Abstract Pulsed dielectric barrier discharges (DBD) in He–H 2 O and He–H 2 O–O 2 mixtures are studied in near atmospheric conditions using temporally and spatially resolved quantitative 2D imaging of the hydroxyl radical (OH) and hydrogen peroxide (H 2 O 2 ). The primary goal was to detect and quantify the production of these strongly oxidative species in water-laden helium discharges in a DBD jet configuration, which is of interest for biomedical applications such as disinfection of surfaces and treatment of biological samples. Hydroxyl profiles are obtained by laser-induced fluorescence (LIF) measurements using 282 nm laser excitation. Hydrogen peroxide profiles are measured by photo-fragmentation LIF (PF-LIF), which involves photo-dissociating H 2 O 2 into OH with a 212.8 nm laser sheet and detecting the OH fragments by LIF. The H 2 O 2 profiles are calibrated by measuring PF-LIF profiles in a reference mixture of He seeded with a known amount of H 2 O 2 . OH profiles are calibrated by measuring OH-radical decay times and comparing these with predictions from a chemical kinetics model. Two different burst discharge modes with five and ten pulses per burst are studied, both with a burst repetition rate of 50 Hz. In both cases, dynamics of OH and H 2 O 2 distributions in the afterglow of the discharge are investigated. Gas temperatures determined from the OH-LIF spectra indicate that gas heating due to the plasma is insignificant. The addition of 5% O 2 in the He admixture decreases the OH densities and increases the H 2 O 2 densities. The increased coupled energy in the ten-pulse discharge increases OH and H 2 O 2 mole fractions, except for the H 2 O 2 in the He–H 2 O–O 2 mixture which is relatively insensitive to the additional pulses.
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Pulsed Laser-Assisted Helium Ion Nanomachining of Monolayer Graphene—Direct-Write Kirigami Patterns
A helium gas field ion source has been demonstrated to be capable of realizing higher milling resolution relative to liquid gallium ion sources. One drawback, however, is that the helium ion mass is prohibitively low for reasonable sputtering rates of bulk materials, requiring a dosage that may lead to significant subsurface damage. Manipulation of suspended graphene is, therefore, a logical application for He+ milling. We demonstrate that competitive ion beam-induced deposition from residual carbonaceous contamination can be thermally mitigated via a pulsed laser-assisted He+ milling. By optimizing pulsed laser power density, frequency, and pulse width, we reduce the carbonaceous byproducts and mill graphene gaps down to sub 10 nm in highly complex kiragami patterns.
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- Award ID(s):
- 1853201
- NSF-PAR ID:
- 10124062
- Date Published:
- Journal Name:
- Nanomaterials
- Volume:
- 9
- Issue:
- 10
- ISSN:
- 2079-4991
- Page Range / eLocation ID:
- 1394
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N2mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N2), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters ([Formula: see text]) and polyacetylene cations (HC2 nH+) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected [Formula: see text] and Au2Ag+clusters.
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Helium-4 in the superfluid phase (He II) is a two-fluid system that exhibits fascinating quantum hydrodynamics with important scientific and engineering applications. However, the lack of high-precision flow measurement tools in He II has impeded the progress in understanding and utilizing its hydrodynamics. In recent years, there have been extensive efforts in developing quantitative flow visualization techniques applicable to He II. In particular, a powerful molecular tagging velocimetry (MTV) technique, based on tracking thin lines of He2 excimer molecules created via femtosecond laser-field ionization in helium, has been developed in our laboratory. This technique allows unambiguous measurement of the normal fluid velocity field in the two-fluid system. Nevertheless, there are two limitations to this technique: (1) only the velocity component perpendicular to the tracer line can be measured; and (2) there is an inherent error in determining the perpendicular velocity. In this paper, we discuss how these issues can be resolved by advancing the MTV technique. We also discuss two novel schemes for tagging and producing He2 tracers. The first method allows the creation of a tagged He2 tracer line without the use of an expensive femtosecond laser. The second method enables full-space velocity field measurement through tracking small clouds of He2 molecules created via neutron-3He absorption reactions in He II.more » « less
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Abstract The typically high U and Th contents of xenotime ([Y,HREE]PO4) make this accessory mineral a promising candidate for (U‐Th)/He thermochronometry if the4He diffusivity can be constrained well enough to estimate its closure temperature. We report new results for two4He step‐heating experiments on different‐sized fragments of a natural xenotime crystal from the Torghar district of Pakistan (FPX‐1). This material, which has a composition within the range of most natural xenotimes (72 mol % YPO4), yields a laser ablation238U/206Pb date of 28.82 ± 0.13 Ma and a (U‐Th)/He date of 15.32 ± 0.61 Ma (2
σ ). Results for our more detailed diffusion experiment display excellent linearity on an Arrhenius diagram and indicate kinetic parameters ofE = 131.4 ± 1.1 kJ/mol and ln(D 0/a 2) = 10.61 ± 0.20 ln(s −1). These results suggest that the bulk closure temperature for4He in the degassed crystal fragment is ∼75 °C for the nominal cooling rate of 10 °C/Ma. At equivalent cooling rates and for crystals with equivalent diffusion dimensions, the closure temperature for helium in xenotime is ∼20 °C lower than the closure temperature for helium in apatite. Because xenotime typically has high U and Th contents, it may serve as a high‐precision method for dating young, low‐temperature cooling histories of rocks in which it crystallized. Helium diffusion in xenotime is likely to be moderately anisotropic and moderately dependent on crystal chemistry, so closure temperature interpretations should be made cautiously. -
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Abstract We present an analysis of helium ion (He+) fraction in an altitude range from about 400 km to around 700 km and its relationship to the ion temperature (
T i ) and the vertical ion drift under solar maximum conditions. The data were obtained from the Arecibo incoherent scatter radar during 27 September to 1 October 2014 and 16–20 December 2014. The large He+fraction (>10%) lasts 15 hr per day during the winter solstice, which is 3 times larger than during fall equinox. This difference is caused by the more persistent downward ion drift in the winter. The incremental He+fraction and incrementalT i are well anticorrelated, and the anticorrelation is more prominent during the daytime. These characteristics are associated with whether O+and He+are in diffusive equilibrium. During nighttime, we show that the vertical ion flow is downward causing the He+layer peak altitude to move to an altitude of 500 km from above 650 km. According to our analysis, He+fraction has to be larger than two thirds for diffusive equilibrium to occur above the He+peak height. Therefore, above the He+peak altitude, O+and He+cannot be in diffusive equilibrium with He+being the minor species. The vertical ion flow plays an important role in determining the diurnal variation and seasonal difference of He+distribution and whether He+is in a diffusive equilibrium with O+.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/nano9101394
