Supersonic Beam Research Articles

Valence-shell photoelectron circular dichroism of ruthenium(III)-tris-(acetylacetonato) gas-phase enantiomers.

Chiral transition-metal complexes are of interest in many fields ranging from asymmetric catalysis and molecular materials science to optoelectronic applications or fundamental physics including parity violation effects. We present here a combined theoretical and experimental investigation of gas-phase valence-shell photoelectron circular dichroism (PECD) on the challenging open-shell ruthenium(III)-tris-(acetylacetonato) complex, Ru(acac)3. Enantiomerically pure Δ- or Λ-Ru(acac)3, characterized by electronic circular dichroism (ECD), were vaporized and adiabatically expanded to produce a supersonic beam and photoionized by circularly-polarized VUV light from the DESIRS beamline at Synchrotron SOLEIL. Photoelectron spectroscopy (PES) and PECD experiments were conducted using a double imaging electron/ion coincidence spectrometer, and compared to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The open-shell character of Ru(acac)3, which is not taken into account in our DFT approach, is expected to give rise to a wide multiplet structure, which is not resolved in our PES signals but whose presence might be inferred from the additional striking features observed in the PECD curves. Nevertheless, the DFT-based assignment of the electronic bands leads to the characterisation of the ionized orbitals. In line with other recent works, the results confirm that PECD persists independently on the localization and/or on the achiral or chiral nature of the initial orbital, but is rather a probe of the molecular potential as a whole. Overall, the measured PECD signals on Ru(acac)3, a system exhibiting D3 propeller-type chirality, are of similar magnitude compared to those on asymmetric-carbon-based chiral organic molecules which constitute the vast majority of species investigated so far, thus suggesting that PECD is a universal mechanism, inherent to any type of chirality.

Ion-Molecule Reactions below 1K: Strong Enhancement of the Reaction Rate of the Ion-Dipole Reaction He^{+}+CH_{3}F.

The reaction between He^{+} and CH_{3}F forming predominantly CH_{2}^{+} and CHF^{+} has been studied at collision energies E_{coll} between 0 and k_{B}·10 K in a merged-beam apparatus. To avoid heating of the ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron. Supersonic beams of CH_{3}F and He(n) Rydberg atoms with principal quantum number n=30 and 35 were merged and their relative velocity tuned using a Rydberg-Stark decelerator and deflector, allowing an energy resolution of 150mK. A strong enhancement of the reaction rate was observed below E_{coll}/k_{B}=1 K. The experimental results are interpreted with an adiabatic capture model that accounts for the state-dependent orientation of the polar CH_{3}F molecules by the Stark effect as they approach the He^{+} ion. The enhancement of the reaction rate at low collision energies is primarily attributed to para-CH_{3}F molecules in the J=1, KM=1 high-field-seeking states, which represent about 8% of the population at the 6K rotational temperature of the supersonic beam.

Ag Functionalization of Al-Doped ZnO Nanostructured Coatings on PLA Substrate for Antibacterial Applications

Developing smart, environmentally friendly, and effective antibacterial surfaces is fundamental to contrast the diffusion of human infections and diseases for applications in the biomedical and food packaging sectors. To this purpose, here we combine aluminum-doped zinc oxide (AZO) and Ag to grow nanostructured composite coatings on bioplastic polylactide (PLA) substrates. The AZO layers are grown by RF magnetron sputtering, and then functionalized with Ag in atomic form by RF magnetron sputtering and in form of nanoparticles by supersonic cluster beam deposition. We compare the morphology, wettability, and antimicrobial performance of the nanostructured coatings obtained by the two methods. The different growth modes in the two techniques used for Ag functionalization are found to produce some differences in the surface morphology, which, however, do not induce significant differences in the wettability and antimicrobial response of the coatings. The antibacterial activity is investigated against Escherichia coli and Staphylococcus aureus as representatives of Gram-negative and Gram-positive bacteria, respectively. A preferential antimicrobial action of Ag on the first species and of AZO on the second one is evidenced. Through their combination, we obtain a hybrid composite coating taking advantage of the synergistic dual action of the two materials deposited, with a total bacterial suppression within few minutes for the first species and few hours for the second one, thus representing a valuable solution as a wide-spectrum bactericidal device.

