Volume 82, Issue 8, August 2011
Index of content:
We have developed an efficient spectrometer capable of performing a wide variety of coherent multidimensional measurements at optical wavelengths. The two major components of the largely automated device are a spatial beam shaper which controls the beam geometry and a spatiotemporal pulse shaper which controls the temporal waveform of the femtosecond pulse in each beam. We describe how to construct, calibrate, and operate the device, and we discuss its limitations. We use the exciton states of a semiconductor nanostructure as a working example. A series of complex multidimensional spectra—displayed in amplitude and real parts—reveals increasingly intricate correlations among the excitons.
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
82(2011); http://dx.doi.org/10.1063/1.3617456View Description Hide Description
We describe a reliable fabrication procedure of silver tips for scanning tunneling microscope (STM) induced luminescence experiments. The tip was first etched electrochemically to yield a sharp cone shape using selected electrolyte solutions and then sputter cleaned in ultrahigh vacuum to remove surface oxidation. The tip status, in particular the tip induced plasmon mode and its emission intensity, can be further tuned through field emission and voltage pulse. The quality of silver tips thus fabricated not only offers atomically resolved STM imaging, but more importantly, also allows us to perform challenging “color” photon mapping with emission spectra taken at each pixel simultaneously during the STM scan under relatively small tunnel currents and relatively short exposure time.
82(2011); http://dx.doi.org/10.1063/1.3615309View Description Hide Description
Single molecule force clamp experiments are widely used to investigate how enzymes, molecular motors, and other molecular mechanisms work. We developed a dual-trap optical tweezers instrument with real-time (200 kHz update rate) force clamp control that can exert 0–100 pN forces on trapped beads. A model for force clamp experiments in the dumbbell-geometry is presented. We observe good agreement between predicted and observed power spectra of bead position and force fluctuations. The model can be used to predict and optimize the dynamics of real-time force clamp optical tweezers instruments. The results from a proof-of-principle experiment in which lambda exonuclease converts a double-stranded DNA tether, held at constant tension, into its single-stranded form, show that the developed instrument is suitable for experiments in single molecule biology.
82(2011); http://dx.doi.org/10.1063/1.3618686View Description Hide Description
An extreme ultravioletspectrometer has been developed for spectroscopic studies of highly charged ions with an electron beam ion trap. It has a slit-less configuration with a spherical varied-line-spacing grating that provides a flat focal plane for grazing incidence light. Alternative use of two different gratings enables us to cover the wavelength range 1–25 nm. Test observations with the Tokyo electron beam ion trap demonstrate the high performance of the present spectrometer such as a resolving power of above 1000.
82(2011); http://dx.doi.org/10.1063/1.3613958View Description Hide Description
A detector with high dynamic range designed for combined small- and wide-angle x-ray scattering experiments has been developed. It allows measurements on single events and reactive systems, such as particle formation in flames and evaporation of levitating drops. The detector consists of 26 channels covering a region from 0.5° to 60° and it provides continuous monitoring of the sampled signal without readout dead time. The time resolution for fast single events is about 40 μs and for substances undergoing slower dynamics, the time resolution is set to 0.1 or 1 s with hours of continuous sampling. The detector has been used to measure soot particle formation in a flame, burning magnesium and evaporation of a toluene drop in a levitator. The results show that the detector can be used for many different applications with good outcomes and large potential.
Interface for time-resolved electrochemical infrared microspectroscopy using synchrotron infrared radiation82(2011); http://dx.doi.org/10.1063/1.3624693View Description Hide Description
A description of a coupled electrochemical and spectrometer interface using synchrotron infrared radiation is provided. The interface described allows for the precise and accurate timing needed for time-resolved IR spectroscopic studies of electrochemical systems. The overall interface uses a series of transistor-transistor logic trigger signals generated from the commercial FTIRspectrometer to regulate the recording of control, electrochemical, and IR signals with reproducible and adjustable timing. The instrument has been tested using a thin-layer electrochemical cell with synchrotron light focused through microscope optics. The time-resolved response of the benzoquinone/dihydroxybenzoquinone redox couple is illustrated as an example of the instrument's capability.
