Using a newly-developed, magnetically confined source, low-energy, ground state oxygen negative ions and neutral atoms are generated. The energy range is variable, and atom and neutrals have been generated at energies varying from 2 eV to 40 eV and higher. It was found that the interaction of these low-energy species with a solid magnesium fluoride target leads to optical emissions in the (at least) visible and infrared regions of the spectrum. Researchers describe y details of the photodetachment source, and present spectra of the neutral and ion glows in the wavelength range 250 to 850 nm (for O(-)) and 600 to 850 nm (for O), and discuss the variability of the emissions for incident energies between 4 and 40 eV

Chutjian, A.

Murad, E.

Orient, O. J.

English

NASA Technical Reports Server

N )0" 25530
EXPERIMENTAL INVESTIGATIONS OF LOW-ENERGY (4-40 eY)
COLLISIONS
OF O-(=P) IONS AND O(Sp) ATOMS WITH SURFACES
A. ChutJian,* O. J. Orient' and E. Murad +
'Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
+Air Force Geophysics Laboratory, Hanscom Air Force Base, MA 01731-5000
ABSTRACT
Using a newly-developed, magnetically confined source low-energy, ground state oxy-
gen negative ions and neutral atoms are generated. The energy range is variable, and atom
and neutrals have been generated at energies varying from 2 eV to 40 eV and higher. We
find that the interaction of these low-energy species with a solid magnesium fluoride target
leads to optical emissions in the (at least) visible and infrared regions of the spectrum. We
will discuss briefly details of the photodetachment source. We will then present spectra of
the neutral and ion Uglows" in the wavelength range 250-850 nm (for O-) and 600-850 nm
(for O), and discuss the variability of the emissions for incident energies between 4 and 40
eV.
l INTRODUCTION
The problem of vehicle Uglow" emission in low-earth orbit (LEO) has been documented
in some detail over the past several years. [See Ref.1 for a recent review of missions and
observations.] By way of summary, the optical emissions observed from the the Shuttle bay
window are from Shuttle surfaces oriented in the _ram" direction (along the Shuttle velocity
vector), and have been associated with interaction of 5 eV ground-state oxygen atoms with
the various surface materials. The emission spectrum in LEO has been measured, at the
3.4 nm resolution level, and found to be continuous in the range 450-800 nm, with a
broad maximum at about 680 nm. Higher resolution spectra in LEO, extending to shorter
wavelengths, are not available and would clearly be helpful in revealing any latent band
structure and in identifying the emitting species.
There is thus far no definite explanation for the glow process. However, various
scenarios have been suggested to account for the glow, with the most promising being
that of Swenson, ct a/ 12] in which the emission has been proposed to arise from the
NO2 recombination continuum. The excited NO t molecules are formed on the surface by
successive surface-mediated reactions of N and O by the so-called Langmuir-Hinshelwood
mechanism. Desorbed, excited NO t molecules emit in the A2BI _ X2A1 continuum. The
lifetime of the _. state (which determines the spatial extent of the glow above the Shuttle
skin), and the wavelengths of emission are both consistent with Shuttle observations.
Because of the difficulty in generating fast, ground-state atomic oxygen in the lab-
oratory, simulation followed by a clear understanding of the mechanism for the surface
glow has been a tedious process. Methods for generating oxygen atoms usually suffer
from one or more of the following properties: the atoms are too slow - less than 1 eV
in energy (plasma discharges); there is an unknown admixture of excited molecules and
atoms (positive-ion charge exchange, or hlgh-power laser sources that use either a solid
target or a dense gaseous target); or peak pulsed O-atom fluxes are so high (order of l02°
atoms/cmLsec for so-called "accelerated testing" by high-power pulsed laser sources) that
one _burns plastic _ rather than bathing the surface in an LEO-encountered flux of 1013-10 is
atoms/cm_-sec.