Photoionization of polarized argon atoms in a magnetic field

Ionization of three-photon excited polarized states of argon atoms by circularly polarized probe light was studied in a supersonic beam. The revealed oscillation structure in the observed photoionization signals due to the Larmor precession of excited atomic states in a magnetic field allowed finding novel data for the Landé g-factors. We derived analytical formulas for the photoelectron current and explained the various types of the detected oscillations in terms of the principal lines among multiplet components of optical transitions. Our results indicate the possibility of implementing Doppler-free spectroscopy involving bound-free transitions.

Sensitive detection of metastable NO and N2 by reactive collisions with laser-excited Li

In a proof-of-principle experiment, we demonstrate that metastable nitric oxide molecules, NO(a4Π i ), generated inside a pulsed, supersonic beam, can be detected by reactive gas-phase collisions with electronically excited Li atoms in the 22P3/2 state. Since the internal energy of NO(a4Π i , v ⩽ 4) is lower than the ionization potential of Li in the 22S1/2 electronic ground state, we observe that the product ion yield arising from autoionizing NO(a4Π i ) + Li(22S1/2) collisions is a factor of 21 lower than the ion yield from NO(a4Π i ) + Li(22P3/2) collisions. We also compare our findings with measurements of relative rates for metastable N2 + Li(22S1/2) and metastable N2 + Li(22P3/2) reactive collisions. Using this detection method, we infer densities of ≈600 NO(a4Π i ) molecules/cm3 and ≈6 ⋅ 104 metastable N2 molecules/cm3 in the interaction region. Our results also allow for an estimate of the fractional population of NO(a4Π i , v ⩾ 5) prior to the collision process. The production of NO(a4Π i ) in selected vibrational states using laser excitation from the X2Π r ground state will open possibilities for the detailed study of vibrational-state-selected NO(a4Π i )–Li(22P3/2) collisions.

Soft and flexible gold microelectrodes by supersonic cluster beam deposition and femtosecond laser processing

Soft and flexible microelectrodes are fabricated and patterned in a novel approach integrating supersonic cluster beam deposition (SCBD) of gold nanoparticles onto Polydimethylsiloxane (PDMS) followed by femtosecond (fs) laser processing. SCBD of gold nanoparticles in PDMS forms a nanocomposite film with mechanical properties similar to those of the elastomeric substrate. Electrically neutral metallic nanoparticles penetrate the polymeric matrix and distribute randomly up to a depth of a few hundred nanometers forming a path of ohmic conduction. High resolution patterning on soft substrates is a major challenge that was overcome by femtosecond laser ablation of micrometer lines with relatively very little thermal damage. Microelectrodes were fabricated with a width down to 3 μm. The printed metal line sheet resistance was 480 Ohm/square for the 100 nm thick film and 210 Ohm/square for the 200 nm thick film. The calculated average film specific resistivity was found to be 2.5–2.6mΩ·cm. It is ~3 orders of magnitude higher than that of physical vapor deposited gold; therefore, it is not suitable for high current interconnects but sufficient for low-current signal transmission such as for electrical and electrochemical biosensors.

Anomalous electrical conduction and negative temperature coefficient of resistance in nanostructured gold resistive switching films

We report the observation of non-metallic electrical conduction, resistive switching, and a negative temperature coefficient of resistance in nanostructured gold films above the electrical percolation and in strong-coupling regime, from room down to cryogenic temperatures (24 K). Nanostructured continuous gold films are assembled by supersonic cluster beam deposition of Au aggregates formed in the gas phase. The structure of the cluster-assembled films is characterized by an extremely high density of randomly oriented crystalline nanodomains, separated by grain boundaries and with a large number of lattice defects. Our data indicates that space charge limited conduction and Coulomb blockade are at the origin of the anomalous electrical behavior. The high density of extended defects and grain boundaries causes the localization of conduction electrons over the entire investigated temperature range.

A Source of a Seeded Supersonic Beam of Alkali Halide Molecules

The design of a source of a seeded supersonic beam of alkali halide molecules or other vaporized solids is described. At a carrier gas (hydrogen or helium) stagnation pressure of 0.5–5 atm, a stagnation temperature of 1000 K, and a partial seed species pressure of 10–2 Torr, the intensity of the salt beam was 1014 sr–1 s–1 and the energy of molecules in the beam varied from 5.5 to 7.5 eV for a mixture with helium and from 7.5 to 15.5 eV for a mixture with hydrogen. The modular design of the source provides the easy replacement of its functional units.

