Gas Phase Lab

measuring total cross sections for collisional processes

		Ion-molecule reactions play a pivotal role in determining the macroscopic properties of many environments where they are found. Examples of such environments include, but are by no means exclusive to, planetary atmospheres, interstellar media, and electrical discharges. A somewhat more pragmatic environment in which ion-molecule reactions are important involves electrical discharges which are currently being employed in ion implantation devices, semiconductor etching and microcircuit fabrication processes, chemical vapor deposition of thin films, and in air purification devices used in enclosed environments with recycled atmospheres such as space shuttle and submarines.
In electrical discharges, ion-molecule reactions affect the nature of the discharge by changing the translational energy, mobility, and electron-ion recombination of the charge carriers. In addition, ion-molecule reactions are a significant source of secondary neutral and ionic products. The primary motivation behind the ion-molecule studies conducted in this laboratory is predicated on the need to predict the macroscopic properties, such as temperature and reactive species concentrations, of a discharge based on ongoing reactions in the discharge. Such characterization of these plasmas could facilitate the modeling and design of discharges specific to certain tasks, a goal which if realized will be extremely valuable to the semiconductor etching and chemical vapor deposition industries. If modeling of various molecular discharges and extraterrestrial gas-phase phenomena is to be successful, a database of physical properties describing formation and transport of various ionic and neutral species contained within these discharges is clearly required. We have recently expanded that database by determining measured cross sections for ion-molecule reactions pertinent to hydrogen [1,2], methane [3] and Freon [4] discharges. Specifically, cross sections resulting from collisions of positive ions and various neutral targets have been measured as a function of energy, for collision energies relevant to these discharges.

Most recent experiments focused on measuring total cross sections for ion molecule reaction SF₆^- + N₂. Sulfur hexafluoride (SF₆) is widely used as a gaseous dielectric in high-voltage applications due to its extremely large cross section for electron attachment. It is also recognized as a greenhouse gas and it has been suggested that a mixture of SF₆ and N₂ might serve as a substitute for pure SF₆ for certain applications which require gaseous dielectrics.

     Collisions of SF₆^- with N₂ are obviously involved in such mixtures when used as gaseous insulator. Measured cross sections for electron detachment and collision-induced dissociation (CID) of SF₆^- can, in part, characterize the arc-quenching properties of the mixture.
     Sulfur hexafluoride is also of great interest to the researchers due to the fact that ions, such as SF₅⁺, formed in its discharges are known to be very effective components of etching plasmas. Therefore, to model and optimize the properties of such plasmas cross sections for ion-molecule collisions involving these ions and certain neutrals need to be measured. This work is planned for the nearest future.

     Currently, in cooperation with Johns Hopkins University Applied Physics Laboratory we participate in development of a miniature plasma spectrometer for NASA. Our role is to evaluate and test its prototype using our versatile ion-beam apparatus.

     To learn about graduate research opportunities at AMO physics labs, please contact Dr. Champion or visit our laboratories in rooms 321 and 313 of Small Hall (Physics Department).

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[1] B.L. Peko, R.L Champion, Y. Wang, J. Chem. Phys. 104 (1996) 6149.

[2] B.L. Peko, R.L. Champion, J. Chem. Phys. 107 (1997) 1156.

[3] B.L. Peko, I.V. Dyakov, R.L. Champion, J. Chem. Phys. (109) (1998) 5269.

[4] B.L. Peko, I.V. Dyakov, R.L Champion, M.V.V.S. Rao, J.K. Olthoff, Phys. Rev. E 60 (1999)
7449.

Front page - Introduction - Surface - Gas phase - Physics Dept.