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  • 1995-1999  (9)
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  • 1
    Digitale Medien
    Digitale Medien
    Vibrational dynamics of large clusters from helium atom scattering: Calculations for Ar55 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9067-9077 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Vibrational excitation of an Ar55 cluster in collisions with He (Ekin=25 meV) is investigated by using classical trajectories and a highly approximate quantum mechanical method (vibrationally sudden approximation). The energy transfer from the helium atom to the cluster (ΔE) is calculated as a function of the scattering angle θ. It is found (i) that predominantly the modes corresponding to the cluster atoms in the outer shell are excited and (ii) that the probability for multi-phonon (Δn≥2) excitations steadily increases with the scattering angle. The results of both sets of calculations are generally in good agreement with measured energy loss spectra over the entire range of scattering angles. In the region θ(approximate)15°–30°, which—because single-phonon transitions dominate—is most important for determining the frequency distribution of the cluster, the quantum mechanical calculations are superior to classical mechanics; the latter fails to conserve zero-point energy and therefore leads to unrealistic energy transfer below ΔE(approximately-less-than)2 meV. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.474011
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  • 2
    Digitale Medien
    Digitale Medien
    A new intermolecular potential for hydrazine clusters: Structures and spectra (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6795-6805 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The structures of small hydrazine clusters from the dimer to the hexamer have been calculated using a standard site-site intermolecular potential and a newly developed systematic approach which is essentially based on monomer properties. Aside from the repulsive and the attractive dispersion and induction interaction special care is taken for the determination of the electrostatic interaction which is represented by a distributed multipole expansion and a penetration correction. Based on these potentials the vibrational spectra of the N-N stretching and the asymmetric NH2 wagging mode are calculated using degenerate perturbation theory. While the small shifts of the N-N stretching mode are fairly well reproduced by both potential models, large differences are predicted for the asymmetric NH2 wagging mode. Here, redshifts of –30 cm−1 are calculated for the standard and blueshifts of 100 cm−1 are obtained for the systematic potential in agreement with experiment. The analysis shows that the reason for this behavior is the careful treatment of the electrostatic term in this model. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473706
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  • 3
    Digitale Medien
    Digitale Medien
    Vibrational excitation in He+(H2O)11 collisions: Quantum calculations and experiment (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10069-10076 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Helium atoms are scattered from a beam of water clusters with mean size n¯ = 10 in an angular and velocity resolved collision experiment. The measured peaks are identified as elastic scattering, rotationally inelastic scattering of monomers, and vibrational excitation of the clusters. To interpret the latter processes quantum calculations are performed for He+(H2O)11 collisions using the TDSCF approximation which includes the anharmonic force field of the water clusters and energy transfer between the modes. By comparison of the calculated and experimental results, the most probable excitations correspond to energy transfer for around 7 meV and, with smaller intensities, up to 20 meV. The excitations correspond to shearing modes of the outer rings and the middle ring of the highly nonrigid cluster against each other. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480358
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  • 4
    Digitale Medien
    Digitale Medien
    Surface vibrations of argon clusters by helium atom scattering (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 3205-3215 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are generated in the size range of n¯=23 to n¯=4600 by adiabatic expansion with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of better than 1 meV. The angular dependence allows to distinguish between single and multiphonon excitations. The measured peaks of the averaged transferred energy in the single phonon regime decrease with increasing average cluster size and converges to the values of the surface phonons of solid argon. This behavior is explained by treating the data as collective breathing vibrations of a solid sphere. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473060
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  • 5
    Digitale Medien
    Digitale Medien
