ALBERT

Notizen: Recently obtained spectroscopic constants for excited vibrational states of HCN–HF with up to five quanta in the ν7 intermolecular bending mode, which can be characterized as being mostly HCN hindered rotation, and one quanta in the ν4 intermolecular stretching mode were combined with earlier experimental data on this system to obtain a reduced intermolecular potential energy surface where the HF fragment was treated as a pseudoatom and the HCN fragment was treated as a rigid rotor. A functional form was assumed for the interaction potential and the vibrational dynamics was studied using a vibrational configuration interaction method. The parameters in the potential were varied to obtain the best fit to all available experimental data. The resulting interaction potential was found to have a strong coupling between the ν4 and ν7 vibrational modes. The locus of constrained minima on this surface was in excellent agreement with the hard-sphere assumption used in the Buckingham–Fowler model [A. D. Buckingham and P. W. Fowler, J. Chem. Phys. 79, 6426 (1983)] for weakly interacting molecular dimers. © 1997 American Institute of Physics.

Notizen: We have studied the collisional energy transfer, and the sticking probability, between molybdenum clusters and the rare-gas atoms Ne, Ar, and Xe. We have chosen these systems as nontrivial models of the thermalization process of metal clusters in a background gas. The mean energy transfer cross sections and the energy transfer rate constants, and the sticking probability of molybdenum clusters (Mo4,9,14) with rare-gas atoms are computed as functions of relative collision energy (gas temperature) and reduced mass. The dynamics of gas phase molybdenum clusters are simulated by molecular dynamics trajectories whose initial conditions are sampled from a distribution appropriate to thermal collisions. The simulation shows that the energy transfer rate constants are dominated by the collision frequency. The mean energy transfer cross sections are coupled to the collision mass as well as to the actual interaction force. The coupling is nonlinear, and there is some evidence that in the energy transfer, for small clusters, complex collisions are involved. The sticking probability at equilibrium temperature is far below 1.

Notizen: A theoretical study is presented of the dynamics of resonant electron scattering from N2–Ar and N2–Ar2 van der Waals clusters. Using the results of quantum electron-molecule scattering calculations we analyze the effects of adjacent Ar atoms on the width and position of the low-energy 2Πg electron-scattering resonance of N2. The results indicate that the presence of the Ar atoms leads to positive energy shifts and an increase in the width of the resonance. The magnitude of these changes depended on the orientation of N2 relative to the Ar atoms and on the number of Ar atoms. Additionally, in some arrangements, the degenerate 2Πg resonance was split into two distinct resonances. Implications for electron scattering from N2 adsorbed on solid Ar surfaces are also discussed. © 1997 American Institute of Physics.

Notizen: The structure and ground state dynamics of the atom–diatom dimer interaction between Ar and HI has been investigated by microwave and near infrared supersonic jet spectroscopy. Ab initio molecular orbital calculations were used to provide greater insight into the nature of the interaction. The ground state is shown to be in the isomeric form Ar–IH with Rcm=3.9975(1) Å, θ=149.33(1)° for the normal isotopomer and Rcm=3.9483(1) Å, θ=157.11(1)° for Ar–ID. The potential surface from an ab initio molecular orbital calculation was scaled and shifted to yield a nonlinear least-squares fit of the rovibrational state energies to the experimental data. The ground state potential energy surface obtained in this manner has a barrier between the Ar–IH and Ar–HI isomers of 88.5 cm−1 with respect to the global minimum. Such calculations are also used to predict the presence of localized states in the secondary minimum associated with isomers Ar–HI and Ar–DI. Attempts to experimentally identify transitions associated with the latter were unsuccessful. The ground state, Ar–IH isomeric structure, contrasts with the corresponding ground state of the other members of the homologous series Ar–HX (X=F, Cl, and Br) in which the Ar is bound to the proton. © 1999 American Institute of Physics.

