837
J. Phys. Chem. 1994, 98, 837-842
Reactions of 02(a1A,) with 0- and 0 2 B. L. Upschulte, W. J. Marinelli, and B. D. Green* Physical Sciences Inc., 20 New England Business Center, Andover, Massachusetts 01810 Received: June 9, 1993"
+
We have measured the rate coefficients for the processes 0- + 02(a1Ag) and 0 2 - 02(a1A,) with improved and calibrated optical diagnostics and additional diagnosis of neutral metastable species. These measurements are an order of magnitude smaller than earlier results, and the uncertainty in the rate coefficients has been reduced from a factor of 1 0 to less than +loo%/-50%. A fast reaction of the negative ions is observed with an unknown species arising in the discharge source of 02(a1A,). The analysis suggests 0 2 (X,a states with vibrational levels > 1) could be the neutral reactant partner. Measurement of the 0- C02 rate is also reported.
+
Introduction The lowest-lying singlet state of molecular oxygen, 02(a1A,), is prevalent in both the quiescent' and aurorally e ~ c i t e dlower ~,~ ionosphere. This excited state is formed by a combination of photodissociation of 0 3 and direct electron excitation. Since transitions to the ground state 02(X32) are spin-forbidden,this metastable 02(a1A,) is capable of surviving for long periods, especially at night, due to its low radiative rate? 2.83 X l e 3s-l, and its apparent inertness to reactions with the dominant ionospheric species N2 (k 5 1t20cm3s-'),~ 0 2 (k = 2.2 X 10-18(T/300)0.78 cm3s-'),~ and 0 (k = 1.3 X 1 C 1 6cm3s-l).' During sunlit periods, Oz(aIA,) becomes photoionized by solar UV radiation to generate 0 2 + . Auroral observations have suggested anomalously large populations of both 02(alA,)233 and vibrationally excited 03,S and thus it was inevitable that a mechanistic link would be suggested. The ion-molecule reaction
ko
0-+ 02(a1Ag) e-
+ 03*(v)
(1)
appears to be a likely candidate for this link mechanism, owing to the large increase in 0- production from secondary electron dissociative attachment to 0 2 and the simultaneous production increases in 02(alA,) by secondary electron inelastic collisional excitation of 0 2 that occur during auroral precipitation. Although production of 03* was not verified, the rate of this ion-molecule reaction was measured by Fehsenfeld et al. in 1969.9 The quoted rate coefficient, 3 x cm3 s-l, was accompanied by a factor of 10 uncertainty. This uncertainty does not allow the relative importance of this reaction to be determined in modeling of the disturbed upper atmosphere. The flowing afterglow technique used to measure this rate was well-established in 1969 and typically allowed a measurement precision of *SO% for an ion-molecule reaction. The large uncertainty ascribed to this measurement arises in the determination of the 02(a1Ag)density in the flow tube, which relies upon an absolute radiometric measurement of the 02(a-X;O-O) emission at 1.27 pm. The calibration of this measurement is difficult as it requires access to standard radiometriclight sources, careful evaluation of the field of view of the optical detection system, and knowledge of the quenching rate of 02(a1A,) with different wall materials. The primary goal of this effort was to remeasure this rate coefficient with a factor of 2 or less uncertainty. This was made possible due to advances in semiconductor IR detector technology and improved knowledge regarding the formation and destructionlo of Oz(aIA,). The determination of this rate will establish e Abstract
published in Advance ACS Abstracts, December 15, 1993.
0022-3654/94/2098-0837$04.50/0
the importance of this process in a variety of plasma processes and in ionospheric physics. For comparison to the previous results, we also chose to measure the rate coefficient for the reaction
ko,
0; + 02(a1Ag)
products
(2)
In this paper we present the experimental technique, theanalysis utilized to evaluate the rate coefficient, the results of our measurements, and an error analysis of the uncertainty in the measured rate coefficient. Because the rate was determined to be slow, product determination was not possible.
