The active environment of a radio-frequency (RF) plasma generator, with and without low-pressure oxygen, has been characterized through the identification of emission lines in the spectral region from 250 to 900 nm. The environment is shown to be dependent on the partial pressure of oxygen and the power applied to the RF generator. Atomic oxygen has been found in significant amounts as well as atomic hydrogen and the molecular oxygen species O2((sup 1)Sigma). The only charged species observed was the singly charged molecular ion O2(+). With a polymer specimen in the plasma chamber, carbon monoxide was also observed. The significance of these observations with respect to previous studies using this type of generator to stimulate material degradation in space is discussed. The possibility of using these generators as atomic oxygen sources in the development of oxygen atom fluorescence sensors is explored

Batten, Carmen E.

Brown, Kenneth G.

Lewis, Beverley W.

English

NASA Technical Reports Server

NASA Technical Memorandum 4612
A Spectral Study of a Radio-Frequency
Plasma-Generated Flux of Atomic Oxygen
Carmen E. Batten
Langley Research Center • Hampton, Virginia
Kenneth G. Brown and Beverley W. Lewis
Old Dominion University • Norfolk, Virginia
National Aeronautics and Space Administration
Langley Research Center • Hampton, Virginia 23681-0001
December 1994
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Abstract
The active environment of a radio-frequency (RF) plasma generator,
with and without low-pressure oxygen, has been characterized through
the identification of emission lines in the spectral region from 250 to
900 nm. The environment is shown to be dependent on the partial
pressure of oxygen and the power applied to the RF generator. Atomic
oxygen has been found in significant amounts as well as atomic hydrogen
and the molecular oxygen species O2(IE). The only charged species
observed was the singly charged molecular ion 0 +. With a polymer
specimen in the plasma chamber, carbon monoxide was also observed.
The significance of these observations with respect to previous studies
using this type of generator to simulate material degradation in space
is discussed. The possibility of using these generators as atomic oxygen
sources in the development of oxygen atom fluorescence sensors is
explored.
Introduction
Radio-frequency (RF) generators have been used
by several groups for the generation of oxygen atoms
in order to study the oxygen degradation of materials
in a simulated low-Earth-orbit (LEO) environment.
(See refs. 1-5.) The rate of degradation in these
facilities is much greater (by as much as a factor of
7) than that observed in LEO (ref. 1). One possible
explanation of this higher rate of degradation is
the interaction of the material with the reaction
products, which further degrade the material (ref. 4).
Such a mechanism would not be possible in LEO
because the products are being continuously removed
in what is essentially an open system in contrast to
the semiclosed, low-flow system present in the RF
generator studies. The studies in reference 4 were
conducted in the afterglow region of the RF generator
in an effort to isolate the sample from other possible
deleterious effects of the RF environment such as
ultraviolet (UV) radiation and free electrons.
The studies in reference 4 do not rule out the pos-
sible presence of other interfering species that might
also enhance the reaction rate and provide an addi-
tional explanation of the discrepancy between labora-
tory studies and LEO results. These species are gen-
erated by the RF discharge and can still be present in
the afterglow region. In this paper we determined the
chemical nature of some of these species and what, if
any, effect they might have in the overall degradation
reaction. We are limited by experimental design to
those species that have emission spectra between the
wavelengths of 250 and 900 nm. The ultimate goal
of this investigation was to understand the dominant
processes in an RF-generated plasma and to deter-
mine if an RF generator can be used in the devel-
opment of fluorescence sensors for the detection of
atomic and molecular species in LEO and in other
rarefied low-flow environments.
Experimental Methods
The experimental system (fig. 1) consisted of
a fused quartz cylindrical reaction chamber with
two semicircular electrodes surrounding the cham-
ber. An RF generator with associated tuning cir-
cuits provided up to 100 W of power at 13.56 MHz.
The chamber was evacuated with an external pump.
The internal pressure was controlled between 13 and
54 Pa, depending upon the adjustable flow rate of
the entering gases. The chamber was provided with
a removable flat window at one end, and an optical
fiber bundle was located in front of this window. The
other end of the fiber bundle was placed at the en-
trance slit of a single-beam monochromator, and then
the fiber bundle acted as a waveguide for the light
emitted by the RF chamber. The monochromator
had a 1-m path length with an effective aperture of
f/8.7. The detector was a photomultiplier tube and
the resultant signal was amplified and then digitized
for computer storage and subsequent analysis.
The gas mixtures were controlled by an external
metering and mixing system. The emission spectra
were determined for a variety of mixtures of oxygen
with other gases to characterize the various species
formed and the conditions under which they are pro_
duced. An early observation was that the discharge
