H2 Beams Research Articles

Characterizing MICROMEGAS ion detection capabilities at the INFN Pisa ion beam facility

Researchers at INFN Pisa are currently developing a detector based on MICROMEGAS technology, sensitive to atoms with a kinetic energy in the 1–100keV range and operating under low-pressure conditions (100hPa or less). For the detector test, an Ion Beam Facility (IBF) was set up at the INFN laboratory in Pisa in order to have a controlled test bench capable of guaranteeing the stability and repeatability of the measurements. The group effort dedicated to the development of the IBF led to detailed characterization measurements of the MICROMEGAS performance using H2 and He ion beams.

Temperature dependent stereodynamics in surface scattering measured through subtle changes in the molecular wave function.

A magnetically manipulated molecular beam technique is used to change the rotational orientation of H2 molecules which collide with a stepped Cu(511) surface and explore how the polarisation dependence of molecules scattering into the specular channel changes as a function of surface temperature. At all temperatures, H2 molecules that are rotating like cartwheels are more likely to be scattered into the specular channel than those that are rotating like helicopters. Furthermore, the scattered molecules are more likely to be rotating like cartwheels, regardless of their state before the collision. Increasing the temperature of the Cu(511) surface causes the polarisation effects to become stronger, with the scattering becoming more selective for H2 with cartwheel like rotation. Therefore, scattering a molecular beam of H2 from a Cu(511) surface and taking the molecules scattered into the specular channel provides a method to create a rotationally polarised beam of H2, where the polarisation can be tuned by changing the surface temperature. In contrast, the rotational orientation dependence observed for specular scattering from a flat Cu(111) surface is independent of surface temperature within the same temperature range.

Hydrogen ion beam transport in an energy range lower than 50 eV: Ion beam species.

Low energy beam transport in the energy range below 50 eV is studied using a compact ion source with a high temperature tungsten cathode that can be put into a vacuum system. A 2.6 cm diameter, 5.2 cm long compact ion source successfully produced more than 2.4 nA beam of H2 + ions at 50 eV beam energy. The source also supplied a stable beam of H3 + ions at 2.5 eV energy with more than 100 pA mass-separated current. The inside wall of the ion source was covered by thin sheets of titanium, nickel, and molybdenum to see if the species composition of the ion beam can be controlled by changing the ion source wall material. Usually, the H2 + ion peak was the highest in the mass spectrum, and the H2 + ion ratio (H2 +/Hn +) was measured for a source operation with hydrogen gas. The ion species ratio drifted against time, and Mo showed the most stable ion ratio.

Low-Energy Water-Hydrogen Inelastic Collisions.

New molecular beam scattering experiments are reported for the water-hydrogen system. Integral cross sections of the first rotational excitations of para- and ortho-H2O by inelastic collisions with normal-H2 were determined by crossing a beam of H2O seeded in He with a beam of H2. H2O and H2 were cooled in the supersonic expansion down to their lowest rotational levels. Crossed-beam scattering experiments were performed at collision energies from 15 cm-1 (below the threshold for the excitation to the lowest excited rotational state of H2O: 18.6 cm-1) up to 105 cm-1 by varying the beam crossing angle. The measured state-to-state cross-sections were compared to the theoretical cross-sections (close-coupling quantum scattering calculations): the good agreement found further validates both the employed potential energy surface describing the H2O-H2 van der Waals interaction and the state-to-state rate coefficients calculated with this potential in the very low temperature range needed for the modeling of interstellar media.

Quadrupole lens alignment with improved STIM and secondary electron imaging for Proton Beam Writing

Minimal proximity effect coupled with uniform energy deposition in thin polymer layers make Proton Beam Writing (PBW) an intuitive direct-write lithographic technique. Feature sizes matching the focused beam spot size have been fabricated in photoresists down to 19nm. Reproducible sub-10nm beam focusing will make PBW a promising contender for sub-10nm lithography. In this paper, we present beam size characterization by imaging a PBW resolution standard using transmitted/scattered ions and secondary electrons. Using Scanning Transmission Ion Microscopy (STIM) spectra for 1 and 2MeV H2+ beams, we experimentally measure the thickness of the resolution standard to be 0.9±0.1μm, applying two independent calibration methods, which match the original intended thickness during fabrication. Through bias optimization of a Micro-Channel Plate (MCP), we show a tuneable secondary electron detection per proton for imaging with a maximum of 75% e/p for a beam of 1MeV H2+. Based on STIM mode beam size measurement, we discuss considerations for quadrupole system alignment in order to remove higher order translational and rotational misalignments critical to achieve sub-40nm spot sizes. A spot size of 13×32nm2 (STIM) was achieved using a newly developed interface, capable of autofocusing ion beams and performing PBW.

