ALBERT

Notes: The phenomenon of aerodynamic enrichment of heavy molecules seeded in supersonic free jets has been known since 1955. But its systematic exploitation in the generation of intensely focused molecular beams has been prevented by the lack of a quantitative and realistic explanation of the observed facts. Here, the aerodynamic focusing of CBr4, W(CO)6, and C2Cl6 molecules seeded in jets of He or H2 is studied experimentally, and found to be most singular under conditions similar to those known to produce sharply focused beams of microscopic spheres suspended in air jets. The gas mixture expands through thin-plate orifice into a vacuum chamber, forming a supersonic free jet. The spacial distribution of the heavy molecules in the jet is measured at varying distances L to the nozzle by scanning a thermocouple probe across a jet diameter. The probe is sufficiently small to interfere negligibly with the flow. The increment DV in the thermocouple voltage resulting from seeding the heavy gas on a given flow of He or H2 is seen to be a sensitive indicator of the local concentration of seed molecules in the jet. The following behavior is observed in terms of the same Stokes number or inertia parameter S that governs the simpler and better understood phenomenon of aerosol focusing. Below S=0.4 for H2 and S=0.2 for He, heavy molecule and aerosol beam widths are practically identical, and the boundary of the jet of heavy molecules is rather sharp. At higher values of S, aerosol beams show further reductionsin cross section, down to less than 10% of a nozzle throat diameter dn. In contrast, the measured heavy species minimal beam widths or waists at a distance L∼dn from the throat are around 0.5dn and 0.35 dn for jets of He and H2, respectively. In units of dn, these widths are several times larger than expected from elementary considerations on the defocusing effects due to Brownian motion (of the order of the square root of the molecular mass ratio between light and heavy molecules). Nonetheless, the thin-plate orifice nozzle yields considerably more concentrated jets of heavy gases than previously seen, with far-field enrichment factors for the seed species close to 50 in thecase of H2 jets. This technique, thus, appears to provide a greatly improved source for intense molecular beams. Aerodynamically focused beams have a sharp distribution of kinetic energies, being ideally suited for cross beam and beam surface studies. But they are not quite so optimal for spectroscopic studies because they require moderate source Reynolds numbers (of order 100), at which the heavy gas undergoes very little translational, rotational, or vibrational cooling.

Notes: The large inertia of heavy molecules suspended in a light carrier gas is exploited in the impingement of seeded free jets against surfaces at near-continuum conditions. It emerges that the kinetic energy at impact can be close to the free stream convective kinetic energy, even at near-continuum densities, i.e., at Knudsen numbers of 10−2 or less. Thus, one can simulate collision energies previously obtained only under molecular beam conditions but with the much higher flux densities characteristic of free jets upstream of the transition to free-molecule flow. The principal design parameter in such "high density seeded free jet'' experiments is the heavy molecule Knudsen number, analogous to the Stokes number governing the inertial impaction of aerosol particles. When the ratio of masses mp/m for the heavy and the light gases is large, the interspecies transfer of momentum and energy is rather slow, and the Stokes number is of the order of mp/m times larger than the light gas Knudsen number. Thus, there is a region in which the light gas still behaves as a continuum fluid while the heavy component penetrates through it under effectively free-molecular conditions. A hydrodynamic theory capable of predicting the distribution of impact energies for such free jets impinging on solid objects at Stokes numbers of order unity is developed.

