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  • 1
    Digitale Medien
    Digitale Medien
    Hydrophobic hydration of inert gases: Thermodynamic properties, inherent structures, and normal-mode analysis (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3719-3727 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Computer simulations on water and aqueous solutions of noble gases have been carried out in order to study the structures of water around a solute. The hydration energy and free energy evaluated for neon (Ne) and xenon (Xe) solutions in the present study were in good agreement with those by experiments. The detailed hydration structures were investigated by means of the so called inherent structures and normal-mode analyses. It was found that the positive excess free energy in the hydration of Xe arises from a decrease in the number of distinct potential-energy minima in configuration space and that the free energy increase in the Ne solution is due partly to the decrease in the number of the potential minima and partly to the anharmonic modes which are harder than those in pure water. The soft anharmonic modes in the Xe solution were almost equivalent to those in pure water. The introduction of a Xe solute gives rise to a change in water structure to a clathrate-like structure and yields an increase in population of the cyclic pentamer connected by hydrogen bonds, which leads to the exothermic hydration.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460822
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  • 2
    Digitale Medien
    Digitale Medien
    Large local energy fluctuations in water. II. Cooperative motions and fluctuations (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5852-5860 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Large local energy fluctuations in liquid water and their physical origin are investigated by using classical molecular dynamics (MD) calculation and quenching techniques. Performing a trajectory calculation of 100 ps, it is found that large rotational motions of individual water molecules, which are always associated with potential energy destabilization of 10–20 kcal/mol, occur once in about 10 ps. The stabilization and destabilization of the individual water molecules are induced by cooperative motions. In order to analyze these cooperative motions in the liquid water, the water structures are quenched to their local minima (called the inherent structures). Comparing the inherent structures successively visited by the system, it is found that collective motions of about 10–40 molecules localized in space occur in unstable regions. The potential energy fluctuation of an individual molecule can reach up to 15 kcal/mol even in the inherent structures. The strong potential energy correlation among neighboring molecules indicates these cooperative motions cause the "flip–flop''-type energy exchanges; as a molecule is stabilized, another is to be unstabilized and vice versa. A flip-flop motion does not involve a (large) energy barrier but causes large energy fluctuations of the individual molecules. A large portion of potential energy fluctuations of the individual water molecules is accounted for as the superposition of fluctuations in the inherent structures and those in the normal modes build upon these structures.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455536
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  • 3
    Digitale Medien
    Digitale Medien
    Structure and thermodynamic properties of water–methanol mixtures: Role of the water–water interaction (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2626-2634 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Thermodynamic properties and structures of water–methanol mixtures at various temperatures have been investigated by means of Monte Carlo simulations and subsequent analyses. The OPLS model by Jorgensen was used for the methanol–methanol interaction and both the Caravetta–Clementi (CC) potential and TIP4P potential by Jorgensen et al. were used for the water–water interaction. We show that the role of water–water interaction is very important in discussing aqueous solutions of alcohols, and examine the origin of the exothermic mixing processes. We have investigated the sensitivity of the temperature dependence of the enthalpy of mixing to the water–water interaction. The CC potential is able to reproduce the temperature dependence observed in experiments, although the absolute values of the mixing enthalpy were larger than the experimental ones. While the TIP4P potential results in better agreement for the excess enthalpy and volume near room temperature, the temperature dependence of the excess enthalpy did not agree with experiment. The difference in the magnitude of the exothermic hydration for different water–water interactions is explained in terms of the energetic stability of the clathrate hydrate compared with ice, on the basis that the structure of water in the vicinity of a methanol molecule is similar to the clathrate hydrate. It is found that the energetic stability of the clathrate hydrate for the CC model is higher than that for TIP4P, and this is responsible for the larger exothermic hydration. The higher stability of the clathrate hydrate structure for the CC potential, in turn, arises from the difference in the pair interaction energy surface between two kinds of potential functions; the minimum energy structure and the flexibility of the hydrogen bonded pair.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463051
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  • 4
    Digitale Medien
    Digitale Medien
    Potential energy surfaces for water dynamics. II. Vibrational mode excitations, mixing, and relaxations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8138-8147 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Dynamical behavior of liquid water is investigated by analyzing the potential energy surface involved. Multidimensional properties of the potential energy surface are explored in terms of vibrational mode excitations at its local energy minima, called inherent structures. The vibrational mode dynamics, especially mechanism of mode relaxation and structure transitions, is analyzed. It shows very strong excitation energy dependence and mode dependence. There are three kinds of vibrational coupling among modes. For excitations of energy near the room temperature, most modes (more than 90% of total modes) individually interact with only one or two other modes, and yield near recurrence of the mode energy in a few tens picoseconds (very slow relaxation). Spatially localized modes in the intermediate frequency range couple with many delocalized modes, yielding fast relaxation. The coupling is governed by atomic displacement overlaps and frequency matching. Each mode couples with nearby frequency or double frequency modes through the Fermi resonance. Lowest frequency modes almost always lead to transitions from a potential energy well to neighbor potential wells, called inherent structure transitions. In high energy excitation, some intermediate frequency modes also yield such transitions. There exist very low energy paths involving single or few water molecule displacements at almost every inherent structure, indicating that certain facile molecular movements occur even in very low temperature states. Different energy excitations of a low frequency mode result in different inherent structure transitions; transitions caused by high energy excitations involve many large molecular displacements. These inherent structure transitions are the source of the water binding structural reorganization dynamics. Significance of these vibrational mode dynamics in the water dynamics is discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459344
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  • 5
    Digitale Medien
    Digitale Medien