A new H2 (+) source: Conceptual study and experimental test of an upgraded version of the VIS-Versatile ion source.

The versatile ion source is an off-resonance microwave discharge ion source which produces a slightly overdense plasma at 2.45 GHz of pumping wave frequency extracting more than 60 mA proton beams and 50 mA He(+) beams. DAEδALUS and IsoDAR experiments require high intensities for H2 (+) beams to be accelerated by high power cyclotrons for neutrinos generation. In order to fulfill the new requirements, a new plasma chamber and injection system has been designed and manufactured for increasing the H2 (+) beam intensity. In this paper the studies for the increasing of the H2 (+)/p ratio and for the design of the new plasma chamber and injection system will be shown and discussed together with the experimental tests carried out at Istituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) and at Best Cyclotron Systems test-bench in Vancouver, Canada.

Experimental results of a dual-beam ion source for 200 keV ion implanter

A dual beam ion source for 200 keV ion implanter aimed to produce 200 keV H2 (+) and He(+) beams simultaneously has been developed. Not suitable to use the analyzing magnet, the purity of beam extracted from the source becomes important to the performance of implanter. The performance of ion source was measured. The results of experiments show that the materials of inlet tube of ion source, the time of arc ionization in ion source, and the amount of gas flow have significant influence on the purity of beam. The measures by using copper as inlet tube material, long time of arc ionization, and increasing the inlet of gas flow could effectively reduce the impurity of beam. And the method using the gas mass flow controller to adjust the proportion of H2 (+) and He(+) is feasible.

A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams

We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H2 beams (23% and 7% of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopy.

Proton beam writing nanoprobe facility design and first test results

A second generation proton beam writing line has been installed at the Centre for Ion Beam Applications at the National University of Singapore. This system features a high demagnification lens system based on compact OM52 lenses with reduced working distance. The system can operate in a variety of high demagnification configurations, including the spaced Oxford triplet and the double-crossover Russian quadruplet. It features an electrostatic scanning system and a new collimator slit aperture design. Here we present the first results with this system operating in a spaced Oxford triplet configuration with a beam focus in one direction down to 20nm. One of the limitations of the current system is the inferior quality of the high-voltage supply driving the electrostatic scanning system. The second focus of this paper is the performance of the new collimator slit aperture design for MeV nanoprobe forming systems. Cylindrically strained electroplated Ni surfaces with a bending radius of 50mm are used as slit edges. An incident molecular beam of 2MeV H2+ ions was used for the slit scattering measurements. Any scattering by the slits resulting in dissociation of the incident molecular beam into 1MeV protons facilitated the measurements of scattered and non-scattered components by the passivated implanted planar silicon (PIPS) detector. The studies showed the expected correlation between the aperture size and scattering. The quality of the beam was satisfactory even for small aperture dimensions with a very small percentage of the beam showing energy loss.

The new Versatile Ion Source (VIS) for high power proton accelerators

The characteristics of the ideal injector for high power proton accelerators have been studied in the last two years with the TRIPS ion source built at INFN-LNS, Catania and now installed at INFN-LNL, Legnaro. The beam production must obey to the request of high brightness, stability and reliability. The new Versatile Ion Source (VIS) is a permanent magnet version of the TRIPS source with a simpler extraction system and mechanics. Its design permits to operate at 80 kV without a bulky high voltage platform, in order to produce multi-mA beams of protons and H2+. The description of the source design and the expected performances will be presented.

In-Plane and Out-of-Plane Diffraction ofH2from Metal Surfaces

We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by reactive Pd(111) and nonreactive NiAl(110) surfaces at 140-150 meV. A comparison with six-dimensional quantum dynamics and classical trajectory calculations shows for the first time that accurate diffraction patterns can be obtained from state-of-the-art potential energy surfaces based on density functional theory. Our measurements show that, at general incidence conditions, out-of-plane diffraction is much more important than was assumed in previous experiments.

