Jets Of He Research Articles

Comparative study on the atmospheric pressure plasma jets of helium and argon

Formation mechanisms for atmospheric pressure plasma jets (APPJ) of He and Ar are investigated by comparing the discharge current, light emission from jet, and time-resolved image of the discharge. A longer jet of He (Ar) is available with active (ground) electrode sitting at downstream side. The jet of He outside active electrode arises from corona discharge, while that of Ar outside ground electrode results from charge overflow, and can be diffusive or filamentous in different phases of the applied voltage. The underlying mechanisms are discussed. These results can be helpful for the further mechanism investigation and implementation of APPJs.

Inactivation of Listeria monocytogenes on agar and processed meat surfaces by atmospheric pressure plasma jets

An apparatus for generating atmospheric pressure plasma (APP) jet was used to investigate the inactivation of Listeria monocytogenes on the surface of agar plates and slices of cooked chicken breast and ham. He, N2 (both 7 L/min), and mixtures of each with O2 (0.07 L/min) were used to produce the plasma jets. After treatment for 2 min with APP jets of He, He + O2, N2, or N2 + O2, the numbers of L. monocytogenes on agar plates were reduced by 0.87, 4.19, 4.26, and 7.59 log units, respectively. Similar treatments reduced the L. monocytogenes inoculated onto sliced chicken breast and ham by 1.37 to 4.73 and 1.94 to 6.52 log units, respectively, according to the input gas used with the N2 + O2 mixture being the most effective. Most APP jets reduced the numbers of aerobic bacteria on the meat surfaces to <102 CFU/g, and the numbers remained below that level of detection after storage at 10 °C for 7 days. The results indicate that APP jets are effective for the inactivation of L. monocytogenes on sliced meats and for prolonging the shelf-life of such foods.

Rotational relaxation of nitrogen in helium

Rotational level populations of N 2 have been measured in free jets of He + N 2 mixtures by an electron beam technique. The experimental data are compared with kinetic calculations, based on theoretical state-to-state rate constants, obtained by the IOS method with an M3SV potential energy surface. Reasonable agreement of the data substantiates the IOS method down to low (≈ 10 K) temperatures. Other available calculations of rotational relaxation times are compared with the present experimental data.

Rotational relaxation time in free jets of He + N 2 mixtures

Non-equilibrium rotational energy distributions of N 2 in free jets of He + N 2 (<5%) were measured by the electron beam technique. The rotational temperature was derived from the spectrum of the first negative system of N 2 + induced by high energy ( E ∼ 10 keV) electrons. The experimental data were compared with kinetic calculations of rotational energy in free jets based on the linear relaxation equation: dE R dt = −(E R −E t) τ . The rotational relaxation times τ were derived from available transport collision integrals, evaluated with three different potential energy surfaces: HFD1, BTT and M3SV. The HFD1 surface leads to an overestimated rate of rotational relaxation. Two other surfaces agree satisfactorily with experiment, similarly as observed by Dickinson et al. for transport coefficients. To decide between BTT and M3SV, more precise measurements should be carried out over a wide temperature range.

Aerodynamic focusing of heavy molecules in seeded supersonic jets

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 reductions in 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 0.5 d n 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 the case 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.

Pulsed laser desorption near a jet orifice: Concentration profiles of entrained perylene vapor

We have investigated the entrainment in a jet expansion of material vaporized with a laser pulse from a surface below but closely adjacent to the jet orifice. Jets of He and Ar were used as carriers, and perylene was the test substance. Its distribution in space and time far from the nozzle was measured by laser-induced fluorescence and one-dimensional imaging with an optical multichannel analyzer. The width of the perylene concentration profile was found to be much narrower than the width of the gas expansion, especially in He. This makes it possible to extract a useful portion of the entrained material through a skimmer. Depending on the values of experimental parameters, the center of the profile first appears above, on, or below the jet axis and then moves lower with time. Effects of several parameters on shape and time-dependent position of the profile were investigated. The results can be used to optimize the overlap of the concentration pulse with a skimmer or other probe.

