Volume 81, Issue 6, June 2010
Index of content:
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
81(2010); http://dx.doi.org/10.1063/1.3373977View Description Hide Description
A novel laser desorption system, with improved signal stability and extraordinary long lifetime, is presented for the study of jet-cooled biomolecules in the gas phase using vibrationally resolved photoionization spectroscopy. As a test substance tryptophane is used to characterize this desorptionsource. A usable lifetime of above 1 month (for a laser desorption repetition rate of 20 Hz) has been observed by optimizing the pellets (graphite/tryptophane, 3 mm diameter and 6 mm length) from which the substance is laser-desorbed. Additionally, the stability and signal-to-noise ratio has been improved by averaging the signal over the entire sample pellet by synchronizing the data acquisition with the rotation of the sample rod. The results demonstrate how a combination of the above helps to produce stable and conclusive spectra of tryptophane using one-color and two-color resonant two-photonionization studies.
81(2010); http://dx.doi.org/10.1063/1.3427357View Description Hide Description
A new instrument has been constructed that couples a supersonic expansion source to a continuous wave cavity ringdown spectrometer using a Fabry–Perot quantum cascade laser(QCL). The purpose of the instrument is to enable the acquisition of a cold, rotationally resolved gas phase spectrum of buckminsterfullerene . As a first test of the system, high resolution spectra of the vibrational band of have been acquired at . To our knowledge, this is the first time that a vibrational band not previously recorded with rotational resolution has been acquired with a QCL-based ringdown spectrometer. 62 transitions of the three isotopologues of were assigned and fit to effective Hamiltonians with a standard deviation of 14 MHz, which is smaller than the laser frequency step size. The spectra have a noise equivalent absorption coefficient of . Spectral simulations of the band indicate that the supersonic source produces rotationally cold molecules.
81(2010); http://dx.doi.org/10.1063/1.3430538View Description Hide Description
We describe our experimental setup for creating stable Bose–Einstein condensates (BECs) of with tunable interparticle interactions. We use sympathetic cooling with in two stages, initially in a tight Ioffe–Pritchard magnetic trap and subsequently in a weak, large-volume, crossed optical dipole trap, using the 155 G Feshbach resonance to manipulate the elastic and inelastic scattering properties of the atoms. Typical condensates contain atoms with a scattering length of . Many aspects of the design presented here could be adapted to other dual-species BEC machines, including those involving degenerate Fermi–Bose mixtures. Our minimalist apparatus is well suited to experiments on dual-species and spinor Rb condensates, and has several simplifications over the BEC machine at JILA, which we discuss at the end of this article.
Data processing correction of the irising effect of a fast-gating intensified charge-coupled device on laser-pulse-excited luminescence spectra81(2010); http://dx.doi.org/10.1063/1.3431536View Description Hide Description
Intensified charge-coupled devices (ICCDs) comprise the advantages of both fast gating detectors and spectrally broad CCDs into one device that enables temporally and spectrally resolved measurements with a few nanosecond resolution. Gating of the measured signal occurs in the image intensifier tube, where a high voltage is applied between the detectorphotocathode and a microchannel plate electron multiplier. An issue arises in time-resolved luminescencespectroscopy when signal onset characterization is required. In this case, the transient gate closing process that causes the detected signal always arises in the middle of the ICCD chip regardless of the spectraldetection window—the so-called irising effect. We demonstrate that in case when the detection gate width is comparable to the opening/closing time and the gate is pretriggered with respect to the signal onset, the irising effect causes the obtained data to be strongly distorted. At the same time, we propose a software procedure that leads to the spectral correction of the irising effect and demonstrate its validity on the distorted data.
A 12.5 GHz-spaced optical frequency comb spanning for near-infrared astronomical spectrograph calibration81(2010); http://dx.doi.org/10.1063/1.3436638View Description Hide Description
A 12.5 GHz-spaced optical frequency comb locked to a global positioning system disciplined oscillator for near-infrared (IR) spectrographcalibration is presented. The comb is generated via filtering a 250 MHz-spaced comb. Subsequent nonlinear broadening of the 12.5 GHz comb extends the wavelength range to cover 1380–1820 nm, providing complete coverage over the H-band transmission window of earth’s atmosphere. Finite suppression of spurious sidemodes, optical linewidth, and instability of the comb has been examined to estimate potential wavelength biases in spectrographcalibration. Sidemode suppression varies between 20 and 45 dB, and the optical linewidth is at 1550 nm. The comb frequency uncertainty is bounded by (corresponding to a radial velocity of ), limited by the global positioning system disciplined oscillator reference. These results indicate that this comb can readily support radial velocity measurements below 1 m/s in the near IR.