Notes: When concentrated solutions of NaI in formamide with electrical conductivities K larger than 1.1 S/m are electrosprayed from a Taylor cone-jet in a vacuum, ions are evaporated at substantial rates from the surface of the meniscus and the drops. This constitutes a new source of ions and nanoparticles, where the relative importance of these two contributions is adjustable. The currents of ions are measured independently from those associated with drops by a combination of stopping voltage analysis and preferential scattering in a gas background. The magnitude E of the electric field at the surface of the drops and at the apex of the cone-jet is controlled through the electrical conductivity K of the liquid and its flow rate Q through the jet. E is related through available scaling laws for Taylor cone-jets to the ratios K/Q or I/Q, where I is the current of drops emitted by the jet. Ion currents are very small or null at typical K/Q values used in the past. A relatively small initial ion current is attributed to a few particularly sharp features present, perhaps associated with small satellite drops. At still higher K/Q this first ionization source saturates, and ion evaporation from the main drops begins to dominate (E∼1 V/nm). E can then be determined with little ambiguity, and the associated ion current is also measured over a broad enough range of electric fields to determine the ionization kinetics. At still higher K/Q the ion current from the drops approaches saturation, and ion evaporation directly from the meniscus becomes dominant. The total spray current then presents the anomaly of increasing rapidly at decreasing liquid flow rate. The ion current from the meniscus can also be measured in this regime over a broad range of K/Q, with qualitative agreement with the ionization measurements from the drops. But the relation established between K/Q and E becomes suspect because ion and drop currents are now comparable. A third approach to infer the ionization rate is based on the related disappearance of Coulomb explosions of the drops above a critical K/Q. These results are congruent with the model of Iribarne and Thomson, with an activation barrier for ion evaporation equal to 1.7 eV−(e3E/4πε0)1/2. © 2000 American Institute of Physics.

Notes: The phenomenon of ion evaporation from charged liquid surfaces is at the basis of electrospray ionization, a source of a stunning variety of gas phase ions. It is studied here by producing a monodisperse cloud of charged droplets and measuring the charge q and diameter dr of the residue particles left after complete evaporation of the solvent. When the droplets contain small monovalent dissolved ions, the electric field E on the surface of their solid residues is found to be independent of dr. One can thus argue that the source of small ions in electrospray ionization is field-emission, and not other proposed mechanisms such as Dole's charged residue model. A consequence of the observed independence of E on dr is that the rate of ion ejection is simply related to the rate of solvent evaporation, estimated here as that for a clean surface of pure solvent.The reduction G(E) brought about by the electric field E in the activation energy for ion evaporation has thus been inferred as a function of the measured field E in the range 1.5〈E(V/nm)〈3.25. It agrees surprisingly well with the so-called Schottky hump from the image potential model (IPM), GIPM=(e3E/4πε0)1/2. This remarkably simple result is paradoxical in view of two major objections raised earlier against the use of the IPM for ion evaporation from liquids. However, the correct mechanism (first introduced by Iribarne and Thomson) leading to an attractive interaction between the liquid surface and the escaping ion is not the creation of an image charge, but the polarization of the dielectric liquid by the ion. In the limit of a large dielectric constant ε(very-much-greater-than)1, the image force and the polarization force coincide numerically, though the later sets in much faster and is apparently free from the paradox raised by Röllgen. Also, the dielectric nature of the liquid and its strong screening of the net charges near its surface resolves another paradox raised by Fenn regarding the discrete distribution of charges. This screening also introduces a correction in the model proposed by Iribarne and Thomson for G(E), making its predictions virtually indistinguishable from those of GIPM(E). In conclusion, small ions observed in electrospray ionization are produced by field-emission. Measured ionization rates are well represented by results from a "polarization potential model'' which appears to be physically sound. These predictions coincide with those from the IPM in the limit ε(very-much-greater-than)1, the only case studied so far. © 1995 American Institute of Physics.

Notes: The frequency shifting of an ultrashort (femtosecond) low intensity laser pulse in the presence of a plasma wave is investigated using particle simulations. One-dimensional simulations confirm the existence of photon trajectories similar to the trajectories of trapped or untrapped charged particles in a plasma wave. In the case when a plasma wake is produced by a relativistically intense laser pulse with a duration of the order of one plasma period, some full oscillations of the photon frequency take place only for the untrapped photons moving backward in the plasma wave frame before the intense laser pulse depletion due to the energy transfer to the plasma wake occurs. An analytical estimate of the probe pulse phase and frequency shift in two-dimensional (2-D) axially symmetrical plasma wake is performed. Axially symmetrical particle simulations in 2-D, with experimentally attainable parameters, display a frequency shift of the probe pulse of a few percent. In these analytical estimates and 2-D simulations special attention has been given to the modeling of the probe pulse optical collecting line that is typically used in plasma diagnostics. © 1999 American Institute of Physics.