Computer simulation and calibration of the charge particle spectrometer-telescope ⪡STEP-F⪢

The spectrometer-telescope “STEP-F” is intended for detecting high-energy charged particle fluxes in near-Earth space. It has been developed to study solar cosmic rays and dynamics of the Earth's radiation belts onboard the Russian spacecraft ⪡Coronas-Photon⪢. The results of simulations by the CERN GEANT4.2 Code under OS LINUX Red Hat 6.2 for some types of particles, which will be detected by the “STEP-F” instrument, are presented. The visualization of primary and secondary particle tracks in each detector of the telescope has allowed us to verify the energy ranges of the recorded particles and to ascertain the possibility to determine the direction of incidence of primary electrons. The detectors have been tested in a high-energy ion cyclotron accelerator at the Institute of Physical and Chemical Research (RIKEN, Japan) in order to determine the response of each of the detectors to primary α-particle and hydrogen ion H2 beams. The experimental data obtained are in good agreement with the results of Monte-Carlo simulations. The basic parameters of the “STEP-F” instrument, such as the energy range of recorded electrons, protons and α-particles, the geometric factor, the field of view and the angular resolution, are presented.

Rotationally Resolved (1 + 1‘) Resonance-Enhanced Multiphoton Ionization (REMPI) of CaR (R = H, D) in Supersonic Beams: CaR X 2Σ+ (υ‘ ‘ = 0) → CaR* B 2Σ+ (υ‘= 0, 1) → CaR+ X Σ+

We report on a laser-spectroscopic study of the CaH and CaD radicals in a supersonic beam, generated in a reaction of laser-sputtered Ca atoms and a pulsed beam of H2 or D2, seeded in helium. The rotational levels of the electronic ground-state X 2Σ+ (υ‘ ‘ = 0, N‘ ‘ ≤ 10) were probed by two-color resonance ionization mass spectrometry(1 + 1‘) resonance-enhanced multiphonon ionization (REMPI)via the intermediate B 2Σ+ (υ‘ = 0, 1) rovibronic levels. Rotational constants for the B-state were derived, including the spin splitting parameters γυ‘, which have been determined in the case of CaD for the first time: γ0 = −0.409(7) cm-1 and γ1 = −0.398(8) cm-1 for CaD (B 2Σ+), and γ0 = −0.792(15) cm-1 and γ1 = −0.765(16) cm-1 for CaH (B 2Σ+). The comparison between the rotational level energies of the F1 and F2 spin substates of the two isotopomers revealed the onset of perturbation in CaH B 2Σ+ (υ‘ = 0), at N‘ ≅ 0, by the presence of the vibronic state A 2Π (υ‘ = 1). Although this perturbation is documented for υ‘...

Generation of intense pulsed low-kinetic-energy molecular beams

A method for the generation of intense pulsed low-kinetic-energy molecular beams is described. The method is based on the formation of a cold (≈77 K) pressure shock as a result of interaction between an intense pulsed gas-dynamically cooled molecular beam with a solid surface. The pressure shock is used as a source of a secondary beam for generating low-energy molecules. The suggested method was used to obtain intense molecular beams of H2, He, CH4, N2, and Kr with kinetic energies lower than or equal to 10 meV and H2/Kr and He/Kr molecular beams with kinetic energies of H2 and He molecules lower than 1 meV. The energy (velocity) of molecules in low-energy beams can be controlled by varying the intensity of the initial beam or temperature in the pressure shock.

Generating intense beams of low-energy molecules

A method for obtaining intense pulsed beams of molecules possessing low kinetic energies is proposed. The method is based on the formation of a cold pressure shock (shock wave) in an intense pulsed molecular beam interacting with a solid surface, which serves as a source of the secondary beam of low-energy molecules. The proposed method was successfully used to obtain intense beams of H2, He, CH4, and Kr molecules with kinetic energies not exceeding 10 meV, and H2/Kr and He/Kr beams with kinetic energies of H2 and He molecules below 1 meV.