Rydberg–Stark deceleration and trapping of helium in magnetic fields

Triplet (S = 1) He Rydberg atoms in supersonic beams with an initial velocity of 350 m s−1 have been decelerated to zero velocity and loaded into an off-axis electric trap in the presence and absence of magnetic fields. Comparing the deceleration efficiencies and the radiative decay of the population of trapped He Rydberg atoms to the (1s)1(2s)1 3S1 metastable level in the two sets of deceleration and trapping experiments revealed that the effects of magnetic fields up to 30 mT are negligible provided that a background dc electric field is maintained in the decelerator. A magnetic quadrupole trap of 30 mT depth corresponds to a He temperature of about 40 mK. The results thus represent an important step towards achieving high densities of cold paramagnetic samples following successive cycles of Rydberg–Stark deceleration, trapping, and radiative decay in overlaid electric and magnetic traps.

Characterization of a supersonic gas jet via laser-induced photoelectron ionization

We describe the characterization of a pulsed supersonic rare gas beam which is intended to serve as an ultracold neutral atom target for the production of an ultrashort ion pulse via femtosecond photoionization. The velocity distribution of atoms entrained in the beam is measured and characterized by temperatures TII and T⊥ in directions along and perpendicular to the beam propagation, respectively. It is shown that T⊥ values in the mK regime are achieved at distances of the order of 1000 mm from the nozzle. Moreover, the center beam density at this position is measured to be of the order of 1011 atoms/cm3. Both findings are essential for the intended application and confirm the targeted beam specifications. Comparison with theoretical estimates reveals the well-known skimmer interference effect, which is found to reduce the beam density by more than one order of magnitude.

Slow Decay Processes of Electrostatically Trapped Rydberg NO Molecules.

Nitric oxide (NO) molecules initially traveling at 795 m/s in pulsed supersonic beams have been photoexcited to long-lived hydrogenic Rydberg-Stark states, decelerated and electrostatically trapped in a cryogenically cooled, chip-based transmission-line Rydberg-Stark decelerator. The decelerated and trapped molecules were detected insitu by pulsed electric field ionization. The operation of the decelerator was validated by comparison of the experimental data with the results of numerical calculations of particle trajectories. Studies of the decay of the trapped molecules on timescales up to 1ms provide new insights into the lifetimes of, and effects of blackbody radiation on, Rydberg states of NO.

Self-acceleration and energy channeling in the saturation of the ion-sound instability in a bounded plasma

A novel regime of the saturation of the Pierce-type ion-sound instability in a bounded ion-beam-plasma system is revealed in 1D particle-in-cell simulations. It is found that the saturation of the instability is mediated by the oscillating virtual anode potential structure. The periodically oscillating potential barrier separates the incoming beam ions into two groups. One component forms a supersonic beam, which is accelerated to an energy exceeding the energy of the initial cold ion beam. The other component is organized as a self-consistent phase space structure of trapped ions with a wide energy spread—the ion hole. The effective temperature (energy spread) of the ions trapped in the hole is lower than the initial beam energy. In the final stage, the ion hole expands over the whole system length.

Proteomic Analysis Reveals a Mitochondrial Remodeling of βTC3 Cells in Response to Nanotopography.

Recently, using cluster-assembled zirconia substrates with tailored roughness produced by supersonic cluster beam deposition, we demonstrated that β cells can sense nanoscale features of the substrate and can translate these stimuli into a mechanotransductive pathway capable of preserveing β-cell differentiation and function in vitro in long-term cultures of human islets. Using the same proteomic approach, we now focused on the mitochondrial fraction of βTC3 cells grown on the same zirconia substrates and characterized the morphological and proteomic modifications induced by the nanostructure. The results suggest that, in βTC3 cells, mitochondria are perturbed by the nanotopography and activate a program involving metabolism modification and modulation of their interplay with other organelles. Data were confirmed in INS1E, a different β-cell model. The change induced by the nanostructure can be pro-survival and prime mitochondria for a metabolic switch to match the new cell needs.