82(2011); http://dx.doi.org/10.1063/1.3624694View Description Hide Description
We present a tomographic characterization of gas jets employed for high-intensity laser-plasma interaction experiments where the shape can be non-symmetrically. With a Mach-Zehnder interferometer we measured the phase shift for different directions through the neutral density distribution of the gas jet. From the recorded interferograms it is possible to retrieve 3-dimensional neutral density distributions by tomographic reconstruction based on the filtered back projections. We report on criteria for the smallest number of recorded interferograms as well as a comparison with the widely used phase retrieval based on an Abel inversion. As an example for the performance of our approach, we present the characterization of nozzles with rectangular openings or gas jets with shock waves. With our setup we obtained a spatial resolution of less than 60 μm for an Argon density as low as 2 × 1017 cm−3.
An LIF characterization of supersonic BO (X2Σ+) and CN (X2Σ+) radical sources for crossed beam studies82(2011); http://dx.doi.org/10.1063/1.3624695View Description Hide Description
Various ablation sources generating supersonic boron monoxide (BO; X2Σ+) radical beams utilizing oxygen (O2), carbon dioxide (CO2), methanol (CH3OH), and water (H2O) as seeding gases were characterized in a crossed molecular beams setup by mass resolved time-of-flight spectroscopy and spectroscopically via laser induced fluorescence. Intensities of the sources as well as rovibrational energy distributions were analyzed. The molecular oxygen source was found to produce excessive amount of an unwanted BO2 byproduct. Internal vibrational energy of boron monoxide generated in the water and methanol sources was too high to be considered for the study of dynamics of ground state radicals. The best combination of intensity, purity, and low internal energy was found in the carbon dioxide source to generate boron monoxide. We successfully tested the boron monoxide (BO; X2Σ+) radical beam source in crossed beamsreactions with acetylene (C2H2) and ethylene (C2H4). The source was also compared with supersonic beams of the isoelectronic cyano (CN; X2Σ+) radical.
82(2011); http://dx.doi.org/10.1063/1.3626903View Description Hide Description
We use a double-passed acousto-opticmodulator (AOM), driven by an arbitrary waveform generator to produce multiple frequency components for a laser with arbitrary frequency spacings. A programmed sequence containing various sections of radio-frequency sinusoidal signal at different frequency is applied to drive the AOM. The diffracted light is used to injection-lock a diode laser. The combined techniques allow us to generate the multi-line spectra for the diode laser with arbitrary frequency spacings in the range of 100 MHz at a relatively high output power of 80 mW and a small power variation of 2%. Such a light source can be used in the application for laser cooling of molecules.
82(2011); http://dx.doi.org/10.1063/1.3627444View Description Hide Description
The use of Raman measurements to examine molecular changes associated with shock-induced structural and chemical changes in condensed materials often poses two challenging requirements: high spectral resolution and significantly reduced background light. Here, we describe an experimental method that addresses these requirements and provides better quality data than the time resolved approach used previously. Representative measurements are presented for shock compression of two energetic crystals: pentaerythritol tetranitrate and cyclotrimethylene trinitramine. The high spectral resolution data have provided insight into molecular changes that could not be obtained from lower-resolution, time-resolved methods.
82(2011); http://dx.doi.org/10.1063/1.3627535View Description Hide Description
For heterodyne phase locking, frequency division of the beat note between two oscillators can improve the reliability of the phase lock and the quality of the phase synchronization. Frequency division can also reduce the size, weight, power, and cost of the instrument by excluding the microwavesynthesizer from the control loop when the heterodyne offset frequency is large (5 to 10 GHz). We have experimentally tested the use of a frequency divider in an optical phase-lock loop and compared the achieved level of residual phase fluctuations between two diode lasers with that achieved without the use of a frequency divider. The two methods achieve comparable phase stability provided that sufficient loop gain is maintained after frequency division to preserve the required bandwidth. We have also numerically analyzed the noise properties and internal dynamics of phase-locked loops subjected to a high level of phase fluctuations, and our modeling confirms the expected benefits of having an in-loop frequency divider.