The turbulent diffusion of gas jets with different densities from the surroundings.

This experimental study aims at investigating the influences of buoyancy on the characteristics of turbulent free jets. Concentration and velocity measurements are carried out in vertically up or down jets of He, CO2 and air into still air. As a result, it is clear that the distance from the nozzle outlet to the starting point of the buoyant jet region depends upon the Froude number, and that, in the case of a large difference in density between the jet and the surroundings, the axial and radial distributions of concentration and velocity are different because of the interaction of the momentum and mass diffusion.

Spectroscopy of large molecular complexes in supersonic jets

In this paper we report the results of an experimental study of laser-induced fluorescence spectroscopy of M⋅Xn van der Waals complexes, consisting of medium-sized molecules (X=methane, carbon tetrachloride, ammonia, water, methanol, acetonitrile, benzene, and toluene) bound to a large aromatic molecule (M=tetracene and fluorene). These large complexes were synthesized in pulsed supersonic jets of He seeded with M and X. The spectral features assigned to S0→S1 electronic-vibrational excitations of these large M⋅X1 complexes, which are characterized by narrow linewidths (FWHM) of 2–4 cm−1, were attributed to three distinct categories: (i) A vibrationless excitation of M⋅X1, (ii) vibrational excitations of M in M⋅X1, and (iii) excitations of the vibrations involving relative motion of M and X in the M⋅X1 complex.

Complexing of porphyrins in supersonic jets

The complexing of porphyrins' is a subject of considerable interest in the areas of physical chemistry and biophysical diagnosis. The attachment of various ligands (L) to metal porphyrins (MP) in s ~ l u t i o n ~ * ~ and in lowtemperature solids4 is manifested in the appearance of new spectral features of the MP-L complexes. We would like to draw attention to the potential of supersonic expans i o n ~ ~ for the production of internally cold, collision-free, large, molecular complexes. We have applied the techniques of laser spectroscopy in seeded supersonic expans i o n ~ ~ for the synthesis, identification, and exploration of excited-state energetics of isolated, ultracold complexes of porphyrins with medium-sized molecules (L = water, methanol, acetonitrile, benzene, and pyridine), providing a new approach for the elucidation of solvent perturbat i o n ~ ~ on porphyrins, as explored from the microscopic point of view. We have studied the laser-induced fluorescence (LIF) excitation spectra of the complexes of zinc octaethylporphyrin (ZnOEP) with medium-sized molecules. The ZnOEP-L complexes were synthesized in pulsed supersonic jets of He seeded with the two molecules which form the complex. A solid sample of ZnOEP was heated in the nozzle chamber to 350 C to yield a vapor pressure of 0.1 torr and seeded into a mixture of He + L. The partial pressure of L was pL = 0.1-1 torr, while the stagnation pressure of He was p = 1200-2000 tom. The He + ZnOEP + L mixture was expanded through a circular pulsed nozzle (diameter D = 600 pm)*. A nitrogen-pumped dye laser (spectral resolution 0.3 cm-') crossed with supersonic expansion at the distance X = 15 mm (X/D = 25) down the nozzle. The LIF spectra were measured as previously described.6 Figure 1 shows the LIF spectrum of ZnOEP in a jet of He. The spectral features, whose energies and relative intensities are independent of the stagnation pressure in the range p = 1200-2500 torr, are assigned to the electronic-vibrational excitation of the So S1 transition (the Q band)g of the bare ZnOEP. No spectral features are exhibited at wavelengths above 5600 A. The lowest-energy spectral features consist of a triplet (Figure 1) exhibiting

Vacancy cascade spectroscopy: A convenient method of studying highly ionized atoms and molecules

Vacancy cascades have been created in gas jets of He, N 2, O 2, CO 2, Cl 2 and Xe by bombardment with a monoenergetic beam of 2–10 keV electrons. The resulting spectra, observed at 500 – 2500 Å are comparable to beam-foil spectra, but have several orders of magnitude higher intensity, with a broad distribution of ionization stages. This technique can be used to measure atomic and molecular lifetimes, and other applications are discussed as well.