Supersonic Beam Research Articles

Structural and morphological phase control by supersonic beams on titanyl phthalocyanine: An investigation on the growth

Titanyl Phthalocyanine (TiOPc) is a cyclic oligomer widely used in organic electronics for its good stability, optical properties and charge mobility. Several polymorphs of TiOPc are known, showing different physical properties and consequently, in particular in the growth of thin films, the full control over the crystal phase formation is critical to effectively tune the conductive properties of the material. As a consequence, the availability of growth techniques allowing a precise control of both the morphology and crystal phase of the obtained films is crucial for the production of devices. In this work the structural properties and polymorphism of titanyl phthalocyanine thin films, grown on silicon substrates, have been systematically studied by synchrotron radiation grazing incidence X-rays diffraction and atomic force microscopy. Films obtained by the use of hyperthermal seeded supersonic beams technique allowed to unveil the key role played by the kinetic energy of the molecules in stabilizing specific polymorphs of TiOPc. The different growth conditions lead to grain dimensions in a range from the nanometric to the micrometric scale, depending on the substrate temperature and on the kinetic energy of the beam, while a high degree of fiber-like crystallographic order is observed in all the analyzed samples. The excellent control over phase selection, grain size and shape together with the production of well oriented high quality crystals makes of the hyperthermal seeded supersonic beams technique a promising tool for the realization of TiOPc thin films with structural and morphologic properties suitable for electronic application.

Simple method to gas cluster size determination based on molecular beam cross-section

A new experimental method of gas cluster size determination based on measuring the transverse profile of the total intensity of a molecular beam was developed. Since the supersonic beam has a high intensity, the proposed method not requires high-sensitivity detecting equipment. By using this method it can determine the mean cluster size in situ not only of any pure gases, but in gas mixtures under certain conditions as well. Experimental measurements were carried out in jets of pure gases (Ar, CO2, N2) and gas mixture (10% С2Н4 + 90% Не) expansions through supersonic conical nozzles. The Hagena scaling parameter of the source conditions Г* varied from 102 to 2 × 104. The mean cluster sizes defined by new method were in the range of 50–1500 molecules/cluster. Comparison of the experimental data to the results of numerical simulations and estimates using the empirical formulae developed elsewhere shows a satisfactory agreement.

Velocity-selected magnetic guiding of Zeeman-decelerated hydrogen atoms

An original design of magnetic guide is presented, suitable for use with Zeeman-decelerated supersonic beams of ground-state hydrogen atoms and other light paramagnetic species. Three-dimensional particle trajectory simulations show that, by combining a series of permanent-magnet Halbach arrays with pulsed high-current wire electromagnets, this guide can be used to efficiently transmit the slow, decelerated atoms and discard the faster, undecelerated atoms and other species in the gas beam. The curved guide would be suitable for guiding hydrogen atoms into an ion trap to investigate low temperature ion-molecule collisions. It is also shown that the device could be used for the guiding or velocity selection from an undecelerated supersonic or effusive beam.

Doping He droplets by laser ablation with a pulsed supersonic jet source.

Laser ablation offers the possibility to study a rich number of atoms, molecules, and clusters in the gas phase. By attaching laser ablated materials to helium nanodroplets, one can gain highly resolved spectra of isolated species in a cold, weakly perturbed system. Here, we present a new setup for doping pulsed helium nanodroplet beams by means of laser ablation. In comparison to more well-established techniques using a continuous nozzle, pulsed nozzles show significant differences in the doping efficiency depending on certain experimental parameters (e.g., position of the ablation plume with respect to the droplet formation, nozzle design, and expansion conditions). In particular, we demonstrate that when the ablation region overlaps with the droplet formation region, one also creates a supersonic beam of helium atoms seeded with the sample material. The processes are characterized using a surface ionization detector. The overall doping signal is compared to that of conventional oven cell doping showing very similar dependence on helium stagnation conditions, indicating a comparable doping process. Finally, the ablated material was spectroscopically studied via laser induced fluorescence.

Nanomanufacturing of titania interfaces with controlled structural and functional properties by supersonic cluster beam deposition

Great emphasis is placed on the development of integrated approaches for the synthesis and the characterization of ad hoc nanostructured platforms, to be used as templates with controlled morphology and chemical properties for the investigation of specific phenomena of great relevance in interdisciplinary fields such as biotechnology, medicine, and advanced materials. Here, we discuss the crucial role and the advantages of thin film deposition strategies based on cluster-assembling from supersonic cluster beams. We select cluster-assembled nanostructured titania (ns-TiO2) as a case study to demonstrate that accurate control over morphological parameters can be routinely achieved, and consequently, over several relevant interfacial properties and phenomena, like surface charging in a liquid electrolyte, and proteins and nanoparticles adsorption. In particular, we show that the very good control of nanoscale morphology is obtained by taking advantage of simple scaling laws governing the ballistic deposition regime of low-energy, mass-dispersed clusters with reduced surface mobility.

