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  • 11
    Electronic Resource
    Electronic Resource
    Temperature dependent relaxation and recombination dynamics of the hydrated electron (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1126-1134 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast solvation and recombination dynamics of the hydrated electron generated by two-photon ionization of water at 4.65 eV is studied by transient absorption spectroscopy as a function of temperature in the range from 277 K to 355 K. The part of the spectral blue shift which is observed in the absorption spectrum of the hydrated electron after 1 ps is purely continuous and is accurately described by the well known analytical expression for the temperature dependent absorption spectrum of the ground state hydrated electron. This indicates that thermal relaxation or more likely solvation of the hydrated electron predominantly causes the blue shift. The survival probability of the hydrated electron shows a strong temperature dependence, which is satisfactory explained by the temperature dependent mobility and reaction rates of the species involved in the recombination. This implies that the average initial separation between the hydrated electron and the ionization site of 〈r0〉=1.0±0.1 nm does not depend significantly on the bulk water temperature. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481891
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  • 12
    Electronic Resource
    Electronic Resource
    Femtosecond spectroscopy of the dissociation and geminate recombination of aqueous CS2 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 703-710 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photolysis of aqueous CS2 has been studied using subpicosecond transient absorption spectroscopy. CS2 was photolyzed at 6.2 eV and the fast formation and decay of the photoproducts were monitored from 6.2 to 3.96 eV. Upon excitation, aqueous CS2 dissociates into CS+S. However, 93%±2% of the fragments geminately recombine on the electronic ground state potential of CS2 within a few picosecond leaving only 7%±2% of the CS+S fragments separated 100 ps after the excitation. In the gas phase, most of the dissociation occurs on a triplet-state potential surface, and the high recombination yield observed in aqueous solution therefore indicates a strong, solvent-assisted coupling between this state and the singlet ground state of CS2. The vibrationally excited CS2 molecule formed by the recombination transfers its high excess energy to the surrounding water molecules in two processes with time constants 8.4±1 and 33±7 ps. The rotational reorientation time of ground state CS2 is 6±1 ps, suggesting a surprisingly strong interaction between the neutral, nonpolar CS2 and the surrounding water molecules. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479364
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  • 13
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of aqueous CS2 (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 4099-4106 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational relaxation of aqueous CS2 formed by geminate recombination of CS+S after photolysis of CS2(aq) at 200 nm is studied by subpicosecond transient absorption spectroscopy. Comparison of the transient absorption measured from 200 to 313 nm with calculated absorption transients pertaining to vibrationally relaxing CS2 shows that relaxation to the vibrational ground state of CS2 predominantly occurs in the asymmetric stretch on a 5 ps time scale. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1344611
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  • 14
    Electronic Resource
    Electronic Resource
    Femtosecond photolysis of aqueous HOCl (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9361-9369 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1413964
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  • 15
    Electronic Resource
    Electronic Resource
    Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers (1998)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 5828-5830 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.368853
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  • 16
    Electronic Resource
    Electronic Resource
    Nitrogen-related local vibrational modes in ZnO:N (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 80 (2002), S. 1909-1911 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. A series of samples grown by chemical vapor deposition (CVD) containing different nitrogen concentrations, as determined by secondary ion mass spectroscopy (SIMS), was investigated. The Raman spectra revealed vibrational modes at 275, 510, 582, 643, and 856 cm−1 in addition to the host phonons of ZnO. The intensity of these additional modes correlates linearly with the nitrogen concentration and can be used as a quantitative measure of nitrogen in ZnO. These modes are interpreted as local vibrational modes. Furthermore, SIMS showed a correlation between the concentration of incorporated nitrogen and unintentional hydrogen, similar to the incorporation of the p-dopant magnesium and hydrogen in GaN during metalorganic CVD. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1461903
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  • 17
    Electronic Resource
    Electronic Resource
    Temperature and pressure dependence of Mg local modes in GaN (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 78 (2001), S. 198-200 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temperature and pressure dependence of the zone-center phonons and of the local vibrational modes of magnesium in the hexagonal modification of GaN was studied within a valence-force model. The contribution caused by thermal expansion was calculated and compared with the experiment. We find that the frequency shift of the local vibrational modes arises to a much larger extent from anharmonic decay into lower energy phonons than the shift of the GaN host modes. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1339848
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  • 18
    Electronic Resource
    Electronic Resource
    Temporal evolution of resonant Raman-scattering in ZnCdSe quantum dots (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 76 (2000), S. 2662-2664 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We investigated ZnCdSe/ZnSe quantum-dot structures which include planar and coherently strained three-dimensional islands with different sizes. Optical excitation of these islands well below the ZnSe band gap leads to a resonant enhancement of the Zn0.7Cd0.3Se longitudinal-optical (LO) phonon-scattering efficiency and makes the 2LO and 3LO multiphonon emission observable. Resonant excitation with a power density of about 1.3 MW/cm2 using a micro-Raman setup results in an exponential decrease of the 1LO, 2LO, and 3LO intensity with irradiation time. This decay behavior is not observed for pure ZnSe crystals and can be avoided for the ZnCdSe/ZnSe structures using much lower excitation densities. The decrease in intensity is accompanied by a shift of the LO mode to higher frequencies resulting from a lower cadmium concentration in the alloy. From these experimental findings, we conclude that resonant excitation at a certain power density leads to cadmium out-diffusion from the planar quantum dots, which shifts the resonance away from the excitation energy. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.126436
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  • 19
    Electronic Resource
    Electronic Resource
    Lattice dynamics of hexagonal and cubic InN: Raman-scattering experiments and calculations (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 76 (2000), S. 2122-2124 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present results of first- and second-order Raman-scattering experiments on hexagonal and cubic InN covering the acoustic and optical phonon and overtone region. Using a modified valence-force model, we calculated the phonon dispersion curves and the density of states in both InN modifications. The observed Raman shifts agree well the calculated Γ-point frequencies and the corresponding overtone density of states. A tentative assignment to particular phonon branches is given. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.126273
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  • 20
    Electronic Resource
    Electronic Resource
    Micro-Raman and cathodoluminescence studies of epitaxial laterally overgrown GaN with tungsten masks: A method to map the free-carrier concentration of thick GaN samples (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 76 (2000), S. 3418-3420 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Cathodoluminescence (CL) and micro-Raman spectroscopy were applied to study microscopically the optical and structural properties of two epitaxial-laterally overgrown GaN structures with tungsten masks in 〈11(underbar)00〉 and 〈112(underbar)0〉 direction, respectively. A free-carrier concentration higher than 1019 cm−3 was observed right above the masks, leading to a gradient in free-carrier concentration over the whole layer thickness. We used the normalized longitudinal-optical-phonon intensity and the broad band around 650 cm−1 as a measure for the free-electron concentration, which is a promising method to determine the free-carrier concentration justified by the good correlation with CL results. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.126665
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