Generation of tunable narrow bandwidth nanosecond pulses in the deep ultraviolet for efficient optical pumping and high resolution spectroscopy81(2010); http://dx.doi.org/10.1063/1.3436973View Description Hide Description
Nanosecond optical pulses with high power and spectral brightness in the deep ultraviolet (UV) region have been produced by sum frequency mixing of nearly transform-limited-bandwidth IR light originating from a home-built injection-seeded ring cavityoptical parametric oscillator(OPO) and the fourth harmonic beam of an injection-seeded Nd:YAG laser used simultaneously to pump the OPO with the second harmonic. We demonstrate UV output, tunable from 204 to 207 nm, which exhibits pulse energies up to 5 mJ with a bandwidth better than . We describe how the approach shown in this paper can be extended to wavelengths shorter than 185 nm. The injection-seeded OPO provides high conversion efficiency ( overall energy conversion) and superior beam quality required for highly efficient downstream mixing where sum frequencies are generated in the UV. The frequency stability of the system is excellent, making it highly suitable for optical pumping. We demonstrate high resolution spectroscopy as well as optical pumping using laser-induced fluorescence and stimulated emission pumping, respectively, in supersonic pulsed molecular beams of nitric oxide.
Double conical crystal x-ray spectrometer for high resolution ultrafast x-ray absorption near-edge spectroscopy of edge81(2010); http://dx.doi.org/10.1063/1.3441983View Description Hide Description
An x-ray spectrometer devoted to dynamical studies of transient systems using the x-rayabsorption fine spectroscopy technique is presented in this article. Using an ultrafast laser-induced x-ray source, this optical device based on a set of two potassium acid phthalate conical crystals allows the extraction of x-rayabsorption near-edge spectroscopy structures following the Al absorption edge. The proposed experimental protocol leads to a measurement of the absorption spectra free from any crystal reflectivity defaults and shot-to-shot x-rayspectral fluctuation. According to the detailed analysis of the experimental results, a spectral resolution of 0.7 eV rms and relative fluctuation lower than 1% rms are achieved, demonstrated to be limited by the statistics of photon counting on the x-ray detector.
A new spectroscopic method for resolving the electronic symmetry properties of the highly excited molecules produced in photoexcitation81(2010); http://dx.doi.org/10.1063/1.3436653View Description Hide Description
A novel method of spectroscopy for highly excited states of molecules in the valence excitation range has been established through the detection of metastable hydrogen atoms in the state formed by photoexcitation. The detector for the metastable hydrogen atom is composed of a stack of parallel plate electrodes that creates a localized electric field and triggers the emission of the Lyman-photon from the atom and a chevron pair of microchannel plates that detects the photon. For linear molecules, the angle-resolved detection of the metastable hydrogen atom enables us to measure cross sections in which electronic symmetries of highly excited molecular states are resolved. Such symmetry-resolved cross section measurements were carried out for doubly excited states of .
Nanograting-based compact vacuum ultraviolet spectrometer and beam profiler for in situ characterization of high-order harmonic generation light sources81(2010); http://dx.doi.org/10.1063/1.3443575View Description Hide Description
A compact, versatile device for vacuum ultraviolet (VUV) beam characterization is presented. It combines the functionalities of a VUVspectrometer and a VUV beam profiler in one unit and is entirely supported by a standard DN200 CF flange. The spectrometer employs a silicon nitride transmission nanograting in combination with a microchannel plate-based imaging detector. This enables the simultaneous recording of wavelengths ranging from 10 to 80 nm with a resolution of 0.25–0.13 nm. Spatial beam profiles with diameters up to 10 mm are imaged with 0.1 mm resolution. The setup is equipped with an in-vacuum translation stage that allows for in situ switching between the spectrometer and beam profiler modes and for moving the setup out of the beam. The simple, robust design of the device is well suited for nonintrusive routine characterization of emerging laboratory- and accelerator-based VUV light sources. Operation of the device is demonstrated by characterizing the output of a femtosecond high-order harmonic generation light source.
