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  • 2000-2004  (27)
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  • 1
    Electronic Resource
    Electronic Resource
    Angular resolved studies of the Lyman-α photodissociation of HCN and DCN: New dynamical insights (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 994-1004 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photochemistries of HCN and DCN at the H(D) Lyman-α wavelength have been reinvestigated using the technique of H(D) Rydberg atom time-of-flight spectroscopy, with angular resolution of the H/D atom signal about the polarization vector of the photolysis radiation. In the case of HCN photodissociation, the previous assignment of substantial branching to H+CN(A 2Π)v=0 products is confirmed. Analysis of the profile taken under parallel polarization of the Lyman-α radiation relative to the time of flight axis reveals additional structure attributable both to a progression in CN(A) products with high rovibrational excitation (v=4–9, with N∼26–41, for all v), and to various rotationally excited levels associated with CN(B 2Σ+)v=0,1. From these various assignments an improved value for the dissociation energy, D0(H–CN)=43 710±70 cm−1, is obtained. The determined β parameter, which is a measure of the angular part of the photofragment velocity distribution about the polarization vector of the photolysis radiation, shows an increasingly parallel distribution of H atom velocities with increasing CN internal energy. DCN photolysis at the D Lyman-α wavelength yields both CN(A)v=0 and a range of rovibrationally excited CN(A) products but, in contrast to HCN, no significant branching to CN(B) products is observed. The corresponding β parameter, which is found to be relatively invariant with CN internal energy, indicates a near limiting perpendicular distribution of D atom velocities about the photolysis radiation. These results are interpreted with reference to the available ab initio calculated potential energy surfaces of A′ and A″ symmetry, and the relative propensities for excitation to, and the likely dynamics on, these various excited states is discussed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481880
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  • 2
    Electronic Resource
    Electronic Resource
    Low temperature diamond growth using CO2/CH4 plasmas: Molecular beam mass spectrometry and computer simulation investigations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 1484-1492 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Microwave plasma enhanced chemical vapor deposition has been used to grow diamond films at substrate temperatures down to 435 °C using CO2/CH4 gas mixtures. An Arrhenius plot of growth rate as a function of substrate temperature yields a value for the activation energy for the growth step of 28 kJ mol−1. This is lower than that measured previously for CH4/H2 systems and hints at a different gas-surface chemistry when using CH4/CO2 plasmas. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of plasma gas mixtures (0%–80% CH4, balance CO2). The CHEMKIN computer package has also been used to simulate the experimental results in order to gain insight into the major reactions occurring within the microwave plasma. The calculated trends for all species agree well with the experimental observations. Using these data, the model for the gas phase chemistry can be reduced to only four overall reactions. Our findings suggest that CH3 radicals are likely to be the key growth species when using CO2/CH4 plasmas and provide a qualitative explanation for the narrow concentration window for diamond growth. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1333031
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  • 3
    Electronic Resource
    Electronic Resource
    Investigations of the plume accompanying pulsed ultraviolet laser ablation of graphite in vacuum (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 697-709 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The plume accompanying 193 nm pulsed laser ablation of graphite in vacuum has been studied using wavelength, time and spatially resolved optical emission spectroscopy and by complementary Faraday cup measurements of the positively charged ions. The temporal and spatial extent of the optical emissions are taken as evidence that the emitting species result from electron–ion recombination processes, and subsequent radiative cascade from the high n,l Rydberg states that result. The distribution of C neutral emission is symmetric about the surface normal, while the observed C+ emission appears localized in the solid angle between the laser propagation axis and the surface normal. However, Faraday cup measurements of the ion yield and velocity distributions, taken as a function of scattering angle and incident pulse energy, indicate that the total ion flux distribution is peaked along the surface normal. The derived ion velocity distributions are used as input for a two-dimensional model which explains the observed anisotropy of the C+ emission in terms of preferential multiphoton excitation and ionization of C species in the leading part of the expanding plasma ball that are exposed to the greatest incident 193 nm photon flux, prior to electron–ion recombination and subsequent radiative decay. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1330548
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  • 4
