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  • 1995-1999  (22)
  • 1985-1989  (6)
  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2436-2443 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared molecular beam depletion spectroscopy of small methanol and acetonitrile clusters embedded in large helium clusters has been studied in the spectral region of the CO stretch and the CH3 rock mode from 1023 to 1059 cm−1. The results are compared with the experimental spectra of the corresponding free clusters generated in adiabatic expansions and calculations based on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the tetramer and pentamer, namely a monomer and dimer attached to a cyclic trimer. For acetonitrile clusters in helium, aside from the dimer, different structures are observed. The spectra from the trimer to the hexamer are dominated by structures which contain the antiparallel dimer as building block with D2d symmetry for the tetramer. They do not correspond to the minimum configurations observed for the free clusters. The fragmentation of the two cluster groups in helium droplets by electron impact ionization is discussed. © 1999 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2595-2605 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around n¯=9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (β=−0.96±0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (β*=1.96±0.05) with a branching ratio of R=0.20±0.03. At the wavelength of 193 nm, about the same value of R=0.18±0.03 is found, but both channels show a mainly perpendicular character with β=−0.90±0.10 for Br and β*=0.00±0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster. © 1999 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 10649-10652 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational OH stretch spectra have been measured for the size-selected pure water clusters (H2O)7. In contrast to (H2O)n, n=8–10 clusters, which exhibit three distinct bands corresponding to three distinct types of OH bonds, the heptamer spectrum displays seven bands spanning the range from 2935 to 3720 cm−1. Calculations suggest that the spectra originate from two isomers, derived from the S4 octamer cube by removal of either one double donor or one double acceptor water molecule. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5914-5920 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared molecular beam depletion spectroscopy of ammonia molecules embedded in large He clusters has been studied in the spectral region of the ammonia umbrella mode from 940 to 1060 cm−1. The transitions between the lowest rotational energy levels could be resolved. By fitting the spectrum with the Hamiltonian of a free rotor, we observe a large shift of the vibrational frequency of 17.4 cm−1 to the blue, an effective decrease of the rotational constant B by 24%, and a reduction of the tunneling splitting by 31% in the excited and 75% in the ground state. This implies an unusually large interaction between the host cluster and the umbrella motion of the ammonia molecule, an intermediate influence on the rotation and also a large perturbation of the inversion by the helium environment. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 33-38 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational predissociation spectra of methanol (CH3OH)n clusters have been measured in the frequency range of the CO stretch mode for n=7 and n=8 and in the range of the OH stretch mode from n=4 to n=9. The clusters are completely size selected in a scattering experiment with helium atoms. For the IR excitation a line-tunable CO2-laser (1020 to 1080 cm−1) and an Optical Parametric Oscillator (3000 to 3800 cm−1) were used, respectively. The spectra of the CO stretch mode are slightly blue shifted by about 12 cm−1 and show a two peak structure for n=8 and a broad unstructured behavior for n=7 as is predicted in the theoretical calculations based on symmetric and asymmetric cyclic structures. In contrast, the spectra of the OH stretch mode exhibit large red shifts up to 500 cm−1, but also show the change of structured and unstructured behavior for even and odd sizes, respectively. The calculations based on the new potential model of the preceding paper are able to reproduce the general structures and the gross features of the spectra but fail for the absolute magnitude of the shifts. © 1998 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 7179-7186 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared molecular beam depletion spectroscopy has been used to measure the spectrum of ammonia dimers embedded in large helium clusters after the excitation of the umbrella mode. The spectral region from 940 to 1010 cm−1 was covered by a line-tunable CO2-laser. By measuring the dependence of the depletion signal on the pressure of the pick-up cell, we confirmed that all lines are caused by the dimer. The spectrum exhibits two groups of two lines at 978/983 and 992/997 cm−1 separated by 14 cm−1 with a line splitting of 5 cm−1 each. The infrared transitions are only marginally shifted compared to the free ammonia dimer. Also the inversion tunneling splittings are equal to those of the free dimer. Differences in the amplitudes are attributed to relaxation effects of the different inversion components of the same symmetry at the low temperature of 0.4 K. The interchange tunneling splitting, however, which dominates the line splittings in the free dimer is considerably quenched in the helium environment. In addition, the difference in the excitation of the two non-equivalent monomers is remarkably reduced in support of a more cyclic structure in the helium environment. © 1997 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 20-32 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures and vibrational spectra of small methanol clusters from dimer to decamer have been calculated using a newly developed intermolecular potential which is essentially based on monomer wave functions. Special care has been taken for the description of the electrostatic interaction using a distributed multipole representation and including a penetration term. In addition, the potential model consists of repulsion, dispersion, and induction terms. Based on this potential model cluster structures have been calculated. The lowest energy dimer configuration is linear, while from trimer to decamer for the most stable structures ring configurations were found. Tetramer, hexamer, and octamer have S4-, S6-, and S8-symmetry, respectively. Vibrational spectra of the CO stretch and the OH stretch mode have been determined in the harmonic and in the anharmonic approximation using perturbation theory and variational calculations. Up to the tetramer the experimental spectra of the CO stretch mode are well reproduced, for larger clusters an increasing blueshift with respect to the experimental evidence is found. The experimental data for the OH stretch mode of the dimer are fairly well reproduced in all approximations, however, the spectrum of the trimer can only be reproduced using the variational calculation which includes Darling–Dennison resonance terms. © 1998 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2866-2880 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: New high-resolution differential scattering cross sections are reported for the HeNe interaction. These experimental results are combined with Hartree–Fock calculations in constructing a highly accurate interatomic potential. The new potential is capable of reproducing all available experimental data judged to be sufficiently reliable. This includes properties that are highly sensitive to the very weak attractive well and its outer bowl, in addition to the weakly repulsive wall. The potential is compared to those previously proposed for HeNe, particularly to one obtained by direct inversion of differential cross section data of similarly high quality. The potential crosses through zero at σ=2.699 A(ring); its minimum occurs at rm=3.029 A(ring) with a depth of ε=1.83 meV.
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  • 9
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 92 (1988), S. 5561-5562 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5408-5415 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are produced in an averaged size range of n¯=6 to n¯=90 by adiabatic expansion through sonic and conical nozzles. The diffraction oscillations in the total differential cross section are used to derive information on the size distribution of the clusters by comparison with quantum mechanical calculations based on a model potential. In the size range covered by the measurements, the average cluster size is given by n¯=38.4(Γ*/1000)1.64, where Γ* is the scaling parameter of the source conditions introduced by Hagena [Z. Phys. D 4, 291 (1987)]. The results are in agreement with recent measurements of corrected mass spectra but disagree with the results obtained from electron diffraction. General relations are recommended which connect the scaling parameter with the averaged size. © 1996 American Institute of Physics.
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