Supersonic Beam Research Articles

Mean Gas Cluster Size Determination from Cluster Beam Cross-Section

This paper describes the simple experimental method of size determination of gas clusters in molecular beams formed from supersonic jets. Mean cluster size N is calculated from broadening of the transverse profile of beam intensity at a fixed distance behind the skimmer. The described method allows determining the mean sizes of the clusters of any pure gases. It does not require the building of some special models, or determination of empirical constants. Due to the high intensity of the supersonic beams, the measurements do not require any complex highly sensitive equipment. The effectiveness of the present method is validated by measurements in a cluster beams of test gases (easily condensable CO2, Ar, and weakly condensable N2) and the beam of C2H4 (ethylene), formed from a supersonic jet behind conical nozzles. The certainty of measured characteristics is confirmed by the results of numerical simulations. By using the described method the mean cluster sizes from 50 to 2000 molecules per cluster were determined. The correctness of the obtained cluster sizes of CO2 and Ar is proved by comparison with results of other authors, obtained by other experimental methods, and estimations according to the empirical correlations using condensation scaling parameter Г*.

Imaging characterization of the rapid adiabatic passage in a source-rotatable, crossed-beam scattering experiment.

In order to achieve a more efficient preparation of a specific ro-vibrationally excited reactant state for reactive scattering experiments, we implemented the rapid adiabatic passage (RAP) scheme to our pulsed crossed-beam machine, using a single-mode, continuous-wave mid-infrared laser. The challenge for this source-rotatable apparatus lies in the non-orthogonal geometry between the molecular beam and the laser propagation directions. As such, the velocity spread of the supersonic beam results in a significantly broader Doppler distribution that needs to be activated for RAP to occur than the conventional orthogonal configuration. In this report, we detail our approach to shifting, locking, and stabilizing the absolute mid-infrared frequency. We exploited the imaging detection technique to characterize the RAP process and to quantify the excitation efficiency. We showed that with appropriate focusing of the IR laser, a nearly complete population transfer can still be achieved in favorable cases. Compared to our previous setup-a pulsed optical parametric oscillator/amplifier in combination with a multipass ring reflector for saturated absorption, the present RAP scheme with a single-pass, continuous-wave laser yields noticeably higher population-transfer efficiency.

Supersonic cluster beam printing of carbon microsupercapacitors on paper

Paper is a cheap, lightweight and renewable material with increasing applicative interest as a substrate for disposable and flexible electronics. The integration of planar energy storage devices on paper is a necessary and challenging step for the development of smart and autonomous flexible electronic platforms. Here we report the one-step, room temperature, fabrication of planar microsupercapacitors where nanostructured current collectors and carbon electrodes are deposited by supersonic cluster beam deposition (SCBD) on plain paper sheets and 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([Emim][NTf2]) is used as ionic liquid electrolyte. As-prepared microsupercapacitors showed a high capacitance density of about 7 F cm−3 and good capacity retention upon prolonged cycling. The encapsulation of SCBD-made microsupercapacitors by means of a polydimethylsiloxane layer and their usability in driving a low power temperature sensor are demonstrated.

Oxidative Destruction of Multilayer Diisopropyl Methylphosphonate Films by O(3P) Atomic Oxygen.

We present work detailing the oxidative destruction of the nerve agent simulant diisopropyl methylphosphonate (DIMP) with O(3P) using time-resolved, in situ reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). Thermally annealed DIMP films deposited on Au(111) are observed to react upon exposure to a supersonic beam containing O(3P) with average translational energies of 0.12 eV. The reaction is initiated by a hydrogen abstraction from one of three possible sites on DIMP, and then progresses through various secondary reactions with resultant hydroxyl radicals, carbon-centered DIMP-derived radicals, and nondissociated O2 in the beam. These reactions are accompanied by uptake of oxygen into the film, leading to new hydrogen bonding with the DIMP phosphoryl group. The generated product also presents greater thermal stability than pristine DIMP, suggesting the formation of a distribution of oligomeric and polymeric products. As reactivity is observed to decrease upon continued O(3P) exposure, this product likely forms a protective layer at the vacuum-film interface, hindering destruction of thicker films. Importantly, the rate of reaction and general reactivity trends are the same between DIMP and the smaller simulant dimethyl methylphosphonate (DMMP). The comparable reaction rates of the two molecules coupled with oxygen's inability to erode thick films all the way down to the substrate have specific implications for the development of oxidation-based decontamination strategies for these and other organophosphates in the solid phase. The findings presented in this paper add significant new fundamental understanding of the oxidative chemistry of such species, knowledge needed in order to develop efficacious nerve agent decontamination strategies as well as the refinement of existing models for the dispersal, adsorption, persistence, and destruction of organophosphates in the environment.

