ALBERT

Notes: Spectroscopic data was inverted to generate simplified atom–diatom intermolecular potentials for the hydrogen bonded dimer HCN–HF. Both the HF ν1=0 and ν1=1 adiabatic surfaces of this complex involving the hydrogen bond stretch (ν4) and the high frequency HF bend (ν6) degrees of freedom were considered. Adiabatic separation of angular and radial degrees of freedom allowed a modified vibrational Rydberg–Klein–Rees (RKR) inversion to give effective radial potentials for each bending state. The final potential at a sequence of intermolecular separations was obtained by requiring agreement between the eigenvalues of the angular Hamiltonian and the effective radial potentials which were obtained from the RKR inversion. Potentials obtained from the inversion procedure were tested by a variational calculation with a basis set that consisted of products of preoptimized radial and angular eigenfunctions. Transition frequencies included in the inversion procedure were reproduced to better than 1 cm−1, respective rotational constants were predicted within 0.15% of the experimental value and predicted intensities were in qualitative agreement with experimental results. Ab initio potentials were also calculated using second order Møller–Plesset perturbation theory to treat electron correlation effects and with a triple-zeta-valence basis set plus two sets of polarization functions. The energies were computed at geometries where the potential had been determined by the inversion procedure. A total of 264 geometries were considered. A correction for basis set superposition errors led to good agreement between ab initio and experimental values of the well depth. We calculated the bound states of the ab initio surfaces assuming the adiabatic separation of the ν1 mode from the ν4 and the ν6 modes. The transition frequencies calculated from the ab initio surface differed from the experimental energies by less than 20 cm−1 even for highly excited overtones. Both the potentials obtained from the inversion procedure and from the ab initio calculations were modified to predict the spectrum of HCN–DF.

Notes: The effects of gas-phase collisions in mixtures of gases rapidly desorbed from surfaces are studied using direct Monte Carlo techniques. The results are compared with the effects observed in the desorption of pure gases under similar conditions. The translational energy distribution of the desorbed particles are found to deviate from the Boltzmann distribution and are found to be well represented by ellipsoidal Boltzmann distributions. In this respect the rapid desorption process is found to have similarities to the expansion of gases in nozzle sources. The influence of mass, internal degrees of freedom, and surface coverage of the adsorbates on the focusing, accelerating, and cooling effects due to gas-phase collisions are analyzed. The presence of molecules with active internal degrees of freedom is found to increase the average number of collisions experienced by the rapidly desorbed molecules. However, the influence of this increased number of collisions on the focusing effects due to gas-phase collisions is less pronounced compared to the focusing effects due to collisions between the desorbed atoms. In a gas mixture containing molecules as the minor constituents (10%) and atoms as the major constituents (90%), atoms are found to be more focused towards the surface normal than the molecules and the mean translational energies of the molecules are found to be less than those calculated in the desorption of pure molecules under similar conditions. The presence of atoms in the desorbed gas mixture is found to increase the most probable speed of the desorbing molecules and this accelerating effect increases with decrease in the mass of the coadsorbed atoms. The light atoms are found to be more efficient than heavy atoms in cooling the internal degrees of freedom.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: A new computational approach has been used to evaluate the rotationally summed, vibronically elastic integral cross sections from the scattering of slow electrons (energy ranging from 1.0 eV up to 40.0 eV) by tetrafluoromethane molecules in the gas phase. The various symmetry components have been analyzed using the exact static exchange approximation and also by including a nonempirical, model polarization potential employed before by our group. A comparison with earlier calculations and with existing experiments allows us to assign the symmetries of the shape resonances in the 5–30 eV energy region which are seen by experiments and are also shown by the present calculations.