Chen K. Lutrus

DWBA predicted relative product rotational state distribution for H + D2 → HD + D

Abstract Relative product rotational state distributions for H + D 2 → HD + D are reported based on the DWBA of Suck Salk, and are shown to agree reasonably well with observations. The rotational peak positions are found to have good correlation with computed “semiclassical” angular momentum.

Potential role of nuclei in cloud element formation at high altitudes

Abstract The role of aerosols in providing a heterogeneous nucleation contribution to cirrus cloud ice crystal formation has long been an unresolved question. A mobile aerosol sampling facility was employed on the NCAR Sabreliner research aircraft to take aerosol samples from the vicinity of cirrus clouds over Colorado and Wyoming. Tandem electrostatic classifier and impactor technique were used to measure aerosol size distribution, hydration capability and composition information. The results suggest that aerosol related heterogeneous nucleation processes are active in cirrus cloud processes.

A study of hydrogen bond strengths of neutral water clusters (H2O)n using modified MNDO

Semi-empirical molecular orbital methods proposed up to now seriously fail to describe hydrogen bonded systems associated with (H2O)n. A new scheme of parametrization using a semi-empirical method is proposed. We tested hydrogen bonding associated with the water clusters (H2O)n. The results are found to be close to ab initio Hartree-Fock quality, indicating a good promise for studying hydrogen-bonding systems other than O-H...O moiety.

Intercomparison Between Commercial Condensation Nucleus Counters and an Alternating Temperature Gradient Cloud Chamber

Three commercial CNC counters (TSI models 3010, 3022A, and 3025A) are compared with an alternating temperature gradient cloud chamber (ALGR). Electrically size classified aerosols of sodium chloride and silver are used. Diffusional losses within the ALGR are much larger than for the TSI instruments: therefore the ALGR concentrations are corrected for internal diffusional losses, but the TSI instruments are not. The particle size range tested is 4–90 nm. For sodium chloride, the TSI concentrations agreed fairly well with the corrected ALGR at larger sizes, and were below the corrected ALGR at smaller sizes. The electrical aerosol classifier (EAC) setting at which a particular TSI instrument concentration is 50% of the corrected ALGR concentration is denoted D 50%. For the 3010 and 3022A, the values of D 50% are 14.5 and 7.9 nm, respectively. For the 3025A the extrapolated D 50% is 3.5 nm. These D 50% values are close to those reported for comparisons between concentrations measured by the TSI instruments a...

An effective Hamiltonian study of molecular clusters

Earlier we presented only a qualitative description of a semiempirical effective Hamiltonian method which was successfully applied to the molecular clusters of hydrogen‐bonded (H2O)n. Here a comprehensive analysis of this method is introduced by means of its generalization. Unlike the earlier study, emphasis is placed upon the physical properties of small clusters concerning the energy of formation and the entropy of formation, in order to find propensity rules associated with prenucleation process. It is pointed out that the predicted energy of formation of the molecular clusters at 0 K increases with size, showing slow convergence to the bulk value. Qualitative estimations of entropy changes for small clusters are made, in order to examine entropy effects on the stability of the clusters at finite temperatures. In addition, it is found that the first ionization potential rapidly converges to the work function of ice and the dipole moments of the clusters of open structures tend to linearly increase with...

Temperature and Supersaturation Dependent Nucleation Rates of Water by Molecular Cluster Model Calculations

Abstract The homogeneous nucleation process for water has been the subject of great interest, despite the fact that water droplet formation in the atmosphere occurs by heterogeneous nucleation. This is because the homogeneous nucleation process is the simplest of the nucleation processes to treat, both theoretically and experimentally, and because it is strongly dependent on, and hence can be used to illuminate, the molecular interactions and processes that are important for heterogeneous nucleation and aerosol hydration. Using a microphysical approach to nucleation, we present an extensive study of water nucleation rates for wide ranges of both temperature and supersaturation ratio. Based on the fundamental molecular properties of clusters instead of bulk properties, the microphysical approach is demonstrated to predict good agreement with measured nucleation rates over this broad range of conditions. Predicted critical sizes for nucleation are found to be relatively small, and are in the molecular clust...

Temperature and supersaturation dependent nucleation rates of heterogeneous water by molecular cluster model calculation

A statistical mechanical method to evaluate the energy of formation of water clusters attached to a foreign particle surface is described, with the binding energy being evaluated on a molecular level, using semiempirical modified neglect of diatomic overlap (MNDO) theory. The model is applied to water nucleation on a silicon oxide surface. The binding energy contribution, which represents the energy of formation at T=0 K, is found to slightly (but not negligibly in the thermal sense) increase with the number of hydrogen bonds between the water cluster and the condensation nucleus whose surface is made of silicon oxide. An analytic expression is developed to fit the binding energy contribution as a function of cluster size. At lower temperatures, a linear relationship is found between the log of the nucleation rate and reciprocal temperature for fixed saturation ratio. However, at higher temperatures, this relationship deviates from linearity. The deviation is sufficient to suggest the existence of a criti...

Comparison between approximate (perturbation) and exact (close-coupling) three-dimensional quantal methods in reactive scattering

Presently the validity of DWBA methods has been largely unchecked with respect to exact methods. In the present study, direct comparison between the DWBA and exact close‐coupling calculations is made by examining reactive scattering angular distributions of H+H2 → H2+H for the same range of collision energy as Schatz and Kuppermann chose. We have found from the DWBA study that (1) the structures of angular distributions between these two methods are remarkably similar at most collision energies; (2) the effect of coupling strongly affects the absolute magnitude of cross sections but not the structures of normalized angular distributions; and (3) the DWBA predicted state‐to‐state integrated (total) cross sections σDWBA are much smaller than the exact close‐coupling results σEXACT, due to the dominant effect of coupling which is found to rapidly increase with collision energy EK, observing the relationship of σEXACT∝E2/3K σDWBA. In addition, the cause of agreement between the approximate DWBA and exact clos...

Quantal estimates of binding energies of large silicon clusters (Si)n with n>10

Abstract We employ a computationally accessible semi-empirical effective Hamiltonian method to calculate binding energies for large silicon clusters (Si) n , for n > 10. The differences in binding energy between different geometric structures at a given cluster size generally tend to decrease with cluster size. This implies the possibility of cluster growth close to crystalline structure with Td symmetry at certain finite temperatures.

Calculation of properties of ionic water clusters by the MNDO method

Abstract Airborne ions created by cosmic rays and earth radioactivity can be subject to hydration. Hydrated ion clusters are of great atmospheric importance, e.g. in regards to atmospheric electricity and ion nucleation. Computed results for the heats of formation and the entropy changes for small ion water clusters H 3 O + (H 2 O) n are reported in comparison with observations and ab initio calculations. Despite the exclusion of hydrated ion clusters in the procedure of parameterization in our earlier modified treatment of MNDO (modified neglect of differential overlap) specifically for treating water clusters (H 2 O) n , computed results of the hydrated ion clusters are in reasonable agreement with observation and ab initio calculations.