    Potential energy surfaces for water dynamics: Reaction coordinates, transition states, and normal mode analyses (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6318-6327 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Dynamics of water binding structure reorganization is investigated by analyzing the potential energy surfaces involved. The water structures in a trajectory are quenched to their local minima, called the inherent structures. The reaction coordinates, which connect the inherent structures successively visited by the system, are determined. It is found that the energy barrier heights, the transition state energies, along the reaction coordinates are mostly distributed in the range of 0.2–6 kcal/mol. The classification of inherent structures is made to groups of "overall-inherent structures''; successive inherent structures are most often not so geometrically distinct. It is found that transitions between the overall-inherent structures, involving large collective motions, occur in the subpicosecond time scale. Individual molecular motions in these collective motions are stongly correlated, not yielding large transition energies. The transition state energy sometimes reaches up to 20 kcal/mol, when the system goes through the ridge between deep minima, yielding ballistic dynamical behavior. Temperature dependence of the collective motions is also investigated.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457399
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  • 6
    Digitale Medien
    Digitale Medien
    Acetonitrile pair formation in aqueous solution (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6935-6940 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Extended reference interaction site model (RISM) theoretical calculations and molecular dynamics simulation have been carried out for dilute aqueous solution of acetonitrile. Potential of mean force between two solute molecules was calculated. Two solute molecules tend to associate with each other by an attractive interaction between two negatively charged nitrogen atoms. It is found that "bifurcated hydrogen bonds'' between a hydrogen atom on a water molecule and nitrogen atoms on acetonitrile molecules play an important role in the solute–solute interaction.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465838
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  • 7
    Digitale Medien
    Digitale Medien
    On the thermodynamic stability of clathrate hydrate. I (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4098-4109 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The thermodynamic stability of a clathrate hydrate has been investigated by examining the free energy of formation of clathrate hydrate II encaging propane. The total free energy has been divided into several contributions—the interaction between water and guest propane molecules, the entropic contribution arising from the combinations of cage occupancy, and also the free energy due to intermolecular vibrations. The present method avoids some of the fundamental assumptions in the van der Waals and Platteeuw theory. This enables us to assess separately the factors which have a bearing on the thermodynamic stability of the hydrate. Kinetic stability has also been investigated by calculating molecular dynamics trajectories having initially excited several characteristic vibrational modes. We show, for propane in large cages, that the potential energy surface of the guest molecule in a cage has a single minimum and molecular motions can be approximated accurately to a collection of harmonic oscillators. It is found that the intermolecular vibrational modes for water molecules shift toward the higher frequency regions in the presence of the guest molecules. This shift of the vibrational motions of water molecules gives rise to an increase in the chemical potential of water compared with that evaluated under the fixed lattice sites approximation where only vibrations of the guest molecules are taken into account. The empty clathrate hydrate structures melt into liquid water within a few picoseconds when several of the lowest frequency modes are excited. On the other hand, the guest molecules are found to prevent the clathrate structures from melting even 10 ps after the excitation of the same frequency modes.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465045
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  • 8
    Digitale Medien
    Digitale Medien
    High-current metal ion beam extraction from a multicusp ion source : International conference on ion sources (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 538-540 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: Improvements have been made in a multicusp ion source, which made it possible to produce metal–vapor plasma and extract a high-current metal ion beam. In the discharge chamber, double radiation shields were set and the inner shields were heated to 1860 K. Therefore, it became possible to maintain enough metal–vapor density to produce plasma without the use of support gas. For the extraction of a high-current metal ion beam, we used multislit electrodes in the accel–decel electrode structure. Experiments of the metal ion beam extraction were performed with several kinds of metals (Al, Cr, Si). For example, the extracted Al ion beam current reached 65 mA. As a result of mass analysis, it was found that more than 90% of an Al ion beam is Al+. Furthermore, stable operation could be achieved. For Al (58 mA) and Si (28 mA) ion beam extraction, the temporal drift of each ion beam current was within ±5% for 3 h.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1141244
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  • 9
    Digitale Medien
    Digitale Medien
    As4 to Ga flux ratio dependence of the optical and electrical properties of Ge-doped GaAs grown by molecular-beam epitaxy (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 248-251 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A drastic change of the conduction type from p to n with an increase of the As4 to Ga flux ratio, γ, was observed for the first time in the photoluminescence spectra of amphoteric impurity (Ge)-doped GaAs made by molecular-beam epitaxy. The sample with the lowest γ (γ=1.0) presented a purely p-type emission associated with pairs between the excited states of acceptors. The sample with the highest γ (γ=10.6) indicated a totally n-type emission reflecting an increase of quasi Fermi energy. Results show that by precisely controlling the flux ratio, γ, one can reliably make use of substantially amphoteric atoms of Ge both as p- and n-type impurities for the fabrication of GaAs by molecular-beam epitaxy.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.342580
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  • 10
    Digitale Medien
    Digitale Medien
    Liquid-phase epitaxy of heavily Mg-doped GaAs: Formation of a new near-band-edge emission exclusively pertinent to acceptor impurities (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 3212-3215 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Liquid-phase epitaxy (LPE) of Mg-doped GaAs was carried out. An extremely heavy doping was accomplished with a maximum net hole concentration of 1.31×1019 cm−3. A systematic investigation of photoluminescence spectra was performed as a function of hole concentration, which revealed that a newly discovered emission characterizing acceptor impurities can be obviously recognized also in LPE-grown samples in spite of the fact that the selective optical compensation effect among doped acceptors and residual donors presumably coming from Si of the LPE quartz tube is supposed to be comparatively strong. The red shift of this emission energy with increasing hole concentration and its locking at high hole concentration were discussed theoretically by introducing a preliminary model, i.e., a pair between excited-state acceptors.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.339322
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