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  • American Chemical Society  (46)
  • American Institute of Physics (AIP)  (33)
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  • 1
    Digitale Medien
    Digitale Medien
    A three-dimensional potential energy surface for He+Cl2 (B 3Π0u+): Ab initio calculations and a multiproperty fit (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 997-1007 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High quality ab initio calculations for the interaction of He with the B 3Π0u+ state of Cl2 for three r(Cl–Cl) distances, and for the He(1S)+Cl(2P) interaction are used to obtain a three-dimensional potential energy surface for the system. The surface was used to calculate HeCl2 excitation spectra, predissociation lifetimes, and product state distributions for comparison with experimental data, and yields a remarkably good agreement. The largest discrepancy is in the dependence of the lifetime on the excited state vibrational level. The calculated lifetimes are too short for the lowest measured vibrational levels. To investigate how the surface could be modified to obtain even better agreement, a microgenetic algorithm was used to adjust the potential parameters to improve the fit. The adjusted surface has a softer repulsive wall for small Cl–Cl separations which helps to lengthen the excited state lifetimes and yields better agreement with the data. Also, the shape of the well region is adjusted somewhat in the fitting process, which yields a stronger dependence of lifetime on vibrational level. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.479190
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  • 2
    Digitale Medien
    Digitale Medien
    Fragmentation of ionized liquid helium droplets: A new interpretation (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10195-10200 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Pure liquid helium droplets of mean size 〈N〉=100–15 000 atoms ionized by electron impact show surprisingly similar ion fragment distributions. For all cluster sizes He2+ is the most probable cluster ion fragment, accounting for 30%–70% of the total ion yield. The high relative intensity of He2+ for the larger clusters shows that the droplets dissipate the ionization energy through an impulsive process, which ejects He2+ from the cluster, rather than by thermal evaporation. The other helium ion fragments that have been the focus of previous studies are most likely formed by a similar mechanism. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477713
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  • 3
    Digitale Medien
    Digitale Medien
    Capture and ionization of argon within liquid helium droplets (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9371-9382 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Liquid helium droplets of initial mean cluster size, 〈N〉, ranging from 600 to 8000 atoms are doped with argon using the pick-up technique. The doped clusters are ionized by electron impact, and the resulting fragment ions are monitored as a function of argon pressure in the pick-up volume. Analysis of the pressure dependent ion signals is used to determine (1) the probability for charge transfer from He+ to the Ar atoms within the droplet, and (2) the probability for fragmentation of the Ark subclusters upon ionization. The measured charge transfer probability from He+ to Ar ranges from 0.05±0.02 for clusters of mean original size 〈N〉=8000 to 0.26±0.05 for 〈N〉=600. Charge transfer to the Ark constituent results in the following qualitative trends; a single Ar atom yields HenAr+ ions; Ar2 mainly yields Ar2+, and Ar3 mainly fragments to yield Ar2+. Simulations of the results are performed to extract information on how the charge transfer and fragmentation processes within the ionized droplet dependent on the size of the helium droplet and the number of argon atoms captured. We use the positive-hole resonant-hopping mechanism to determine that the He+ hops 3–4 times prior to localization with either the Ar dopant or another He atom to form He2+. This corresponds to a time scale for He2+ formation of 60–80 fs. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.476389
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  • 4
    Digitale Medien
    Digitale Medien
    Ab initio calculations of the interaction of He with the B 3Π0u+ state of Cl2 as a function of the Cl2 internuclear separation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2685-2694 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ab initio calculations using unrestricted Møller–Plesset perturbation theory to fourth order (UMP-4) were carried out for the interaction of He with the B 3Π0u+ state of Cl2. Also, more reliable unrestricted coupled cluster singles, doubles, and noniterative triples (UCCSD(t)) calculations were performed for several points on the B electronic state surface and were used to scale the UMP-4 points. Exp-6 type two center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potential energy surface. An r (Cl–Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of HeCl2 properties in different vibrational states so that vibrational predissociation rates could be calculated. Excitation spectra, predissociation lifetimes, and rotational product distributions were calculated and compared to the available experimental data. It was found that the calculated B←X, 8←0 spectrum is in good agreement with the experimental one, and the calculated blueshifts for ν=8, 10, 12 show the right trend when compared to the experimental findings, i.e., the blueshifts get larger with increasing ν. The blueshift values are generally too small which suggests that the ab initio calculations underestimate the van der Waals interactions in the B state less than they do in the X state. The calculated vibrational predissociation lifetimes τ are in good agreement with the experiment, as are the rotational product distributions for ν=8, 10, and 12.© 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473413
