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  • 1
    Digitale Medien
    Digitale Medien
    Vibrationally enhanced inelastic atom–molecule cross sections at very low energies (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 8819-8829 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mean collision energy of the order of 1 meV in a supersonic seeded beam arrangement. The initial vibrational state v and rotational state j is varied in the range 4≤v≤35 and 5≤j≤25. The Na2 molecules are excited by Franck–Condon pumping near the transition region to free molecular flow and the flux of molecules which survive in the level (v,j) is monitored far downstream behind a small aperture. It is shown that elastic processes do not contribute to the observed signal. The magnitude of the cross section for the sum of all inelastic processes is as large as several hundred A(ring)2. It is found that the vibrational motion of the molecule, which carries an energy of up to 500 meV, strongly influences the collision dynamics. The rate constant increases with v by about 35% within the range of vibrational levels studied here. Some state-to-state rate constants are also reported. The experimental results are in good agreement with data from quasiclassical trajectory calculations on a semiempirical bond-distance-dependent potential surface. The observations can be rationalized by realizing that the long range attraction increases with vibrational excitation since the polarizability of the molecule increases. Furthermore, the vibrational excitation in combination with the relatively long interaction time due to the low collision energy enhances the probability for a change of the initial quantum state of the molecule.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462239
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  • 2
    Digitale Medien
    Digitale Medien
    Angularly resolved rotational energy transfer in highly vibrationally excited states: Na2(v=31)–Ne (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4252-4259 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The scattering of high vibrationally excited sodium molecules Na2(v=31) with Ne atoms at 180 meV collision energy is investigated in a crossed molecular beam arrangement using laser optical methods. Angularly resolved rotationally inelastic and vibrationally elastic cross sections ji→jf are measured for ji =5, 7, and 9≤jf ≤25. Pronounced rotational rainbow maxima are observed, the angular position of which indicates a significantly larger anisotropy of the interaction potential compared to that of Na2(v=0)–Ne. Except for the increase of the anisotropy the vibrational excitation has little effect on the dynamics of rotational energy transfer. Good agreement with a new semiempirical Na2–Ne potential surface V(r,R,γ) is found.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460611
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  • 3
    Digitale Medien
    Digitale Medien
    Models for bond distance dependent alkali dimer–rare gas potentials (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5384-5396 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Analytic expressions for Li2–Rg and Na2–Rg electronic ground state potentials (Rg=He,Ne,Ar,Kr,Xe) that depend on all three Jacobian coordinates, atom–molecule distance R, bond length r, and orientation angle γ, are presented. The potentials are obtained by expanding the repulsive and attractive parts in a two-term Legendre expansion and connecting them via generalized damping functions as proposed by Tang and Toennies [J. Chem. Phys. 80, 3726 (1984)]. The bond distance dependence (req=5.051 a.u.≤r≤6.838 a.u. for Li2 and req=5.809 a.u.≤r≤7.607 a.u. for Na2) is introduced in the repulsive part via the r-dependent shift of the repulsive barrier and in the attractive part via the r dependence of the multipole polarizabilities and effective excitation energies. These potentials are shown to agree reasonably with ab initio data. The resulting values of potential well depth ε and equilibrium distance Rm are used to check the validity of recently proposed direct combining rules for r-dependent potentials.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458516
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  • 4
    Digitale Medien
    Digitale Medien
    A molecular beam study of the potential anisotropy of laser vibrationally excited Li2(v=0,20) scattered from Kr (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 287-294 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A beam of highly vibrationally excited (v≤20) Li2 molecules has been generated via Franck–Condon pumping using a cw ring dye laser. The linearly polarized light from a second cw dye laser is used to determine the polarization of the remaining unscattered molecules of the beam after it has passed through a scattering cell filled with krypton gas. The dependence of the state selective total integral cross section on orientation is obtained from the anisotropy of the angular momentum distribution of the beam molecules described by 1+a2P2(cos θ), where θ is the angle between the angular momentum vector and the velocity vector. The measured polarization increases from a2=−0.019±0.007 for Li2(v=0)–Kr collisions to a2=−0.074±0.016 for Li2(v=20)–Kr collisions. The measurements are interpreted in a vibrationally adiabatic approximation in terms of the potential anisotropy parameter q2 for an assumed simple anisotropic potential model V=±(Cs/Rs) [1+q2⋅P2(cos γ)], where γ is the angle between bond distance r and distance between center of mass R. For the v=20 vibrationally excited molecules we obtain q2=1.38±0.31 compared to q2=0.47±0.15 for the v=0 molecules. This large increase in anisotropy with vibrational excitation is compared with that obtained from a previously proposed model for Li2(v)–He [J. Chem. Phys. 85, 7062 (1986)].
