experiments. It is a single beam device with a diode array detector for simultaneous multiwavelength measurement of optical spectra. The time resolution of the microspectrophotometer is 2.5 ms, which matches the ms time resolution of the x‐ray experiments obtained at existing synchrotron sources. Optical densities of single crystals can be reliably measured up to 2.5 OD, with the linear range extending to 2 OD. The tight focusing of the light beam allows monitoring of crystals as small as 30–50 &mgr;m, an important feature since successful reaction initiation and optical monitoring of crystals in time‐resolved x‐ray diffraction experiments often require very small crystals. The compact design of the microspectrophotometer and the long objective‐to‐crystal distance make it compatible with the crowded environment of time‐resolved x‐ray diffraction experiments.

ISSN:0034-6748    

DOI:10.1063/1.1144883

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The leaf photosynthetic O2evolution detected by the open photoacoustic cell (OPC) technique is compared to the measurements using a Clark‐type O2electrode detector.Invivoandinsitumeasurements were carried out with undetached maize leaves. From the correlation between these two methods the possibility of a calibration is proposed for the oxygen evolution OPC signal.  

ISSN:0034-6748    

DOI:10.1063/1.1144884

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A design is presented for a millimeter/submillimeter direct absorption spectrometer for studies of the pure rotational spectra of metal‐bearing free radicals. The spectrometer operates in the frequency range of 65–550 GHz with an instrumental resolution of 200–1000 kHz and an absorption sensitivity of a few ppm. The instrument utilizes phase‐locked Gunn oscillators as the tunable, coherent source of radiation from 65–140 GHz. Higher source frequencies are obtained with Schottky diode multipliers. The gas cell and optics path are designed utilizing Gaussian beam optics to achieve maximum interaction between molecules and the mm‐wave radiation in the reaction region. Scalar feedhorns and a series of PTFE lenses are used to propagate the source signal. The gas cell is a cylindrical tube 0.5 m in length with a detachable Broida‐type oven. The detector for the spectrometer is a helium‐cooled InSb hot electron bolometer. Phase‐sensitive detection is achieved by FM modulation of the Gunn oscillators and use of a lock‐in amplifier. Spectra are recorded by electrical tuning of the Gunn oscillator, which is done under computer control. The millimeter and sub‐mm rotational spectra of several free radicals have been observed for the first time using this instrument, including CaOH, MgOH, CaH, MgF, and BaOH.

ISSN:0034-6748    

DOI:10.1063/1.1144885

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A static cell has been developed for routine gas phase spectroscopy under high vacuum conditions. Its design provides both reusability and convenience using parts readily available from commercial vendors at an economical price. The cell features a warm‐up cycle that keeps the windows free of condensation and an operating range of 300–700 K that offers good uniformity of temperature throughout the cell for extended periods of time. The operation of the cell is illustrated with absorption spectra of a zinc metalloporphyrin in the gas phase.

ISSN:0034-6748    

DOI:10.1063/1.1144886

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new visualization technique is presented for microscopic particle trajectories displaying interaction and agglomeration phenomena in a high intensity acoustic field. The experiments are carried out in a small‐scale observation chamber using a CCD camera in conjunction with a high resolution video system. A homogeneous acoustic velocity field is generated by two square, flat‐membrane loudspeakers which comprise two opposing walls in the observation chamber. Glass microspheres (diameters 8.1 and 22.1 &mgr;m) and arbitrarily shaped quartz particles (diameter<50 &mgr;m) are used for the observation of interaction and agglomeration trajectories under the influence of an intense acoustic velocity field (1.2–0.53 m/s@xa400–900 Hz). The new technique allows the observation of the particles’ general motion as well as acoustically induced oscillations. The direction of propagation of a particle can be extracted by following its trajectory in a 2D laser lightsheet. Image processing of the digitized data allows the reconstruction of particle trajectories for time spans up to 0.5 s. From the images, the particle size can be estimated based on measurements of the acoustic entrainment factor. Most importantly, with the new experimental technique it is possible to resolve particle interaction and agglomeration processes caused by the acoustic field. The recorded digitized images show a number of different interaction phenomena as well as one distinct pattern that resembles the shape of a tuning fork (thus called the tuning fork agglomeration). The latter appears to be the predominant agglomeration mechanism leading to rapid particle approach and multiple, subsequent particle interactions at high frequencies and large acoustic velocities.

ISSN:0034-6748    

DOI:10.1063/1.1144887

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A correlation spectrometer for the IRAM Plateau de Bure radio astronomy observatory is presented. It can process up to 960 MHz wide IF signals coming from the 4‐antenna connected array. An IF processor selects the frequency window of interest, both in width and central frequency. A digital correlator produces channelized amplitude and phase information for the six pairs of antennas, which is later used for aperture synthesis mapping. Some original technical features are detailed. Some improvements suggested by observing results are discussed.

ISSN:0034-6748    

DOI:10.1063/1.1144888

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The laser intensity modulation method (LIMM), developed by Lang in the early eighties, is a useful technique for determining the spatial polarization and/or space charge distributions in pyroelectric and/or piezoelectric materials, for instance in thin polymer electret films. The advantage of the LIMM is the high spatial resolution near the irradiated side. Therefore, we introduced the logarithmic spatial grid distribution into the deconvolution process. For the first time the self‐consistency method developed by Honerkamp and Weese is applied to the deconvolution program with respect to LIMM. Using multilayer systems with known spatial distributions our new equipment is tested with regard to providing reasonable results as well as the deconvolution program to represent the known spatial distributions.

ISSN:0034-6748    

DOI:10.1063/1.1144889

出版商:AIP    

年代:1994

数据来源: AIP