263
https://ntrs.nasa.gov/search.jsp?R=19900016214 2020-03-19T22:39:21+00:00Z
We present in Sec. 2 a description of the JPL atomic oxygen source which generates
ground-state O-atoms of quantum state, energy, flux, and beam purity simulating that
encountered in LEO. It has been used to obtain first spectra of the glow phenomenon in
the laboratory. The IVigF2 surface chemiluminescence spectra using low-energy O- ions is
described in Sec. 3, and chemiluminescence spectra using low-energy O-atoms are given in
Sec. 4.
2 OXYGEN-ATOM SOURCE DESCRIPTION
The JPL atomic oxygen source was designed specifically to study the mechanism of the
spacecraft glow phenomenon, degradation of materials in LEO, and to carry out basic fast-
atom collision studies with neutral gaseous targets. Techniques used in the source involve
the following steps carried out in a uniform, high-intensity solenoidal magnetic field (see
Fig. I): (a) generate O-(_P) ions via dissociative attachment to NO at 8 eV electron energy
(point G); (b) accelerate the O- ions and electrons to the desired final energy (5 eV, say);
(c) by trochoidal deflection (T1) separate the higher-velocity electrons from the slower O-
ions, and trap the electrons in their Faraday cup; (d) detach the electron from O- by a cw
laser and a multiple-pass mirror (M) geometry; andre) direct the O- and O beams toward
the target, and reflect the undetached O- beam by biasing the target negative with respect
to the O- kinetic energy. Alternatively, a second trochoidal deflector, located immediately
after the mirrors M, was sometimes used to separate the ground-state O(SP) atoms from
undetached 0- ions, and trap the latter in a separate Faraday cup for analysis of the O-
be_[n.
The laser wavelengths are restrictedto visiblelines of an argon-ion laser so that
v,!F O murtlple mirror C
Figure 1. Schematic dlagram of the magne_caliy-confined, photodetachment o'atom source.
detachment results exclusively in ground-state atoms [3]. For a 20 watt laser, 5 eV ions,
and 100 passes across the confined ion beam, one obtains about a 15% detachment efficiency
into O(sP). Furthermore, if wavelengths shorter than about 360 nm are used, one may also
generate a mixture of O(1D) and O(sp) atoms. This capability is useful in studying
differential reaction effects of electronically-excited atoms, such as O-atom addition to
double-bonds in epoxies and organic molecules.
264
In orderto minimize rapid divergence of the O-atom beam after detachment, care
was taken in the source to restrict the energy of the O- ions perpendicular to the B field.
This involves choosing the target gas (at G) to provide small O- kinetic energy at onset,
and giving due regard to the space-charge repulsion between the ions, especially at hlgh
O- currents.
For both the O- and O-atom studies,the same sample of MgFi was used as the target.
A 95°A transmitting tungsten gauze covered the surface to prevent surface charging by the
O- beam; and alsoto allow repulsionof the O- beam during the O-atom measurements.
A large,high-reflectivitymirror at C (Fig. l)was used to collecta broad spatialextent of
optical emissions from the target. These emissions were focused onto the entrance plane of
a fastf/3.5 double-grating monochromator capable (forthe O-atom case) of attenuating by
a factor 10_ laserlinesat 500 nrn from spectral emissions at 600 nm. The detection system
used an RCA phototube with a gallium arsenide photocathode and a manufacturer's stated
long-wavelength cutoffof 900 nm. Fast,pulse-counting electronicswere used, and spectra
accumulated by multichannel scaling.
3 THE NZGATrVE ZON [O-pP)] GLOW
We show in Fig. 2 spectra of the O-(2P) glow from a MgFz surface in the wavelength
range 250-850 nm, and at four O- energies ranging from 5-40 eV. The currents were in
the range [.0-5.0/_a, or fluxesof (0.32 -1.6) x i014ions/cmZ-secl The spectra in Fig. 2
were normalized to a common flux of ].0 x 1014. Care was taken in these measurements
to prevent surface charging of the dielectrictarget by use of a transparent, conducting
tungsten gauze on the target surface. This gauze itselfwas shown not to contribute to
SO
A
1
0
60
4O
40 ev
0 I _ I _ I
250 350 450 550 650 750
WAVELENGTH(nm)
85O
Figure 2. Glow signal of negative oxygen ions O-(IP) from a MgFi surface, at the indicated
O- collisionenergies.
