ESR on adsorbed doubly spin-polarized atomic hydrogen

Studies of the ESR spectrum of spin-polarized atomic hydrogen adsorbed on a liquid helium film are presented. The absorption peaks associated with the surface atoms are displaced relative to that of the gaseous atoms due to the electronic dipole-dipole interaction, which does not average to zero for atoms moving in two dimensions. This phenomenon, first observed by Reynolds et al., allows the surface atom density to be measured directly and, through the lineshape, information on the dynamics of the 2-D gas can in principle be obtained. Here we present a more detailed study, with a better characterized substrate for the helium film.Using only the assumption that the ideal gas approximation is valid for the experimental conditions, we find that the binding energy of hydrogen atoms to the liquid helium surface is 1.03 (2) K. Although these measurements of the binding energy are not the most accurate, they are the most direct.The ESR lineshape of the absorption peak of the bulk atoms is determined by the inhomogeneity of the applied magnetic fields, whereas the resonance lineshape of the adsorbed atoms, which is very asymmetric and much broader than the main resonance, is clearly due to some other mechanism. In spite of a considerable effort to explain the observed lineshapes, we have not reached satisfactory conclusions.     

Thermal detection of ESR on spin-polarized hydrogen: Study of surface recombination

We introduce a new method for the detection of electron-spin resonance in spin-polarized atomic hydrogen gas (H). Instead of observing the microwave power absorbed in the ESR transition, we monitor the recombination heat deposited by ESR-induced spin-up atoms (H) onto a liquid-helium coated carbon bolometer. The signal from this sensor reproduces well the ESR absorption lineshape registered by our 128 GHz homodyne spectrometer. Using the recombination detection we have achieved a density detection limit of n=3·10¹⁰ atoms/cm³ for 0.2 nW microwave power incident on the resonant cavity at the temperature T=150 mK. We have studied the decay rate of recombination heat absorbed by the bolometer after an ESR excitation pulse and the dependence of this rate on T, n and nuclear polarization of the H sample. The bolometer signal is found to be related mainly to second-order H + H recombination to ortho-H₂ on the surfaces of the sample cell. From the signals we have determined the rate constant K _bc ^c =3.2(5)·10^–5T cm²/s·K^–1/2 in the interval from T=250 to 425 mK in a field of 4.5 T.     

Raman spectroscopy using a spatial heterodyne spectrometer: proof of concept

The use of a spatial heterodyne interferometer-based spectrometer (SHS) for Raman spectroscopy is described. The motivation for this work is to develop a small, rugged, high-resolution ultraviolet (UV) Raman spectrometer that is compatible with pulsed laser sources and that is suitable for planetary space missions. UV Raman is a particular technical challenge for space applications because dispersive (grating) approaches require large spectrographs and very narrow slits to achieve the spectral resolution required to maximize the potential of Raman spectroscopy. The heterodyne approach of the SHS has only a weak coupling of resolution and throughput, so a high-resolution UV SHS can both be small and employ a wide slit to maximize throughput. The SHS measures all optical path differences in its interferogram simultaneously with a detector array, so the technique is compatible with gated detection using pulsed lasers, important to reject ambient background and mitigate fluorescence (already low in the UV) that might be encountered on a planetary surface where samples are uncontrolled. The SHS has no moving parts, and as the spectrum is heterodyned around the laser wavelength, it is particularly suitable for Raman measurements. In this preliminary report we demonstrate the ability to measure visible wavelength Raman spectra of liquid and solid materials using an SHS Raman spectrometer and a visible laser. Spectral resolution and bandpass are also discussed. Separation of anti-Stokes and Stokes Raman bands is demonstrated using two different approaches. Finally spectral bandpass doubling is demonstrated by forming an interference pattern in both directions on the ICCD detector followed by analysis using a two-dimensional Fourier transform.     

空间外差光谱仪的干涉图校正

空间外差光谱技术(SHS)作为一种新型超光谱分辨率的光谱分析技术近年来得到了快速发展和广泛应用。根据SHS的基本结构和原理,本文对SHS应用系统中能对干涉图产生影响的各种干扰和畸变进行了分析,并针对这些干扰提出了一种SHS干涉图校正方案。实验结果表明,该方案不仅可以对干涉图进行有效校正,而且复原光谱能够良好地反映输入光谱信息,提高SHS的反演精度。     

Modeling of the generic spatial heterodyne spectrometer and comparison with conventional spectrometer

We describe the modeling of the generic spatial heterodyne spectrometer. This instrument resembles a somewhat modified Michelson interferometer, in which the power spectrum of the input source is determined by performing a one-dimensional Fourier transform on the output intensity profile. Code has been developed to analyze the performance of this type of spectrometer by determining the dependence of both spectral resolution and throughput on parameters such as aperture and field of view. An example of a heterodyne spectrometer is developed to illustrate the techniques employed in the modeling and a comparison undertaken between its performance and that of a conventional spectrometer. Unlike the traditional Fourier transform infrared system, the heterodyne spectrometer has the very desirable feature of having no moving components.     

