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  • Digitale Medien  (231)
  • 11
    Digitale Medien
    Digitale Medien
    Photodissociation of intraadsorbate bonds at adsorbate-metal interfaces (1990)
    s.l. : American Chemical Society
    Accounts of chemical research 23 (1990), S. 327-332 
    Details
    ISSN: 1520-4898
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ar00178a002
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  • 12
    Digitale Medien
    Digitale Medien
    Homodyne photon-correlation spectroscopy of a supercooled liquid: 1,3,5-Tri-α-naphthyl benzene (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 6086-6090 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Homodyne depolarized photon-correlation functions of 1,3,5-tri-α-naphthyl benzene (TαNB) have been measured at the 90° scattering angle in the temperature range of 75–110 °C. The correlation functions are analyzed in terms of a fractional exponential function and a modified constrained regularization method originally developed by Provencher. The results from both methods of data analysis are in agreement. The temperature dependence of the mean orientational relaxation time when fitted to the Arrhenius equation, gives an apparent activation energy E=85.3 kcal/mol. The Antoine–WLF equation provides a better fit of the mean reorientational relaxation time data. It is also found that orientational time τ¯ is linearly proportional to η/T, indicating that the Debye–Stokes–Einstein equation is applicable to describe molecular reorientation in the supercooled TαNB liquid.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.450797
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  • 13
    Digitale Medien
    Digitale Medien
    Laser pointing stability measured by an oblique-incidence optical transmittance difference technique (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 3714-3717 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: We describe an oblique-incidence optical transmittance technique for determining the pointing stability of a laser. In this technique, we follow the angular drift of a monochromatic laser beam by measuring the relative changes in transmittance through a parallel fused quartz window for s and p polarized components of the beam in response to the drift. This method is shown in the present experiment to have the sensitivity to detect angular changes in the range of 2 μ radians (1 μ radian=10−6 radian). To demonstrate this technique, we measured the angular drifts of two commercial intensity-stabilized He–Ne lasers. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1394187
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  • 14
    Digitale Medien
    Digitale Medien
    Hot carrier induced photodesorption dynamics of SO2 from Ag(111) (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 10021-10033 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The photon-driven (2.5 to 6.4 eV) desorption of SO2 (coverages up to 2.7 monolayers) on Ag(111) has been studied by time-of-flight mass spectrometry in combination with temperature programmed desorption and Auger electron spectroscopy. The photodesorption yields per incident photon increase with photon energy. Consistent with substrate-mediated excitation, the mean translational energy 〈Etrans/2k〉 is constant (650 K) for photon energies between 3.5 and 6.4 eV, but decreases at longer wavelengths (480 K at 2.5 eV). The decrease is attributed to changes in the energy distribution of the hot substrate carriers responsible for desorption. The photodesorption yields vary with the initial coverage, the method of preparing the initial coverage, and the extent of photolysis. These variations are attributed, in part, to changes in the SO2 orientation on the surface. Even at 6.4 eV, where unimolecular photodissociation occurs in the gas phase and multilayer, there is no photodissociation in monolayers. This effect is attributed to strong substrate quenching.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465506
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  • 15
    Digitale Medien
    Digitale Medien
    Surface photochemistry 15: On the role of substrate excitation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2681-2682 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The rates of photodissociation and photodesorption of dioxygen adsorbed on Ag (110) under ultraviolet irradiation have been measured as a function of polarization at normal incidence. No dependence of the rate was found on the angle between the O2 bond axis and the electric vector of the incident light. This result supports a mechanism for surface photochemistry in which substrate excitation is the dominant process.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457964
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  • 16
    Digitale Medien
    Digitale Medien
    Photochemical decomposition of AsH3 on GaAs(100) (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 605-615 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Molecular AsH3, adsorbed on Ga-rich GaAs(100) at 115 K, dissociates readily upon uv irradiation with 193, 248, and 351 nm excimer laser light. In the initial photodissociation step one As–H bond cleaves, leaving all the AsH2, and a large fraction of the H, adsorbed to As. The AsH2 further photodissociates to give As–H and Ga–H. The final steps, photochemical removal of hydrogen from Ga–H and As–H, lead to As deposition. The photodissociation cross section decreases sharply with the extent of photolysis. The wavelength dependence, compared to the gas-phase absorption cross section, extends to much lower photon energies and indicates that substrate-mediated excitation dominates the observed chemistry. There are strong isotope effects in all the cross sections; these are related to mass-dependent substrate-mediated quenching of the excited states. Implications for photon-assisted organometallic chemical vapor deposition are discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463556