Notizen: The effect of two different interaction potentials, a two-body and a many-body potential, on thermal cluster reaction rates was studied for 2–13 atom nickel clusters using the classical trajectory method. The reaction rates were computed for cluster–monomer and cluster–cluster collisions at T=1200 K, using the bulk and dimer parametrized Lennard-Jones (LJ) potentials and were compared with the rates previously obtained for these collisional events by using a more realistic many-body tight-binding second moment approximation (TB-SMA) potential. For cluster–monomer collisions, close agreement exists between the reaction cross section results for dimer fitted LJ (LJD) potential and TB-SMA potential suggesting that the cluster–monomer collisions may be dominated by pairwise interactions. The bulk fitted LJ potential (LJB) underestimates the sticking cross section results of the other two potentials for most cluster sizes. This discrepancy however appears to be due to the relatively smaller cluster binding energies obtained for this potential as a result of which a larger cross section for dissociation is observed. For cluster–cluster collisions, for most cluster sizes, no agreement exists between the reaction cross section results for the three potentials. The discrepancy between the cross section results for the LJ potentials and the TB-SMA potential appears to lie in the difference in the scaling of cluster energy with cluster coordination for these two types of potentials (i.e., linear for LJ vs square root dependence for TB-SMA). Some characteristics of the cross section results of both LJB and LJD potentials correlate with the relative cluster stability pattern for the LJ clusters. For TB-SMA case, no such correlation exists, which however is consistent with the smooth and featureless size distributions observed experimentally for nickel and other transition metals. The cut-off used in the TB-SMA potential appears to lead to a significant underestimation of the total reaction cross section for N=13, in the case of the cluster–cluster collisions. The results of this study indicate that the rate calculations may be sensitive to both the nature and parametrization of the simulation potential depending on the temperature range considered and cluster growth process simulated. © 1996 American Institute of Physics.

Notizen: Low-energy quantum calculations are carried out for electrons scattering by CF4 molecules in their ground electronic states. The corresponding elastic cross sections (rotationally summed) are obtained as integral quantities and as angular distributions, i.e., differential cross sections (DCS), over a range of collision energies from ≈3 eV up to 35 eV. The exact static exchange (ESE) results compare well with experiments and with previous calculations. The inclusion of a model polarization potential is shown to generally improve results, especially at low collision energies and in the small-angle scattering region. © 1996 American Institute of Physics.

Notizen: The collisions of small nickel clusters of size 2–14 atoms were studied using the classical trajectory method. Three cases were considered: cluster–monomer, cluster–dimer, and cluster–cluster collisions. The interaction between the nickel atoms was modeled by a semiempirical many-body potential based on the second moment approximation of the tight-binding scheme. This potential, which previously has been shown to reproduce a wide range of bulk properties including finite temperature behavior for nickel, was also found to describe the cluster properties very well. Both the internal temperatures of the colliding clusters and the collision temperature were set equal to 1200 K. In each of the cases studied, sticking was the dominant channel of reaction for clusters other than dimer and trimer. The sticking cross section was further found to be well approximated by the geometric cross section obtained using a simple hard sphere model for clusters larger than pentamer in the case of cluster–monomer and cluster–dimer collisions. For cluster–cluster collisions, the hard sphere approximation overestimates the sticking cross section by about 40% for even the largest clusters considered.However in this case also, the observed trend suggests a better agreement for cluster sizes somewhat larger than the sizes considered in this study. The other significant reaction channel observed was monomer evaporation which becomes more frequent and persists for larger target cluster sizes as the size of the projectile cluster is increased. The cross section results in all three cases do not exhibit any dramatic dependence on cluster size, consistent with the experimental observation of smooth and featureless size distributions for nickel and other transition metal clusters. The cluster–monomer collision calculations were repeated by setting the internal temperature of the cluster to 0 K. The lowering of temperature did not lead to any dramatic size dependence. For the 0 K case, the sticking cross section is underestimated by the hard sphere cross section even for the larger clusters. However, the observed trend indicates a better agreement between the two cross sections for cluster sizes outside the size regime considered. For all of the above cases considered, the hard sphere cross section appears to be easily parametrizable in terms of the cluster size. For a limited number of cluster sizes, the collision calculations were repeated using different integration times and from these calculations it appears that the collisionally formed clusters decay roughly in an exponential manner. This suggests that the cluster decay rates may be obtained using a simple statistical theory such as the RRK theory. Also, these calculations suggest that even the smallest of the collisionally formed clusters survives long enough to be cooled by collisions with background gas molecules. As a consequence, cluster growth may be determined by coagulation-type reactions, unless monomer is supplied continuously. The implications of the results of this study to cluster growth models are discussed. The results of this study may be improved by the inclusion of two factors, directional bonding and (particularly) long range interactions in the potential. © 1995 American Institute of Physics.