Notes: The dispersion relation for electronic parametric instabilities of a circularly polarized laser wave is solved in the case where the distribution function is supposed to be cold in the transverse direction and to be a linear combination of a cold distribution function and of a Maxwellian in momentum in the longitudinal direction. Only densities below the critical density are considered. It is shown that the longitudinal temperature as expected reduces the growth rate, but that the existence of a hot tail is not sufficient to significantly reduce the instability. It is the bulk of the distribution function that must be heated to efficiently stabilize the system. Another important effect of the heating is to reduce the backscattered component of the instability. An example of a one-dimensional particle simulation performed in the exact conditions of validity of the theory is discussed. © 2001 American Institute of Physics.

Notes: In this paper a thermodynamic model for a polymerized unary liquid is presented. The model assumes there is a distribution of species as a result of a polymerization equilibrium which involves chain formation and scission. According to the model, the distribution of species depends on the Gibbs energy of bond formation and on the temperature of the liquid. Such a polymerization equilibrium is widely believed to occur in the Se-Te melts. The paper discusses the heat capacity, which is compared with the experimental heat capacity of the Se-Te melts. A very close behavior is found and an explanation from the point of view of a polymerization equilibrium is suggested. © 1997 American Institute of Physics.

Notes: Approximate scaling laws for the charge and size of the drops ejected from the apex of Taylor cones run in the cone-jet mode (electrospray) are now available for highly conducting electrolytes (10−4 S/m〈K〈1 S/m) electrosprayed at atmospheric pressure. In order to confirm that such laws do also apply to Taylor cones in vacuo, the current versus liquid flow rate curves I(Q) characteristic of a given liquid are investigated both in vacuum and in atmospheric pressure air. Although the sprays of drops differ profoundly in both cases, the two corresponding I(Q) curves are nearly identical for relatively involatile liquids such as tributyl phosphate. A discussion on the possible relation between the behavior of Taylor cones of electrolytes of organic liquids and liquid metal ion sources (K∼106 S/m) is attempted, yielding insights on the role of space charge. However, the electrical conductivity variable which dominates the behavior of liquid cones of electrolytes appears to be irrelevant in liquid metals. © 1998 American Institute of Physics.

Notes: In this paper we present calculations of levitation forces between a cylindrical permanent magnet and a cylindrical superconductor using a commercial finite element program. Force limits for zero field cooled and field cooled processes have been obtained using the Meissner effect and the perfect pinning hypothesis, respectively. Comparison of the experimentally determined forces with respect to these limits provides a simple estimation of the sample quality. The hysteretical behavior of the forces has been reproduced assuming a critical state model for the superconductor. Results are compared with experimental data. Excellent agreement has been found for forces measured after zero field cooled process thus allowing us to estimate the critical current of the samples. As a further exploitation of the software capabilities we have investigated the effects of the superconducting sample geometry and the effects of different strategies of flux conditioning to optimize the levitation forces. © 1997 American Institute of Physics.

Notes: YBa2Cu3O7 cylinders with 10%–20% of Y2BaCuO5 and 1% CeO2 additions have been directionally solidified under a temperature gradient. It is shown that a steady growth regime of domains, typically 1 cm in diameter, is established after polynucleation at the bottom of the cylinders on the substrate interface. The length of the region where a steady growth proceeds is limited by the liquid loss, which induces an enrichment in unreacted Y2BaCuO5 in the upper part of the cylinder and a polycrystalline structure. The vertical and lateral magnetic levitation forces and flux trapping profiles have been measured and a direct correlation with the size and location of the domains has been found. © 1996 American Institute of Physics.