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  • 17
    Digitale Medien
    Digitale Medien
    The role of direct and substrate excitation in ultraviolet photolysis of phosgene on Pt(111) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1555-1563 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The photodissociation rate of phosgene (Cl2CO) adsorbed on Pt(111) has been measured as a function of photon energy at normal incidence and as a function of incident angle using p-polarized light. Compared to the gas phase, the wavelength dependence of the initial photolysis cross section on the surface is redshifted. The angular response to p-polarized light is wavelength dependent. Above 315 nm, the angular dependence correlates with calculated metal absorption. At 280 nm, the angular dependence is much too strong to be accounted for solely by substrate excitation. A combination of substrate and direct excitation is adequate. This is the first direct evidence, for monolayers on metals, that both direct and substrate excitation contribute to surface photochemistry but dominate at different wavelengths.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460013
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  • 18
    Digitale Medien
    Digitale Medien
    Surface photochemistry. II. Wavelength dependences of photoinduced dissociation, desorption, and rearrangement of O2 on Pt(111) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5011-5020 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A Pt(111) surface saturated with molecular oxygen was irradiated by UV light from a mercury arc lamp with various cutoff filters. High-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption mass spectroscopy (TPD) were used to characterize the products retained on the surface. Upon UV irradiation at 95 K, chemisorbed O2 (peroxo) undergoes dissociation, desorption and rearrangement. Different wavelength dependences were observed for the three processes: dissociation was not observed at wavelengths longer than 295 nm, in agreement with gas phase photodissociation of the O–O bond in hydrogen peroxide; desorption and rearrangement became negligible at wavelengths longer than 420 nm, in agreement with the photolysis of an organometallic peroxoplatinum complex. For wavelengths between 230 and 315 nm, the average cross sections of dissociation and desorption were estimated to be 5.7×10−20 and 1.2×10−19 cm2 , respectively. The possible origins of the three processes are discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456741
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  • 19
    Digitale Medien
    Digitale Medien
    Direct and precursor dynamics in dissociative hydrogen chemisorption on Ni(100) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 7442-7447 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We report evidence for the coexistence of direct and precursor dynamics in the dissociative chemisorption of H2 on Ni(l00). Hydrogen and deuterium uptakes on Ni(l00) were measured at various surface temperatures by following the secondary ion ratios, Ni2 H+/Ni+ and Ni2 D+/Ni+ , which are proportional to surface hydrogen and deuterium coverage on both clean and carbon-covered Ni(100). Between 100 and 200 K on clean Ni(l00), the initial sticking coefficient of hydrogen decreases, but only slightly, as the surface temperature increases. The decrease is more pronounced both in the presence of predosed carbon and for deuterium adsorption on Ni(100). This is interpreted as due to the involvement at low temperatures of a molecular precursor which mediates dissociative adsorption at low temperatures. The precursor probably involves surface defects and sites formed in the presence of carbon. In addition to the precursor channel, a direct dissociation channel also operates, and dominates for T≥200 K.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456224
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  • 20
    Digitale Medien
    Digitale Medien
    Isotope effect in vibration-mediated ultraviolet photodesorption: A short-time-limit harmonic approximation (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 3410-3414 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Excitation and de-excitation of the electronic states of an adsorbed molecule on a solid surface result in intramolecular vibrational excitation. The internal vibrational energy can couple to the adsorbate–substrate bond and lead to molecular desorption. In this mechanism, the isotope effect in molecular desorption is determined not by the mass ratio of the leaving particles, but by that in the internal coordinate. This process is modeled within the context of semiclassical wave packet dynamics, which, in the short-time-limit harmonic approximation, allows analytical solution. Results are obtained for the ammonia/GaAs(100) system and are compared with experimental observations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465048
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