AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Calculations and Measurements of the Collision Cross Sections of Sub-2.0 nm Metal Iodide Clusters in Air
HUI OUYANG, Carlos Larriba-Andaluz, Derek Oberreit, Christopher Hogan Jr., University of Minnesota
Abstract Number: 53 Working Group: Aerosol Physics
Abstract In the free molecular regime, electrical mobility can be related to the collision cross section (CCS) of charged particles and ions. To distinguish possible particle candidate structures from one another, calculations of CCS with proper gas molecule scattering model are required for comparison with measurements. The elastic hard sphere scattering (EHSS) model is widely used in CCSs calculations for comparison to CCS measurements made in Helium background gas, while the gas molecule scattering law in polyatomic gas background remains under controversy. In this work, a diffusive hard sphere scattering (DHSS) model is proposed for CCS measurements in polyatomic gas, which, when applied to a spherical particles, leads to results in excellent agreement with the free molecular limit of the semi-empirical Stokes-Millikan equation. To examine this scattering law, the electrical mobility in air of singly and doubly charged Sodium, Potassium, Rubidium, and Cesium Iodide cluster ions (positively charged) are measured via a high resolution parallel-plate differential mobility analyzer coupled to a time of flight mass spectrometer (DMA-MS). Candidate structures corresponding to the measured ions are generated through density functional theory (DFT) calculations. CCSs calculations for these candidate structures are performed with both EHSS and DHSS models accounting for the influence of gas molecule polarization, and compared to the measured CCSs. In general, DHSS predictions are found in better agreement with measurements than are EHSS predictions, though the measurement agreement with DHSS predictions and disagreement with EHSS predictions both decrease with increasing cation mass in cluster ions, which suggests neither EHSS and DHSS can fully describe the scattering rule for all four salt ion clusters. Further, the influence of polarization in air is found to have a non-negligible effect on CCSs of the examined cluster ions.