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  • 1
    Digitale Medien
    Digitale Medien
    Combination bands versus overtone stretch excitation and rotational effects in vibrationally mediated photodissociation of acetylene (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 8959-8967 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Rovibrational excitation combined with promotion of C2H2 molecules to the excited electronic trans-bent states A˜ 1Au/B˜ 1Bu and photofragment ionization are used to generate action spectra, H Doppler profiles, and time-of-flight mass spectra. Rovibrational states of C2H2 in the 15 480–15 723 cm−1 region are photodissociated by 243.135 nm photons that subsequently tag the H fragments. The H photofragment yield is greatly enhanced upon rovibrational excitation. In the action spectra, the intensities of the combination bands that involve high stretch and low bend excitation, (1410020) and (2031100), are close to that of the fourth overtone of the C–H stretch, (2030000), while in the absorption spectra the intensities of the combination bands are much weaker. In addition, the effect of rotation on fragment yield and dissociation channels is demonstrated. Several pairs of rotational transitions (sharing similar J′) stand out from the rotational band contour and the R(13) line of (2030000) shows an anomalously high intensity. The origin of the intensity enhancement for the combination bands is the better Franck–Condon (FC) overlap of states containing bend excitation with the dissociative wave functions. The source of the high intensity for the pairs of rotational transitions is the interaction with zero-order dark states that also give rise to a better FC factor, whereas in R(13) excitation an additional bent state is prepared, followed by two photon ultraviolet absorption, leading to a different photodissociation pathway. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477567
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  • 2
    Digitale Medien
    Digitale Medien
    C–Cl and C–H bond cleavage in 193 nm photodissociation of CH3CF2Cl and CH3CFCl2 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 8476-8482 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The relative amounts of Cl(2P3/2) [Cl], Cl(2P1/2) [Cl*], H, and HCl produced by 193 nm photodissociation of CH3CF2Cl and CH3CFCl2 were studied by a laser pump-and-probe technique. An excimer laser photodissociated the parent molecules and a frequency doubled tunable dye laser probed the photofragments via (2+1) resonantly enhanced multiphoton ionization coupled with a time-of-flight mass spectrometer. Only atomic products were observed; no HCl was found. The profiles were well fitted to Gaussians. The Cl*/Cl branching ratios were almost identical, 0.18±0.04 and 0.16±0.04, while the H/(Cl+Cl*) ratios were 0.67±0.21 and 0.18±0.07, for CH3CF2Cl and CH3CFCl2, respectively. The mechanism for the production of the atomic species is discussed. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.475048
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  • 3
    Digitale Medien
    Digitale Medien
    Photodissociation of rovibrationally excited C2H2: Observation of two pathways (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 385-391 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: C2H2 is prepared in the 2030000 (five quanta of C–H stretch) vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via (2+1) resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a Σu+–Σg+ band and mimics the absorption spectrum, except that the R(13) line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67±0.10, whereas for R(13) it is 1.34±0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of C2H2, where the C2H fragment is produced in two different states. In the R(13) pathway an additional bent state is prepared, or an accidental coincidence resonance is involved. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.474399
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  • 4
    Digitale Medien
    Digitale Medien
    Dynamics of the detonation products of lead azide. I. Hydrodynamics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 4693-4708 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The present paper is the first of a series reporting on a comprehensive study of the hydrodynamics, kinetics, and spectroscopy of the transient species formed following the detonation of lead azide (LA). The spatial and temporal behavior of the detonation products expanding into vacuum is obtained via high-speed framing photography, transmission of a HeNe laser beam, and chemiluminescence from excited Pb atoms. The photography reveals that following the initiation of LA the products form an expanding, bell-shaped cloud. The HeNe beam is attenuated when the cloud of products traverses its route. The attenuation starts 4–15 μs after initiation and depends on the height of the beam above the LA sample. The chemiluminescence consists of two components: the first, appearing 1–2 μs after initiation, is obtained from excited products formed by the detonation near the surface of the sample, while the second, starting 2–14 μs after initiation, originates from the expanding cloud of products.The intensity and the temporal behavior of the second component of chemiluminescence depend on the distance to a barrier placed above the LA sample. The cloud contains gaseous products and solid particles which propagate perpendicular to the LA surface with a maximum velocity of 4.48±0.10 km/s and 3.78±0.18 km/s, respectively. To reproduce the experimental results, two alternative hydrodynamic models are applied: Stanyukovich's model [K. P. Stanyukovich, Unsteady Motion of Continuous Media (Pergamon, London, 1960), pp. 498–501] for isentropic expansion and London and Rosen's model [R. A. London and M. D. Rosen, Phys. Fluids 29, 3813 (1986)] for exploding foil. The latter model is preferred and when incorporating Beer's law and Mie's theory [C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light in Small Particles (Wiley, New York, 1983), p. 77] it reproduces very well both the temporal behavior of the second component of chemiluminescence and the attenuation of the HeNe beam and suggests that lead particles with radius of 0.05–0.15 μm are involved in the attenuation. The model also provides an estimate of the composition of the product cloud and of the density of the gaseous and solid species as a function of time and distance from the LA sample.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.350660
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  • 5
    Digitale Medien
    Digitale Medien