Inelastic scattering dynamics of naphthalene and 2-octanone on highly oriented pyrolytic graphite.

The inelastic scattering dynamics of the isobaric molecules, naphthalene (C10H8) and 2-octanone (C8H16O), on highly oriented pyrolytic graphite (HOPG) have been investigated as part of a broader effort to inform the inlet design of a mass spectrometer for the analysis of atmospheric gases during a flyby mission through the atmosphere of a planet or moon. Molecular beam-surface scattering experiments were conducted, and the scattered products were detected with the use of a rotatable mass spectrometer detector. Continuous, supersonic beams were prepared, with average incident translational energies, ⟨Ei⟩, of 247.3 kJ mol-1 and 538.2 kJ mol-1 for naphthalene and 268.6 kJ mol-1 and 433.8 kJ mol-1 for 2-octanone. These beams were directed toward an HOPG surface, held at 530 K, at incident angles, θi, of 30°, 45°, and 70°, and scattered products were detected as functions of their translational energies and scattering angles. The scattering dynamics of both molecules are very similar and mimic the scattering of atoms and small molecules on rough surfaces, where parallel momentum is not conserved, suggesting that the dynamics are dominated by a corrugated interaction potential between the incident molecule and the surface. The effective corrugation of the molecule-surface interaction is apparently caused by the structure of the incident molecule and the consequent myriad available energy transfer pathways between the molecule and the surface during a complex collision event. In addition, the HOPG surface contributes to the corrugation of the interaction potential because it can absorb significant energy from collisions with incident molecules that have high mass and incident energy. Small differences in the scattering dynamics of the two molecules are inferred to arise from the details of the molecule-surface interaction potential, with 2-octanone exhibiting dynamics that suggest a slightly stronger interaction with the surface than naphthalene. These results add to a growing body of work on the scattering dynamics of organic molecules on HOPG, from which insight into the hypervelocity sampling and analysis of such molecules may be obtained.

Coupling Rydberg Atoms to Microwave Fields in a Superconducting Coplanar Waveguide Resonator.

Rydberg helium atoms traveling in pulsed supersonic beams have been coupled to microwave fields in a superconducting coplanar waveguide (CPW) resonator. The atoms were initially prepared in the 1s55s ^{3}S_{1} Rydberg level by two-color two-photon laser excitation from the metastable 1s2s ^{3}S_{1} level. Two-photon microwave transitions between the 1s55s ^{3}S_{1} and 1s56s ^{3}S_{1} levels were then driven by the 19.556GHz third-harmonic microwave field in a quarter-wave CPW resonator. This superconducting microwave resonator was fabricated from niobium nitride on a silicon substrate and operated at temperatures between 3.65 and 4.30K. The populations of the Rydberg levels in the experiments were determined by state-selective pulsed electric field ionization. The coherence of the atom-resonator coupling was studied by time-domain measurements of Rabi oscillations.

(Invited) Isolation and Structure Determination of Trifluoromethylated Gadoliniummetallofullerenes

Endohedral metallofullerenes have been extensively studied since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, most of these studies have focused on metallofullerenes larger than C60 such as Ln@C82 (Ln: lanthanide). There are few examples for isolation of small-cage (C60, C70, C72, and C74) lanthanide metallofullerenes because of their extremely high chemical reactivity. Recently, we developed an in situ trifluoromethylation method for the extraction and purification of these missing metallofullerenes [1−3]. Here we report the isolation and structure determination of trifluoromethylated gadolinium metallofullerenes (Gd@C60(CF3)3, Gd@C60(CF3)5, Gd@C70(CF3)3, Gd@C74(CF3), and Gd@C74(CF3)3). Gd-metallofullerenes are promising as magnetic resonance imaging (MRI) contrast agents. The fully enclosing carbon cage completely prevents leaching of the Gd atoms, resulting in lower toxicity than commercially available metal chelate reagents such as Gd-DTPA.Trifluoromethylated Gd-metallofullerenes were synthesized by the modified arc-discharge method. PTFE rods are placed near the discharge area as CF3 source. During arc discharge, PTFE was decomposed and evaporated to produce CF3 radicals. Gd-metallofullerenes and empty fullerenes were extracted from the raw soot with o-xylene. Rapid separation of Gd-metallofullerenes from empty fullerenes was carried out by the TiCl4 Lewis acid method. High-performance liquid chromatography (HPLC) purification was conducted for isolation of trifluoromethylated Gd-metallofullerenes. Single crystals of trifluoromethylated Gd-metallofullerenes were obtained from solution. The single-crystal X-ray diffraction data for structure determination were collected at SPring-8 large synchrotron radiation facility.Figures show the molecular structures of Gd@C74(CF3) (I), Gd@C74(CF3) (II), and Gd@C74(CF3)3 derived from the X-ray crystal structure analysis. The closed-shell molecular structures are theoretically stable and have wide HOMO-LUMO energy gaps.[1] Z. Wang et al. Angew. Chem. Int. Ed. 55, 199 (2016).[2] A. Nakagawa et al. Nature. Commun. 9, 3073 (2018).[3] A. Nakagawa et al. R. Soc. Open Sci. 5, 181015 (2018). Figure 1