Primary cortical neurons on PMCS TiO2 films towards bio-hybrid memristive device: A morpho-functional study

We report a comprehensive study of the biocompatibility and neurocompatibility of titanium dioxide films (TiO2) prepared by Pulsed Microplasma Cluster Source (PMCS). This technique uses supersonic pulsed beams seeded by clusters of the metal oxide synthesized in a plasma discharge. The final stoichiometry of the TiO2 thin films is tuned changing the gas mixture, achieving stoichiometric or oxygen overstoichiometric films. All the films showed consistent biocompatibility and a spontaneous absorption of poly-d-lysine (PDL) that favors the adhesion and growth of murine cortical neurons. Moreover, the bioelectrical activity of the neuronal culture grown on the TiO2 film can be modulated by changing the chemistry of the surface. This work paves the way to develop a bio-hybrid neuromorphic device, where viable nerve cells are grown directly over a titanium dioxide film showing a network of memristors.

Electrically tuned Förster resonances in collisions of NH3 with Rydberg He atoms

The effects of weak electric fields on resonant energy transfer between ${\mathrm{NH}}_{3}$ in the $X$ ${^{1}A}_{1}$ ground electronic state and Rydberg He atoms in triplet states with principal quantum numbers $n=36$--41 have been studied in a crossed-beam apparatus. For these values of $n$, electric dipole transitions between the Rydberg states that evolve adiabatically to the $|ns\ensuremath{\rangle}$ and $|np\ensuremath{\rangle}$ states in zero electric field can be tuned into resonance with the ground-state inversion transitions in ${\mathrm{NH}}_{3}$ using electric fields, with energy transfer occurring via F\"orster resonance. In the experiments the Rydberg He atoms, traveling in pulsed supersonic beams, were prepared by resonant two-photon excitation from the metastable $1s2s\phantom{\rule{0.16em}{0ex}}{^{3}S}_{1}$ level and crossed an effusive beam of ${\mathrm{NH}}_{3}$ before being detected by state-selective pulsed-electric-field ionization. The resonant-energy-transfer process was identified by monitoring changes in the ionization signal from the $|ns\ensuremath{\rangle}$ and $|np\ensuremath{\rangle}$ Rydberg states for each value of $n$. The electric-field dependence of the experimental data is in good agreement with the results of calculations in which the resonant dipole-dipole coupling between the collision partners was accounted for.

Electroactive Ionic Soft Actuators with Monolithically Integrated Gold Nanocomposite Electrodes

Electroactive ionic gel/metal nanocomposites are produced by implanting supersonically accelerated neutral gold nanoparticles into a novel chemically crosslinked ion conductive soft polymer. The ionic gel consists of chemically crosslinked poly(acrylic acid) and polyacrylonitrile networks, blended with halloysite nanoclays and imidazolium-based ionic liquid. The material exhibits mechanical properties similar to that of elastomers (Young's modulus ≈ 0.35 MPa) together with high ionic conductivity. The fabrication of thin (≈100 nm thick) nanostructured compliant electrodes by means of supersonic cluster beam implantation (SCBI) does not significantly alter the mechanical properties of the soft polymer and provides controlled electrical properties and large surface area for ions storage. SCBI is cost effective and suitable for the scaleup manufacturing of electroactive soft actuators. This study reports the high-strain electromechanical actuation performance of the novel ionic gel/metal nanocomposites in a low-voltage regime (from 0.1 to 5 V), with long-term stability up to 76 000 cycles with no electrode delamination or deterioration. The observed behavior is due to both the intrinsic features of the ionic gel (elasticity and ionic transport capability) and the electrical and morphological features of the electrodes, providing low specific resistance (<100 Ω cm-2 ), high electrochemical capacitance (≈mF g-1 ), and minimal mechanical stress at the polymer/metal composite interface upon deformation.