“The Even-Lavie valve as a source for high intensity supersonic beam”

We use extensive computer simulation to design (and test) a high pressure, fast acting, pulsed, valve that can produce short pulses of gas. We use flow simulations from shaped nozzles to optimize the beam density and finally use low density simulation to decide on the best skimmer shape and placement. All these details are crucial to operate a high intensity beam machine.

State purified deceleration of SD radicals by a Stark decelerator

ABSTRACTDeceleration of supersonic beams of SD radicals by a linear Stark decelerator was demonstrated for the first time. SD radicals produced by an electric discharge of supersonic beams mainly occupied the lowest two rotational states of and in the 2Π3/2 state. Laser-induced fluorescence of the decelerated beam showed that, due to the proximity of the Stark shift between these two rotational states, the final radical packet still contained both the and rotational states when the decelerator was operated at a constant phase below 50°. We show that by controlling the timing sequence of the Stark decelerator, it is possible to create a decelerated molecular packet containing only a single rotational state of either or of the SD radicals with a tunable velocity. The creation of a cold radical beam containing a single rotation–vibration state will be a useful tool for the study of cold collisions and reactions in the quantum regime.

Efforts to achieve high-performance long-pulse operations in the EAST

To achieve long-pulse tokamak operation, sufficient current drive and self-generated current are required, with the challenges of the exhaust of the heat from the divertor plates. Experiments have proven that lower hybrid current drive (LHCD) can broaden the divertor power footprint and cause the splitting of the strike point current and hence reduce the peak heat flux on the divertors. Edge localized mode (ELM) mitigation can be realized by supersonic molecule beam injection (SMBI), modulated LHCD, lithium granule and aerosol injection, as well as resonant magnetic perturbation (RMP). Enhanced transport by an electrostatic edge coherent mode at the pedestal region is observed in the ELM mitigated plasmas by LHCD. Long-pulse H-mode plasmas in the small ELMy regime have been demonstrated by a combination of ELM mitigation techniques and the optimization of the plasma confinement performance. These newly achieved H-mode scenarios by using features of LHCD in the control of steady-state peak heat flux and transient heat flux due to ELMs may offer a promising regime for further EAST long-pulse high-performance operation and be applicable to ITER.

Rydberg–Stark states in oscillating electric fields

Experimental and theoretical studies of the effects of weak radio-frequency electric fields on Rydberg–Stark states with electric dipole moments as large as 10,000 D are reported. High-resolution laser spectroscopic studies of Rydberg states with principal quantum number n = 52 and 53 were performed in pulsed supersonic beams of metastable helium with the excited atoms detected by pulsed electric field ionisation. Experiments were carried out in the presence of sinusoidally oscillating electric fields with frequencies of 20 MHz, amplitudes of up to 120 mV/cm, and dc offsets of up to 4.4 V/cm. In weak fields, the experimentally recorded spectra are in excellent agreement with the results of calculations carried out using Floquet methods to account for electric dipole couplings in the oscillating fields. This highlights the validity of these techniques for the accurate calculation of the Stark energy level structure in such fields, and the limitations of the calculations in stronger fields where n-mixing and higher order contributions become important.

Trapping molecules on chips

In the last years, it was demonstrated that neutral molecules can be loaded on a microchip directly from a supersonic beam. The molecules are confined in microscopic traps that can be moved smoothly over the surface of the chip. Once the molecules are trapped, they can be decelerated to a standstill, for instance, or pumped into selected quantum states by laser light or microwaves. Molecules are detected on the chip by time-resolved spatial imaging, which allows for the study of the distribution in the phase space of the molecular ensemble.

A Terahertz VRT spectrometer employing quantum cascade lasers

The first application of a commercial Terahertz quantum cascade laser (QCL) system for high resolution spectroscopy of supersonic beams is presented. The QCLs exhibited continuous linear voltage tuning over a 2GHz range about a center frequency of 3.762THz with ∼1ppm resolution. A sensitivity of ∼1ppm fractional absorption was measured with a single pass optical system. Multipass operation at the quantum noise limit of the stressed photoconductor detector would produce a 100-fold improvement.