Fast-beam study of H2+ ions in an intense femtosecond laser field

A fast beam of H2+ ions, produced from a low energy ionaccelerator, has been used for the first time in intense laser fieldexperiments. The technique has enabled neutral dissociation products tobe analysed and detected for the first time in such studies. Energyspectra of neutral and ionized fragments, product yields as a functionof focused laser intensity and angular distributions of neutraldissociation products have been measured. Significant differences areobserved between the present results and those obtained from experimentsinvolving neutral H2 molecules. These differences are indicative ofthe precursor H2 molecule playing an important and hitherto neglectedformative role in the laser-induced fragmentation processes.

A review of ion projection lithography

Although optical lithography has been extended to far smaller dimensions than was predicted 15 years ago, there are definite physical barriers to extending it to the minimum dimensions of 70 nm that are projected to be required 15 years from now. Both focused, point electron beams and ion beams have been used to write dimensions in resist well below 20 nm, albeit at speeds far too slow for production lithography. Projection systems, which employ a mask and, in effect, produce a large array of beams, can provide both small minimum dimensions and high throughput. Ions are particularly well suited for this because they suffer little or no scattering in the resist, the linewidth is not a strong function of dose (good process latitude), and the resist sensitivity is relatively independent to resist thickness or ion energy. IMS in Vienna, Austria has built two generations of ion projection lithography systems which have demonstrated many of the features needed for high throughput lithography. In these systems a stencil mask is irradiated with a uniform beam of light ions, H+, H2+, or He+, and the transmitted pattern is demagnified (by 10× to 3×) and focused on a resist covered wafer at energies in the 70–150 keV range. So far, minimum dimensions down to 70 nm line-space pairs have been demonstrated, drift has been eliminated with a “pattern lock” servo system, and field distortion of less than 0.15 μm over 8×8 mm has been measured in agreement with calculations. Based on these achievements a new generation ion lithography machine has been designed which uses 3× demagnification and will expose a 20×20 mm field at 0.12 μm minimum dimensions with less than 10 nm of distortion introduced by the ion optics. Global and stochastic space charge effects have been modeled and, in some cases, measured with the existing machines. Stochastic space charge effects will not cause unacceptable blur in the new design if the total ion current is kept below 3 μA. At this total current the time to expose one chip is still of order 0.5 s so that the calculated throughput is about 70 wafers (200 mm) per hour. Ion sources with low energy spread (∼2 eV) have been developed and will provide uniform illumination of the mask. Stencil mask fabrication on membranes of 2.5-μm-thick silicon has been developed. Distortion of the pattern cut in the membrane due to stress relief has been modeled, and with proper mask design can be kept below 20 nm (6.7 nm on the wafer). According to calculations and measurements mask distortion due to ion beam heating can be reduced to a negligible level if a radiation cooling cylinder is used. As a result of the building and evaluation of the existing machines and the design of the next generation, significant progress has been made in ion projection lithography which we will review in this article. The next step in the development of ion projection lithography is being conducted by the MEDEA program in Europe, which will develop a full field processing tool.