was much brighter at low pressures. This intense
background made it difficult to discern the presence
or absence of discrete molecular and atomic species.
To eliminate this background glow we employed a va-
riety of shield configurations, including an aluminum
tunnel which surrounded the observation area insid@
the chamber. This latter design was an attempt
to produceanenvironmentthat mightemulatethe
afterglowenvironmentdescribedin reference4.
A glassrackwasalsoconstructedto allowsamples
to be mountedinsidethe reactionchamber.This
rackwasconstructedsothat the front faceof the
samplewasheldperpendicularto the observation
axisof the fiberbundle. As a reference,we used
Du Pont Kapton,a materialwhoseoxygenatom
degradationhasbeenfully characterized.(Seerefs.1
and5.) Sampleswerecutinto1-in.squares,weighed
priorto beingplacedin thechamber,andheldunder
vacuumovernight.Thesampleswerethenexposed
to theplasmaforaperiodof timewhiletheemission
spectrumwasrecorded,afterwhichthesampIeswere
removedfrom the reactionchamberandreweighed
to determinethenetweightloss.A mass-normalized
mass-rateloss(D1)couldthenbedeterminedas
D1 = --Am min- 1 (1)
mot
where mo denotes the original mass, Am denotes the
mass loss, and t denotes the time (given in minutes).
The values of D1 could be compared for all the gas
mixtures. The mass loss of the material normalized
by the exposed sample area (D2), is defined as
Dl mo g
D2 = 2A cm 2- min (2)
where A denotes the sample area. The factor of 2
occurs in the denominator because both faces of the
sample were exposed. The sample is so thin (_1 mil)
that collisions with the edges could be ignored. The
oxygen atom flux, which is the number of oxygen
atoms colliding with the surface per second, is shown
in equation (3) as
D2 atoms
J0 = p(3 × 10 -24) cm 2- rain (3)
where Jo denotes the oxygen atom flux and p denotes
the sample density. Here, J0 is computed by using a
reaction efficiency (3 x 10 -24 cm3/atom) quoted in
reference 1 from LEO data.
Results and Discussion
The main spectral features observed from 02
plasma in the reaction chamber at low 02 pressures
(17 Pa) are shown in figure 2. The two strong fea-
tures due to atomic oxygen are at 777 and 845 nm
(ref. 6). These two features persist under all ex-
perimental conditions with the 845-nm peak becom-
ing very difficult to observe at the lowest pressures
of oxygen. The other pronounced features are the
strong band at approximately 656 nm and a weaker
feature at 486 nm (not shown in fig. 2), both of which
are due to the presence of atomic hydrogen. The
656-nm band persists under all experimental condi-
tions and arises from either residual adsorbed water
or surface hydroxyl groups.
As the pressure of oxygen changed, other strong
features began to appear in the spectrum. The spec-
tral changes that occurred as the O2 pressure (p)
varied from 32 to 16 Pa are summarized in figures 3
and 4 for the species O2(1_E), O(3p), and O + (refs. 6
and 7). The molecular feature centered at approx-
imately 760 nm is due to the presence of O2(IE)
(ref. 6). This feature persisted as a typical molecular
band with structure until the oxygen pressure de-
creased below 23 Pa. At lower pressures of 02, some
new molecular features emerged, shown in figure 4, at
523 and 557 nm. These features persisted through-
out the low-pressure range and are assigned to O_-.
When the concentration of O2 was varied by diluting
the gas with helium, the O + features reappeared at
low partial pressures of oxygen (i.e., less than 19 Pa)
as shown in figure 5. The disappearance of the sin-
glet 02 species when the gas was diluted with helium
paralleled that of the pure gas when the 760-nm band
disappeared at 02 pressures below 23 Pa (fig. 3).
The presence of hydrogen emission lines was in-
dependent of any gas mixture examined. After the
chamber had been exposed to ambient conditions,
the hydrogen line was quite strong and decreased
gradually, but it never disappeared. The hydro-
gen emission was quite likely the result of environ-
mental contamination, with the first strong oc-
currence due to loosely associated water and the
long-term hydrogen emission due to bound water
and hydroxyl groups located upon the surface of the
chamber.
When the reference material Kapton was placed
in the chamber and exposed to the RF field in the
presence of O2, spectra similar to those shown in
figure 6 were observed. They were, in basic fea-
tures, similar to those observed for the chamber with
only oxygen present, except that no spectral feature
assignable to singlet oxygen O2(1_) was observed at
any pressure of O2. Apparently, those bands that are
assignable to O + persisted to higher pressures of oxy-
gen. This assignment was difficult to make because
of overlapping CO emission bands, which are due to
the presence of oxidation products of Kapton. The
CO bands, however, have a different shape from the
bands due to O_-; at lower pressures, the CO bands
can be distinguished and uniquely assigned at 519,
483, and 451 nm (refi 7). A CO band also occurs
_l: li I
at 559nm, but it is obscuredby thecoincidentO_-
band.Asmightbeexpected,wealsoobserveda sig-
nificantincreasein the hydrogenfeatureat 486nm
due,in part, to the increasedpresenceof H20 as
anoxidationproductin thechamber.Theobserved
assignableplasmabandsfor theemptychamberand
for the chamberwith Kaptonin placearesumma-
rizedin tableI.
TableI. AssignableBandsinRFPlasmaChamber
FromReferences5and6
Spectrallines,nm
Species WithoutKapton WithKapton