Slowing dynamics of a supersonic beam, simulation and experiments

In this paper we present numerical and experimental methods aimed to study the evolution in space and time of a slowed supersonic beam. These generic methods are applicable to a variety of beams and decelerating techniques. The present implemented experimental set up is based upon Zeeman slowing of a metastable atom beam. The detection uses a channel-electron multiplier and a delay-line detector allowing time-of-flight analysis and numerical image reconstruction. In particular a depopulation effect at the centre of the beam is evidenced. In view of quantifying the slowing process, Monte Carlo calculations based on rate-equations are detailed.

Probing interactions between Rydberg atoms with large electric dipole moments in amplitude-modulated electric fields

Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number $n=53$ and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\sim10^9$ cm$^{-3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to 4 atoms. The use of these Rydberg-Stark states as sensors for non-resonant broadband radio-frequency electrical noise is also discussed.

Smoluchowski Equations for Agglomeration in Conditions of Variable Temperature and Pressure and a New Scaling of Rate Constants: Application to Nozzle-Beam Expansion.

The Smoluchowski equations provide a rigorous and efficient means for including multiple kinetic pathways when modeling coalescence growth systems. Originally written for a constant temperature and volume system, the equations must be modified if temperature and pressure vary during the coalescence time. In this paper, the equations are generalized, and adaptations appropriate to the situation presented by supersonic nozzle beam expansions are described. Given rate constants for all the cluster-cluster reactions, solution of the Smoluchowski equations would yield the abundances of clusters of all sizes at all times. This is unlikely, but we show that if these rate constants scale with the sizes of the reacting partners, the asymptotic (large size and large time) form of the cluster size distribution can be predicted. Experimentally determined distributions for He fit the predicted asymptotic distribution very well. Deviations between predicted and observed distributions allow identification of special cluster sizes that is, magic numbers. Furthermore, fitting an observed distribution to the theoretical form yields the base agglomeration cross section, from which all cluster-cluster rate constants may be obtained by scaling. Comparing the base cross section to measures of size and reactivity gives information about the coalescence process.

The Interaction of C60 on Si(111) 7 × 7 Studied by Supersonic Molecular Beams: Interplay between Precursor Kinetic Energy and Substrate Temperature in Surface Activated Processes

Buckminsterfullerene (C60) is a molecule fully formed of carbon that can be used, owing to its electronic and mechanical properties, as “clean” precursor for the growth of carbon-based materials, ranging from -conjugated systems (graphenes) to synthesized species, e.g. carbides such as silicon carbide (SiC). To this goal, C60 cage rupture is the main physical process that triggers material growth. Cage breaking can be obtained either thermally by heating up the substrate to high temperatures (630°C), after C60 physisorption, or kinetically by using Supersonic Molecular Beam Epitaxy (SuMBE) techniques. In this work, aiming at demonstrating the growth of SiC thin films by C60 supersonic beams, we present the experimental investigation of C60 impacts on Si(111) 7x7 kept at 500°C for translational kinetic energies ranging from 18 to 30 eV. The attained kinetically activated synthesis of SiC submonolayer films is probed by in-situ surface electron spectroscopies (XPS and UPS). Furthermore, in these experimental conditions the C60-Si(111) 7×7 collision has been studied by computer simulations based on a tight-binding approximation to Density Functional Theory, DFT. Our theoretical and experimental findings point towards a kinetically driven growth of SiC on Si, where C60 precursor kinetic energy plays a crucial role, while temperature is relevant only after cage rupture to enhance Si and carbon reactivity. In particular, we observe a counterintuitive effect in which for low kinetic energy (below 22 eV), C60 bounces back without breaking more effectively at high temperature due to energy transfer from excited phonons. At higher kinetic energy (22 < K < 30 eV), for which cage rupture occurs, temperature enhances reactivity without playing a major role in the cage break. These results are in good agreement with ab-initio molecular dynamics simulations. SuMBE is thus a technique able to drive materials growth at low temperature regime.

Simultaneous deceleration of atoms and molecules in a supersonic beam

A unique property of Zeeman effect based manipulation of paramagnetic particle’s motion is the ability to control velocities of both atoms and molecules. In particular the moving magnetic trap decelerator is capable of slowing and eventually trapping mixtures of both cold atoms and cold molecules generated in a supersonic expansion. Here we report the deceleration of molecular oxygen together with metastable argon atoms. The cold mixture with temperature below 1 K is slowed from an initial velocity of 430 m s−1 down to 100 m s−1. Our decelerator spans 2.4 m and consists of 480 quadrupole traps. Our results pave the way for the study of sympathetic cooling of molecules by laser cooled atoms.