    Electronic Resource
    Electronic Resource
    Sulfur doping of diamond films: Spectroscopic, electronic, and gas-phase studies (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3605-3613 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Chemical vapor deposition (CVD) has been used to grow sulfur doped diamond films on undoped Si and single crystal HPHT diamond as substrates, using a 1% CH4/H2 gas mixture with various levels of H2S addition (100–5000 ppm), using both microwave (MW) plasma enhanced CVD and hot filament (HF) CVD. The two deposition techniques yield very different results. HFCVD produces diamond films containing only trace amounts of S (as analyzed by x-ray photoelectron spectroscopy), the film crystallinity is virtually unaffected by gas phase H2S concentration, and the films remain highly resistive. In contrast, MWCVD produces diamond films with S incorporated at levels of up to 0.2%, and the amount of S incorporation is directly proportional to the H2S concentration in the gas phase. Secondary electron microscopy observations show that the crystal quality of these films reduces with increasing S incorporation. Four point probe measurements gave the room temperature resistivities of these S-doped and MW grown films as ∼200 Ω cm, which makes them ∼3 times more conductive than undoped diamond grown under similar conditions. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for H2S doping levels (1000–10 000 ppm in the gas phase) in 1% CH4/H2 mixtures, and for 1% CS2/H2 gas mixtures, for both MW and HF activation. CS2 and CS have both been detected in significant concentrations in all of the MW plasmas that yield S-doped diamond films, whereas CS was not detected in the gas phase during HF growth. This suggests that CS may be an important intermediary facilitating S incorporation into diamond. Furthermore, deposition of yellow S was observed on the cold chamber walls when using H2S concentrations 〉5000 ppm in the MW system, but very little S deposition was observed for the HF system under similar conditions. All of these results are rationalized by a model of the important gas phase chemical reactions, which recognizes the very different gas temperature profiles within the two different types of deposition reactor.© 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1448679
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  • 5
    Electronic Resource
    Electronic Resource
    Modeling of the gas-phase chemistry in C–H–O gas mixtures for diamond chemical vapor deposition (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 5219-5223 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The boundaries of the diamond deposition region in the C–H–O (Bachmann) atomic phase composition diagram have been reproduced successfully for 38 different C, H, and O containing gas mixtures using the CHEMKIN computer package, together with just two criteria—a minimum mole fraction of methyl radicals [CH3] and a limiting value of the [H]/[C2H2] ratio. The diamond growth/no-growth boundary coincides with the line along which the input mole fractions of C and O are equal. For every gas mixture studied, no-growth regions are found to coincide with a negligible (〈10−10) mole fraction of CH3 radicals, while for gas mixtures lying within the diamond growth region the CH3 mole fraction is ∼10−7. Each no-growth→diamond growth boundary is seen to be accompanied by a 2–3 order of magnitude step in CH3 mole fraction. The boundary between diamond and nondiamond growth is less clearly defined, but can be reproduced by assuming a critical, temperature dependent [H]/[C2H2] ratio (0.2, in the case that Tgas=2000 K) that reflects the crucial role of H atoms in the etching of nondiamond phases. The analysis allows prediction of the composition process window for good quality diamond growth for all stable input gas mixtures considered in this study. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1360221
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  • 6
    Electronic Resource
    Electronic Resource
    Effects of NH3 and N2 additions to hot filament activated CH4/H2 gas mixtures (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 92 (2002), S. 672-681 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Resonance enhanced multiphoton ionization and cavity ring down spectroscopies have been used to provide spatially resolved measurements of relative H atom and CH3 radical number densities, and NH column densities, in a hot filament (HF) reactor designed for diamond chemical vapor deposition and here operating with a 1% CH4/n/H2 gas mixture—where n represents defined additions of N2 or NH3. Three-dimensional modeling of the H/C/N chemistry prevailing in such HF activated gas mixtures allows the relative number density measurements to be placed on an absolute scale. Experiment and theory both indicate that N2 is largely unreactive under the prevailing experimental conditions, but NH3 additions are shown to have a major effect on the gas phase chemistry and composition. Specifically, NH3 additions introduce an additional series of "H-shift" reactions of the form NHx+H(r harp over l)NHx−1+H2 which result in the formation of N atoms with calculated steady state number densities 〉1013 cm−3 in the case of 1% NH3 additions in the hotter regions of the reactor. These react, irreversibly, with C1 hydrocarbon species forming HCN products, thereby reducing the concentration of free hydrocarbon species (notably CH3) available to participate in diamond growth. The deduced reduction in CH3 number density due to competing gas phase chemistry is shown to be compounded by NH3 induced modifications to the hot filament surface, which reduce its efficiency as a catalyst for H2 dissociation, thus lowering the steady state gas phase H atom concentrations and the extent and efficiency of all subsequent gas phase transformations. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1481961