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  • 5
    Digitale Medien
    Digitale Medien
    Short-time charge motion in Hen+ clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10873-10884 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The mechanism of short-time charge delocalization in Hen+ clusters, n=3, 14, 40, and 112, is studied using time-dependent wave packets for the electronic states for fixed nuclear positions chosen randomly from quantum Monte Carlo distributions for the neutral clusters. A DIM (diatomics-in-molecules) potential is used to calculate electronic eigenvalues and eigenvectors for the positively charged clusters, and a coherent superposition of states is created to initially localize the charge on a single atom. The charge distribution as a function of time is then calculated by propagating the initial wave function for 200 fs, during which little nuclear motion will occur. The charge motion is separated into three categories. For initial ionization of relatively isolated atoms in the cluster, the charge is only weakly coupled to other atoms, and there is little charge motion in 200 fs. For initial positions at which two or three atoms are coupled, but isolated from the rest of the cluster, the charge resonates among these atoms. For initial ionization positions at which more than three atoms are coupled, the charge can effectively delocalize over that part of the cluster. For the first two types of positions, which tend to occur near the surface of the cluster, He2+ is likely to form and the cluster will fragment before significant charge migration can occur. Charge delocalization tends to occur for initial ionization at the third type of location in the core of the cluster. For the initial positions that lead to charge delocalization, the distance and direction of the delocalization was characterized. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477784
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  • 6
    Digitale Medien
    Digitale Medien
    A combined experimental-theoretical study of the vibrational predissociation and product rotational distributions for high vibrational levels of He79Br2 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 256-266 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Pump-probe spectra of HeBr2 in vibrational states v′=10 and 39 through 48 of the B electronic state are reported and the fragment rotational distributions from vibrational predissociation of the cluster are extracted from the measured E(0g+)←B(3Π0u+) spectra of Br2. The experimental results are compared to theoretical calculations on the B←X spectra using atom-atom model potentials and performing a thermal average over transitions that contribute to the net excitation. Very good agreement between experiment and theory is obtained, except in the region of v′=44, where the Δv=−1 channel closes, and in the region of v′=48 where the Δv=−2 channel closes. For v′=43, and v′=44, the agreement is less satisfactory because the dynamics are extremely sensitive to details of the potential energy surface due to threshold effects associated with the Δv=−1 channel closing. Similar sensitivity to the potential due to the Δv=−2 channel closing impairs the agreement between experiment and theory for v′=48. Below v′=43, the rotational distributions for Δv=−1 and Δv=−2 are quite similar. Above v′=43 the peaks of the rotational distributions for Δv=−2 move to higher values of j. These results are compatible with the theoretical conclusion that dissociation shifts from a direct mechanism to one involving intramolecular vibrational distribution in the region of the closing of the Δv=−1 channel. Although the simple additive potential model used in this work succeeds in reproducing most of the experimental data for this system, further improvements in the potential energy surface will be required to achieve precise agreement between experiment and theory for large Br-Br separations. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.478101
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  • 7
    Digitale Medien
    Digitale Medien