Harnessing the Power of Adiabatic Curve Crossing to Populate the Highly Vibrationally Excited H_{2} (v=7, j=0) Level.

A large ensemble of ∼10^{9} H_{2} (v=7, j=0) molecules is prepared in the collision-free environment of a supersonic beam by transferring nearly the entire H_{2} (v=0, j=0) ground-state population, where v and j are the vibrational and rotational quantum numbers, respectively. This is accomplished by controlling the crossing of the optically dressed adiabatic states using a pair of phase coherent laser pulses. The preparation of highly vibrationally excited H_{2} molecules opens new opportunities to test fundamental physical principles using two loosely bound yet entangled H atoms.

Single-Source, Collinear Merged-Beam Experiment for the Study of Reactive Neutral-Neutral Collisions.

We present two methods for studying reactive collisions between two atomic or molecular species: a collinear merged-beam method in which two gas pulses from a single supersonic beam source are coalesced and an intrabeam-scattering technique in which a single gas pulse is used. Both approaches, which rely on the laser cooling and deceleration of a laser-coolable species inside a Zeeman slower, can be used for a wide range of scattering studies. Possible experimental implementations of the proposed methods are outlined for autoionizing collisions between helium atoms in the metastable 23S1 state and a second, atomic or molecular species. Using numerical trajectory calculations, we provide estimates of the expected on-axis detection efficiency, collision-energy range, and collision-energy resolution of the approaches. We have experimentally tested the feasibility of such an experiment by producing two gas pulses at very short time intervals, and the results of these measurements are also detailed.

Antimicrobial Nanostructured Coatings: A Gas Phase Deposition and Magnetron Sputtering Perspective.

Counteracting the spreading of multi-drug-resistant pathogens, taking place through surface-mediated cross-contamination, is amongst the higher priorities in public health policies. For these reason an appropriate design of antimicrobial nanostructured coatings may allow to exploit different antimicrobial mechanisms pathways, to be specifically activated by tailoring the coatings composition and morphology. Furthermore, their mechanical properties are of the utmost importance in view of the antimicrobial surface durability. Indeed, the coating properties might be tuned differently according to the specific synthesis method. The present review focuses on nanoparticle based bactericidal coatings obtained via magneton-spattering and supersonic cluster beam deposition. The bacteria–NP interaction mechanisms are first reviewed, thus making clear the requirements that a nanoparticle-based film should meet in order to serve as a bactericidal coating. Paradigmatic examples of coatings, obtained by magnetron sputtering and supersonic cluster beam deposition, are discussed. The emphasis is on widening the bactericidal spectrum so as to be effective both against gram-positive and gram-negative bacteria, while ensuring a good adhesion to a variety of substrates and mechanical durability. It is discussed how this goal may be achieved combining different elements into the coating.

A traveling wave Zeeman decelerator

SynopsisWe developed a new-concept Zeeman decelerator which produces a traveling magnetic trap. Atoms and molecules possessing a magnetic dipole moment, in so-called low field seeking states, are trapped around a node of a propagating wave provided that the initial velocity of the wave matches a velocity populated in the supersonic beam. In addition, three dimensional confinement is achieved by controlling the radial orientation of the trap, which can be done fully independently from its longitudinal motion.