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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 148-153 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The initial step of particulate growth in a dust forming low pressure radio-frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser-particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 2959-2961 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nd:YAG-laser-induced evaporation of particulates formed in an Ar-CCl2F2 rf plasma and the subsequent discharge in the vapor have been investigated in situ by means of optical emission spectroscopy. The estimated threshold for discharge formation is 5×106 W/cm2. The maximum laser-induced emission intensity is observed when the laser is operated in the long-pulse mode (about 200 μs pulse duration) at the fundamental frequency. The wavelength integrated intensity of this continuum emission has been compared with light scattering intensity at the same laser energy. It has been found that the laser-induced emission intensity can be more than ten times higher than the scattering intensity, especially for particulates with a diameter much smaller than the wavelength of the laser. Therefore, this effect provides a sensitive particulate detection method.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 4867-4872 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The average electron density and electron density fluctuations in a dusty Ar/SiH4 rf discharge have been studied using a microwave resonance technique. The average electron density increases with rf input power and it has a maximum as a function of pressure at about 30 mTorr. Within the first second of plasma operation the electron density decreases with a factor of ten. This is caused by submicroscopic particles, formed in the discharge, which rapidly absorb electrons. When the particles reach a critical size they are expelled from the plasma. This process is governed by a balance between the Coulomb force, trapping the particles in the positive plasma glow and the neutral drag force, flushing them out. The periodic growth and expulsion of particles, monitored by light scattering, results in an oscillatory behavior of the electron density. From the measured oscillation period (τ), which is in the order of seconds to minutes, and its dependence on the gas flow rate (F) and on the fraction α of SiH4 in the plasma (τ[s]≈4.5×102α−1F−2 [sccm], at 10 W rf power input), the trapping force (FC) on particles can be calculated: FC[N]≈4×10−18r [nm], where r is the radius of a particle. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 116-122 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Electron attachment mass spectrometry (EAMS) has been developed to study mixtures of electronegative gases and plasmas. A quadrupole mass spectrometer (QMS) has been used to detect negative ions, formed from sampled species by attachment of low energy electrons. Varying the electron energy allows to collect the attachment cross section of the considered species. EAMS appears to be a very powerful technique to study the chemistry of electronegative gases. Unlike ionization mass spectrometry, where cross sections are low at the threshold and rather flat over a broad range of electron energies, attachment resonances are sharp and distinct. Also very limited fragmentation of the parent negative ion occurs, so a given molecule yields only a few different negative ions. This facilitates identification of components in a gas mixture. It is particularly advantageous for detection of large, fragile molecules, which break up after ionization, but can be easily transformed into large negative ions. Moreover, sensitive detection of active species is possible due to their relatively high attachment cross sections. A particularly important application of EAMS is the determination of an effective attachment cross section in a plasma. Recording this cross section allows to decide on the actual negative ion formation mechanism in the plasma environment, where active products of plasma conversion can significantly alter the negative ion production channels and consequently the whole balance of charged particles. Examples of EAMS applications to fluorocarbon gases and low-pressure radio-frequency plasmas are discussed. In a CF4 discharge conversion of the parent gas into species like C2F6 and C3F8 is easily visualized. The dominant mechanism of negative ion formation in the plasma is electron attachment to these minority species and not to the parent gas. Also larger polymers are readily formed in fluorocarbon plasmas. In a C2F6 discharge molecules with up to ten carbon atoms (the mass limit of our apparatus) have been detected using EAMS. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 3442-3451 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Formation of MoS2 nanoparticles at pressures between 0.5 and 10 Torr has been studied. Two different chemistries for the particle nucleation are compared: one based on MoCl5 and H2S, and the other based on MoCl5 and S. In both cases particle formation has been studied in a thermal oven and in a radio-frequency discharge. Typically, the reaction rates at low pressures are too low for an efficient thermal particle production. At pressures below 10 Torr no particle production in the oven is achieved in H2S chemistry. In the more reactive chemistry based on sulfur, the optimal conditions for thermal particle growth are found at 10 Torr and low gas flows, using excess of hydrogen. In the radio-frequency discharge, nanoparticles are readily formed in both chemistries at 0.5 Torr and can be detected in situ by laser light scattering. In the H2S chemistry particles smaller than 100 nm diameter have been synthesized, the sulfur chemistry yields somewhat larger grains. Both in thermal and plasma-enhanced particle syntheses, using excess of hydrogen is beneficial for the stability and purity of the particles. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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