- Particle Sources, Optics and Acceleration; Particle Detectors
82(2011); http://dx.doi.org/10.1063/1.3617475View Description Hide Description
Solid-state nuclear track detectors, such as CR-39, are widely used in physics and in many inertial confinement fusion(ICF) experiments. In the ICF experiments, the particles of interest, such as D3He-protons, have ranges of order of the detector thickness. In this case, the dynamic range of the detector can be extended by recording data on both the front and back sides of the detector. Higher energy particles which are undetectable on the front surface can then be measured on the back of the detector. Studies of track formation under the conditions on the front and back of the detector reveal significant differences. Distinct front and back energy calibrations of CR-39 are therefore necessary and are presented for protons. Utilizing multiple surfaces with additional calibrations can extend the range of detectable energies on a single piece of CR-39 by up to 7-8 MeV. The track formation process is explored with a Monte Carlo code, which shows that the track formation difference between front and back is due to the non-uniform ion energy deposition in matter.
An ultra-low energy (30–200 eV) ion-atomic beam source for ion-beam-assisted deposition in ultrahigh vacuum82(2011); http://dx.doi.org/10.1063/1.3622749View Description Hide Description
The paper describes the design and construction of an ion-atomic beamsource with an optimized generation of ions for ion-beam-assisted deposition under ultrahigh vacuum (UHV) conditions. The source combines an effusion cell and an electron impact ion source and produces ion beams with ultra-low energies in the range from 30 eV to 200 eV. Decreasing ion beam energy to hyperthermal values (≈101 eV) without loosing optimum ionization conditions has been mainly achieved by the incorporation of an ionization chamber with a grid transparent enough for electron and ion beams. In this way the energy and current density of nitrogen ion beams in the order of 101 eV and 101 nA/cm2, respectively, have been achieved. The source is capable of growing ultrathin layers or nanostructures at ultra-low energies with a growth rate of several MLs/h. The ion-atomic beamsource will be preferentially applied for the synthesis of GaN under UHV conditions.
A Thomson parabola ion imaging spectrometer designed to probe relativistic intensity ionization dynamics of nanoclusters82(2011); http://dx.doi.org/10.1063/1.3624698View Description Hide Description
Conventional techniques of probing ionizationdynamics at relativistic intensities for extended target systems such as clusters are difficult both due to problems of achieving good charge resolution and signal integration over the focal volume. Simultaneous measurement of arrival time, necessary for these systems, has normally involved complicated methods. We designed and developed a Thomson parabola imaging spectrometer that overcomes these problems. Intensity sampling method evolved in this report is proved to be mandatory for probing ionizationdynamics of clusters at relativistic intensities. We use this method to measurecharge resolved kinetic energy spectra of argon nanoclusters at intensities of 4 × 1018 W cm−2.
Profiling of barrier capacitance and spreading resistance using a transient linearly increasing voltage technique82(2011); http://dx.doi.org/10.1063/1.3626796View Description Hide Description
A technique for the combined measurement of barrier capacitance and spreading resistance profiles using a linearly increasing voltage pulse is presented. The technique is based on the measurement and analysis of current transients, due to the barrier and diffusioncapacitance, and the spreading resistance, between a needle probe and sample. To control the impact of deep traps in the barrier capacitance, a steady state bias illumination with infrared light was employed. Measurements of the spreading resistance and barrier capacitance profiles using a stepwise positioned probe on cross sectioned silicon pin diodes and pnp structures are presented.
82(2011); http://dx.doi.org/10.1063/1.3626932View Description Hide Description
A new type of pulse chopper called an Einzel lens chopper is described. An Einzel lens placed immediately after an electron cyclotron resonance ion source is driven by high-voltage pulses generated by a newly developed solid-state Marx generator. A rectangular negative barrier pulse-voltage is controlled in time, and the barrier pulse is turned on only when a beam pulse is required. The results of successful experiments are reported herein.
