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ABSTRACT: By means of quantum mechanical simulation of the reaction F + H2(v = 0; j = 0, 1, 2) → H + HF(v′, j′) on the Stark-Werner ground state potential energy surface at collision energies of 1.84, 2.74, and 3.42 kcal/mol, we have
analyzed interference of the “partial waves” corresponding to different values of the total angular momentum J. As the vibrational quantum number v′ of the HF(v′, j′) product increases, the interference for the HF forward scattering becomes noticeably more constructive. This is probably
the reason for the maximum in the angular distributions of the HF(v′= 3) molecules at small scattering angles that was discovered experimentally by D.M. Neumark, A.M. Wodtke, G.N. Robinson,
C.C. Hayden, and Y.T. Lee, J. Chem. Phys. 82 (7), 3045 (1985) at the same collision energies. We have also determined the intervals of J values most effective for forward scattering of the HF(v′, j′) molecules.
Russian Journal of Physical Chemistry B 04/2012; 3(6):857-863. · 0.26 Impact Factor
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ABSTRACT: Using the quantum mechanical path integral Monte Carlo method, we simulated parahydrogen clusters (j = 0) and clusters doped with several (up to 6) orthohydrogen molecules (j = 1), with the total number of molecules ranging from 4 to 40 at temperatures of 1.5, 3, and 4.5 K. Some energy parameters
(including the chemical potentials) and spatial characteristics of the clusters are found. At a temperature of 1.5 K, as the
total number N of molecules in the cluster increases, the chemical potential and the rotational energy of the clusters attain local minima
at the same geometrically determined values of N (the magic numbers). The ortho-molecules exhibit a larger probability (compared to the para-molecules) to reside in the central
region of the cluster and a smaller probability to be located near its surface. This effect is enhanced as the number of orthohydrogen
molecules in the cluster increases, the total number N of molecules grows, or the temperature is lowered.
Russian Journal of Physical Chemistry B 04/2012; 3(5):743-752. · 0.26 Impact Factor
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ABSTRACT: We studied the nature and collision energy dependence of the maximum that appears in the angular distributions of the HF (v′ = 3) product of the F + H2 (v = 0; j = 0, 1, 2) → H + HF (v′, j′) reaction at small scattering angles θ in the center-of-mass frame. This maximum and its increase as the collision energy
increased were discovered in the well-known experiment described by D.M. Neumark, A.M. Wodtke, G.N. Robinson, C.C. Hayden,
and Y.T. Lee, J. Chem. Phys. 82 (7), 3045 (1985). In order to determine the nature of the maximum, we performed quantum-mechanical simulation of the reaction
on the Stark-Werner ground state potential energy surface at collision energies of 1.84, 2.74, and 3.42 kcal/mol corresponding
to the above-mentioned experiment and calculated the vibrationally and rotationally resolved differential cross sections dσv′j′/dΩ of the reaction. The maximum under consideration was found to be due to a superposition of two effects, namely, the absence
of HF (v′ = 3; j′) products with large j′ because of energy restrictions and an increase in the relative amplitude of quantum-mechanical oscillations on dσv′j′/dΩ cross sections at small j′ and θ as v′ increased. Oscillations on dσ3j′
/dΩ cross sections with small j′ are responsible for the maximum observed.
Russian Journal of Physical Chemistry B 04/2012; 1(5):452-464. · 0.26 Impact Factor
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ABSTRACT: A high-efficiency ion source for a mass-spectrometer’s detector of molecular beams and their scattering products is described.
The ion source is designed according to a scheme of impact ionization of a beam particle by a longitudinal electron beam in
a magnetic field with a strength of up to 130 mT. The design of the source developed is very flexible and has no limitations
for use in any experiments with molecular beams. An ionization efficiency of particles of an atomic helium beam of 10−3 ions/atom has been achieved. The useful signal-to-background ratio in the detector’s chamber is 3 × 104 during detection of ions with mass-to-charge ratio m/q = 4 amu.
Instruments and Experimental Techniques 09/2006; 49(5):709-713. · 0.36 Impact Factor
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ABSTRACT: The construction of an improved source of intense beams of fluorine atoms is described. The source is ideally suited for crossed beam studies of chemical reactions and also for high aspect-ratio etching of silicon. In the new source the MgF2 single crystal oven chamber used in our earlier source (Faubel et al 1996a J. Phys. D: Appl. Phys. 29 1885) has been replaced by a CaF2 single crystal. The more favourable high temperature mechanical properties and lower rate of evaporation of CaF2 permit higher temperatures of up to 1200 °C compared with 1000 °C previously and a corresponding higher degree of dissociation of 50–60% in a 20% mixture of F2 in Kr at source stagnation pressures of 17 bar. The advantages of the new source are demonstrated with time-of-flight spectra measured for scattering from an He secondary beam and in reactive scattering from H2.
