Volume 79, Issue 12, December 2008
- optics; atoms and molecules; spectroscopy; photon detectors
- particle sources, optics and acceleration; particle detectors
- nuclear physics, fusion and plasmas
- microscopy and imaging
- condensed matter; materials
- biology and medicine
- electronics; electromagnetic technology; microwaves
- general instruments
Index of content:
- OPTICS; ATOMS AND MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
A frequency-stabilized difference frequency generation laser spectrometer for precise line profile studies in the midinfrared79(2008); http://dx.doi.org/10.1063/1.3033164View Description Hide Description
A midinfrared laser spectrometer is built up based on the difference frequency generation (DFG) of a Nd:YAG (yttrium aluminum garnet) laser and a tunable Ti:sapphire (Ti:Sa) laser. Tuning the Ti:Sa laser and operating properly with the periodically poled lithium niobate crystal, the DFG emission is tunable in the spectral range of . The 1064 nm Nd:YAG laser frequency is stabilized to the level on a Doppler-broadened absorption line at 532 nm. As a result, the DFG emission frequency is stabilized within . The measurement of an absorption line of near demonstrates that the DFG spectrometer is very suitable for the molecular absorption line profile studies in the midinfrared region.
79(2008); http://dx.doi.org/10.1063/1.3036976View Description Hide Description
A cavity-dumped erbium laser with a frustrated total internal reflection (FTIR) shutter was investigated and compared with a -switched erbium laser using the FTIR shutter. The -switched and the cavity-dumped laser outputs were obtained with a dichroic coatedmirror with high reflectance at and high transmittance at . For the -switched operation, a maximum peak power of 33.5 kW was achieved, and its pulse width was . For the cavity-dumped operation, the laser pulse energy was optimized by changing the switching time of the FTIR shutter. When the pulse width is reduced to 210 ns, the peak power increases to 154 kW.
79(2008); http://dx.doi.org/10.1063/1.3036979View Description Hide Description
This paper is devoted to the description and characterization of a new electro-opticsensor based on a polarimetric device operating in reflection mode, and on a closed-loop controller. The main features of the proposed sensor are versatility, insensitivity to the laser beam optical power fluctuation, and fully linear working mode. The resolution of the sensor, in terms of phase shift, is shown to be better than and its accuracy to .
79(2008); http://dx.doi.org/10.1063/1.3036982View Description Hide Description
We present a vapor cell design that enables resonant optical spectroscopy in high-density, high-temperature alkali metal vapors. Optical access is provided via reflection from the interface between the vapor and a sapphire window. The cell resists corrosion from the highly reactive alkali vapors, in our case, potassium, up to (number densities up to ). The cells maintain their integrity for an average of 100 h above with about 10 heating/cooling cycles.
79(2008); http://dx.doi.org/10.1063/1.3030776View Description Hide Description
A novel instrument is described for obtaining accurate high-resolution residual stress images of aluminum oxide materials, based on the piezospectroscopy of dopant ions. The instrument employs a charge coupled device camera, a narrow bandpass tunable filter and the use of Tikhonov regularization for reconstruction of the raw spectral data. The experimental accuracy and spectral shift resolution of this method were analyzed with two calibration light sources and with independently measured spectra, and were found to be approximately across the pixel array. This is close to the theoretically obtainable accuracy for the particular filter used, a solid etalon Fabry–Pérot filter with a passband of 0.25 nm, based on an analysis with simulated data. The spectral resolution corresponds to a stress resolution, under biaxial stress conditions, of .
High-resolution Rydberg tagging time-of-flight measurements of atomic photofragments by single-photon vacuum ultraviolet laser excitation79(2008); http://dx.doi.org/10.1063/1.3043427View Description Hide Description
By coupling a comprehensive tunable vacuum ultraviolet (VUV) laser system to a velocity-mapped ion imaging apparatus, we show that high-resolution high-Rydberg tagging time-of-flight(TOF)measurements of nascent atomic photofragments formed by laser photodissociation can be made using single-photon VUV laser photoexcitation. To illustrate this single-photon Rydberg tagging TOF method, we present here the results of the VUV laser high-Rydberg tagging TOFmeasurements of and formed in the photodissociation of and at 193.3 and 202.3 nm, respectively. These results are compared to those obtained by employing the VUV laser photoionization time-sliced velocity-mapped ion imaging technique. The fact that the kinetic energy resolutions achieved in the VUV laser high-Rydberg tagging TOFmeasurements of O and S atoms are found to be higher than those observed in the VUV laser photoionization, time-sliced velocity-mapped ion imaging studies show that the single-photon VUV laser high-Rydberg tagging TOF method is useful and complementary to state-of-the-art time-sliced velocity-mapped ion imaging measurements of heavier atomic photofragments, such as O and S atoms. Furthermore, the general agreement observed between the VUV laser high-Rydberg tagging TOF and velocity-mapped ion imaging experiments supports the conclusion that the lifetimes of the tagged Rydberg states of O and S atoms are sufficiently long to allow the reliable determination of state-resolved UVphotodissociation cross sections of and by using the VUV laser high-Rydberg tagging TOF method.