265
the observed spectra. Also, a glow contribution arising from interaction of the O- beam
with the background gas (mainly NO) was observed. This contribution to the spectrum
was measured by taking spectra with the tungsten gauze biased negatively, so that the O-
beam was reflected just above the MgFz target.
One sees in Fig. 2 that the O- glow from MgF2 is characterized by two broad, un-
structured (at the 10 nm resolution level of the present measurements) spectral emissions:
one peaked near 375 nm, and the second at 600-650 nm. The spectral intensity is a strong
function of O- energy, with emission being strongest at 40 eV, and diminishing rapidly
at energies near 5 eV. No attempts have been made to indentify the origin of these fea-
tures pending experiments to be carried out under more controlled vacuum and surface
conditions (see Se¢. 4).
It isinterestingto note that at high fluxes(order of 1014),the O- glow Iseasilyvisible
to the eye,having a bluish appearance (not resembling, that is,the orangish Shuttle glow!).
4 THE ATOMIC OXYGEN [O(SP)] GLOW
Shown inFig. 3 are spectra in the wavelength range 600-8S0 nm arisingfrom the interaction
of 4, 5 and 40 eV O(SP) oxygen atoms with the same MgF_ surface. Comparison of these
I I I I
, // _'_'_._. NO.O(3p} _ NO2"
70 /i/ \\\
/_ %\ST8 41-D (till, port oftglnl
/ I \,sev ',
!
1°
10
o I f I
600 65O 700 750 800 850
WAVELENGTH (nm)
Figure 3. Glow signal of ground stateatomic oxygen atoms O(SP) from a MgF2 surface,at the
indicated O-atom collisionenergies.
266
spectra are made with glow results from the STS 41-D mission [2], and from laboratory-
measured NO -{- O recombination spectra at thermal oxygen-atom energies [4,5].
The spectra in Fig. 3 were approximately two-to-three orders of magnitude weaker
than the O- glow spectra of Fig. 2. Reasons for this are (a) the efficiency of the pho-
todetachment step is only about i0%, (b) the O-beam, no longer confined by the magnetic
field, diverges as it travels toward the target from a combination of imparted kinetic energy
in the dissociative attachment step, and effects of space-charge repulsion in the 0- beam,
and (c) the conversion efficiency of O-atoms to visible photons is estimated to be quite
low, about 2.5 × 10 -6 [6l.
As a result, a number of other background sources had to be identified and eIimlnated
from these spectra. The major contributors were : (a) a persistent fluorescence signal
BL arising from the detachment mirrors with the laser on, (b) photons Bo- arising from
interaction of the O- beam with the background gas, and (c) photons B_ from the electron-
gun filament. The net glow signal G was obtained over four cycles of signal-gathering.
These were (l) photon counts with laser on and O- beam on, (2) counts with laser on and
O- off, (3) counts with laser off and O- on, and (4) counts with laser off and O- off. The
net glow signal G was then given by (1) - (2) - (3) ÷ (4). The error limits shown in Fig. 3
reflect the sum of the four statistical errors in the individual counting cycles.
Also considered in this sequence was the fact that the background Bo- in steps (l) and
(3) are not the same, due to depletion of O- by detachment. We found that this depletion
had to be about a factor of 2.5 greater than could be accounted for solely by detachment.
The reason for this is not clear. It could lie either in the measured value [3] of the detach-
ment cross section being a factor of 2.5 too low; or that the detachment cross section of
O- is enhanced (by the same factor) via Landau resonances of the detached electron in the
solenoidal magnetic field. Calculations based on works of Crawford [71 and observations
by Krause [8] show that these resonances do persist up to 1 eV above detachment thresh-
old. Enhancement is thus possible in our case, given the homogeneity of magnetic field
(estimated to be better than 0.5% in the detachment region), laser bandwidth (8 GHz),
and spacing of the resonances (169 GHz, neglecting overlapping fine-structure transitions
and Zeeman splittings).