Experimental Technique These experiments were performed in a flow tube system similar to that used previously? The was produced by an ion source upstream of the neutral injection port. These ions were transported down the tube by a large flow of inert buffer gas, Le., helium, and thermalized with this buffer gas. Upon reaching the neutral injection port, the ions begin to react with the O2(a) that has been injected and continue to react as they flow toward the end of the flow tube. The ion density is sampled at the end of the flow tube with a differentially pumped ion mass spectrometer via a 1-mm orifice. The reaction time of the ions is established by the ion flow time between the neutral injection port and the flow tube sampling orifice, typically 5-20 ms. The 0- signal in the mass spectrometer is recorded as the O2(a) injection density is varied. The density of 02(a) in the flow tube is much larger than the ion density and can thus be assumed a constant along thereaction distance/reaction time scale. This pseudo-first-order kinetic assumption allows the rate coefficient to be measured from theslopeof a plot of theln(Osigna1) versus theconcentration of 02(a) injected into the flow tube. Five major subcomponents comprise the flow tube device: the ion source, the metastable discharge source, the ion mass spectrometer/detector/recording system, the Roots pump, and the flow tube. The overall apparatus schematicis shown in Figure 1. The flow tube itself was constructed of modular stainless steel L-400flange sections with a nominal tube 0.d. of 4 in. and are 8 in. long. These modular sections were equipped with a single 2-in. quick flange port located 5 in. from the end. These sections are mounted/vacuum-sealed together with centering O-ring assemblies and double-claw clamps. The ion source attaches to the main flow tube body via a K200 to L400 adapter. The last downstream section of the flow tube mounts to and through the wall of a special Del-Seal four-way cross. Inside this cross the flow connects to the Roots pump-out line and intercepts the ion sampling orifice. The four-way cross is pumped by a Varian VHS-6 diffusion pump equipped with a LN2 trap to prevent oil contamination of the mass spectrometer or flow tube. The 0 1994 American Chemical Society
Upschulte et al.
838 The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
Microwave
TO PressureJ Transducer
I
Spectre p t e r
ITurboI Pump
Figure 1. System schematic.
diffusion pump section acts as the first stage of the differentially pumped mass spectrometer and exhausts the gas load that flows through the 1-mm flow tube sampling orifice. For a nominal flow tube pressureof 0.7 Torr, this differential chamber pressure rises from a base value of 1 X 1 V to 1.6 X 1 V Torr. The second differential pump is a Balzer TMP-50 turbomolecularpump and maintains the electron multiplier of the ion mass spectrometer at pressures below 2 X Torr. The Roots pump exhausts the bulk of the buffer gas and reaction gases. During this effort we also utilized a neutral mass spectrometer and a high-voltage pulsed power supply (not shown in the figure) to establish relative dischargeeffluent concentrationsand ion flow times, respectively. Ion formation can be accomplished by several different processes. For 0- generation, we chose to inject trace amounts of COz past an electron-emittingfilament to dissociatively attach electrons: e-+ CO,
0-+CO
(3) The electron energy and attachment cross sections have been measured previously.1' This process was preferred to dissociative electron attachment to 0 2 because it produces exclusively 0from the parent COz and from the electron impact on the dissociation fragment CO. The COS-signal was not detectable, indicatingvery little excess CO2.l2 ResidualCO or CO2 surviving the ion source does not interfere with the kinetic measurements because of their low density and due to the discharge on versus discharge off ratio technique utilized in the experiments and analysis. For 0 2 - generation we added 10-20 mTorr of 0 2 about 10 cm downstream of the electron-emitting filaments to attach low-energy electrons via the three-body mechanism.' The 02(a1Ag)is generated in a microwave discharge side arm constructed of 0.5-in.-diameter Pyrex glass. The discharge side arm terminates into a Woods horn just after the gases turn a right angle and expand into a 2-in. quick flangecross. The interior of this cross is Teflon-coated to minimize O2(a) deactivation. In addition, this cross is equipped with opposing Pyrex windows for optical analysis of the discharge species. A Teflon-coated'/4-in. stainless steel finger type injector allows the discharge effluent to be introduced into the flow tube. Discharge power is normally held at 10 W and the 0 2 flow increased to generate more +
