Date of Award

8-2002

Level of Access

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor

François G. Amar

Second Committee Member

Jayendran C. Rasaiah

Third Committee Member

Brian G. Frederick

Abstract

The nitrogen-argon system provides a useful paradigm for studying segregation and mixing behavior in heterogeneous clusters. Using three realistic pair potentials corresponding to argon-argon, nitrogen-nitrogen and argon-nitrogen interactions, the structures and thermodynamics of Arm(N2)n, clusters are investigated. Comparison of these results to previous simulation results on neat argon and nitrogen clusters is made. The initial focus of the study is the series of thirteen particle clusters of Arm(N2)13-m (0 ≤ m ≤ 13); molecular dynamics in the microcanonical ensemble is the principle simulation tool. The icosahedral nitrogen-argon clusters display systematic changes in energetics when argon is substituted by nitrogen in the central position. Comparisons between argon centered and nitrogen centered clusters are made. The consequences of these observations for cluster stability and for dynamical behavior, such as melting and evaporation, are investigated. These results are interpreted in terms of frustration effects due to anisotropy in the N2-N2 and N2-Ar potentials. Study of larger clusters reveals that the argon core clusters are more stable than nitrogen core clusters showing the preference of argon atoms to be in the middle regardless of the size. The relative stability of argon centered clusters over nitrogen centered clusters is further investigated by defining and calculating a "species-centric" order parameter, which can be monitored during a MD simulation. This study aids in the interpretation of experimental electron diffraction diagrams of Torchet and coworkers by comparing the experimental diagrams with our predicted electron diffraction patterns for mixed clusters of different sizes and stoichiometries. By calibrating our patterns against known work on neat argon and nitrogen clusters, we are able to predict the average cluster size and stoichiometry (Ar/N2 ratio) as a function of experimental conditions (total nozzle pressure and Ar partial pressure). While the thirteen-mers mentioned above provide clues as to the structural motif to be observed in the electron diffraction diagrams, our results indicate that even at the lowest pressures studied experimentally, the observed clusters have more than 13 particles. Diffraction patterns for polyicosahedral clusters containing between 15 and 67 particles are compared with experimental diagrams and representative clusters for lo%, 11.2%, 20% and 30% argon molar fractions at 10 bar are given. At higher argon mole fractions, larger clusters (above 130 particles) with a multi-icosahedral shell structure provide the best fit to the experimental patterns.

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