Supersonic Atomic Beam Research Articles

The influence of experimental conditions on absolute beam density measurements for NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} and H

In order to establish important reaction properties, such as rate coefficients, it is often necessary to know the number of reactants that are present in an interaction region. The absolute number densities of pulsed supersonic molecular (NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document}) and atomic (H) beams are reported using a laser-based detection method, under a range of experimental conditions including photolysis, Zeeman deceleration, and magnetic focusing. Time-averaged densities of (3.6 ± 2.7) ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^4$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document} are reported for successfully Zeeman-decelerated and magnetically focused H atoms, generated by the photodissociation of precursor NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} molecules. Without the magnetic guide components in the beamline, the density of the target radicals of interest is somewhat lower, at (2.5 ± 1.8) ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^4$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}. The average density of the undecelerated H atom beam is approximately an order of magnitude higher (2.9 ± 1.9) ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}, with the average density of the molecular ammonia beam over two orders of magnitude higher again (5.1 ± 2.9) ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 107\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^7$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}. The average number densities measured for the two different species of interest in this work span more than three orders of magnitude. These findings highlight the need for accurate and precise experimental measurements of number densities—for each species of interest, under the appropriate experimental conditions—before doing absolute rate coefficient calculations.Graphical abstract

Isotope-selective laser photoionization of tin in supersonic atomic beam

The isotope 124Sn enriched to better than 50% is needed in substantial quantity as detector material for the proposed neutrinoless double beta decay (0νββ) experiment in India (TIN.TIN). Present work investigates isotope-selective laser photoionization scheme for 124Sn. Transition to the first excited state from the ground state lies at 286.3 nm having short lifetime (~ 5 ns) and isotope shift (IS), Δν(124–120) = 441 MHz. However, the availability of high repetition rate (kHz), narrowband tunable pulsed laser in UV with low temporal jitter is a technological impediment for achieving isotopic selectivity in this transition. An alternate three-color photoionization scheme is proposed for separation of 124Sn. Laser vaporization of tin followed by supersonic expansion in a molecular beam apparatus provided the tin atomic beam. Tin atoms were first excited to the 5p6s,3P1 state from the 5p2,3P0 ground state by a broadband pulsed dye laser (λ1 = 286.3 nm). Isotopic selectivity was achieved in the second excitation step at λ2 = 855.2 nm by a narrowband continuous wave laser. Subsequently, resonant photoionization at λ3 = 694.7 nm to a newly observed autoionizing state at 60,992.9 cm−1 provided efficient photoionization. The IS for all even and hyperfine splitting for odd isotopes have been measured for the 855.2 nm transition. A selectivity factor of 24 is achieved for 124Sn isotope. The absorption cross sections of the three excitation steps is reasonably high to have an efficient photoionization scheme.

Brightening of a supersonic beam of neutral atoms

Well-collimated, high-intensity beams of neutral atoms have many applications ranging from atom microscopy to atom interferometry to ultracold atomic physics. We experimentally demonstrate a method for brightening a supersonic atomic beam and observe an increase in the phase space density by a factor of at least 20. Our scheme relies upon a permanent magnetic hexapole lens to focus a divergent beam of neutral atoms emitted from a supersonic nozzle and transverse laser cooling as the beam converges downstream from the lens so as to create a dense, narrow, Doppler-collimated atomic beam. In principle, this method can be repeated multiple times in series for further beam brightening.

Laser Ablation Atomic Beam Apparatus with Time-Sliced Velocity Map Imaging for Studying State-to-State Metal Reaction Dynamics

We report a newly constructed laser ablation crossed molecular beam apparatus, equipped with time-sliced velocity map imaging technique, to study state-to-state metal atom reaction dynamics. Supersonic metal atomic beam is generated by laser vaporization of metal rod, and free expansion design without gas flow channel has been employed to obtain a good quality of metal atomic beam. We have chosen the crossed-beam reaction Al+O2 to test the performance of the new apparatus. Two-rotational-states selected AlO(X2∑+, v=0, N and N+14) products can be imaged via P(N) and R(N+14) branches of the Δv=1 band at the same wavelength, during (1+1) resonance-enhanced multi-photon ionization through the AlO(D2∑+) intermediate state. In our experiment at 244.145 nm for simultaneous transitions of P(15) and R(29) branch, two rings in slice image were clearly distinguishable, corresponding to the AlO(v=0, N=15) and AlO(v=0, N=29) states respectively. The energy difference between the two rotational levels is 403 cm−1. The success of two states resolved in our apparatus suggests a better collisional energy resolution compared with the recent research study [J. Chem. Phys. 140, 214304 (2014)].