    Dynamics of the detonation products of lead azide. IV. Laser shadowgraphy of expanding species (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 5360-5365 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Laser resonant shadowgraphy (LRS) and laser nonresonant shadowgraphy (LNRS) are used to monitor the detonation products of lead azide. Photographs of the cloud of products are obtained via illumination with a doubled dye laser tuned on-resonance to the 3P1o-3P0 transition of the Pb atom at 283.31 nm, and off-resonance at 284.31 nm. The versatility of the diagnostics and its applicability to detonation products expanding into vacuum and into atmospheric pressure air are demonstrated. The LRS monitors the density gradients of both lead atoms and solid particles formed in the detonation, whereas the LNRS detects only the latter. Expansion into vacuum through a nozzle leads to an increase in the velocity (from ∼4.5 to (approximately-greater-than)5 km/s) and density of the atoms and to a decrease in the density of the particles. The LRS measurements show that the expansion of both products in air is relatively slow (∼0.75 km/s) and leads to production of shock waves. From the shape of the shock waves created by an obstacle when the products expand into vacuum, the Mach number is estimated to be (approximately-greater-than)20 in the outer parts and around 3 in the inner parts of the cloud.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.354238
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  • 6
    Digitale Medien
    Digitale Medien
    Preferential excitation and enhanced emission of Pb atoms following detonation of lead azide (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 322-324 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Preferential excitation of the 3P°1 state of the lead atom and enhanced emission in the Pb(3P°1Pb(3P°1→1D2) transition at 722.9 nm have been observed following the detonation of lead azide, Pb(N3)2. The detonation is initiated by a short laser pulse and the products are expanded through a supersonic nozzle. It is suggested that the enhanced emission is due to preferential excitation of Pb(3P°1) via energy transfer from electronically excited N2 combined with the effect of self-trapping of the emission from 3P°1 to the 3P0,1,2 states. The implications to short-wavelength chemical lasers are discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.104674
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  • 7
    Digitale Medien
    Digitale Medien
    Dynamics of the detonation products of lead azide: III. Laser-induced hole burning and flow visualization (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 45-52 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The cloud of products formed following the detonation of lead azide (LA) contains solid particles. Utilizing a pulsed beam of a neodymium: yttrium aluminum garnet (Nd:YAG) laser, the particles are evaporated and hole burning through the opaque cloud is demonstrated. The characteristics of the hole and of the expanding cloud are monitored in real time by a HeNe beam, high-speed framing photography and emission of excited Pb atoms. The hole is carried with the cloud and propagates at a constant velocity of 0.5–2.8 km/s, depending on the time and location of burning and given by d(h)/t(h), where d(h) is the distance from the LA sample to the center of the Nd:YAG beam and t(h) is the time from detonation to hole burning. The reduction in the number and size of the particles is monitored by scanning electron microscopy of the deposits formed on a substrate following the detonation. The propagation of the Nd:YAG laser pulse through the cloud is numerically modeled and provides an estimate of the increase in the temperature and the decrease of the size of the particles as a result of the interaction with the laser beam.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.355253
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  • 8
    Digitale Medien
    Digitale Medien
    Dynamics of the detonation products of lead azide. II. Formation of charged particles (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2138-2144 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The formation and temporal behavior of charged particles ensuing from the detonation of lead azide (LA) is studied. An ion probe tailored for measurements in the hostile environment produced following the detonation is described. The positive ions (most probably singly ionized lead atoms) and the electrons, which are simultaneously collected on separate electrodes, are embedded in the outer half of the expanding product cloud resulting from the detonation. They show similar temporal behavior and their maximum velocity is ∼4.5 km/s. The density of each of them at a distance of 2.5 cm from the LA sample is 4.3×1011–1.3×1012 cm−3 for detonation of 5–15 mg of LA, respectively. From the hydrodynamics of the expanding cloud and the density of the electrons, their temperature is estimated to be in the range 2600–3000 K. The results of the measurements are discussed in view of the mechanisms believed to govern the expansion of the product cloud following the detonation.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.353140
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  • 9
    Digitale Medien
    Digitale Medien
    Symmetry and geometry of the first two excited singlet states of dimethylnitrosoamine studied by vector correlations (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 1639-1640 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A study of the geometry and symmetry of two different excited potential surfaces of the same molecule is demonstrated for the first time by analyzing the alignment and Λ doubling of the NO fragment resulting from the photodissociation of dimethylnitrosoamine irradiated at 363.5 nm, [S1(nπ*)←S0], and 250 nm, [S2(ππ*)←S0]. The values obtained for the alignment and Λ doubling indicate that (1) the fragmentation process is fast in both cases; (2) there is some planarity in the fragmentation process in both cases; (3) at 250 nm the transition dipole moment lies in the frame of the molecule, while at 363.5 nm it is perpendicular to the frame of the molecule; (4) the first excited state has an A‘ symmetry and the second excited state has an A' symmetry.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.452206
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  • 10
    Digitale Medien
    Digitale Medien
    Control of fragment alignment via photodissociation from different types of parent rotation (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4218-4221 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The alignment of the OH fragment following dissociation of H2O (1,0,0) at 193 nm has been measured as a function of initially prepared parent rotations. The degree of alignment is almost perfect for an in-plane rotation, but is reduced markedly for a mixture of in- and out-of-plane rotations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.466190
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