Georgia Tech | EPICS Lab

Quantum-Resolved Photodesorption and Photodissociation Studies of Halogenated Compounds (CH₃I, HI, MI) on Ice Surfaces and Interfaces

Although the general importance of surface enhanced processes on Polar Stratospheric Clouds have been implicated as a major source of sustaining ozone depletion in the stratosphere, the fundamental physics and chemistry generating the dissolution, uptake and photochemical transformation in the clean troposphere is not completely understood. In EPICS LAB we have embarked on a series of experiments designed to provide fundamental and quantitative information concerning the chemistry and physics involved in the reactions and photochemistry of acid-ice systems, halogenated hydrocarbon-ice systems and sea salt with photons within the actinic flux to simulate the atmospheric conditions.

Several mechanisms have been proposed for the reaction of halogenated compounds (halogenated hydrocarbons, acids and sea salts) on ice surfaces during tropospheric ozone depletion episodes at polar sunrise, yet the description of the initial reactions that lead to the formation of reactive halogen species remain unknown. It is proposed that the surface enhanced catalytic effect of ice (snow) during these periods could be the long sought link in understanding the mechanism involved in the initial release of these species. To develop a better understanding and improve modeling of ozone depletion episodes at polar sunrise it is important to understand the photochemistry of heterogeneous reactions on ice. Specifically, in EPICS lab we investigate the possible mechanism for the direct release of reactive halogen species involved in ozone depletion episodes at polar sunrise. The research is designed to provide quantitative information concerning non-thermal heterogeneous surface reactions of halogenated hydrocarbons, acid, and sea salt on ice at wavelengths within the actinic flux. Methyl iodide (CH₃I) and hydroiodic acid (HI) adsorbed on ice and sodium iodide (NaI) co-adsorbed with ice is investigated as models for these systems. A UHV chamber developed with the capabilities for performing quantum resolved laser detection of neutrals using resonance enhanced multiphoton ionization (REMPI), photon stimulated desorption (PSD) and temperature programmed desorption (TPD) is employed. The UHV chamber as shown is equipped with rotatable, liquid nitrogen cooled zirconia substrate and a dosing system consisting of two leak valves coupled to a differentially pumped dosing manifold. It is also equipped with a linear time of flight mass spectrometer (ToF-MS) for neutral particle detection by using resonance enhanced multiphoton ionization (REMPI) and a quadrupole mass spectrometer (QMS) to analyze desorbing species during temperature programmed desorption (TPD) analysis. The chamber operates at a base pressure of 5.0×10^-10 Torr.