Evolution from Rydberg gas to ultracold plasma in a supersonic atomic beam of Xe

A Rydberg gas of xenon, entrained in a supersonic atomic beam, evolves slowly to form an ultracold plasma. In the early stages of this evolution, when the free-electron density is low, Rydberg atoms undergo long-range -mixing collisions, yielding states of high orbital angular momentum. The development of high- states promotes dipole–dipole interactions that help to drive Penning ionization. The electron density increases until it reaches the threshold for avalanche. Ninety μs after the production of a Rydberg gas with the initial state, , a 432 V cm−1 electrostatic pulse fails to separate charge in the excited volume, an effect which is ascribed to screening by free electrons. Photoexcitation cross sections, observed rates of -mixing, and a coupled-rate-equation model simulating the onset of the electron-impact avalanche point consistently to an initial Rydberg gas density of .

Magneto-optical cooling of atoms.

We propose an alternative method to laser cooling. Our approach utilizes the extreme brightness of a supersonic atomic beam, and the adiabatic atomic coilgun to slow atoms in the beam or to bring them to rest. We show how internal-state optical pumping and stimulated optical transitions, combined with magnetic forces, can be used to cool the translational motion of atoms. This approach does not rely on momentum transfer from photons to atoms, as in laser cooling. We predict that our method can surpass laser cooling in terms of flux of ultracold atoms and phase-space density, with lower required laser power.

Note: Manipulation of supersonic atomic beams with static magnetic fields

The inhomogeneous magnetic field of a permanent-magnet planar Halbach array is used to either deflect or to specularly reflect a supersonic beam of neutral atoms. Metastable neon and helium beams are tested to experimentally evaluate the performance of this array in a range of configurations. Results are compared with numerical simulations and the device is presented as a high precision tool for the manipulation of neutral atom beams.

Chirped-Pulse Millimeter-Wave Spectroscopy of Rydberg-Rydberg Transitions

Transitions between Rydberg states of Ca atoms, in a pulsed, supersonic atomic beam, are directly detected by chirped-pulse millimeter-wave spectroscopy. Broadband, high-resolution spectra with accurate relative intensities are recorded instantly. Free induction decay (FID) of atoms, polarized by the chirped pulse, at their Rydberg-Rydberg transition frequencies, is heterodyne detected, averaged in the time domain, and Fourier transformed into the frequency domain. Millimeter-wave transient nutations are observed, and the possibility of FID evolving to superradiance is discussed.

Supersonic sodium atomic beam hyperfine structure measurement orthogonally with dye laser R6G

Supersonic sodium atomic beam hyperfine structure measurement orthogonally with dye laser R6G

Nanofabrication by magnetic focusing of supersonic beams

We present a new method for nanoscale atom lithography. We propose the use of a supersonic atomic beam, which provides an extremely high-brightness and cold source of fast atoms. The atoms are to be focused onto a substrate using a thin magnetic film, into which apertures with widths on the order of 100 nm have been etched. Focused spot sizes near or below 10 nm, with focal lengths on the order of 10 microns, are predicted. This scheme is applicable both to precision patterning of surfaces with metastable atomic beams and to direct deposition of material.

An improved high intensity recycling helium-3 beam source

We describe an improved high intensity, recycling, supersonic atomic beam source. Changes address several issues previously limiting performance and reliability of the apparatus, including the use of newly available vacuum pumps and modifications to the recycling system. We achieve a source intensity of 2.5 x 10(19) atoms/s/sr, almost twice that previously achievable during recycling. Current limits on intensity are discussed.

Field ionization of helium in a supersonic beam: Kinetic energy of neutral atoms and probability of their field ionization

High detection efficiency combined with spatial resolution on a nm-scale makes the field ionization process a promising candidate for spatially resolved neutral particles detection. The effective cross-sectional area σ eff can serve as a measure for the effectiveness of such a field ion detector. In the present contribution, we combine quantum-mechanical calculations of the field-modified electron density distribution near the tungsten tip surface and of the resulting local field distributions, performed using the functional integration method, with a classical treatment of the atom trajectories approaching the tip in order to calculate the σ eff values for ionization of free He atoms over an apex of a tungsten field emitter tip. The calculated values are compared with experimental data for supersonic He atomic beams at two different temperatures 95 and 298 K.

