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Fig. 1.7
Branching ratio
PI )asa
function of the particle energy
AI /( AI C
where electrons produced in the channel (Eq. 1.45 ) have the energy 14.6-15.6 eV,
and the spectrum is shifted to higher energies in the channel of Eq. 1.46 . The total
excitation energy of heavy particles in low-temperature plasma is much greater than
the kinetic energy of free electrons. Therefore, PI and AI channels (Eqs. 1.45 , 1.46 ,
1.47 , 1.48 ), called chemo-ionization, play an important role in low-temperature
plasma, and especially in cryogenic plasma and plasma without current. The
possibility of the simultaneous existence of these two channels takes place because
the total energy (kinetic C potential) conservation of the heavy particles before and
after the collision is required (Fig. 1.7 ).
The PI and AI cross sections involving the metastable helium atoms were
calculated by Neynaber et al. ( 1978 ). The resulting AI C PI total cross sec-
tion of the reaction He(2 3 S) C He(2 3 S) in the energy range E D 0.01-0.13 eV
within an experimental error ( ˙ 30%) coincides with the results of the beam
experiments (Garrison et al. 1973 ). However, the value of the branching factor
D AI =. AI C PI / D .4:6 ˙ 0:6/ 10 2 obtained in by Bellissard ( 1991 )atthe
collision energy E D 0.033 eV essentially differed from the calculated data (M uller
et al. 1991 ;Devdarianietal. 1983 )(seeFig. 1.7 ). The foregoing is clearly illustrated
in Table 1.1 .
For inert gases, a part of the spectrum regarding the reasonably fast electrons is
associated with the AI process, but for the slower electrons it mainly results from
the PI process. The spectra of plasma electron spectroscopy is averaged over the
Maxwellian distribution of particles (Fig. 1.8 )at T D 300 K.
The concentration of the metastable atoms in the active phase of the discharge
is of the order of 10 11 cm 3 here, but the electron energy resolution is 0.2 eV.
The origin of the energy is located at the maximum of the electron energy
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