metastables. At the maximumachievable0 2 flow rate, 100pmol s-I, metastable production is increased by raising the discharge power. As power is increased to 20 W, the 02(a) density reaches a maximum and then begins a slight decline. A gas handling manifold provides a 60-70 pmol s-1 flow of helium in combinationwith the 1-100 pmol s-l of 0 2 to generate a discharge tube pressure between 3 and 6 Torr. The pressure is monitored with a MKS baratron capacitance manometer. Gas flows are monitored with calibrated rotameters at a fixed input pressure. The gas handling manifold can permit the gases to pass through a section with a reservoir of mercury. Discharging this flow with mercury vapor deposits a mercuric oxide coating on the discharge side arm. This coating is known to efficiently react13 with the atomic oxygen produced in the discharge. In similar discharge geometries,the atomicoxygen density is reduced by many orders of m a g n i t ~ d e ' due ~ J ~to reaction on the mercuric oxide coating. A single 15-min discharge period with Hg vapor is sufficient to build up a coating that is active for tens of hours of further discharge time. The Oz(alAg)density was determined from its IR atmospheric emission at 1.27 pm. The optical arrangement and the calibration arrangement are shown in Figure 2. Light emission from the discharge cross was chopped at a frequency of about 1 kHz. An intrinsic germanium detector, cooled with liquid nitrogen and equipped with a 1.27-pm narrow band filter (Figure 3), and a visible blocking/IR transmittingfilter viewed a cylindrical volume through the discharge cross. This volume was defined by fiiing the detector at the focal length of a 1-in.-diameter F/1 lens, in conjunction with a 3/4-in. field stop at the lens and another stop at the end of a light baffle between the detector and the chopper. The lens and field stops are mounted and T i e d on the detector housing to guarantee alignment of the optical system. Signals from thedetector were measured with a lock-in amplifier. Signal levels ranged between a few millivolts and 2 V. The detector calibration was performed using a blackbody sourcepositioned opposite the windowed cross and in the far field of the detector/lens/field stop optical system. Several data points at different blackbody temperatures and apertures at a fmcd distance were coupled with the viewing geometry and filter transmission curves to generate the calibration curve. Because
Reactions of 02(a1Ag)with 0-and
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 839
02-
Pyrex-TFE Coated 2 in. QF-Cross
From Discharge Tube I
I
7
1 1
Window
Window
11111111
Field of View
--
.
Baffle and Lens Mount Chopper
SS-Tube
ND Filter (for calibration only)
T Black Glass
Filter 1.27 pm Filter
To Flow Tube Finger Injector Figure 2. Schematic of the optical system.
The solution for this complex differential equation is
0 1250
1280
1270 1280 Wavelength (nm)
1290
1300
Figure 3. Narrow band filter transmission curve.
of the long residence times in the discharge cross and the inert Teflon coating, we have assumed the O2(a) uniformly fills the volume of the cross, Le., the viewed cylindrical volume. The ion detection system is composedof an ion samplingorifice at the end of the flow tube, a quadrupole mass spectrometer (QMS) equipped with entrance ion optics, and a 90' off-axis secondaryelectron multiplier (SEM). A high-voltage dc blocking capacitor circuit, a pulse counting amplifier/discriminator, and a multichannel scaling (MCS) counter and recorded were used to acquire signals from the mass spectrometer. Ion signals are collected by the MCS as the mass spectrometer sweeps through a mass/charge ( m / e )range. The mass peaks and the m / e values were calibrated by identifying the major features, fragments, and isotope ratios of the parent gas utilized in the electron attachment process, i.e., SF;, F-, 0-,CO3-, etc. The QMS was operated with sufficient resolution to obtain full base line (>95% reduction) between peaks separated by 1 amu. For kinetic measurements, mass sweeps from 10 to 63 amu at 10 ms/amu were accumulated in the MCS. Typically 5Cj100 sweeps were co-added. Background levels arising from free electrons,electron detachment, and noise in the counting electronics were typically 102-104 Hz. Typical unattenuated primary ion peaks possessed count rates of 105-107 Hz. The background noise level is subtracted from the peak integrals automatically by the MCS counter. Data Annlysis The determination of rate coefficients using flow tube techniques was first demonstrated by E. Ferguson in the N O M Laboratories in the early 196Os.l6 The data analysis including radial, axial diffusion corrections, corrections for wall slip, and ion flow velocity has been described in detail previously17J*and will not be repeated here.