A time-resolved single-pass technique for measuring optical absorption coefficients of window materials under 100 GPa shock pressures79(2008); http://dx.doi.org/10.1063/1.3046279View Description Hide Description
An experimental method was developed to perform time-resolved, single-pass optical absorption measurements and to determine absorption coefficients of window materials under strong shock compression up to . Experimental details are described of (i) a configuration to generate an in situ dynamic, bright, optical source and (ii) a sample assembly with a lithium fluoride plate to essentially eliminate heat transfer from the hot radiator into the specimen and to maintain a constant optical source within the duration of the experiment. Examples of measurements of optical absorption coefficients of several initially transparent single crystalmaterials at high shock pressures are presented.
Near UV-near IR Fourier transform spectrometer using the beam-folding position-tracking method based on retroreflectors79(2008); http://dx.doi.org/10.1063/1.3046281View Description Hide Description
A near UV-near IR Fourier transform spectrometer based on a beam-folding position-tracking method realized by using retroreflectors is reported. The use of retroreflectors maintains all beams in the beam-fold arrangement in parallel with the incident beams. The beam-folding interferometer used for position tracking is arranged to have optical path symmetry with the measurement interferometer in the zero path difference position of the measurement interferometer, and the vertex of the movable retroreflector in the measurement interferometer is arranged very close to the midpoint of the vertices of two movable retroreflectors in the position-tracking interferometer. These measures keep the equivalent optical axis of the position-tracking interferometer well in line with that of the measurement interferometer even with translational misalignments. Therefore, the change in the optical path difference of the position-tracking interferometer is always synchronous to that of the measurement interferometer during the scanning process. That is, the position-tracking error can be suppressed to very small values during a scan. We have demonstrated a UV-near IR Fourier transform spectrometer with a standard quality ball-bearing translation stage achieving a resolution close to the theoretical resolution of at the He–Ne laser wavelength when the scan distance reaches the travel distance of over 2 cm. This was achieved without the need for elaborate optics, sophisticated detecting electronics, and high-precision servomotion control.
79(2008); http://dx.doi.org/10.1063/1.3048542View Description Hide Description
A new titration system for studying protein-ligand interactions has been developed. In this system, the sample solution is circulated in the route formed by an access path in a split superconducting magnet to maintain a constant protein concentration during the titration experiments. A concentration-control procedure for the ligand/protein ratio is devised, and the ligand/protein ratio is well controlled by this apparatus.
A broadband absorption spectrometer using light emitting diodes for ultrasensitive, in situ trace gas detection79(2008); http://dx.doi.org/10.1063/1.3046282View Description Hide Description
A broadband absorption spectrometer has been developed for highly sensitive and target-selective in situ trace gas measurements. The instrument employs two distinct modes of operation: (i) broadband cavity enhanced absorption spectroscopy (BBCEAS) is used to quantify the concentration of gases in sample mixtures from their characteristic absorption features, and (ii) periodic measurements of the cavity mirrors’ reflectivity are made using step-scan phase shift cavity ringdown spectroscopy (PSCRDS). The latter PSCRDS method provides a stand-alone alternative to the more usual method of determining mirrorreflectivities by measuring BBCEAS absorption spectra for calibration samples of known composition. Moreover, the instrument’s two modes of operation use light from the same light emitting diode transmitted through the cavity in the same optical alignment, hence minimizing the potential for systematic errors between mirrorreflectivity determinations and concentration measurements. The ability of the instrument to quantify absorber concentrations is tested in instrument intercomparison exercises for (versus a laser broadband cavity ringdown spectrometer) and for (versus a commercial hygrometer). A method is also proposed for calculating effective absorption cross sections for fitting the differential structure in BBCEAS spectra due to strong, narrow absorption lines that are under-resolved and hence exhibit non-Beer–Lambert law behavior at the resolution of the BBCEAS measurements. This approach is tested on BBCEAS spectra of water vapor’s absorption bands around 650 nm. The most immediate analytical application of the present instrument is in quantifying the concentration of reactive trace gases in the ambient atmosphere. The instrument’s detection limits for as a function of integration time are considered in detail using an Allan variance analysis. Experiments under laboratory conditions produce a detection limit of 0.25 pptv for a 10 s acquisition time, which improves with further signal averaging to 0.09 pptv in 400 s. Finally, an example of the instrument’s performance under field work conditions is presented, in this case of measurements of the sum of concentrations in the marine boundary layer acquired during the Reactive Halogens in the Marine Boundary Layer field campaign.