81(2010); http://dx.doi.org/10.1063/1.3455809View Description Hide Description
A fast-scan method was developed to obtain time-resolved signals with femtosecond resolution over a picosecond range on the fly and in real time. Traditional fast-scan methods collect data at each probe wavelength one by one, which is time consuming and thus not possible for the study of photofragile materials. In this work, we have developed a system that performs fast scans with multiplex detection. Ultrafast time-resolved spectroscopy was demonstrated using the newly developed system. Femtosecond laser pulses have been used for pump-probe studies of ultrafast processes in various materials, and both electronic relaxation and vibrational dynamics have been studied. However, experiments have been limited in sensitivity and reliability because they are affected by the long-term instability of the ultrashort laser pulses and by the fragility of the samples. The instability of the sources hinders precise determination of electronic decay dynamics and introduces systematic errors. The fragility of the samples reduces their amount or concentration, and can lead to contamination of the materials even if they were pure before the measurement. These effects make it difficult to obtain reproducible and reliable experimental data. In the present work, we have developed a fast-scan pump-probe spectroscopic system that can complete a set of measurements in less than 2 min. Quantitative estimates of the signal reproducibility demonstrate that these measurements provide higher reproducibility and reliability than conventional measurements.
Sample cells for probing solid/liquid interfaces with broadband sum-frequency-generation spectroscopy81(2010); http://dx.doi.org/10.1063/1.3443096View Description Hide Description
Two sample cells designed specifically for sum-frequency-generation (SFG) measurements at the solid/liquid interface were developed: one thin-layer analysis cell allowing measurement of films on reflective metallic surfaces through a micrometer layer of solution and one spectroelectrochemical cell allowing investigation of processes at the indium tin oxide/solution interface. Both sample cells are described in detail and data illustrating the capabilities of each are shown. To further improve measurements at solid/liquid interfaces, the broadband SFG system was modified to include a reference beam which can be measured simultaneously with the sample signal, permitting background correction of SFG spectra in real time. Sensitivity tests of this system yielded a signal-to-noise ratio of 100 at a surface coverage of . Details on data analysis routines, pulse shaping methods of the visible beam, as well as the design of a purging chamber and sample stage setup are presented. These descriptions will be useful to those planning to set up a SFG spectrometer or seeking to optimize their own SFG systems for measurements of solid/liquid interfaces.
81(2010); http://dx.doi.org/10.1063/1.3449333View Description Hide Description
We have developed a rotatable hemispherical spectrometer with good energy and angular resolution, which can be positioned with the lens axis arbitrarily within a solid angle of . The collection angle of the emitted electrons with respect to the polarization axis of the light is set by means of a three-axes goniometer, operating under vacuum. An important requirement for this setup was the possibility to perform coincidences between the electron analyzed by the spectrometer and one or several other particles, such as ions, electrons, or photons. The lens system and the hemispheres have been designed to accommodate such experimental demands, regarding parameters such as the resolving power, the acceptance angle, or the width of the kinetic energy window which can be recorded for a given pass energy. We have chosen to detect the impact position of the electron at the focal plane of the hemispherical analyzer with a delay line detector and a time-to-digital converter as acquisition card rather than using a conventional charge-coupled device camera.
- Particle Sources, Optics and Acceleration; Particle Detectors
81(2010); http://dx.doi.org/10.1063/1.3442514View Description Hide Description
The absolute detection efficiencies of a microchannel plate detector for neutral atoms were measured using the coincidence method for neutralized incident ions and ionized target atoms in electron capturecollisions. This method does not require knowledge of the absolute electron-capture rates for determination of the detection efficiencies. Results for Ne, Ar, and Kr atoms at energies of 0.5–5 keV are reported. The detection efficiencies for all atomic species increase concomitantly with increasing impact energy and plateau at the efficiency of about 50%. For low impact energies, the efficiency decreases with increasing mass of the impact atom at a given energy.
81(2010); http://dx.doi.org/10.1063/1.3436659View Description Hide Description
Pulsed extraction techniques are investigated for a quadrupole ion trap (QIT) interfaced to a linear time-of-flight(TOF) mass analyzer. A nonfocusing short-pulse mode of operation is developed and characterized. The short-pulse mode creates a near-monoenergetic ion packet, which is useful for reaction kinetics experiments and for making diagnostic measurements of the ion cloud size in the trap. Monopolar and bipolar pulsing modes, with the voltage pulses applied to one or both QIT endcaps to extract the ions into the TOF region, are compared. Ion TOF peak distributions are characterized experimentally and by ion trajectory simulations. Also, first-order spatial (Wiley–McLaren) focusing of ions is characterized for the conventional long-pulse extraction mode. The nonparallel fields in the QIT, which serves as the first acceleration region in the linear-TOF mass spectrometer, are shown to degrade spatial focusing and mass resolution.