where Z is the total reaction distance, C, are constants related to diffusion effects, k is the rate coefficient to be determined, [0-]and [Oz(a)] are the densities of reactant ionsandmetastables respectively, and Vion is the ion flow velocity. The rate coefficient is determined by plotting the natural log of the ion signal (Zo-a[O-])versus the injected density of the reactant neutral [Oz(a)]. The rate coefficient is determined from the slope of the plot:
k = -slope.~o,/(C,Z)
(5) The slope is determinedfrom a least-squaresfit to the experimental data points. In our experimentswe measure the exact ion reaction time, v,,,,/Z,using pulsed grids at the position of the neutral gas injector. Time for the ions to reach the QMS is corrected for transit through the QMS. In practice,our experimentswere performed slightlydifferently, but the analysis is nearly identical. We acquire sequential Ointegrals with the microwave discharge on and then off and ratio the signals. This ratio represents the fractional attenuation of the ion signal due solely to the discharge effluents. This ratio is plotted for each 02(a1Ag)injection density, and the least-squares analysis is performed on these plotted ratios. In addition, this procedure minimizes any drift in the ion source or flow tube conditions. The gas flows, temperatures, pressures, and ion production are held stable during the sequence of data acquisition runs that takes approximately 6 min. Critical to the analysis is the determination of the density of 02(a1A,) injected into the flow tube. The injected density is calculated in the following fashion,
where flow(O,(a)) = DT
and the subscripts DT and FT designate the discharge tube side arm and flow tube, respectively. The [ O ~ ( ~ ' A ~ )is] Dmeasured T with the Ge detector/optical system, pressures with a MKS baratron, and flows with a combination of rotameters and electronicflow meters. The pseudo-first-order kinetic assumption
Upschulte et al.
840 The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
I
\ \
\
\ \ \
10-3
t
I
0
'4 I
I
I
I
I
2 4 6 8 10 12 Concentration of 02 (a'$) (1012 moleculescm-3)
Figure 6. Plot of ion attenuation for the reaction 0 2 e- 202.
+
Concentration of CO (x 1011 molecules cm-3) Figure 4. Plot of ion attenuation for the calibration reaction 0- + CO e- C02*(v).
-
+
1
t
2 4 6 8 10 12 Concentration of 02 (al%) (1012moleculesc m 3 )
+ 0 2 (aIAg)
Figure 5. Plot of ion attenuation for the reaction 0-
+ 03.
-
e-
requires this density to remain constant along the reaction distance of the flow tube. The ion density is sufficiently small to not deplete the O2(a) density. We passivate the stainless steel flow tube walls to minimize their Oz(a) deactivation by operating the discharge tube at its maximum production for 30 min prior to beginning the data acquisition. Based on rough diffusion calculations, an Oz(a) molecule will experience between 2 and 10 wall collisions in flow time down the flow tube. The deactivation probability of Oz(a) is not known on stainless steel but is on the order of le3on a clean nickel1°J9 surface. Deactivation on the flow tube walls is thought to be small and will be ignored.
Results and Discussion To test the apparatusand identify any systematic experimental errors, we validated the experimental procedures by measuring
-
TABLE 1: Kinetics and Energetics of 0 2 Discharge Effluent Reactions with 0- and 0 2 - a AHo = +1.02 eV 0- + 02(X,u=O) - < 1 0 - " F " / a 0 + 02AHo = -0.05 eV 0-+ 02(X,u=5) 0 + 0 2 0- + 02(X,u=O) e- + 03 AHo = +0.36 eV 0- + 02(X,u=2) e- + 03 AHo = -0.13 eV 0- + Oz(alA,,u=O) -e- + 0 3 AH" = -0.62 eV 0- + 02(a1A,,u=O) 0 + 0 2 AH" = +0.05 eV 0- + 02(a1Ag,u=l) 0 + 0 2 AH" = -0.23 eV AH" = 4 - 6 0 eV 0-+ 02(blx+) 0 + 0 2 AH" = -1.27 eV 0-+ 02(b'Cg) e- + 03 o-+ o3-8 Xe 1 ~ o c m 3 / 0 s + 03AH" = -0.64 eV 0 3 0 2 +02AHo = -2.96 eV 0-+ 0 -1.9 x 10-10 m 3 / s e- + o2 AH" = -3.62 eV 0 2 - + 0 ~ ( X , u = 0 ) 0 2 + 0 2 + eAEP = +0.4 eV 0 2 - + 02(X,u=2) 0 2 + 0 2 + eAH" = -0.04 eV 0 2 - + Oz(alA,,u=O) 02 + 02 + eAHo = -0.54 eV AH" = -1.19 eV 0 2 - + 02(b'xl) 0 2 + 0 2 + eAH" = -2.06 eV 02-+ O3-6 X cm'/s 03- + 0 2 02-+ 0 -1.5 X I W " ' / s 0-+ O2 AH" = -1.02 eV 02-+ 0 -1.5 x 10-'0cm/s e- + o3 AH" = -0.66 eV 'Electron affinities for 0, 02, and 0 3 from refs 27, 28, and 29, respectively. Term energies were calculated from spectroscopicdata in ref 30. Ozone bond energy is from ref 31.