79(2008); http://dx.doi.org/10.1063/1.3053338View Description Hide Description
A new optical Dopplerlidar receiving system which can measure Doppler-shifted incoherent scattering light has been proposed and tested. This system functions in a manner similar to two edge filters and consists of a single etalon, a polarizationbeam splitter, and mirrors that can guide scattered light into a single etalon with two different angles. These two incident angles are precisely adjusted by the polarizationbeam splitter and mirrors. Using this optical receiving system we can measure a moving target with a high sensitivity. Theoretical calculations show that this system will have a better optical sensitivity than a single edge system and will allow the use of incoherent Dopplerlidar for wind velocity measurements.
79(2008); http://dx.doi.org/10.1063/1.3048544View Description Hide Description
For x-ray spot sizes of a few tens of microns or smaller, a millimeter-sized flat analyzer crystal placed from the sample will exhibit high energy resolution while subtending a collection solid angle comparable to that of a typical spherically bent crystal analyzer (SBCA) at much larger working distances. Based on this observation and a nonfocusing geometry for the analyzeroptic, we have constructed and tested a short working distance (SWD) multicrystal x-ray spectrometer. This prototype instrument has a maximum effective collection solid angle of 0.14 sr, comparable to that of 17 SBCA at 1 m working distance. We find good agreement with prior work for measurements of the x-ray emission and resonant inelastic x-ray scattering for MnO, and also for measurements of the x-rayabsorption near-edge structure for Dy metal using partial-fluorescence yield detection. We discuss future applications at third- and fourth-generation light sources. For concentrated samples, the extremely large collection angle of SWD spectrometers will permit collection of high-resolution x-ray emission spectra with a single pulse of the Linac Coherent Light Source. The range of applications of SWD spectrometers and traditional multi-SBCA instruments has some overlap, but also is significantly complementary.
79(2008); http://dx.doi.org/10.1063/1.3055912View Description Hide Description
Time-correlated single photon counting continues to gain importance in a wide range of applications. Most prominently, it is used for time-resolved fluorescencemeasurements with sensitivity down to the single molecule level. While the primary goal of the method used to be the determination of fluorescence lifetimes upon optical excitation by short light pulses, recent modifications and refinements of instrumentation and methodology allow for the recovery of much more information from the detected photons, and enable entirely new applications. This is achieved most successfully by continuously recording individually detected photons with their arrival time and detection channel information (time tagging), thus avoiding premature data reduction and concomitant loss of information. An important property of the instrumentation used is the number of detection channels and the way they interrelate. Here we present a new instrument architecture that allows scalability in terms of the number of input channels while all channels are synchronized to picoseconds of relative timing and yet operate independent of each other. This is achieved by means of a modular design with independent crystal-locked time digitizers and a central processing unit for sorting and processing of the timing data. The modules communicate through high speed serial links supporting the full throughput rate of the time digitizers. Event processing is implemented in programmable logic, permitting classical histogramming, as well as time tagging of individual photons and their temporally ordered streaming to the host computer. Based on the time-ordered event data, any algorithms and methods for the analysis of fluorescence dynamics can be implemented not only in postprocessing but also in real time. Results from recently emerging single molecule applications are presented to demonstrate the capabilities of the instrument.
79(2008); http://dx.doi.org/10.1063/1.3058603View Description Hide Description
A minute, yet practical, modification of a well-known spectrograph based on a set of wedged narrow band filters is presented. It makes possible two-dimensional-imaging spectral measurements with a potential subpixel precision of a few micrometers. The simple spectroscopic device can evaluate dispersion of quantities which can be transformed into an image, e.g., into a spatial beam distribution. The spectrograph was used to measure angular dispersion of both a laser oscillator producing femtosecond pulses and a simple dispersive optical system, an optical wedge.
79(2008); http://dx.doi.org/10.1063/1.3036978View Description Hide Description
A new variation on time of flight mass spectrometry is presented, which uses a fast framing charge coupled devicecamera to velocity map image multiple product masses in a single acquisition. The technique is demonstrated on two photofragmentation processes, those of and (dimethyldisulfide) at a photolysis wavelength of 193 nm. In both cases, several mass fragments are imaged simultaneously, and speed distributions and anisotropy parameters are extracted that are comparable to those obtained by imaging each fragment separately in conventional velocity map imaging studies.
