chemical physics of discharges - Argonne National Laboratory

As expected from the previous sections, O$4?on appears as the most abundant ion in
this system. Also as expected, 04+ ion appears in the system and as the pressure is
increased, it becomes an important charge carrier and its relative abundance eventually
reaches a plateau. The appearance of the ion 02+(H20), hydrated form of molecular
oxygen ion, in this system, indicates the high affinity of this ion for hydration as
the gas contained less than 5 x 10-3 mole % water in the discharge tube.
Nitric oxide ion was the only oxide of nitrogen found in the system under the
experimental conditions used here. Its relative abundance remainedlconstant as the
discharge pressure was varied between 2 to 40 Torr. The presence of NO+ in these experiments
and its absence in experiments carried out in nitrogen containing 0.1 mole %
of oxygen, indicates that these ions are probably formed through the reaction
N ~ + o2 + NO+ + NO, AH = -4.5ev
+
Assuming that 02' arises eiti4er through charge-exchange with Ng ion or by
direct electron impact on neutral oxygen molecule, one can use the data in these experiments
to obtain a relative ratio of the rate-constants for the charge-exchange
reaction of N2+ with oxygen to that of ion-molecule reaction (8). This ratio is found
to be equal to 8, a value which is lower than the ratio of the published values of
these rate-constants (IO). This indicates that possibly other reactions such as N+ +
02 + NOf 0 and O+ + N2
NO+ + N or others involving neutral atomic species
also contribute to the total yield of NO+ ion.
CONCLUSIONS
Mass spectrometric studies of low pressure positive d.c. corona discharges in
atmospherrc gases containing trace quantities of water vapor show a complex series
of reactions with each component leading to the formation of hydrated protons in the
system. In the case of nitrogen, intermediate species N2* and H20+ are presumably
formed through ion-molecule reaction and charge-exchange of N2+ and N4+ with water
molecules. These species later form the hydrated proton through proton transfer reactions
in subsequent collisions with water molecules. In moist gaseous oxygen, it
appears that the hydrated form of the primary ion 02+(H20)2, plays an important role
in the conversion of the charge carriers to hydrated protons. It is suggested that
this transformation may occur through the formation of the intermediate (H20)2+ which
has been found in this system. In experiments where water vapor is excluded from the
system, a concentration of 1.2~ 10-1 mole % oxygen can transform, through chargeexchange
reactions, all ionic species of nitrogen to 02+ at a discharge pressure of
20 Torr, and that ion-molecule reactions leading to the formation of oxides of
nitrogen are by far less probable within the pressure range investigated.
ACKNOWLEDGMENTS
The author is indebted to Dr. W. Roth for his invaluable comnents during the
course of this work and to A. Friske for his assistance in experimental work.
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