Resistive hydrogen sensing response of Pd-decorated ZnO “nanosponge” film

Hydrogen (H2)-induced resistive response of palladium (Pd)-decorated zinc oxide “nanosponge” (Pd/ns-ZnO) film was studied at different operation temperatures with and without UV illumination. The as-deposited ns-ZnO film, fabricated using a supersonic cluster beam deposition system, was highly porous and composed of ∱/45nm nanoclusters embedded in an amorphous matrix. After annealed at 500°C for 1h, the film was found to contain loosely connected 13nm crystallites and had a porosity of 73%. The Pd/ns-ZnO film sensor at 20°C showed a sensor response of 82 and a response time of 1s for 2% H2. After heated slightly to 80°C, the sensor response was increased by 43 times, and the response and recovery times dropped to 0.3s and 18s, respectively. This performance is superior to those of many other metal oxide nanomaterials operating at temperatures >200°C. The sensing properties of the Pd/ns-ZnO film showed little degradation when UV illumination was applied, but the sensing stability was improved. A reaction model was proposed to give an explanation to the gas sensing process. The influences of increasing the operation temperature and UV illumination are discussed.

Dominant conformer of tetrahydropyran-2-methanol and its clusters in the gas phase explored by the use of VUV photoionization and vibrational spectroscopy.

Tetrahydropyran-2-methanol (THPM) is a typical alcohol containing a six-member cyclic ether, which can be considered as the model molecule of cyclic sugar. Herein, vacuum ultraviolet (VUV) photodissociation spectroscopy is employed to study fragmentation pathways and infrared (IR) plus VUV photoionization spectroscopy to investigate the structures of neutral THPM and its clusters with the size up to the trimer. Qualitative structural assignments are confirmed for the neutral species and ions based on MP2/aug-cc-pVTZ and ωB97X-D/cc-pVTZ calculations. The fragment cations at m/z = 84, 85, and 98 arise by the losing of CH2OH, CH3OH, and H2O from the monomer, respectively, as a result of C-C bond and C-O bond dissociation under the VUV (118 nm) radiation. It is found that the loss of CH3OH and H2O involves hydrogen transfer from the CH2 group to the dissociating CH2 and OH groups. Comparing the observed and calculated spectra of the monomer THPM, it suggests that the conformer containing a chair tetrahydropyran ring and an intramolecular hydrogen bond would be dominantly survived in a supersonic beam. Moreover, the IR spectra of larger clusters n > 1 (n = 2, 3) show only the broad hydrogen bonded OH stretch mode, and thus these larger clusters would form a closed-cyclic structure, where all OH groups are participating in hydrogen bonding. Partially the CH stretch positions of THPM clusters do not change significantly with the increasing of cluster size, thus the CH and CH2 groups are not involved in H-bonding interactions.

Benchmark studies of BOUT++ code and TPSMBI code on neutral transport during SMBI

SMBI (supersonic molecule beam injection) plays an important role in tokamak plasma fuelling, density control and ELM mitigation in magnetic confinement plasma physics, which has been widely used in many tokamaks. The trans_neut module of BOUT++ code is the only large-scale parallel 3D fluid code used to simulate the SMBI fueling process, while the TPSMBI (transport of supersonic molecule beam injection) code is a recent developed 1D fluid code of SMBI. In order to find a method to increase SMBI fueling efficiency in H-mode plasma, especially for ITER, it is significant to first verify the codes. The benchmark study between the trans_neut module of BOUT++ code and the TPSMBI code on radial transport dynamics of neutral during SMBI has been first successfully achieved in both slab and cylindrical coordinates. The simulation results from the trans_neut module of BOUT++ code and TPSMBI code are consistent very well with each other. Different upwind schemes have been compared to deal with the sharp gradient front region during the inward propagation of SMBI for the code stability. The influence of the WENO3 (weighted essentially non-oscillatory) and the third order upwind schemes on the benchmark results has also been discussed.

Magnetic Trapping of Cold Methyl Radicals.

We have demonstrated that a supersonic beam of methyl radicals (CH_{3}) in the ground rotational state of both para and ortho species has been slowed down to standstill with a magnetic molecular decelerator, and successfully captured spatially in an anti-Helmholtz magnetic trap for >1 s. The trapped CH_{3} radicals have a mean translational temperature of about 200mK with an estimated density of >5.0×10^{7} cm^{-3}. The methyl radical is an ideal system for the study of cold molecules not only because of its high reactivities at low temperatures, but also because further cooling below 1mK is plausible via sympathetic cooling with ultracold atoms. The demonstrated trapping capability of methyl radicals opens up various possibilities for realizing ultracold ensembles of molecules towards Bose-Einstein condensation of polyatomic molecules and investigations of reactions governed by quantum statistics.