Journal of Physics D Applied Physics 09/2006; 39(19):4186. · 2.54 Impact Factor
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ABSTRACT: Time-of-flight spectra of HF products in the v′ = 2 vibrational state from reactive scattering of F atoms from para-H2 exhibit at least four smaller peaks which are assigned to the rotational states j′ = 7, 8, 9, and 10. The center-of-mass rotational distributions are in good agreement with accurate quantum mechanical and approximate coupled states calculations. © 1999 American Institute of Physics.
The Journal of Chemical Physics 05/1999; 110(21):10231-10234. · 3.33 Impact Factor
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Phys. Rev. B. 01/1999; 60(16):11707-15.
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ABSTRACT: The important properties of thermal ovens for producing intense beams of F atoms by thermal dissociation of are analysed. After reviewing previous constructions a new source made of a single crystal of is described and characterized. This source has been tested for more than 3000 h at temperatures up to with a mixture of 10% in Ar at pressures up to 12 bar. A degree of dissociation of 50% and a F atom beam velocity spread of about 7% were achieved.
Journal of Physics D Applied Physics 12/1998; 29(7):1885. · 2.54 Impact Factor
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ABSTRACT: Product rotationally state-resolved differential cross sections (DCS) have been determined for the DF(vf, jf) products of the F + D2 (vi = 0, ji = 0, 1, 2) reaction from the detailed analysis of high resolution crossed molecular beam experiments at the collision energies of 140, 180, and 240 meV. An increasing rotational excitation when going from the backward to the sideways and forward scattering regions is observed for all vibrational DF states, except for vf = 4. The DF products in vf = 4 scattered in the forward region (θcm = 0°−20°) are rotationally cooler than those scattered at intermediate scattering angles (θcm = 30°−100°). The experimental results are compared with quasiclassical trajectory (QCT) calculations on the ab initio potential energy surface (PES) of Stark and Werner. Good qualitative agreement is found for the observed trend of the vf, jf state-resolved DCSs. In particular, the behavior of the vf = 4 product state is well accounted for by the QCT calculation. The results are discussed in terms of the quasiclassical state-to-state reaction probabilities as a function of the total angular momentum (opacity functions).
10/1998;
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ABSTRACT: The F+D2→DF+D reaction has been investigated in a high resolution crossed molecular beam scattering experiment at a collision energy of 90 meV (2.07 kcal/mol). Time-of-flight spectra of the DF products have been measured covering the backward hemisphere of center-of-mass scattering angles (θcm = 90°–180°). The energy resolution achieved in the spectra, as good as 20 meV, together with a careful calibration of the beam source intensities and detector sensitivity makes it possible to determine absolute differential and integral cross sections resolved in vibrational and rotational states of the DF products. Interestingly, the backward scattered DF(vf = 2) and DF(vf = 3) vibrational products are found to present double-peaked (i.e., bimodal) rotational distributions. A three dimensional quantum mechanical calculation of the title reaction performed on the recent ab initio potential energy surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)] is presented, which was carried out in the reagents arrangement channel within the coupled-states approximation, employing negative imaginary potentials. The calculated state-to-state differential cross sections show a very encouraging overall agreement with those evaluated from the scattering experiment. In particular, the bimodal structure of the rotational distributions is qualitatively well accounted for in the calculation. However, significant differences still persist between the calculated and the experimental results. © 1998 American Institute of Physics.
The Journal of Chemical Physics 06/1998; 108(23):9694-9710. · 3.33 Impact Factor
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ABSTRACT: Absolute differential and integral cross-sections for the DF(vf=0−4) vibrational products of the F + D2(vi=0,ji=0−2) → DF(vf) + F reaction have been evaluated from the time-of-flight spectra measured in high-resolution crossed molecular beam scattering experiments at five collision energies within the range 90−240 meV. The observed rise of the total reactive cross-section with increasing collision energy can be fitted with an empirical, modified line-of-centers model, yielding an activation energy of 1.8 kcal·mol-1. Remarkably pronounced angular structures are observed in the vibrationally resolved differential reactive cross-sections, especially for the largest accessible vibrations, vf = 3 and vf = 4. The experimental absolute cross-sections are compared with the results of previously reported quantum mechanical and quasiclassical trajectory calculations on the most recent potential energy surfaces for the F + D2 system. An overall excellent agreement is found within the experimental uncertainty. However, some significant differences are also apparent, especially at the lowest collision energies studied.
09/1997;
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Surf. Sci. 01/1997; 385:L958-L964.
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ABSTRACT: The sticking coefficients of oxygen on the Rh(111) surface have been measured using O-2 molecular beams seeded with different rare gases to achieve beam energies in the range E-i = 25 - 600 meV. With increasing O-2 incident beam energy, the adsorption evolves continuously from a molecular precursor-mediated process at low energies to a direct adsorption process described by Langmuir kinetics at high energies. The analysis of the dependence of the initial sticking coefficient on the incident kinetic energy and the angle of incidence as well as on the surface temperature provide information on important parameters of the potential energy surface. Further insight into the nature of the adsorbed species could be obtained from the UPS measurements of the adsorption induced work function changes and from helium atom diffraction measurements of structural modifications as a function of the kinetic energy angle of incidence of the oxygen molecules, and the surface temperature. The "simple washboard" model accounts for the trapping well of the molecular precursor and is able to describe many of the observed features in the precursor region. (C) 1997 American Institute of Physics.