Notizen: A high-resolution FTIR supersonic slit jet absorption spectrometer is described for the investigation of weakly bound dimers and trimers in the near-infrared spectral region. The spectrometer is demonstrated to conservatively have a sensitivity of 6×108 molecules/cc/state and can be operated at an apodized resolution of 0.004 cm−1. To illustrate the performance of the spectrometer, it has been applied to the rovibrational analysis of the band spectra of three weakly bound species in the near infrared: ν1 OC–HF, ν1 N2–HCl, and ν5 (H35Cl)3. The recorded spectra of these species are compared with corresponding investigations using state-of-the-art tunable infrared laser supersonic jet or molecular-beam spectrometers to illustrate the capabilities and limitations of the current FTIR supersonic jet spectrometer. © 1995 American Institute of Physics.

Notizen: We have studied the collisional energy transfer between molybdenum clusters and the rare-gas atoms Ne, Ar, and Xe. We have chosen these systems as nontrivial models of the thermalization process of metal clusters in a background gas. The mean energy transfer cross sections and energy transfer rate constants for collisions of molybdenum clusters (Mo5) with rare gas atoms are computed as functions of relative collision energy (gas temperature) and mass. The dynamics of gas phase molybdenum clusters are simulated by classical trajectories whose initial conditions are sampled from a distribution appropriate to thermal collisions. For the interaction of the molybdenum cluster atoms with the background gas, a Buckingham-type potential for unlike atoms was fitted to energies obtained using standard quantum chemistry techniques. The molybdenum cluster atoms interact among themselves by a Lennard-Jones potential. The simulation shows that the energy transfer rate constants are dominated by the characteristic collision velocity, i.e., within the domain of internal cluster temperature and background gas temperature investigated here, the mass and the interaction force do not change the energy transfer rate constants very much. The mean energy transfer cross sections, however, are coupled to the collision mass as well as to the actual interaction force. The coupling is nonlinear, and there is some evidence that in the energy transfer, for small clusters, complex collisions with more than one cluster atom are involved.

Notizen: The hydrogen bond OC–HI has been characterized using high resolution microwave and infrared spectroscopies in supersonic seeded molecular jets. Ground state molecular parameters of the 16O12C–HI and 16O13C–HI isotopic species determined by the pulsed-nozzle Fourier transform microwave supersonic jet technique include: for 16O12C–HI, B0 (MHz)=900.9522(1), DJ (kHz)=2.519(1), CN (kHz)=0.94(18), χ(MHz)=−1346.238(13), χJ (kHz)=−8.27(31). The corresponding values for 16O13C–HI are 882.5997(2), 2.404(2), 0.87(19), −1349.481(17), and −7.76(28). This analysis is consistent only with a linear equilibrium dimer structure in which the proton is bound to the carbon atom of carbon monoxide. Other derived dimer parameters include: r(C–I)=4.271(2) A(ring), αav=24.8°, kσ(N m−1)=1.713. Infrared diode laser investigations provide a band origin frequency ν0 of 2148.549 040(29) cm−1 for the ν2 C≡O stretching fundamental vibration. This corresponds to a blue shift of 5.277 28(37) cm−1 relative to free monomer CO. Excited state molecular constants B2=898.2728(33) MHz. and DJ(2)=2.614(24) kHz are also determined. Line profiles are consistent with an excited state lifetime ≥0.54 ns.