Communication: Neutral atom imaging using a pulsed electromagnetic lens.

We report on progress towards a neutral atom imaging device that will be used for chemically sensitive surface microscopy and nanofabrication. Our novel technique for improving refractive power and correcting chromatic aberration in atom lenses is based on a fundamental paradigm shift from continuous-beam focusing to a pulsed, three-dimensional approach. Simulations of this system suggest that it will pave the way toward the long-sought goal of true atom imaging on the nanoscale. Using a prototype lens with a supersonic beam of metastable neon, we have imaged complex patterns with lower distortion and higher resolution than has been shown in any previous experiment. Comparison with simulations corroborates the underlying theory and encourages further refinement of the process.

Quantitative Control of Protein and Cell Interaction with Nanostructured Surfaces by Cluster Assembling.

The development of smart prosthetics, scaffolds, and biomaterials for tissue engineering and organ-on-a-chip devices heavily depends on the understanding and control of biotic/abiotic interfaces. In recent years, the nanometer scale emerged as the predominant dimension for processes impacting on protein adsorption and cellular responses on surfaces. In this context, the extracellular matrix (ECM) can be seen as the prototype for an intricate natural structure assembled by nanoscale building blocks forming highly variable nanoscale configurations, dictating cellular behavior and fate. How exactly the ECM nanotopography influences mechanotransduction, that is, the cellular capacity to convert information received from the ECM into appropriate responses, remains partially understood due to the complexity of the involved biological structures, limiting also the attempts to artificially reproduce the nanoscale complexity of the ECM. In this Account, we describe and discuss our strategies for the development of an efficient and large-scale bottom-up approach to fabricate surfaces with multiscale controlled disorder as substrates to study quantitatively the effect of nanoscale topography on biological entities. Our method is based on the use of supersonic cluster beam deposition (SCBD) to assemble, on a substrate, neutral clusters produced in the gas phase and accelerated by a supersonic expansion. The assembling of clusters in the ballistic deposition regime follows simple scaling laws, allowing the quantitative control of surface roughness and asperity layout over large areas. Due to their biocompatibility, we focused on transition metal oxide nanostructured surfaces assembled by titania and zirconia clusters. We demonstrated the engineering of structural and functional properties of the cluster-assembled surfaces with high relevance for interactions at the biotic/abiotic interface. We observed that isoelectric point and wettability, crucial parameters for the adhesion of biological entities on surfaces, are strongly influenced and controlled by the nanoscale roughness. By developing a high-throughput method (protein surface interaction microarray, PSIM), we characterized quantitatively the capacity of the nanostructured surfaces to adsorb proteins, showing that with increasing roughness the adsorption rises beyond what could be expected by the increase in specific area, paralleled by an almost linear decrease in protein binding affinity. We also determined that the spatial layout of the surface asperities effectively perceived by the cells mimics at the nanoscale the topographical ECM characteristics. The interaction with these features consequently regulates parameters significant for cell adhesion and mechanotransductive signaling, such as integrin clustering, focal adhesion maturation, and the correlated cellular mechanobiology, eventually impacting the cellular program and differentiation, as we specifically showed for neuronal cells.