Transferring cold atoms in double magneto-optical trap by a continuous-wave transfer laser beam with large red detuning79(2008); http://dx.doi.org/10.1063/1.3058607View Description Hide Description
A novel scheme of transferring cold atoms in a double magneto-optical trap(MOT) system has been experimentally demonstrated. Cold cesium atoms trapped in a vapor-cell MOT are efficiently transferred to an ultrahigh-vacuum (UHV) MOT by a continuous-wave divergent Gaussian transfer laser beam. When large red detuning and moderate intensity are adopted for the transfer laser beam, enhancement of the recapturing of atoms in the UHV MOT is clearly observed. Using the divergent transfer laser beam (diameter of ~1.60 mm in the vapor-cell MOT region) with typical power of ~20.2 mW, up to ~85% of transfer efficiency is obtained when the frequency detuning is set to around −1.2 GHz, and it is not sensitive to small detuning variation. This transfer is much efficient compared with that in the case of continuous-wave near-resonance weak transfer laser beam (typical power of order of and typical frequency detuning of ~−10 MHz) which is normally used in double-MOT experiment. The enhancement is ascribed to the guiding effect on cold atomic flux by transverse dipole potential of the large red-detuned transfer laser beam.
79(2008); http://dx.doi.org/10.1063/1.3058757View Description Hide Description
A new spin- and angle-resolved photoemission spectrometer was developed adopting the very-low-energy-electron-diffraction (VLEED)-type spin polarimeter. The film grown on MgO(001) crystal for the VLEED target yields significantly high spin-resolving power, the effective Sherman function of , with long lifetime and stability compared to the conventional Fe(001) target. Under the favor of high resolving power, approximately 100 times higher efficiency than that of conventional Mott-type spin polarimeter, the figure of merit of was achieved. Owing to this high efficiency, high-energy resolution can be realized with this new spin-polarized photoemission spectrometer. The simplified ways of target preparation and revitalization make the VLEED spin polarimeter much more convenient and feasible for the spin-polarized photoemissionspectroscopy.
- PARTICLE SOURCES, OPTICS AND ACCELERATION; PARTICLE DETECTORS
79(2008); http://dx.doi.org/10.1063/1.3030858View Description Hide Description
We present a detailed analysis and simulation of solenoidal, magnetically confined electron bombardment ion sources, aimed at molecular beam detection. The aim is to achieve high efficiency for singly ionized species while minimizing multiple ionization. Electron space charge plays a major role and we apply combined ray tracing and finite element simulations to determine the properties of a realistic geometry. The factors controlling electron injection and ion extraction are discussed. The results from simulations are benchmarked against experimental measurements on a prototype source.
79(2008); http://dx.doi.org/10.1063/1.3039976View Description Hide Description
This paper presents the design and analysis of a multipurpose combined function magnet for use in accelerators. This magnet consists of three corrector magnets: (i) skew quadrupole, (ii) horizontal dipole, and (iii) vertical dipole magnets, along with the main sextupole magnet. The strength of the corrector magnets is smaller than that of the main sextupole magnet. The strength of all the four magnets can be varied independently. The excitation strength required to produce skew quadrupole gradient and the presence of various multipole components in the magnet are estimated using first order perturbation theory. The experimental data for the variation of the sextupole strength and its higher order multipoles in the presence of skew quadrupole excitations are presented and compared to the theoretical predictions. Simulation using two-dimensional fine element code, Poisson, is also done. Results obtained from all the above three methods are found to be in good agreement with each other. The variations of skew quadrupole gradient for different sextupole excitations are also measured. The validity of this theory is also checked for various combinations of excitations including the case where magnet gets saturated. The excitation strengths required for producing the horizontal and vertical dipole fields are estimated analytically along with the presence of various multipoles. Theoretical predictions of permissible multipoles are compared to the results obtained from simulation.
79(2008); http://dx.doi.org/10.1063/1.3030857View Description Hide Description
Two methods to correct spherical aberration in electrostatic gridded lenses have been studied using ray-tracing simulations. Both methods are based on modifying the electrostatic field on the radial periphery of the lens. In the simplest case, the modification is done by extending the grid support axially. In the second method, the electric field on a radial periphery of the lens is modified by applying optimum voltage on an isolated correcting electrode. It is demonstrated that, for a given focal length, the voltage on this lens can be optimized for minimum aberration, and also that these lenses reduce the emittance growth of the ion beam.