    Vibrational predissociation spectra of size selected hydrazine clusters: Experiment and calculations (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6806-6812 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Vibrational predissociation spectra of hydrazine (N2H4)n clusters have been measured from the dimer to the tetramer using a linetunable, isotopically substituted CO2-laser in order to fill the frequency gap between 990 and 1010 cm−1. The clusters are size selected in a scattering experiment with helium atoms. The large blue shifts of the asymmetric NH2 wag mode at 937 cm−1 are completely interpreted by calculations based on a recently determined systematic model potential. The gross shifts of 60 cm−1 for the dimer, 80 cm−1 for the trimer, and 110 cm−1 for the larger clusters are explained by the different structures: Cyclic arrangements with two hydrogen bonds per molecule for the dimer, rings with one hydrogen bond per molecule for the trimer, and three-dimensional structures for the larger ones. The peaks in the spectra are caused by characteristic vibrations to which more than one isomer contributes. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473707
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  • 6
    Digitale Medien
    Digitale Medien
    Pseudorandom time-of-flight analysis with a time-of-flight mass spectrometer (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 417-422 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: We describe the advantages of using the pseudorandom time-of-flight (TOF) technique for measuring velocity distributions together with the TOF mass spectrometric detection. The basic idea of the method is the successive measurement of abundance spectra with the TOF mass spectrometer correlated with each channel of the pseudorandom sequence. Extraction of the intensities of each mass and subsequent cross correlation with the pseudorandom sequence leads to the TOF and thus the velocity distributions. The proposed technique can substantially reduce the data acquisition time in systems for which many different reaction products are expected such as in cluster research. An illustration of the efficiency is presented using angular resolved TOF distributions for products of the reaction of sodium clusters Nan with O2. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1146606
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  • 7
    Digitale Medien
    Digitale Medien
    Vibrational excitation of ammonia clusters by helium atom scattering (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 109-115 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In a crossed molecular beam arrangement helium atoms are scattered from ammonia clusters of the averaged sizes n¯=18, 745, and 1040 which are generated by isentropic expansions with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of less than 5 meV at a collision energy of 95 meV. The energy transfer increases with increasing deflection angle and extends to 65 meV. Intensity maxima are observed between 11 and 16 meV, around 27 meV, and at 33 meV. The one in the middle is attributed to the vibration of a specific cluster network, while the other two occur in the energy regime of the translational or librational modes of the solid with a preference for small or large clusters, respectively. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473036
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  • 8
    Digitale Medien
    Digitale Medien
    Cluster sputtering: complete fragmentation of large ammonia clusters by photons and electrons (1999)
    Springer
    Applied physics 69 (1999), S. S209 
    Details
    ISSN: 1432-0630
    Schlagwort(e): PACS: 36.40.Qv
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract. The interaction of large ammonia clusters in the size range from n=20 to 2000 with photons and electrons are investigated in a reflectron time-of-flight mass spectrometer. The clusters are generated in adiabatic expansions through conical nozzles. The clusters are detected by (1+1) resonance-enhanced multiphoton ionization via the a state at 193 nm or electron impact ionization. By varying the power density of the laser between 20 and 1000 kW  cm-2 and the electron energy between 50 and 1000 eV, the clusters were largely fragmented. The operating mechanisms were identified as those responsible for the electronic sputtering of solid matter. The yields, however, were orders of magnitude larger than those for the solid. This result is a consequence of the finite dimensions of the cluster which cannot accommodate the energy.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s003399900390
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  • 9
    Digitale Medien
    Digitale Medien
    A flexible method for calculating vibrational spectra of clusters applied to methanol (1998)
    Springer
    The European physical journal 1 (1998), S. 207-216 
    Details
    ISSN: 1434-6079
    Schlagwort(e): PACS. 36.40.Mr Spectroscopy and geometrical structure of clusters
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract: A method is presented for calculating vibrational spectra of clusters using intra- and intermolecular potential models of different origin. The two types of potentials are tuned and coupled to construct a total cluster potential. The determination of minima on this cluster potential energy surface, and the expansion of the potential about the minimum configurations makes it possible to apply standard spectroscopic methods. Starting from a normal mode analysis, harmonic and anharmonic frequency shifts are calculated using perturbational and variational methods for methanol clusters from the dimer to the hexamer. The results using the empirical OPLS potential model are compared with each other, with the harmonic line shift calculations for a calculated potential in the SCF approximation, and with the experimental data. There is an improvement with respect to the results obtained so far, however, the need for a better description of the intermolecular potential is pointed out.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s100530050082
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