    The HeCl2 potential: Atom–atom and ab initio compared to experiment (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8846-8854 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Two forms for the HeCl2 potential are compared to the available experimental data. First, an atom–atom form that incorporates the recently measured anisotropic He–Cl potential is used. The anisotropy of this potential is slight, and its strengths and weaknesses are similar to previous potentials in which the He–Cl interaction was treated as isotropic. In particular, the fit to the scattering data is poor. Second, a fit to ab initio points calculated using Møller–Plesset perturbation theory to fourth order was performed. The resulting potential is much more anisotropic than any potential previously proposed and tested for HeCl2. This potential fits the rotationally resolved excitation spectra as well as do previous empirical potentials, and is consistent with certain features of the total differential scattering data with which previous potentials were not. Although the ab initio potential has a global minimum in the linear configuration, the probability distribution of the ground vibrational level still maximizes in the perpendicular configuration, accounting for the good fit to the rotationally resolved spectrum. We conclude that noble gas–halogen potentials are much more anisotropic than previously believed, and we suggest several experiments that could help to confirm this anisotropy. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.468938
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  • 8
    Digitale Medien
    Digitale Medien
    Charge transfer within He clusters (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7872-7875 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Mass spectrometric measurements of clusters containing N helium atoms and an NO molecule yield probabilities for charge transfer from He+ to NO that range from 0.8±0.1 for 〈N〉=540 to 0.02±0.02 for 〈N〉=15 000. From this we estimate that the He+ hole hops approximately 70 times before localization at a He+2 core. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.472567
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  • 9
    Digitale Medien
    Digitale Medien
    High resolution spectroscopy of the He79Br2 van der Waals molecule: An experimental and theoretical study (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 3501-3510 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The structure, dissociation dynamics, and intermolecular potential energy surfaces of the He79Br2 van der Waals molecule have been studied using high resolution, two color, pump–probe laser induced fluorescence spectroscopy and three dimensional quantum mechanical calculations. A conical nozzle produces higher centerline cluster densities than a standard nozzle, and allows data collection further downstream from the nozzle. This yields improved signal to noise ratios and lower Doppler widths. He79Br2 is found to have a T-shaped average geometry with He to Br2 center-of-mass distances of 3.98 A(ring) and 4.11 A(ring) for the X and B states, respectively, somewhat longer than previously reported. Spectra were also obtained for excitation to excited bending levels of the van der Waals coordinate. However, these spectra have yet to be rotationally assigned. Vibrational predissociation line widths for the B state of He79Br2 have been measured for three new vibrational levels and range from 0.036 cm−1 for B, v′=8 to 0.062 cm−1 for B, v′=12.These values are somewhat larger than was expected based on previous HeBr2 linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl2 have been applied to the HeBr2 data. For the HeBr2 X state, two potentials are tested. First, a slightly anisotropic, one center Morse–Spline–van der Waals potential with angle dependent parameters is used. Second a much more anisotropic potential obtained from ab initio calculations is tested. The more anisotropic potential produces a significantly better fit to the data. The B state potential is constructed using Morse atom–atom potentials for the short range part of the He–Br interaction. This simple potential is sufficient to fit the main excitation band, but does not yield a fit to spectra involving vibrationally excited van der Waals modes. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471055
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  • 10
    Digitale Medien
    Digitale Medien
    Hyperfine analysis of the mixed A 3Π1 v=28 and X 1Σ+ v=69 states of I35Cl (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 9125-9131 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The A 3Π1 v=28 state of I35Cl is known from previous studies to be strongly mixed with v=69 of the X 1Σ+ state. Spectra of both these mixed states are taken at high resolution to allow accurate measurement of the hyperfine splittings due to the iodine nucleus. The hyperfine structure in both parity components of the A state is found to be perturbed, though the common rotational-electronic induced mixing should only affect the e components. Detailed analysis shows that hyperfine terms in the Hamiltonian also contribute significantly to the rovibronic state mixing. A Hamiltonian for these states is described that accounts for the hyperfine structure in both the A and X states to within the observed linewidth. The hyperfine parameters of this Hamiltonian, including the perturbation parameters, can be accounted for semi-quantitatively in terms of a separated atom model. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.470023
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