chemical physics of discharges - Argonne National Laboratory

-
+
( ~ ~ + 0 H ) ~ O ~ ( H~o)~H+ + OH
(H~o)~+ + o2 (H,o)H+ + OH + o2 (7)
Reaction 5 is expected to be exothermic owing to the heat of hydration of H20+ ion.
It may also be possible, though unlikely, that any required energy fqr the reaction
be supplied through the electric field in the discharge.
One other ionic species which may play a role in this system is 0 . However,
since the I.P. of atomic oxygen (13.61ev) is greater than that of oxygen, this species
is not observed in this system as it most probably undergoes charge exchange with
molecular oxygen at pressures used in these experiments. The reaction of OC with O2
to give rise to 0 , if it occurs at all, is not observed in these experiments since
ozone has also a ?:gher I.P. (12.8ev) than that of oxygen and therefore 03+ i s similarly
expected to undergo a charge-exchange reaction with 02.
Three other ions were observed in minor relative abundance in this system. These
ions are OH+, H20+ and H202+. The relative yield of the O p ion is shown in Fig. 3
while the yields of H20+ and H202+, being somewhat smaller, are not indicated. It is
believed that the first two of these ions arise from an ion-molecule reaction and a
charge-exchange process between O+ and water molecule respectively, and are detected
only at higher discharge pressures where such reactions compete with direct charge
transfer of O+ to oxygen molecules. H202+ ion, however, may appear as a result of an
efficient charge transfer process between 02+ ion and the trace quantities .of H202,
apparently formed in the system by the recombination of hydroxyl free radicals, or the
reaction, H02 + H2 H202 + H, hydrogen being supplied through the decomposition
of water.
In a system of pure oxygen, where the waier content was kept below 5 x mole %,
only two ions, 02' and O4+ were observed within the pressure range investigated. The
abundance of O4+ ion rises sharply as the pressure is increased while that of 02+ decreases
and both reach a plateau beyond a pressure of 20 Torr in the discharge tube.
The relative abundance of O4+ in this plateau region was found to be about 62%.
Discharge in Nitrogen Containing 0.12 mole % Oxygen:
In order to determine the role of oxygen in air discharges in the absence of
water vapor, a number of experiments were carried out in nitrogen containin 0.12 mole %
of oxygen. The lower limit of water vapor in these experiments was 5 x IO' 3 mole %.
Fig. 4 shows the results of the relative abundance of various ionic species which are
detected at the cathode as the pressure of the discharge is increased. These experiments
clearly trace the history of the primary ionic species, namely N2" as it is
converted to N4+ and the latter undergoes charge-exchange with the trace quantity of
oxygen present in this system. Thus, beyond a pressure of 20 Torr in the discharge
tube the charge carriers in this system are almost all 02+. The trace amount of N3+
observed in the system is also all removed beyond a pressure of 15 Torr.
It is significant to note that no nitric oxide ion is observed in this system,
indicating that neither the reaction N4+ + O2 d NO+ + NO + N2, nor 0 + + N2
NO+ + NO, occur to any appreciable extent, in spite of the fact that bott these re-
actions are expected to be exothermic.
Experiments in which water was not excluded showed the appearance of many ionic
intermediates of both pure oxygen and nitrogen and their mixtures. All these ions
could be traced and their final conversion to hydrated protons followed.
Discharge in Air:
Experiments in air in the absence of water vapor (less than 5 x mole %)
were carried out in order to determine the importance of ionic species of oxides Of
nitrogen in these systems. The results of these experiments are shown in Figure 5.
+