Tribological coatings for complex mechanical elements produced by supersonic cluster beam deposition of metal dichalcogenide nanoparticles

Fullerene-like MoS2 and WS2 nanoparticles can be used as building blocks for the fabrication of fluid and solid lubricants. Metal dichalcogenide films have a very low friction coefficient in vacuum, therefore they have mostly been used as solid lubricants in space and vacuum applications. Unfortunately, their use is significantly hampered by the fact that in the presence of humidity, oxygen and moisture, the low-friction properties of these materials rapidly degrade due to oxidation. The use of closed-cage MoS2 and WS2 nanoparticles may eliminate this problem, although the fabrication of lubricant thin films starting from dichalcogenide nanoparticles is, to date, a difficult task. Here we demonstrate the use of supersonic cluster beam deposition for the coating of complex mechanical elements (angular contact ball bearings) with nanostructured MoS2 and WS2 thin films. We report structural and tribological characterization of the coatings in view of the optimization of tribological performances for aerospace applications.

A compact design for a magnetic synchrotron to store beams of hydrogen atoms

We present a design for an atomic synchrotron consisting of 40 hybrid magnetic hexapole lenses arranged in a circle. We show that for realistic parameters, hydrogen atoms with a velocity up to 600 m s−1 can be stored in a 1 m diameter ring, which implies that the atoms can be injected in the ring directly from a pulsed supersonic beam source. This ring can be used to study collisions between stored hydrogen atoms and supersonic beams of many different atoms and molecules. The advantage of using a synchrotron is two-fold: (i) the collision partners move in the same direction as the stored atoms, resulting in a small relative velocity and thus a low collision energy, and (ii) by storing atoms for many round-trips, the sensitivity to collisions is enhanced by a factor of 100–1000. In the proposed ring, the cross-sections for collisions between hydrogen, the most abundant atom in the universe, with any atom or molecule that can be put in a beam, including He, H2, CO, ammonia and OH can be measured at energies below 100 K. We discuss the possibility of using optical transitions to load hydrogen atoms into the ring without influencing the atoms that are already stored. In this way it will be possible to reach high densities of stored hydrogen atoms.

A chopper system for shortening the duration of pulsed supersonic beams seeded with NO or Br2 down to 13 μs.

A chopper wheel construct is used to shorten the duration of a molecular beam to 13 μs. Molecular beams seeded with NO or with Br2 and an initial pulse width of ≥200 μs were passed through a spinning chopper wheel, which was driven by a brushless DC in vacuo motor at a range of speeds, from 3000 rpm to 80,000 rpm. The resulting duration of the molecular-beam pulses measured at the laser detection volume ranged from 80 μs to 13 μs and was the same for both NO and Br2. The duration is consistent with a simple analytical model, and the minimum pulse width measured is limited by the spreading of the beam between the chopper and the detection point as a consequence of the longitudinal velocity distribution of the beam. The setup adopted here effectively eliminates buildup of background gas without the use of a differential pumping stage, and a clean narrow pulse is obtained with low rotational temperature.

Plastic and Flexible Micro-Electrochemical Supercapacitor Based on Cluster-Assembled Carbon

Flexible, thin micro-electrochemical supercapacitors (MES) are attracting much attention in electronics for their capability of delivering short power pulses with high stability over repeated charge/discharge cycling. Supersonic Cluster Beam Deposition (SCBD) is an effective strategy for the development of nanostructured, binder-free porous carbon electrodes. It consists in the deposition of low kinetic energy clusters producing films with very low-density and high porosity. The use of SCBD for carbon cluster deposition allows to obtain very high deposition rates and highly collimated beam (divergence <20 mrad) which are suitable for the room temperature deposition of patterned nanostructured films by using stencil masks on a wide variety of substrates including polymers. We exploited SCBD, for the development of a planar, high voltage MES featuring nanostructured carbon electrodes deposited on a flexible plastic Mylar substrate and ionic liquid-based electrolyte. Here, the electrochemical performance at different temperatures of the MES which operate above 2.5 V with a capacitance density approaching approaching 10 F cm-3,and delivering 9 mWh cm-3 and 2.5 W cm-3 above room temperature is reported. ACKNOWLEDGMENTS RFO, Ricerca Fondamentale Orientata, is acknowledged for financial support.