Journal of Chemical Physics. 01/1997; 106(21):8876-8889.
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ABSTRACT: In this letter we report quantum mechanical integral and differential cross sections for the title reactions as calculated on a new ab initio potential energy surface. The calculations, all carried out in the reagents arrangement channel employing negative imaginary potentials, were done within the coupled‐states approximation. The final vibrational state‐to‐state differential and integral cross sections were compared with experiment. Altogether, a very encouraging agreement was obtained. © 1996 American Institute of Physics.
The Journal of Chemical Physics 02/1996; 104(7):2743-2745. · 3.33 Impact Factor
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ABSTRACT: The structure and vibrational modes of CO adsorbed on Cu(001) have been investigated by helium atom scattering (HAS). Below coverages of θ=0.13, CO adsorbs as isolated molecules, and the time of flight (TOF) spectra of helium atoms scattered from the surface are characterized by energy losses and gains of 3.94 ± 0.07 meV. With the aid of isotope shift measurements, this mode is unambiguously assigned to the frustrated translation mode of the adsorbed CO. In the c(2×2) structure (θ=0.5) this mode is found to show dispersion in both the 〈100〉 and 〈110〉 azimuths with frequencies ranging from 3.9 meV at the zone origin to 5.6 meV at the zone boundary. A simple force constant model with a single nearest neighbor force constant within the layer is found to be sufficient to describe the dispersion. For coverages slightly less than θ=0.5 the angular distributions of scattered helium atoms show features characteristic of vacancies in the overlayer. In the TOF spectra an additional nondispersive mode at about 4 meV is also seen and assigned to the frustrated translation of CO molecules adjacent to the vacancies in the c(2×2) overlayer. The Rayleigh mode is clearly observed in the c(2×2) structure but is found to decrease slightly in frequency while the longitudinal resonance, which is especially prominent on the clean surface, disappears completely. The results are interpreted with the aid of slab calculations. © 1995 American Institute of Physics.
The Journal of Chemical Physics 03/1995; 102(12):5059-5070. · 3.33 Impact Factor
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Surf. Sci. 01/1995; 323:228.
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Journ. Electron. Spectrosc. Rel. Phenom. 01/1993; 64/65:715.
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ABSTRACT: A three-dimensional quantum mechanical study of vibrational state resolved differential cross sections for the direct inelastic and charge transfer channels of the H{sup +}+H{sub 2} system has been carried out at {ital E}{sub cm} =20 eV using the infinite order sudden approximation (IOSA). Steric factors, opacity functions, angular distributions, and integral cross sections are calculated. The integral cross sections are in very good agreement with recent experimental results, whereas the angular distributions agree only partially with the experiments. A further comparison of both the theoretical and experimental results with semi-classical calculations based on the usual trajectory surface hopping method revealed that the present quantum results provide a better description of the experimental observations. The likely shortcomings of the semiclassical method are discussed.
Journal of Chemical Physics; (USA). 09/1989; 91:7.
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ABSTRACT: Well defined two and three phonon excitation bands are observed in time of flight spectra of helium atoms scattered from the CO-Cu(001) surface. The detailed intensity structure of the bands enables multiphonon interaction theories to be tested in a uniquely sensitive way for the first time. The CO-Cu(001) scattering system is well characterised, enabling density of states calculations to be performed for the two phonon excitation process. These can be used to show that phonons polarised out of plane, perpendicular to the scattering wavevector, are being created in contradiction to the current theoretical understanding of the process. A possible explanation is proposed in terms of scattering out of plane by the two phonon process, followed by diffraction from the highly corrugated surface back to the scattering plane.
Journal of Electron Spectroscopy and Related Phenomena · 1.96 Impact Factor
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ABSTRACT: The F + D2(ji) → DF(νf,jf) +D reaction has been studied in a high resolution crossed molecular beam scattering experiment at collision energies of 180 meV (4.1 kcal/mol) and 240 meV (5.5 kcal/mol), the highest collision energies ever achieved for this system in this kind of experiments. The product energy resolution in the time-of-flight (TOF) spectra was typically 30 meV and 35 meV, respectively. The vibrationally resolved center-of-mass (CM) differential cross sections (DCS) have been extracted from the TOF measurements and the products' angular distributions (AD) in the laboratory (LAB) frame. It has been found that, for almost all the final states, the DCS shifts towards lower CM scattering angles as the collision energy increases. At the two collision energies studied, the scattering from the highest vibrational state accessible for the DF, i.e. νf = 4, gives rise to a prominent forward peak in the DCS that seems to increase as the collision energy goes up. Measurements carried out with D2 molecular beams at different rotational temperatures indicate that the large angle (backward) scattering increases with the initial rotation of the D2, in qualitative agreement with theoretical predictions. The rovibrationally state resolved integral reaction cross sections have been also experimentally deduced. All these results are compared with quasi-classical trajectory (QCT) calculations reported in the following paper of this issue.
Chemical Physics. 207:227-243.