81(2010); http://dx.doi.org/10.1063/1.3443319View Description Hide Description
Neutron guides made of supermirror-coated glass are important components of most neutron scattering instruments, thus their quality and possible deterioration due to various deleterious effects (e.g., surface contamination or defects) deserve careful examination. The modification of the reflectivity of supermirrors and the transmission of neutron guides due to surface contamination with hydrocarbon oil has been investigated using neutron reflectometry together with model calculations. A significant loss in the neutronreflectivity was observed for supermirrors covered with thin hydrocarbon oil films, which were confirmed in model calculations. Simulations carried out for several typical arrangements show drastic decreases in the transmitted neutron flux of neutron guides. These simulations show that determining the distortion of the beam profile (using a slit or a pin hole) enables the detection of oil contamination even in an operating neutron guide.
81(2010); http://dx.doi.org/10.1063/1.3433485View Description Hide Description
We have developed an electrostatic, double-focusing 90° deflector for fast ion beams consisting of concentric cylindrical plates of differing heights. In contrast to standard cylindrical deflectors, our design allows for focusing of an incoming parallel beam not only in the plane of deflection but also in the orthogonal direction. The optical properties of our design resemble those of a spherical capacitor deflector while it is much easier and more cost effective to manufacture.
- Nuclear Physics, Fusion and Plasmas
First results obtained from the soft x-ray pulse height analyzer on experimental advanced superconducting tokamak81(2010); http://dx.doi.org/10.1063/1.3443572View Description Hide Description
An assembly of soft x-ray pulse height analyzer system, based on silicon drift detector (SDD), has been successfully established on the experimental advanced superconductingtokamak (EAST) to measure the spectrum of soft x-ray emission . The system, including one 15-channel SDD linear array, is installed on EAST horizontal port C. The time-resolved radial profiles of electron temperature and intensities of metallic impurities have been obtained with a spatial resolution of around 7 cm during a single discharge. It was found that the electron temperatures derived from the system are in good agreement with the values from Thomson scattering measurements. The system can also be applied to the measurement of the long pulse discharge for EAST. The diagnostic system is introduced and some typical experimental results obtained from the system are also presented.
81(2010); http://dx.doi.org/10.1063/1.3429942View Description Hide Description
The first plasma with target values of the plasma current and the pulse duration was finally achieved on June 13, 2008 in the Korea SuperconductingTokamak Advanced Research (KSTAR). The diagnostic systems played an important role in achieving successful first plasma operation for the KSTAR tokamak. The employed plasma diagnostic systems for the KSTAR first plasma including the magnetic diagnostics, millimeter-wave interferometer, inspection illuminator, , visible spectrometer, filterscope, and electron cyclotron emission (ECE) radiometer have provided the main plasma parameters, which are essential for the plasma generation, control, and physics understanding. Improvements to the first diagnostic systems and additional diagnostics including an x-ray imaging crystal spectrometer, reflectometer, ECE radiometer, resistive bolometer, and soft x-ray array are scheduled to be added for the next KSTAR experimental campaign in 2009.
81(2010); http://dx.doi.org/10.1063/1.3455200View Description Hide Description
A fast electrostaticdiagnostic and analysis scheme on nanosecond pulsed beams in the keV energy range has been developed in the Malmberg–Penning trap ELTRAP. Low-noise electronics has been used for the detection of small induced current signals on the trap electrodes. A discrete wavelet-based procedure has been implemented for data postprocessing. The development of an effective electrostaticdiagnostics together with proper data analysis techniques is of general interest in view of deducing the beam properties through comparison of the postprocessed data with the theoretically computed signal shape, which contains beam radius, length, and average density as fit parameters.
81(2010); http://dx.doi.org/10.1063/1.3449541View Description Hide Description
A Faraday probe with three concentric rings was designed and fabricated to assess the effect of gap width and collector diameter in a systematic study of the diagnostic ion collection area. The nested Faraday probe consisted of two concentric collector rings and an outer guard ring, which enabled simultaneous current densitymeasurements on the inner and outer collectors. Two versions of the outer collector were fabricated to create gaps of 0.5 and 1.5 mm between the rings. Distribution of current density in the plume of a low-power Hall thruster ion source was measured in azimuthal sweeps at constant radius from 8 to 20 thruster diameters downstream of the exit plane with variation in facility background pressure. A new analytical technique is proposed to account for ions collected in the gap between the Faraday probe collector and guard ring. This method is shown to exhibit excellent agreement between all nested Faraday probe configurations, and to reduce the magnitude of integrated ion beamcurrent to levels consistent with Hall thruster performance analyses. The technique is further studied by varying the guard ring bias potential with a fixed collector bias potential, thereby controlling ion collection in the gap. Results are in agreement with predictions based on the proposed analytical technique. The method is applied to a past study comparing the measured ion current density profiles of two Faraday probe designs. These findings provide new insight into the nature of ion collection in Faraday probe diagnostics, and lead to improved accuracy with a significant reduction in measurement uncertainty.