Kinematic production of isolated millikelvin molecules

Recently we developed a technique for producing cold molecules from a supersonic molecular beam via single collisions with a supersonic atomic beam [M. S. Elioff, J. J. Valentini, and D. W. Chandler, Science 302, 1940 (2003)]. This cooling technique necessarily produces the cold molecules at the relatively high density crossing of the atomic and molecular beams. In previous reports, secondary glancing collisions with the remnant atomic and molecular beams lead to rapid depletion of the cold molecules limiting the observation time to less than $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ with an estimated temperature of $440\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. Here we present experimental conditions for the kinematic cooling technique which overcome the limitations of the previous experiments. We demonstrate the success of this experiment for the production of cold nitric oxide (NO) in the ground vibrational, $j=7.5$ rotational, state in order to compare with our previously reported data. With the present setup, we are able to extract the cold molecules from the pulsed molecular and atomic beams and we observe these cold $\mathrm{NO}{(X)}_{j=7.5}$ persisting for over $150\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. This long observation time demonstrates our ability to temporally and spatially separate the cold molecules from the parent atomic and molecular beams, simultaneously allowing for a better measurement of the final average velocity for the kinematically cooled molecules. From the data we find a final average velocity of the $\mathrm{NO}{(X)}_{j=7.5}$ of approximately $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$, consistent with simulations. The final observed average velocity is equated to a temperature of approximately $35\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, over an order of magnitude colder than our previous measurements.

Development of Supersonic Metastable Helium Pulsed Beam Source for Plasma Diagnostics

A supersonic metastable helium (2 1S) pulsed beam source, which consists of an electromagnetic valve, a collimation skimmer, and discharge electrodes for production of a plasma, has been constructed, which is essential for the direct measurement of electric fields in plasmas by means of polarization laser-induced fluorescence spectroscopy. A supersonic helium atomic beam with a short pulselength (∼300 µs), narrow divergence (∼1.1°), and high density of ∼1.4×1014 cm-3 has been achieved. In order to generate metastable atoms in this beam source, a Penning-type discharge was employed, which is suitable for producing stable plasma with high temperature, even at low gas densities, due to the particular configuration of electrodes together with a magnetic field. Spectroscopic observations indicated that the temporal behaviors of neutral atom and ion emissions were almost the same as that of the helium atom profile, and on increasing the discharge voltage applied between electrodes, the spectral intensity increased approximately linearly. Moreover, the fact that the ion emission can also be observed showed that it was probable that a high-temperature plasma was generated by the Penning discharge.

The First Layers of Water on Ru(001)

The initial growth of water molecules to form the first bilayer and then ice layers on Ru(001) was studied utilizing work function change (ΔΦ), temperature programmed desorption (TPD), and supersonic atomic beam−collision-induced desorption (CID) measurements. A kinetic model that reproduces the first bilayer growth, as determined by the ΔΦ measurements, was developed. It indicates that monomers dominate the cluster size distribution at low coverages, but at high coverages, tetramers gradually become the dominant clusters. Small contributions to ΔΦ suggest that tetramers are cyclic at the adsorbed state with inclined dipoles. CID measurements of H2O and D2O at coverages near one bilayer reveal strong selectivity to the removal of molecules in the A2 adsorption sites over those in the icelike C sites and the A1 sites. Soft removal rates of thicker ice layers as a result of CID with energetic Kripton atoms were then studied as a function of the ice layer thickness. Near the completion of the third bilayer, ...

Enhanced oxidation rate of Ni(111) by atomic oxygen

The oxidation of the Ni(111) surface by a supersonic atomic oxygen beam has been quantitatively studied using in situ high-resolution electron-energy-loss spectroscopy. For a room temperature substrate, a drastically different oxidation rate is observed for atomic oxygen induced oxidation than for molecular oxygen based oxidation. This rate was found to be two orders of magnitude higher than that for molecular oxygen. The reaction on a 110K substrate indicated oxygen uptake only to the chemisorption saturate, with no further oxidation. This later finding agrees with previous results for low surface temperature oxidation using molecular oxygen, implying that the inhibiting step in low temperature Ni oxidation is not molecular oxygen dissociation, but a more fundamental property of the metallic substrate. The chemisorption region of the atomic oxygen reaction was found to saturate at the same sub-monolayer as results from exposure to molecular oxygen, i.e. a dense (1×1) overlayer as has been seen on other metals does not form on Ni(111).

