chemical physics of discharges - Argonne National Laboratory

21
region in w!iich large, nighlv localized surface 10s; terns could qulcLlv reduce tile
atom concentration.
Experimentally, 1ary.e catalytic effects by :;?, SO, or li2 in tile production of
0-atoms in microwave ciisciiargcs ~iave ~cerl reported.22 Very ptire oxygen qave only u.6~
atoms (still ~ O W ~ yields K oi 0.3';; WCKC latcr obtained), !>ut small adtiitions (U.01 to
0.u5;:) of Sz, >;20, or !;o produced +atoms at tile rate of 4u to 45 per added ;:, and
similar additions of 112 produced 160 to 200 +atoms per added 112. Ih terms of the
!'resent interpretation, tile large catalytic cffrct may he understandable for ti2 additions
as due to t!20 wall effects, !,ut less so for nitrogen compounds which sliould not
be StKOIlGlY adsorlied at tile surface. (:onceivably, XO+ ir ::02+, stron): Lewis acids, my
be involved in poisoiiiny, the surface. Thus, our understanding of 02 discharzes is still
in an unsatisf actory state. Furthcr experiinents are required in wliich particular
attention should he given to tile condition and cliaracterization of the surface as vel1
as to the imnedinte downstream rccion.
111. 4. SitroRcn Disc!iarges.
The great complexity of "active nitro1:en" is probably due to its larger cross
sections for vibrational excitation anci to tile existcncc of metastable electronically
excited states helot) tlie dissociation l i m i t of ground-state 1iz. Consequently, extensive
vibrational excitation persists for times much longer than those spent in tlie discharge
zone, and chemiionizatioll is observed in regions such as tlie "pink glow" well downstream
of the discharge. The absence of the lowest triplet state, A,3Z:, in active nitrogeng3
contaiiiing '.';-atoms indicates that tliesc excited molecules are very efficiently quenched
by S, and that vibrationally liiglily excited ground-s tate molecules arc the principal
carriers of excitation to tlie dowiistreai~! reqioii. Engelliardt, Phelps, and Riskz4 have
determined tlie relevant elastic arid inelastic electron collision cross sections. Some
of the electronically excited states above tne dissociation limit do not lead to pre-
dissociation, and thcrefore only the state with tlireshold energy of 14V was used in
tile estimate of dissociation. Assuming an average electron energy. = 3 ev, and a
maxwellian distribution, one obtains an effective dissociation rate constant, kd, of
3 x lo-'' (bo torr/sec) and a correspondinz ionization rate constant, ki, of 6 x lU-l'.
'The latter is larzer (b torr/sec) than the corresponding ambipolar diffusion loss term
(13.5 to 1 torr/sec). lilt more realistic calculation by Dr. Phelps which simultaneously
fits ck, L/:i, and tlie known cross sections to make the ambipolar diffusion loss equalthe
rate of ionization gave ck = 2.2 ev, E/?: = 1.2 x cm2/molccule. kd = 3 x 10 "
(6 torr/sec), and ki = 3 x This dissociation rate is very much lower than that
of 1iZ or O2 and properly reflects the tiifficulty of producing extensive dissociation of
Hz in glow discharxes. ?io other source terms of comparable magnitude are available.
The principal loss processes include atom recombination at the surface which can be set
equal to those of oxygen, because the molecul K velocities are similar. The catalytic
a atom loss nechanicm by E;,++ + X + i