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  • 7
    Electronic Resource
    Electronic Resource
    Quasiclassical and quantum mechanical modeling of the breakdown of the axial recoil approximation observed in the near threshold photolysis of IBr and Br2 (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6064-6071 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociations of jet-cooled IBr and Br2 molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(2P3/2o)+Br(2P3/2o) and Br(2P3/2o)+Br*(2P1/2o) products from parent molecules in their v″=0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with v″=1 show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of v″=0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br2. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1457439
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  • 8
    Electronic Resource
    Electronic Resource
    An experimental and ab initio reinvestigation of the Lyman-α photodissociation of H2S and D2S (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 1672-1684 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Lyman-α (λH=121.6 nm) photodissociation of both H2S and D2S has been reinvestigated using the experimental technique of H/D atom photofragment translational spectroscopy. Their total kinetic energy release profiles consist of two distinct components. The first, which is highly structured, is assigned to two body dissociation to H/D(2S)+SH/SD(A 2Σ+) fragments, with the latter formed in a range of rovibrational states. By assigning these various levels the dissociation energy of D2S (measured relative to the lowest rovibrational level of the products) is determined to be D0(D-SD)≥31 874±22 cm−1. The second contribution, which is broad and relatively unstructured, is modeled in terms of two likely fragmentation pathways; secondary predissociation of SH/SD(A 2Σ+) partner fragments associated with the structured contour, and primary three-body dissociation to 2H/D(2S)+S(1D) atomic products. The presented data allow determination of the kinetic energy-dependent anisotropy parameter (β), which is positive over both profiles. This indicates a preferentially parallel distribution of H/D atom recoil velocities about the laser polarization axis. These data are presented in tandem with ab initio and classical trajectory calculations which seek to explain the lack of branching to ground state H/D+SH/SD(X 2ΠΩ) molecular products. The analogous channel is important in the Lyman-α dissociation of the lighter homologue, H2O. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1334617
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  • 9
    Electronic Resource
    Electronic Resource
    Ion imaging studies of Cl(2P3/2) fragments arising in the visible photolysis of BrCl: Measurement of orientation, alignment, and alignment-free anisotropy parameters (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2087-2096 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the "alignment-free" recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1487374
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  • 10
    Electronic Resource
    Electronic Resource
    Continuum state spectroscopy: A high resolution ion imaging study of IBr photolysis in the wavelength range 440–685 nm (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2629-2646 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of jet-cooled IBr molecules has been investigated at numerous excitation wavelengths in the range 440–685 nm using a state-of-art ion imaging spectrometer operating under optimal conditions for velocity mapping. Image analysis provides precise threshold energies for the ground, I(2P3/2)+Br(2P3/2), and first excited [I(2P3/2)+Br(2P1/2)] dissociation asymptotes, the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of excitation wavelength. Such experimental data have allowed mapping of the partial cross-sections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorptions and thus deconvolution of the separately measured (room temperature) parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+) and 1Π(1) excited states of IBr. Such analyses of the continuous absorption spectrum of IBr, taken together with previous spectroscopic data for the bound levels supported by the A and B state potentials, has allowed determination of the potential energy curves for, and (R independent) transition moments to, each of these excited states. Further wave packet calculations, which reproduce, quantitatively, the experimentally measured wavelength dependent product channel branching ratios and product recoil anisotropies, serve to confirm the accuracy of the excited state potential energy functions so derived and define the value (120 cm−1) of the strength of the coupling between the bound (B) and dissociative (Y) diabatic states of 0+ symmetry. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1337049
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