Theoretical model of the helium zone plate microscope

Neutral helium microscopy is a new technique currently under development. Its advantages are the low energy, charge neutrality, and inertness of the helium atoms, a potential large depth of field, and the fact that at thermal energies the helium atoms do not penetrate into any solid material. This opens the possibility, among others, for the creation of an instrument that can measure surface topology on the nanoscale, even on surfaces with high aspect ratios. One of the most promising designs for helium microscopy is the zone plate microscope. It consists of a supersonic expansion helium beam collimated by an aperture (skimmer) focused by a Fresnel zone plate onto a sample. The resolution is determined by the focal spot size, which depends on the size of the skimmer, the optics of the system, and the velocity spread of the beam through the chromatic aberrations of the zone plate. An important factor for the optics of the zone plate is the width of the outermost zone, corresponding to the smallest opening in the zone plate. The width of the outermost zone is fabrication limited to around 10 nm with present-day state-of-the-art technology. Due to the high ionization potential of neutral helium atoms, it is difficult to build efficient helium detectors. Therefore, it is crucial to optimize the microscope design to maximize the intensity for a given resolution and width of the outermost zone. Here we present an optimization model for the helium zone plate microscope. Assuming constant resolution and width of the outermost zone, we are able to reduce the problem to a two-variable problem (zone plate radius and object distance) and we show that for a given beam temperature and pressure, there is always a single intensity maximum. We compare our model with the highest-resolution zone plate focusing images published and show that the intensity can be increased seven times. Reducing the width of the outermost zone to 10 nm leads to an increase in intensity of more than 8000 times. Finally, we show that with present-day state-of-the-art detector technology (ionization efficiency $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$), a resolution of the order of 10 nm is possible. In order to make this quantification, we have assumed a Lambertian reflecting surface and calculated the beam spot size that gives a signal 100 cts/s within a solid angle of 0.02 sr, following an existing helium microscope design.

Vacuum-ultraviolet frequency-modulation spectroscopy.

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing (νVUV=2νUV+ν2) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies νUV and ν2. Sidebands generated in the output of the second laser (ν2) using an electro-optical modulator operating at the frequency νmod are directly transferred to the VUV and used to record FM spectra. Demodulation is demonstrated both at νmod and 2νmod. The main advantages of the method compared to VUV absorption spectroscopy are its background-free nature, the fact is that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages, we present VUV FM spectra of Ar, Kr, and N2 in selected regions between 105000 cm-1 and 122000 cm-1.

Effects of width and density of supersonic molecule beam on penetration depth of tokamak

The penetration depth and the fueling efficiency of the supersonic molecular beam injection (SMBI) are affected by both the intrinsic parameters of the SMBI and the parameters of background plasma. The purpose of the present paper is to explore the possible methods of improving the fueling efficiency of SMBI by varying the beam parameters. The penetration depths and the transport processes of SMBI with different beam densities and different beam widths are studied using the trans-neut module of the three-dimensional (3D) edge turbulence simulation code BOUT++. In our present study, the number of the injected molecules per unit time the injection speed and the injected flux are kept constant throughout the SMB fueling process, but the beam density and beam width are adjusted. The simulation is based on the real magnetic configuration of the HL-2A tokamak. Our results indicate that the deeper injection depth can be obtained with a supersonic molecular beam (SMB) with smaller density and larger width. However, the injection depth decreases when the beam density or the beam width increases. The residence time of the beam front can be lengthened by increasing the beam density and widening the beam width. If the beam density increases or the beam width enlarges, not only the injection depth decreases, but also the residence time shortens. The front of the atom density exhibits the behaviors analogous to that of the SMB, namely, both its depth and its residence time decreases with beam density increasing and beam width decreasing. At the same time, the dissociation rate has a larger range in the spatiotemporal coordinate. The global growth of dissociation rate is inhibited by the molecular dissociation localization. However, the localization of the molecular dissociation accelerates the local growth of the dissociation rate, and the global growth of the molecular dissociation rate is promoted. When the promoting effect is dominant, under the condition of constant flux and fixed injection speed, the smaller molecular injection width will lead to the shallower molecular penetration depth. The simulation results suggest that if we attempt to promote the fueling efficiency and to increase the injection depth of SMBI, we should utilize the SMBI with a smaller density and larger beam width. Of course, the concrete influences of the SMBI on injection depth and fueling efficiency should be studied further by varying other relevant parameters of the SMB and the backgroud plasma.

Chiroptical inversion for isolated vibronic transitions of supersonic beam-cooled molecules.

Circular dichroism-resonance-enhanced multiphoton ionization (CD-REMPI) was used for CD measurements on several single vibronic transitions of supersonic beam-cooled (R)-(+)-1-phenylethanol. Due to the low molecular densities within a supersonic beam and the expected small anisotropy factor of 1-phenylethanol in the permille region, the precision of the experimental method had to be significantly improved. Therefore, a single laser pulse evaluation combined with a twin-peak technique enabled within the used supersonic beam setup is presented. For the electronic transition S0 → S1 of (R)-(+)-1-phenylethanol (π → π* transition of the phenyl ring at 266 nm) ten different vibrational modes as well as the 0-transition were investigated with one-color (1 + 1) CD-REMPI. The results deliver new experimental insights on the influence of molecular vibrations on the anisotropy factor. TD-DFT theoretical predictions show how the angle between the electronic and magnetic transition dipole moments of the electronic transition can be modified by different vibrational modes, making even a flip of the sign of the anisotropy factor possible.