----
4
0
+ 0 2 (aIAg)
---
a rate constant that has been well-determined previously. The reaction
0-+ CO
-+ e-
C02*(v)
(8)
is well-documented in the literature and has been measured with a variety of techniques including flowing afterglow,20flow drift tube,21*22 drift ion beam,24J5and tandem mass spectrometerZ6systems. The rate coefficients for the thermal reaction fall in the range 5.5 X 1O-Ioto 7.3 X 1Wocm3s-I. The largest value stands alone and appears very high in the grouping of measurements. Several of the researchers measuring the 5.5 X rate coefficients have revised their values as 10-15% larger21s22due to recent improvements in the analysis techniques. Our data are presented in Figure 4 and span more than 3 decades of ion attenuation. Individual data points exhibit some scatter, but the slope is determined to within a few percent by the least-squares analysis. The rate coefficient calculated from the slopeis 6.4 X l@l0cm3s-I. This excellent agreement between our current measurement and the previous measurements demonstrates the reliabiiity and accuracy of our technique and analysis. Experiments were performed to measure the reaction rate of 0- and 0 2 - with 02(a1A,). The data are presented in Figures 5
Reactions of 02(a1Ag)with 0- and
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 841
02-
TABLE 2: Rate Coefficient Compnrisoas and Recommendations from Average of Current Experimental Measurements
reaction 0-+ CO- C02* + e-
-
0- + 02(alAg) -e- + 0 3 02-+ Oz(alAg) e- + 2 0 2
k (cm3molecule-l s-I)
k (literature)
ref
6.4 X i 30% 3.3 X 1P1'+100%/-50% 2.7 X 10-l' +loo%/-50%
(5.5-7.3) x lcrl* 3.0 X +goo%/-90% 2.0 x 10-10 +900%/-90%
49 9
and 6. The horizontal axis is the flow tube density of Oz(a). The vertical axis is the ratio of ion countswith the dischargeon divided by the ion counts with the dischargeoff. The 02(a) is only present with the discharge on. The discharge off condition acts as a base line of ion production/loss and transport down the flow tube with all the same gases except the metastables generated in the discharge. The reaction time measured with pulsing experiments is 11.7 ms, and the C2 correction factor was 0.98 for our typical flow tube conditions. Our pseudo-first-order kinetic plots exhibit an initial steep slope that must be due to another energetic metastable effluent from the discharge. These initial slopes are inconsistent with even the relative rate constants measured previously? A careful examination of the 1969 data9 shows one data point at the lowest 02(a1Ag)density that hints at the same behavior, Le., an initially fast attenuation; however, the effect is markedly more obvious in our present results. Many excited-state speciescan be generated in the discharge volume; however, the gas residence time in the cross, 1-5 s, is sufficiently long to guarantee that only the most long-lived 0 2 metastable states will be injected into the flow tube. Table 1 presents the likely reactions with long-lived species generated in the discharge. Ions generated in the discharge have been ignored because they are not expected to propagate into the cross. A 760-nm radiometer IO-nm half-width was implemented to identify the presence of 02(b1Z) present in the side arm cross. Noemission in theO2(b1Z,0-0)atmosphericsystemwas observed. Thesensitivityfordetectionof 02(b) is ordersof magnitude better than that forOz(a), suggestingverylittleif any 02(b) was injected into the flow tube. Atomic oxygen generated in the dischargecan have three fates. It may react with the mercuric oxide coating and be destroyed, recombine with molecular oxygen to form ozone, or recombine with another oxygen atom into molecular oxygen. The high pressure and long gas residence time in the cross are sufficient to recombine most of the atomic oxygen surviving the discharge side arm into ozone before injection into the flow tube. The formation of ozone prior to coating the discharge arm with mercuric oxide has been confirmed using a neutral mass spectrometer. Under these conditions, the ozone density was a factor of 20-50 below the 02(a) density. After coating the side arm with mercuric oxide, the ozone density was not detectable with the neutral mass spectrometer and is estimated as below 0.0005of the O2(a) density. The loss of 0-and 02-due to reaction with 0 or O3 generated in the discharge is thus reduced to a negligible level. Furthermore, data acquired after mercuric oxide application to the discharge tube still exhibited a steep initial slope, suggesting that neither 0 nor O3is not responsible for the initial steep slope. Vibrationallyexcited Oz(X,v) or 02(a,v) could be present, but no diagnosticmeasurementof their concentrationwas attempted. The initial slope changes to a shallower slope as 0 2 is added to the discharge to generate more Oz(a). This indicates the metastable responsible for the initial steepion attenuation is being rapidly depleted by the added 0 2 . Both 02(X,v) and O2(a,v) are long-lived and relax at about the same slow, i.e., 5 X cm3 s-l, rate with molecular o ~ y g e n . ~ This ~ . allows ~ ~ us to speculate that the reaction of 0- with either vibrationally excited species may contribute to the fast two-body source of ozone observed in electron-irradiated oxygen-containing environments. It is impossible to confirm these speculations or positively identify the metastable responsible for the initial steep ion attenuation at this time.
