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Cross sections for dissociative attachment range from
values (for electronegative gases) that are several orders of magnitude
larger than cross sections for positive ion formation, to values (for SiF4
and CF4) that are several orders of magnitude smaller than
cross sections for positive ion formation. Even when the dissociative
attachment cross sections are small, these processes are important for
full characterization of the plasma. The negative ions formed may
influence the free-electron density and may also participate in nucleation
of parasitic dust. Although there have been few measurements, recent
exceptions used Fourier transform mass spectrometry techniques and beam
techniques to determine dissociative attachment cross sections for SiF4
and CF4 and SiH4. Some experimental effort has
been devoted to the study of attachment to vibrationally excited
molecules.
There exist virtually no data on electron attachment to
radicals, although such species are produced in large numbers in plasmas.
Only a few research groups are engaged in these types of measurements. |