The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and ...

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

The structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed.

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Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects

The development of wave optics for light brought many new insights into our understanding of physics, driven by fundamental experiments like the ones by Young, Fizeau, Michelson-Morley and others. Quantum mechanics, and especially the de Broglie’s postulate relating the momentum p of a particle to the wave vector k of an matter wave: k = 2 λ = p/ℏ, suggested that wave optical experiments should be also possible with massive particles (see table 1), and over the last 40 years electron and neutron interferometers have demonstrated many fundamental aspects of quantum mechanics [1].

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Optics and interferometry with atoms and molecules

https://arxiv.org/pdf/cond-mat/0410417.pdf

Universality in few-body systems with large scattering length

Magnetically tunable Feshbach resonances were employed to associate cold diatomic molecules in a series of experiments involving both atomic Bose and two-spin-component Fermi gases. This review illustrates theoretical concepts of both the particular nature of the highly excited Feshbach molecules produced and the techniques for their association from unbound atom pairs. Coupled-channels theory provides a rigorous formulation of the microscopic physics of Feshbach resonances in cold gases. Concepts of dressed versus bare energy states, universal properties of Feshbach molecules, and the classification in terms of entrance- and closed-channel-dominated resonances are introduced on the basis of practical two-channel approaches. Their significance is illustrated for several experimental observations, such as binding energies and lifetimes with respect to collisional relaxation. Molecular association and dissociation are discussed in the context of techniques involving linear magnetic-field sweeps in cold Bose and Fermi gases and pulse sequences leading to Ramsey-type interference fringes. Their descriptions in terms of Landau-Zener, two-level mean-field, as well as beyond mean-field approaches are reviewed in detail, including the associated ranges of validity.

Production of cold molecules via magnetically tunable Feshbach resonances

We describe a pulsed gas valve which we have developed for use as a molecular beam source. In order to observe the performance of the pulsed beam source, we also have developed an ionization detector with a rise time of about 1 μs. The pulsed valve produces very intense supersonic molecular beam pulses of about 10 μs duration for light gases such as H2 and He, and of somewhat longer duration for heavier gases. As a new tool for the study of molecular collisions, the pulsed beam technique offers substantial advantages over the conventional continuous‐beam method for experiments which are limited by the signal‐to background ratio for scattered products.

Ten‐microsecond pulsed molecular beam source and a fast ionization detector

Helium dimer ion was observed after electron impact ionization of a supersonic expansion of helium with translational temperature near 1 mK The dependence of the ion signal on source pressure, distance from the source, and electron kinetic energy was measured The signal was determined to arise from ionization of neutral helium dimer

/pdf/the-weakest-bond-experimental-observation-of-helium-dimer-4oklk2lnlv.pdf

The weakest bond: Experimental observation of helium dimer

The relative transmissions of helium dimer and helium atom beams through a set of nanoscale sieves were measured as a function of hole size in the range from 98–410 nm. From the relative transmission coefficients, the mean internuclear distance of helium dimer was determined to be 〈r〉=62±10 A. This enormous bond length—by far the largest known—confirms recent theoretical estimates of the potential energy well depth e and the extremely small binding energy Eb of helium dimer. The ranges of these parameters corresponding to the experimental uncertainty in 〈r〉 are e/k=10.88–10.98 K and Eb/k=0.65–1.30 mK, where k is the Boltzmann constant.

Direct measurement of the size of the helium dimer

Classical trajectory treatment of inelastic scattering in collisions of H + with H 2 , HD, and D 2

Molecular beam experiments have been carried out on the infrared photodissociation of the (C2H4)2, (C2D4)2, and (C2H4)⋅ (C2D4) van der Waals molecules. Measurements of the frequency and fluence dependences of the photodissociation cross sections were performed over the CO2 laser frequency range from 900 to 1100 cm−1, which covers the region of the ν7 out‐of‐plane vibration of C2H4 and the ν12 in‐plane vibration of C2D4. In addition, the monomer product speed and angle distributions were measured in order to obtain information on the final energy disposition in the products. Absorption in either vibrational mode induces dissociation of the cluster, but the linewidth associated with excitation of the ν7 mode of C2H4 is about four times larger than that associated with excitation of the ν12 mode of C2D4. Two‐laser ‘‘hole‐burning’’ experiments demonstrate the homogeneity of the ground‐state populations. The absorption intensities are not simply proportional to the number of molecules in the cluster capable of...

W. Ronald Gentry

Papers

Ten‐microsecond pulsed molecular beam source and a fast ionization detector

The weakest bond: Experimental observation of helium dimer

Direct measurement of the size of the helium dimer

Classical trajectory treatment of inelastic scattering in collisions of H + with H 2 , HD, and D 2

The dynamics of ethylene dimer infrared photodissociation in pulsed molecular beamsa)