chemical physics of discharges - Argonne National Laboratory

44
whose rate constant, 1.1 x 10- cm'/sec, from flowing afterglow experiments2 agrees
with an earlier value 9 x lo-' cm3/sec measured in a mass spectrometer ion SOUpCe
by Franklin and Munson"; the reaction
+ +
0 + co2 - o2 + co (12)
with a rate constant of 1.2 x lo-' cm3/sec from both flowing afterglow' and mass
spectrometer ion source measurements7 ; and
C+ + co2 - co+ + co I (13)
with a rate constant 1.9 x lo-' cm3/sec.
Such reactions are more often fast than slow. One of the slowest exothermic
ion-molecule reactions (again barring cases where charge-transfer comptes) is
o+ + N2 -t NO+ + N, (14)
with a rate constant - m3/sec3. Elis rate constant increases3 to about
3 x cm3/sec for an N2 vibrational temperature of 5000' K and also increases
with 0' kinetic energy"'.
No case of a fas? ion-atom interchange reaction involving the breaking of two
bonds has so far been reported. The exothermic reaction
02+ + N2 +
has a rate constant less than 1O-l' cm3/sec, for example.
NO+ + NO (15)
IV. Associative Detachment Reactions
A number of associative detachment reactions have been recently measured to be
fast in the flowing afterglow system (i.e. k > lo-' an3/ see), including:
and
0- + 0 + O2 + e ~
OH- + o -t H O ~ + e
0- + H2 - H20 + e
(16)
H- + H - H2 + e (17)
(18)
(20)
OH- + H - H20 + e
0- + co - C O + ~ e
Phelps and Moruzzil' have been making similar measurements in drift tube expriments
at Westinghouse and have measured reactions, (20), (21), and (22). The flaring
afterglow and drift tube results agree within better than a factor of two in each
case. Several exothermic associative detachment reactions do not occur at measurable
rates (k < 10-l' cm3/sec) , an example being
0- + N2 - N20 + e + 0.2 eV. (23) :
V. Ion-Molecule Reactions in Discharges
he obvious application of measured rate constants to the qualitative interpretation
of the ion composition of a gas discharge is the case of the COZ discharge ion
composition studied by barnon and Ticher''. Dawson and Tickner observed the d d -
nant ions in a glow discharge in COZ to be Oa+ and COZ+. This is quite reasonable in 1
xLew of the known occurrence of reactions (ll), (12) , and (13) above, together with
the fast charge-transfer of CO+ vith C& to produce COZ'.
The results of these
reactions are graphically illustrated in Fig. 2 (from Fehsenfeld, et al., J. Chem.
Rys. 5 23 (1966)), which shars that dl of the ions, C+, O+, and CO+, do convert
to 02+ and Cb+ by reaction with COa in the - 6 milliseconds reaction tbne in the
PlOWhg afterglow. If molecular oxygen were added, one would expect the dominant
ion to become Oa+ alone, since CoZ+ is horn to charge-transfer rapidly with &la.
(19)
(21)
,' '
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