Search for Off-Diagonal Density Matrix Elements for Atoms in a Supersonic Beam

We demonstrate the absence of off-diagonal elements for the density matrix of a supersonic Na atomic beam, thus showing that there are no coherent wave packets emerging from this source. We used a differentially detuned separated oscillatory field longitudinal interferometer to search for off-diagonal density matrix elements in the longitudinal energy/momentum basis. Our study places a stringent lower bound on their possible size over an off-diagonal energy range from 0 to 100 kHz.

Collision induced desorption of N2 from Ru(001)

The dynamics of collision-induced desorption (CID) of N2 from Ru(001) exposed to hyperthermal rare gas colliders generated in a supersonic atomic beam source have been studied. Low coverage of 0.01 ML N215 at crystal temperature of 96 K was chosen to represent a CID process of a practically isolated molecule, neglecting the effect of lateral N2–N2 interactions. The cross sections for CID of nitrogen molecules, σdes(Ei,θi), as a function of the kinetic energy and angle of incidence of Ar and Kr colliders have been measured. It was found that σdes(Ei,θi=0°) changes monotonically in the range 0–25 Å2 for beam energy in the range of 0.5–5.5 eV and is insensitive to the type of collider (Ar, Kr) as well as to the adsorbate isotope (14N2, 15N2). The threshold energy for desorption has been determined to be 0.50±0.10 eV, which is twice the binding energy of N2 to Ru(001). The cross section for CID at a fixed collider’s energy rises approximately four times as the incidence angle θi increases from 0° to 70° relative to the surface normal. Neither normal nor total energy scaling of the cross section could describe the results. The σdes(θi) scales reasonably well, however, with the tangential energy of the collider for angles above 30°. Classical molecular dynamics simulations were performed to gain better understanding of the CID process. Threshold energy and angular dependence of the cross section were reproduced very well. The predominant CID mechanism was concluded to originate from a direct rare gas–nitrogen collision, in which impulsive-bending and the motion along the surface are coupled to the adsorbate motion which leads to desorption.

Micrometer-sized nozzles and skimmers for the production of supersonic He atom beams

Micrometer-sized nozzles and skimmers made from drawn glass tubes are described and tested for the production of highly monoenergetic He atom beams. Glass nozzles with diameters between 1 μm and 4 μm when operated at He source stagnation pressures of up to 1000 atm provide intense beams with measured speed ratios of S=50–100, in good agreement with the predicted behavior scaled from nozzles with larger openings. Miniature glass skimmers with diameters as small as 3 μm were also successfully tested with conventional 10 μm diameter nozzles. These miniature nozzle-beam sources can be used to greatly reduce the size of present-day He-atom surface-scattering time-of-flight spectrometers and to reduce the number of vacuum stages and the size of vacuum pumps. They also open up new experimental possibilities as illustrated by measurements of the spatial profiles of seeded nozzle beams.

Mass-spectrometry of cluster molecular beams

An apparatus with gas-dyhamical molecular beam source (GMBS) has been developed providing formation of cool supersonic atomic and molecular beams with 10 18 part/cm 2 s intensity and angular divergence of < 1.5°. The apparatus design enables one to generate cluster beams of various substances. All parameters of the apparatus are continuously controlled in an automated mode. An oil-free vacuum pumping provides a limiting pressure from 2 × 10 −7 up to < 10 −9 Torr allowing one to study the interaction of gas flows with solid surfaces with subsequent structural analysis by means of a photoelectron X-ray spectroscopy. The cluster beams of the Ar atoms, CO 2 molecules and Ar(CO 2) mixture have been studied by means of a mass-spectrometric method. The production of the Ar N X-like and (CO 2) N X-like associated clusters (where X = H, H 2 and OH) has been observed. In the mass-spectrum for the argon + carbon dioxide mixture, besides the basic (parent) Ar N and (CO 2) N clusters, the composite [Ar n (CO 2) m ] N -like clusters have been found comprising the mass peaks related to the various combinations of daughter Ar n and (CO 2) m clusters with ( n + m = N) total size of incorporated molecules.