- Nuclear Physics, Fusion and Plasmas
Development of a real time monitor and multivariate method for long term diagnostics of atmospheric pressure dielectric barrier discharges: Application to He, He/N2, and He/O2 discharges82(2011); http://dx.doi.org/10.1063/1.3624743View Description Hide Description
In this paper we present the development and application of a real time atmospheric pressuredischarge monitoring diagnostic. The software based diagnostic is designed to extract latent electrical and optical information associated with the operation of an atmospheric pressuredielectric barrier discharge (APDBD) over long time scales. Given that little is known about long term temporal effects in such discharges, the diagnostic methodology is applied to the monitoring of an APDBD in helium and helium with both 0.1% nitrogen and 0.1% oxygen gas admixtures over periods of tens of minutes. Given the large datasets associated with the experiments, it is shown that this process is much expedited through the novel application of multivariate correlations between the electrical and optical parameters of the corresponding chemistries which, in turn, facilitates comparisons between each individual chemistry also. The results of these studies show that the electrical and optical parameters of the discharge in helium and upon the addition of gas admixtures evolve over time scales far longer than the gas residence time and have been compared to current modelling works. It is envisaged that the diagnostic together with the application of multivariate correlations will be applied to rapid system identification and prototyping in both experimental and industrial APDBD systems in the future.
82(2011); http://dx.doi.org/10.1063/1.3622747View Description Hide Description
Reflectometry profile measurement requires an accurate determination of the plasma reflected signal. Along with a good resolution and a high signal to noise ratio of the phase measurement, adequate data analysis is required. A new data processing based on time-frequency tomographic representation is used. It provides a clearer separation between multiple components and improves isolation of the relevant signals. In this paper, this data processing technique is applied to two sets of signals coming from two different reflectometer devices used on the Tore Supra tokamak. For the standard density profile reflectometry, it improves the initialization process and its reliability, providing a more accurate profile determination in the far scrape-off layer with density measurements as low as 1016 m−1. For a second reflectometer, which provides measurements in front of a lower hybrid launcher, this method improves the separation of the relevant plasma signal from multi-reflection processes due to the proximity of the plasma.
82(2011); http://dx.doi.org/10.1063/1.3627609View Description Hide Description
The tritium plasma experiment (TPE) is a unique facility devoted to experiments on the behavior of deuterium/tritium in toxic (e.g., beryllium) and radioactive materials for fusion plasma-wall interaction studies. A Langmuir probe was added to the system to characterize the plasma conditions in TPE. With this new diagnostic, we found the achievable electron temperature ranged from 5.0 to 10.0 eV, the electron density varied from 5.0 × 1016 to 2.5 × 1018 m−3, and the ion flux density varied between 5.0 × 1020 to 2.5 × 1022 m−2 s−1 along the centerline of the plasma. A comparison of these plasma parameters with the conditions expected for the plasma facing components (PFCs) in ITER shows that TPE is capable of achieving most (∼800 m2 of 850 m2 total PFCs area) of the expected ion flux density and electron density conditions.
- Microscopy and Imaging
Compact device for cleaning scanner-mounted scanning tunneling microscope tips using electron bombardment82(2011); http://dx.doi.org/10.1063/1.3622519View Description Hide Description
Most scanning probe techniques rely on the assumption that both sample and tip are free from adsorbates, residues, and oxide not deposited intentionally. Getting a clean sample surface can be readily accomplished by applying ion sputtering and subsequent annealing, whereas finding an adequate treatment for tips is much more complicated. The method of choice would effectively desorb undesired compounds without reducing the sharpness or the general geometry of the tip. Several devices which employ accelerated electrons to achieve this are described in the literature. To minimize both the effort to implement this technique in a UHV chamber and the overall duration of the cleaning procedure, we constructed a compact electron source fitted into a sample holder, which can be operated in a standard Omicron variable-temperature (VT)-STM while the tip stays in place. This way a maximum of compatibility with existing systems is achieved and short turnaround times are possible for tip cleaning.
Systematic analyses of vibration noise of a vibration isolation system for high-resolution scanning tunneling microscopes82(2011); http://dx.doi.org/10.1063/1.3622507View Description Hide Description
We designed and constructed an effective vibration isolation system for stable scanning tunneling microscopymeasurements using a separate foundation and two vibration isolation stages (i.e., a combination of passive and active vibration isolation dampers). Systematic analyses of vibration data along the horizontal and vertical directions are present, including the vibration transfer functions of each stage and the overall vibration isolation system. To demonstrate the performance of the system, tunneling current noise measurements are conducted with and without the vibration isolation. Combining passive and active vibration isolation dampers successfully removes most of the vibration noise in the tunneling current up to 100 Hz. These comprehensive vibration noise data, along with details of the entire system, can be used to establish a clear guideline for building an effective vibration isolation system for various scanning probe microscopes and electron microscopes.