Ion-implanted resist removal using atomic hydrogen

We removed B-, P-, and As-ion-implanted positive-tone novolak resists with an implantation dose of 5 × 10¹² to 5 × 10¹⁵ atoms/cm² at 70 keV, using atomic hydrogen. Though the removal rate decreased with increase in the implantation dose, all of the ion-implanted resists were removed. The rates of thickness of the surface-hardened layer/all resist layer of B, P, and As implanted resists were 0.38, 0.26, and 0.16, respectively, by SEM observation. The removal rate decreased with increasing the rate of the surface-hardened layer. The energy supplied from the ions to the resist concentrated on the surface side in the increasing order of B-P-As.     

High-sensitivity mid-infrared heterodyne spectrometer with a tunable diode laser as a local oscillator

A new mid-IR heterodyne spectrometer, which is intended to be applied for atmospheric and astrophysical studies, is presented. The spectrometer uses a frequency-stabilized tunable diode laser as a local oscillator. Owing to the low output power of available single-mode diode lasers, a newly developed confocal-ring resonator, the diplexer, is used to superimpose the source signal efficiently with that of the local oscillator. Additionally, the diplexer serves as an optical filter that establishes controlled optical feedback between the laser diode and the detector, which allows stable laser operation with linewidths of the order of 1 MHz. The heterodyne signal from the HgCdTe detector is analyzed by means of a 1.4-GHz acousto-optical spectrometer. With this setup we find system temperatures as low as 4400 K (double sideband), that is, approximately a factor of 6 of the quantum limit.     

SHIMMER: a spatial heterodyne spectrometer for remote sensing of earth's middle atmosphere

It is well known and demonstrated that interference spectroscopy offers capabilities to obtain passive remote optical sensing spectra of high precision and also achieves economies in size, cost, and ease of deployment compared with more conventional systems. We describe the development of a near-ultraviolet spatial heterodyne spectrometer designed for remote sensing of the global distribution of the hydroxyl radical OH in the Earth's middle atmosphere. The instrument, known as SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals), is expected to obtain its first OH measurement from space in early 2002 from the Space Shuttle.     

Optically pumped 3He NMR and atomic hydrogen ESR at 0.6T

We report the successful observation of high-fields optically pumped nuclear magnetic resonance (NMR) of helium-3 and electron spin resonance (ESR) of atomic hydrogen, both at 0.6 T. Also described are our findings on the relative optical pumping efficiencies of the various components of the 1083 nm line in helium-3. These are a precursor to a measurement of the magnetic moment of the helion in Bohr magnetons. For this measurement the two signal frequencies must be measured sucessively in the same apparatus. This work is aimed at overcoming the limitations for high-accuracy magnetometry of using water as an NMR substance     

Development of a digital micromirror spectrometer for analytical atomic spectrometry

A digital micromirror device (DMD) has been incorporated into a novel spectrometer for use in analytical atomic spectrometry. The device can be taken from a commercial computer projector. A protective glass window covering the DMD chip limits the viewable wavelengths to the visible range. The DMD is used to project an image of the light source onto the exit plane of a flat-field spectrograph. A single photomultiplier tube is used for detection. The high switching rate of the micromirrors (15 μs) enables rapid full-spectrum capture, wavelength-modulation, source-modulation, fast narrow-wavelength window scans, and rapid-wavelength "jumping." Calcium, sodium, and potassium have been determined in several standard reference materials (tomato leaves, bovine liver, rice flour, total diet) by flame atomic absorption and emission spectrometry. Absorption sensitivities for each element are near the 0.02 μg/mL level, and detection limits for both absorption and emission are near the 0.01 μg/mL level. Elemental recoveries were within 10% of certified values for most reference materials.     

Organ dose conversions from ESR measurements using tooth enamel of atomic bomb survivors

Dose conversions were studied for dosimetry of atomic bomb survivors based upon electron spin resonance (ESR) measurements of tooth enamel. Previously analysed data had clarified that the tooth enamel dose could be much larger than other organ doses from a low-energy photon exposure. The radiation doses to other organs or whole-body doses, however, are assumed to be near the tooth enamel dose for photon energies which are dominant in the leakage spectrum of the Hiroshima atomic bomb assumed in DS02. In addition, the thyroid can be a candidate for a surrogate organ in cases where the tooth enamel dose is not available in organ dosimetry. This paper also suggests the application of new Japanese voxel phantoms to derive tooth enamel doses by numerical analyses.     