O
02(1E)
H
CO
H20
615,777, 845
760
523, 557, 635
486, 656
Absent
Absent
615, 777, 845
Absent
523, 557, 635
486, 656
451,483, 519, 559
822, 900
From the measured weight loss, we can determine
an effective oxygen atom concentration in the reac-
tion chamber by assuming a Kapton erosion efficiency
of 3 x 10 -24 cm3/O atom (ref. 1). Figure 7 shows
that the value of D1, normalized by the 02 partial
pressure, varied for different mixtures of gases in the
chamber. The line intensity and therefore the oxygen
atom density also depended on RF power, as shown
in figure 8. However, above 50-W RF power, the
emission line strength did not change significantly,
thus indicating that we were operating in a power
regime where oxygen atom production reached a
plateau and was not sensitive to variations in power.
Therefore, we determined an average oxygen atom
density of approximately 1012 O atoms/cm 3 at an
RF power of 100 W. This average density is much
higher than the atomic oxygen density determined in
LEO at 200 km (109 O atoms/cm 3) and at 400 km
(108 O atoms/cm 3) (ref. 8). It is also important to
note that the density was determined at a chamber
pressure of approximately 40 Pa in pure oxygen. As
shown in figure 9, atomic oxygen density increased
at compositions below 10 percent O2 in He.
Concluding Remarks
A technique has been developed to character-
ize the production of atomic oxygen by using a
radio-frequency (RF) generator. This system can
be used in future studies of candidate fluorescent
materials for the detection of atomic oxygen. The
generator has been characterized to enable a deter-
mination of the amount of atomic oxygen produced
under a variety of experimental conditions. Other
species (O2(1E), O +, and H) were also observed that2
may react with materials in the chamber and inter-
fere with fluorescence studies on candidate materials.
Possibly, these other species could, in part, account
for the high polymer degradation rates produced in
ground experiments when compared with the degra-
dation rates observed in a low Earth orbit (LEO).
NASA Langley Research Center
Hampton, VA 23681-0001
October 17, 1994
References
1. Koontz, S. L.; Albyn, K.; and Leger, L.: Materials Selec-
tion for Long Life in Low Earth Orbit--A Critical Eval-
uation of Atomic Oxygen Testing With Thermal Atom
Systems. J. IES, vot. 33, Mar.-Apr. 1990, pp. 50-59.
2. Hansen, R. H.; Pascale, J. V.; De Benedictis, T.; and
Rentzepis, P. M.: Effect of Atomic Oxygen on Polymers.
J. Polym. Sci.: Part A, voh 3, 1965, pp. 2205-2214.
3. Lerner, N. R.; and Wydeven, T.: Polymer Etching in
the Oxygen Afterglow--Increased Etch Rates With In-
creased Reactor Loading. J. Electrochem. Soc., vol. 136,
May 1989, pp. 1426-1430.
4. Lerner,. N. R.; and Vv_ydeven, T.: Decrease in the Etch
of Polymers in the Oxygen Afterglow With Increasing
Gas Flow Rate. J. Appl. Polym. Sci., vol. 35, 1988,
pp. 1903 1908.
5.
6,
7.
8.
Miller, W. L.: Mass Loss of Shuttle Space Suit Orthofabric
Under Simulated Ionospheric Atomic Oxygen Bombard-
ment. NASA TM-87149, 1985.
Weast, Robert C.; and Astle, Melvin J.: CRC Handbook
of Chemistry and Physics. 60th ed., CRC Press, 1979.
Pearse, R. W. B.; and Gaydon, A. G.: The Identification
of Molecular Spectra, Third ed., Chapman & Hall, Ltd.,
1963.
Peplinski, D. R.; Arnold, G. S.; and Borson, E. N.:
Introduction to Simulation of Upper Atmosphere Oxygen
Satellite Exposed to Atomic Oxygen in Low Earth Orbit.
Thirteenth Space Simulation Conference--The Payload:
Testing For Success, J. Stecher, ed., NASA CP-2340,
1984, pp. 133-145.
4
i|i | I
Gas
mixer _-- I f
Vacuum __ga ge
generator I
Plasma/quartz
chamber
Vacuum
pump
UV
source
I Amplifier
Detector
Optical fiber
H Recorder H Computer
Monochromator
Figure 1. Sensor evaluation system.
5
50-
¢$
656 nm H
"N_ 777 nm 0
845 nm O j
650 700 750 800 850
Wavelength, nm
Figure 2. Identification of spectral lines for atomic O and H. Pure oxygen with no sample in chamber;
p= 17 Pa.
50
p = 32 Pa
O
02
I I
750 780
p = 27 Pa
O
02
I I
750 780
p = 23 Pa
O
O2
I 1
750 780
Wavelength, nm
p= 19Pa
O
02
I .1_
750 780
p = 16 Pa
O
O2
I
750
Figure 3. Pressure dependence of O2(]_3) and O(3p) in 100-percent oxygen plasm_
I
780
71; • I
"6
5O
p = 20 Pa
4- +
0 2 0 2
I I
525 560
p= 17.3 Pa
4- 4-
I 1
525 560
Wavelength, nm
p= 16Pa
4-
02
525 560
Figure 4. Pressure dependence of O_- in 100-percent oxygen plasma.
"6
1.5
1.0
.5
760 nm 02(Iz)--,,
I
557 nm O_
I I
500 750
Wavelength, nm
Figure 5. Molecular oxygen species in 40-percent oxygen in helium plasma with a total pressure of 49.3 Pa.
7
50--
.E
_9
°_
486 nm H
+ 2, CO
483nmCO--_ 519nmCO---_ F524nmO2 _-559nmO+
0'_ _ 451 nm CO \
I I I I I
450 500 550 600
Wavelength, nm
(a) Range of wavelength from 450 to 600 nm.
50--
,m
°>_.
,v
777 nm O
1
750
845 nm O
822 nm H20-" _ 900 nm H20
I I I
800 850 900
Wavelength, nm
(b) Range of wavelength from 750 to 900 nm.
Figure 6. Spectrum of oxygen plasma with Kapton in chamber.
8
!IF II I
I I I
I I I I I I
5
4
t9
3
2
1
0
l0
0 .001 .(D2 .003 .004 .005 .006 .007 .008 .009
D1, rain -1
Figure 7. Kapton weight-loss rate (D1) corrected for pressure.
,L
-- f /--- 845 nmO
707 nm He -_ / /
_ I --/ I I I I I I I
20 30 40 50 60 70 80 90 I00
RF power, W
Figure 8. Effect of RF power on emission lines of a mixture of 50 percent 02 in He. p = 33.3 Pa; flow rate is
5.00 standard cm3/min.
9
.OOO6
.0005
.0(}04
I
.=_
E .O0O3
v
©
.0001
o 5 _o 2o 30 40 50 loo
Percent of oxygen in helium
Figure 9. Effect of 02 pressure in the plasma on weight-loss rate of Kapton.
10