Inelastic focusing effects in atom-surface scattering

An inelastic focusing mechanism has been identified in the angular distributions of helium atoms scattered from single-crystal NaCl~001! surfaces as being due to energy exchange with a Rayleigh phonon mode. This effect, called inelastic focusing, occurs under special conditions in which the small spread of energies in the incident beam is sharply focused into a very narrow range of final angles. For a fixed initial scattering angle this is mathematically described as a singularity associated with the density of states in the incident momentum as a function of final scattering angle. This effect is an intrinsic feature of any scattering event of particles from surfaces and could be used to greatly enhance the intensity of weak single-phonon features in inelastic timeof-flight spectra. @S0163-1829~98!50416-X# The scattering of atoms and molecules from single-crystal surfaces has provided a wealth of information both on the physical properties of surfaces 1 and also on the interactions of the particles with the surface. Initially, most experiments concentrated on diffraction studies of the surface structure, but during the last two decades, the development of highintensity nearly monoenergetic helium-atom beams has provided sufficiently high intensities so that dynamical features of the surface such as phonon dispersion curves could also be explored. 2 In the ensuing period many of the unexpected features observed in these measurements 2‐6 were explained in terms of selective adsorption resonances, both elastic and those involving phonons, as well as kinematic focusing. 7 More recently, focused inelastic resonance ~FIR !~ Refs. 8 and 9! and diffuse elastic 10 resonance ~DER! effects have also been identified as causing distinct features in heliumatom scattering experiments. Since the interaction potential between He atoms and the NaCl~001! surface is reasonably well known, 9‐11 many of these new resonance mechanisms and focusing phenomena have first been studied on this surface. In parallel with experiments, the theory of atom-surface scattering has also undergone significant development. However, the energy resolution of the incident He-atom beam has usually been considered to have no substantial effect on the distribution of the scattering intensity. Thus, for example, in the calculations of the differential reflection coefficients for elastic and inelastic scattering the incident beam of the He atoms is usually considered to be sharp in both energy and angles, i.e., the finite energetic width of the incident beam is neglected and it is usually represented by a well-defined plane wave. In practice, this would appear to be justified since in most He-atom scattering experiments the energy resolution is usually as small as DEi /Ei’2%. If the finite angular width of the incident beam is included in the calculations several singularities appear that strongly modify the final scattering intensity. One of these new singularities, the inelastic focusing ~IF! singularity discussed here, was observed previously in the rotationally inelastic diffraction of molecular beams of H2 and its isotopes from single crystals of MgO~001 !~ Ref. 12! as well as in the inelastic scattering of D2 from NaF~001!. 13 In this communication, experimental evidence for the atomic IF effect in the scattering of He from NaCl~001! is presented. Moreover, in atomic scattering from other surfaces such as, for example, He/LiF~001!, 6 certain features were previously observed that can now be attributed to IF. In this work, the IF singularity is reexamined and found to have more far-reaching physical implications than previously envisaged. We note that the IF is a quite distinct effect from the previously investigated FIR ~Ref. 9! and DER ~Ref. 10! resonances. Both FIR and DER involve a resonant transition through a bound state of the interaction potential, whereas the IF is a kinematic enhancement of any selected inelastic feature and does not require the presence of a bound state. To experimentally demonstrate the IF singularity, highresolution helium-atom scattering experiments were carried on the well-studied NaCl~001! surface. In the scattering apparatus, which has been described in detail previously, 14 a nearly monoenergetic helium-atom beam (DEi /Ei<2%) is scattered through a fixed angle of 90° between source and detector in a plane perpendicular to the crystal surface. The parallel wave-vector transfer DK is changed by rotating the crystal and is given by 15

Understanding gas-surface interactions from direct force measurements using a specialized torsion balance

Measurements of the magnitude and direction of the forces exerted on surfaces by molecular beams are used to obtain information about the microscopic properties of gas-surface interactions. The reduced force coefficients are introduced as a new set of parameters that completely describe the macroscopic average momentum transferred to a surface by incident molecules and can be used to obtain the flux-weighted average velocity and translational energy of the scattered molecules. This approach avoids the complications associated with obtaining macroscopic average quantities from averages of microscopic measurements and can be used to determine microscopic details concerning gas-surface interactions. Using a specialized torsion balance, the reduced force coefficients were determined from measurements of the forces exerted on surfaces of a solar panel array material, Kapton, SiO2-coated Kapton, and Z-93-coated Al by molecular beams of H2, N2, CO, and CO2 as a function of the angle of incidence.

Determination of Λ-doublet resolved cross-sections for inelastic scattering of OH by para- and normal-H2

In this paper we report the measurement of Λ-doublet resolved state-to-state cross sections for inelastic collisions of OH by H2 at a translational energy of 595 cm−1. Experimental values are obtained for transitions from both the upper (f) and the lower (e) Λ-doublet substates of the lowest rotational state (J=3/2) of OH(2Π3/2) to almost all other states within the range of the collision energy. Cross sections for scattering by both para- and normal-H2 have been determined. The main difference between para- and normal-H2 scattering is seen in the Λ-doublet cross section. The results are compared to He-scattering which shows that H2 (J=0) scattering behaves similar to He scattering. When averaged over the Λ-doublet states, the cross sections are in good agreement with the measurements of Andresen et al. [J. Chem. Phys. 81, 571 (1984)], although the conclusions with regard to collisional pumping of interstellar OH masers are different. The quantum calculations of Offer et al. [J. Chem. Phys. 100, 362 (1994)] show a surprisingly good correspondence with the measured values. In this comparison the full initial state distribution of the OH and H2 beams has been taken into account.