Detection of atomic hydrogen in flames using picosecond two-color two-photon-resonant six-wave-mixing spectroscopy

We report an investigation of two-color six-wave-mixing spectroscopy techniques using picosecond lasers for the detection of atomic hydrogen in an atmospheric-pressure hydrogen-air flame. An ultraviolet laser at 243 nm was two-photon-resonant with the 2S(1/2) <-- <-- 1S(1/2) transition, and a visible probe laser at 656 nm was resonant with H(alpha) transitions (n=3 <-- n=2). The signal dependence on the polarization of the pump laser was investigated for a two- beam polarization-spectroscopy experimental configuration and for a four- beam grating configuration. A direct comparison of the absolute signal and background levels in the two experimental geometries demonstrated a significant advantage to using the four-beam grating geometry over the simpler two-beam configuration. Picosecond laser pulses provided sufficient time resolution to investigate hydrogen collisions in the atmospheric-pressure flame. Time-resolved two-color laser-induced fluorescence was used to measure an n=2 population lifetime of 110 ps, and time-resolved two-color six-wave-mixing spectroscopy was used to measure a coherence lifetime of 76 ps. Based on the collisional time scale, we expect that the six-wave-mixing signal dependence on collisions is significantly reduced with picosecond laser pulses when compared to laser pulse durations on the nanosecond time scale.     

On-board aircrew dosimetry using a semiconductor spectrometer

Radiation fields on board aircraft contain particles with energies up to a few hundred MeV. Many instruments have been tested to characterise these fields. This paper presents the results of studies on the use of an Si diode spectrometer to characterise these fields. The spectrometer has been in use since spring 2000 on more than 130 return flights to monitor and characterise the on-board field. During a Czech Airlines flight from Prague to New York it was possible to register the effects of an intense solar flare, (ground level event, GLE 60), which occurred on 15 April 2001. It was found that the number of deposition events registered was increased by about 70% and the dose in Si by a factor of 2.0 when compared with the presence of galactic cosmic rays alone. Directly measured data are interpreted with respect to on-earth reference field calibration (photons, CERN high-energy particles): it was found that this approach leads to encouraging results and should be followed up.     

OH absorption spectroscopy in a flame using spatial heterodyne spectroscopy

We demonstrate measurements of OH absorption spectra in the post-flame zone of a McKenna burner using spatial heterodyne spectroscopy (SHS). SHS permits high-resolution, high-throughput measurements. In this case the spectra span approximately 308-310 nm with a resolution of 0.03 nm, even though an extended source (extent of approximately 2x10(-7) m(2) rad(2)) was used. The high spectral resolution is important for interpreting spectra when multiple absorbers are present for inferring accurate gas temperatures from measured spectra and for monitoring weak absorbers. The present measurement paves the way for absorption spectroscopy by SHS in practical combustion devices, such as reciprocating and gas-turbine engines.     

Polarimetry of scattered light using heterodyne detection

Abstract

Heterodyne detection has been used to measure the polarization state of light back-scattered from various targets (including flame-sprayed aluminium, sandpaper and painted surfaces). The samples are illuminated with a linearly polarized single-frequency continuous-wave CO₂ laser operating at a wavelength of 10.6 μm. The back-scattered co-polarized and cross-polarized components are both coherently detected by beating with an optical local oscillator. This process allows the relative amplitudes and phases of the two components to be measured and hence the light's polarization state can be evaluated. When the target undergoes movement, the scattered light demonstrates the usual properties of dynamic speckle, and the technique allows observation of the time evolution of the polarization ellipse.     

Detailed studies of molecular conductance using atomic resolution scanning tunneling microscopy

Atomically resolved scanning tunneling microscopy and spectroscopy (STM/STS) have been used to carefully examine the relationship between molecular conductivity and the adsorption state of various organic molecules on silicon surfaces. We show that the particular configuration of styrene and cyclopentene molecules on silicon affects the conductivity of the molecules. Detailed correlation of STM images with point specific current-voltage spectroscopy reveal that observed negative peaks are due to random configuration changes driven by inelastically scattered electrons and not due to tuned alignment of molecule and electrode levels. These random processes, which include molecular rearrangement, desorption, and/or decomposition occur with increasing frequency at larger voltage and current settings.     

Design, fabrication and performance evaluation of a 22-channel direct reading atomic emission spectrometer using inductively coupled plasma as a source of excitation

The indigenous design, fabrication and performance evaluation of a polychromator, using inductively coupled plasma (ICP) as a source of excitation, are described. A concave holographic grating is used as the dispersing element and a Paschen — Runge mount is chosen to focus the spectra over a wide range along the Rowland circle. Twenty-two exit slits, mounted along the circle, precisely correspond to the wavelengths used for determination of up to twenty elements present in the plasma. Radiations emerging from the exit slits are detected by photomultiplier tubes placed behind them. The photomultiplier signal is recorded by an electronic system consisting of an integrator and a PC-based data acquisition system. The performance of the spectrometer has been evaluated with an ICP excitation source. Synthetic standards in deionized water containing a mixture of twenty impurities have been analysed. Typical determination limits observed for elements range from sub-ppm to ppm levels. All the elements present as impurities can be detected simultaneously. It is also observed that each element has a different emitting region in the ICP flame for which the maximum signal to the background is obtained. The determination limits obtained corresponding to these zones are the lowest. A study of the sensitive emitting zones for several elements has been carried out and the results are demonstrated by photographs of the ICP flame. The study will help in achieving the minimum value of determination limit for an impurity element.