Analysis of the vibronic structure of the trans-stilbene fluorescence and excitation spectra: the S0 and S1 PES along the Ce[double bond, length as m-dash]Ce and Ce-Cph torsions.

We analyze the highly resolved vibronic structure of the low energy (≤200 cm-1) region of the fluorescence and fluorescence excitation spectra of trans-stilbene in supersonic beams. In this spectral region the vibronic structure is associated mainly with vibrational levels of the Ce-Ce torsion (τ) and the au combination of the two Ce-Cph bond twisting (ϕ). We base this analysis on the well-established S0(τ, ϕ) two-dimensional potential energy surface (PES) and on a newly refined S1(τ, ϕ) PES. We obtain vibrational eigenvalues and eigenvectors of the anharmonic S0(τ, ϕ) and S1(τ, ϕ) PES using a numerical procedure based on the Meyer's flexible model [R. Meyer, J. Mol. Spectrosc., 1979, 76, 266]. Then we derive Franck-Condon factors and therefore intensities of the relevant vibronic bands for the S0 → S1 excitation and S1 → S0 fluorescence spectra. Furthermore, we assess the role of the bg combination of the two Ce-Cph bond twisting (ν48) in the structure of the S1 → S0 fluorescence spectra. By the use of these results we are able to assign most of the low energy vibrational levels of the S0 → S1 excitation spectra and of the fluorescence spectra of the emission from several low energy S1 vibronic levels. The good agreement between the observed and the computed vibrational structure of the S0 → S1 and S1 → S0 spectra suggests that the proposed picture of the E1(τ, ϕ) and E0(τ, ϕ) PES, in particular along the coordinate τ governing trans-cis photo-isomerization in S1, is accurate. In S0, the barriers for the Ce[double bond, length as m-dash]Ce torsion and for the au type Ce-Cph bond twisting are 16 080 cm-1 and 3125 cm-1, respectively, while in S1, where the bond orders of the Ce[double bond, length as m-dash]Ce and Ce-Cph bonds are reversed, the two barriers become 1350 cm-1 and 8780 cm-1, respectively.

Theoretical model of the helium pinhole microscope

In recent years, the development of neutral helium microscopes has gained increasing interest. The low energy, charge neutrality, and inertness of the helium atoms makes helium microscopy an attractive candidate for the imaging of a range of samples. The simplest neutral helium microscope is the so-called pinhole microscope. It consists of a supersonic expansion helium beam collimated by two consecutive apertures (skimmer and pinhole), which together determine the beam spot size and hence the resolution at a given working distance to the sample. Due to the high ionization potential of neutral helium atoms, it is difficult to build efficient helium detectors. Therefore, it is crucial to optimize the microscope design to maximize the intensity for a given resolution and working distance. Here we present an optimization model for the helium pinhole microscope system. We show that for a given resolution and working distance, there is a single intensity maximum. Further we show that with present-day state-of-the-art detector technology (ionization efficiency $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$), a resolution of the order of 600 nm at a working distance of 3 mm is possible. In order to make this quantification, we have assumed a Lambertian reflecting surface and calculated the beam spot size that gives a signal 100 cts/s within a solid angle of $0.02\ensuremath{\pi}$ sr, following an existing design. Reducing the working distance to the micron range leads to an improved resolution of around 40 nm.

Mean-field energy-level shifts and dielectric properties of strongly polarized Rydberg gases

Mean-field energy-level shifts arising as a result of strong electrostatic dipole interactions within dilute gases of polarized helium Rydberg atoms have been probed by microwave spectroscopy. The Rydberg states studied had principal quantum numbers $n=70$ and 72, and electric dipole moments of up to 14050 D, and were prepared in pulsed supersonic beams at particle number densities on the order of $10^{8}$ cm$^{-3}$. Comparisons of the experimental data with the results of Monte Carlo calculations highlight effects of the distribution of nearest-neighbor spacings in the pulsed supersonic beams, and the dielectric properties of the strongly polarized Rydberg gases, on the microwave spectra. These observations reflect the emergence of macroscopic electrical properties of the atomic samples when strongly polarized.