9
Rate coefficients for the reaction of 0- and 02-, with 02(alAg) have been measured from the shallow slopes as 3 X cm3s-l with a factor of 2 uncertainty. The rate coefficients quoted in Table 2 are the average of three kinetic measurementswhich fall within the typical f30% errors standard for a flow tube system. The larger factor of 2 error arises due to uncertainty in the optical system calibration and determination of the 02(a1$) density. These rate coefficients are at the lower limit of the 1969 measurements by Fehsenfeld et al.9 Our recent measurements exhibit relative rate constants of 0- and 0 2 - with Oz(aIA,) which areconsistentwith the relative ratecoefficients measuredin 1969. We suggest that the density of 02(alAg)determined in 1969 may have been in error by a factor of 5-10 due to the difficulties associated with the absolute optical calibration. The systematic uncertainty in our rate coefficients measurements stems from the uncertainty in the measured parameters, which include gas pressures, gas flow rates, reaction time, flow velocity, ion source stability, and counting statistics, among others. A complete propagation of error analysis using partial differentiation with respect to each measured parameter has been performedon a nearly identical flow kinetic apparatus previously.M The results of that analysis show that the major contributor to the uncertainty in the rate coefficient arises from the uncertainty in the determination of the reactant neutral species density. Of course, this uncertainty is related once again to the measurement accuracy of several flow rates and pressures. In our kinetic experiments we have already demonstrated the accuracy and precision of this apparatus and technique via the reaction of 0with CO, whose rate constant determinations falls well within the typicaluncertaintyrange of f30%. Thedominant uncertainty in the rate Coefficients for reaction with 02(a1Ag)is indeed the Oz(alA,) density in the flow tube. Although this densitydepends upon several gas flow rates, the discharge tube pressure, and the flow tube pressure, the major uncertainty arises from the absolute calibration of the optical system and determination of the 02(a'$) density in the discharge tube cross. A conservative evaluation of the uncertainty in these parameters has lead us to assign a factor of 2 uncertaintyin the rate coefficientsfor reactions with 02(alA,). These rate constantsdeterminedfrom our data will be valuable input to aeronomicmodels in an attempt to bound the production of ozone vibrational emissions observed at 9.6 pm during auroral electron precipitation and to understand the anomalously large IR atmosphere emissions. The ozone production in this reaction cannot be confirmed using this experimental configuration due to the relatively small ion density levels and reaction rate coefficient. These small rate coefficients suggest the infrared radiances may not be linked to this particular ion-molecule reaction; however, the initial steep slopes exhibited in the data suggest another mechanism is present. We will be pursuing new experiments to diagnose vibrationally excited species generated in the discharge and to identify the products formed as the 0and 0 2 - are attenuated. Acknowledgment. The authors acknowledge useful discussions with Drs. William A. M. Blumberg (PL/GPOS) and R. Forrest Gilmore (RDA) in defining these experiments. This effort was supported by the Air Force Office of Scientific Research (Task 2303EP and 23 10G4) and the Defense Nuclear Agency (MIPR No. 92-586, workunit 00002) through a contract with the Phillips Laboratory, Geophysical Directorate, F19628-88-C-0069.
842
The Journal of Physical Chemistry, Vol. 98, No. 3, I994
References and Notes ( 1) Arnold, F.; Krankowsky, J. Mid-Latitude Lower Ionosphere Structure
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