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455523
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  • 5
    Digitale Medien
    Digitale Medien
    Laser spectroscopical investigation of atomic sodium kept at defined distances from Au(111) single crystal surfaces via Langmuir–Blodgett films (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6805-6807 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456301
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  • 6
    Digitale Medien
    Digitale Medien
    Laser investigation of Na atoms deposited via inert spacer layers close to metal surfaces (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 7625-7635 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Na atoms have been observed on top of inert spacer layer/metal substrates at room temperature in an ultrahigh-vacuum environment via two-photon laser-induced fluorescence. As spacers, 1, 5, and 11 cadmium-arachidate (LB) and single dodecanthiol (SAM) layers were used, thereby changing the distance between atom and metal between 26 and 290 A(ring). A low sticking probability ((very-much-less-than)10−6) was observed, resulting in a coverage of the spacer layers with less than 10−3 of a monolayer Na. This low coverage is due to the low binding energy of Na at the CH3 end groups of the spacers (millielectron volts). The Na intensity showed a biexponential decay as a function of time with a small time constant (40 s) due to direct evaporation of Na adsorbed on top of the layers and a large time constant (650 s) due to diffusion of the Na atoms through the fatty acid layers. The two-photon detection scheme allowed a Doppler-free (high-resolution) measurement of line broadening and frequency shift of the spectral lines due to the presence of the metal surface. A moderate broadening was observed for Na on top of LB multilayers on Pt/mica substrates, denoting that the Na atoms diffuse through the fatty acid chains with their mean distance from the surface decreasing from 290 to 28 A(ring). In addition, we found a red shift of the spectral lines by a few hundred Megahertz. The results are compared with previous data of sticking and line broadening of Na on a SAM/Au/Si substrate.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464703
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  • 7
    Digitale Medien
    Digitale Medien
    Laser-induced density changes in supersonic Ar beams (1989)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 344-346 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: The nonresonant heating of a supersonic beam nozzle induced by antiparallel coaxial irradiation by a cw Ar+ laser has been investigated time dependent using a mass spectrometer and an Ar beam. The resulting laser-induced decrease of beam density as a function of laser power and source pressure is quantitatively modeled, leading to an accurate measure of effective nozzle temperature and beam heating.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1140435
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  • 8
    Digitale Medien
    Digitale Medien
    Dipole-assisted self-assembly of light-emitting p-nP needles on mica (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 3860-3862 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report on dipole-assisted, self-assembled formation of p-6P and p-5P needles on cleaved and heated mica (0001) surfaces. Low-energy electron diffraction (LEED) reveals that the needles are single crystalline with the (11¯1¯) face parallel to the surface, consisting of parallel stacks of laying molecules oriented along the direction of microscopic dipoles on the mica surface. They have submicrometer cross-sectional dimensions and lengths as large as millimeters. Moreover, due to the strong dipole confinement of individual molecules, the needles form large domains with parallel oriented entities. A pronounced optical dichroism agrees with the findings from LEED. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1424071
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  • 9
    Digitale Medien
    Digitale Medien
    Observation of an anomalous increase in total cross sections with high vibrational excitation in the Li2 (v=0–21)+Na reactive system (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1262-1271 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The effect of initial vibrational excitation on Li2(v,J)+Na collisions was studied experimentally and theoretically. The relative integral cross sections σ(v,J) were measured in a crossed beam experiment for vibrational states v populated thermally or by optical pumping, and for final states detected by laser induced fluorescence. It is found that σ(v,J) increases by 35±7% between v=0 and v=20, while the dependence on the initial rotational state J is insignificant, within ±5%. The effect of vibrational enhancement is about twice as large as that found for the Li2–rare gas systems for the same amount of vibrational excitation. The effect is about an order of magnitude larger than predicted by our quasiclassical trajectory calculations on a LEPS potential energy surface, thus suggesting that additional long-range potential terms ought to be included. We also attempted to measure state-selectively the LiNa (v',J') products from the thermal reaction and found an upper limit of the reactive cross section of 1 A(ring)2. This contrasts with the trajectory calculations that predict a 20 times larger cross section; we thus conclude that the reactive part of the potential also needs to be re-evaluated.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.467818
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  • 10
    Digitale Medien
    Digitale Medien
    Anisotropic integral cross sections for lithium dimer(Li2)(v = 0, v = 20)-sodium collisions (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8215-8219 
    Details
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/j100174a038
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