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December 1994 Technical Memorandum
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
A Spectral Study of a Radio-Frequency Plasma-Generated Flux
of Atomic Oxygen
6. AUTHOR(S)
Carmen E. Batten, Kenneth G. Brown, and Beverley W. Lewis
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
NASA Langley Research Center
Hampton, VA 23681-0001
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546-0001
WU 307-51-12-04
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REPORT NUMBER
L-17351
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
NASA TM-4612
11. SUPPLEMENTARY NOTES
Batten: Langley Research Center, Hampton, VA; Brown and Lewis: Old Dominion University, Norfolk, VA.
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12b. DISTRIBUTION CODE
13. ABSTRACT (Maximum 200 words)
The active environment of a radio-frequency (RF) plasma generator, with and without low-pressure oxygen,
has been characterized through the identification of emission lines in the spectral region from 250 to 900 nm.
The environment is shown to be dependent on the partial pressure of oxygen and the power applied to the RF
generator. Atomic oxygen has been found in significant amounts as well as atomic hydrogen and the molecular
oxygen species O2(1T_). The only charged species observed was the singly charged molecular ion O_-. With
a polymer specimen in the plasma chamber, carbon monoxide was also observed. The significance of these
observations with respect to previous studies using this type of generator to simulate material degradation in
space is discussed. The possibility of using these generators as atomic oxygen sources in the development of
oxygen atom fluorescence sensors is explored.
14. SUBJECT TERMS
Radio-frequency plasma; Oxygen atom flux; Spectral analysis
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A spectral study of a radio-frequency plasma-generated flux of atomic oxygen

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