One striking feature in Fig. 3 is that the glow is continuous (10 nm resolution level)
and shows two prominent maxima, one at 650 nm and the second at 800-825 nm. The first
maximum is in good agreement with the STS 41-D data and laboratory recombination
spectra. No evidence is seen for a second maximum in these last data. However, the NO2
recombination continuum may take on a different shape, depending on the recombining
species and the available internal energy; and on the type of Shuttle surface material and
its temperature. It is interesting to note recent laboratory glow results wherein a single
maximum at 820 nm in glow signal was seen in an O (0.16 eV energy} + NO ÷ Ni surface
experiment [9].
The second striking feature is that the glow signal falls below our present detection
limit at an energy of 4 eV, and this represents an upper energy bound to the true threshold.
To obtain an estimate of the threshold, we use the following pieces of information: Ca) the O
÷ NO (surface) system gains 3.1 eV by the ON-O bond formation, (b) one requires about
0.5 eV to dislodge the electronically-excited NO_ from the surface, (c) the NO_ leaves
the surface with about !.3 eV kinetic energy, and (d) the maximum internal electronic
excitation energy of NO_ corresponds to the 400 nm (3.1 eV) onset of the LEO-observed
glow. Thus, the center-of-mass threshold energy T_,_ is given by T_,, ÷ (a) = (b) ÷ (c) +
(d), or T,,_ = 1.8 eV, and T_b ---- 1.8 x (16/10.4) ----2.8 eV (neglecting the small thermal
energy of the surface-bonded NO). This is consistent with our upper limit of 4 eV.
267
As in the O- case, no identification of the emission continua is possible without un-
derstanding the mechanism of the glow process. To this end, work is currently underway to
carry out the glow measurements under ultrahigh vacuum conditions, by playing different
gases over the solid target surface and identifying which gases, if any, enhance the glow.
Work is also planned to explore the spectral region below 600 nm. While some of this
region is obscured by the detaching argon-ion laser lines, wavelengths below 450 nm are
accessible.
5 ACKNOWLEDGEMENTS
We thank Mr. A. Timpe for his assistance in machining many of the components of the
source. This work was carried out at the Jet Propulsion Laboratory, California Institute
of Technology, and was supported by the Air Force Geophysics Laboratory and DARPA
through agreement with NASA.
6 REFERENCES
1. H. B. Garrett, A. Chutjian and S. Gabriel, _Space Vehicle Glow and Its Impact on
Spacecraft Systems," J. Spacecraft and Rockets 25, 321-340 (1988).
2. G.R. Swenson, S.B. Mende, and K.S. Clifton, _Ram Vehicle Glow Spectrum: Impli-
cation of NO2 Recombination Continuum," Geophysical Research Letters 1P, 97-100
(1985).
3. L.M. Branscomb, S.J. Smith, and G. Tisone, "Oxygen Metastable Production Through
Photodetachment," J. Chem. Phys. _S, 2906-7 (1965).
4. A. Fontijn, C.B. Meyer, and H.I. Schiff, "Absolute Quantum Yield Measurements of
the NO-O Reaction and its use as a Standard for Chemi!uminescent Reactions," J.
Chem. Phys. _0, 64 (1964).
5. D.E. Paulson, W.F. Sheridan and R.E. Huffman, "Thermal and Recombination Emis-
sion of NO_," J. Chem. Phys. 58, 647 (1970).
6. J.-H Yee and A. Dalgarno, _Radiative Lifetime Analysis of the Shuttle Optical Glow,"
d. Spacecraft and Rockets 28, 635-40 (1986).
7. O. H. Crawford, _Oscillations in Photodetachment Cross Sections for Negative Ions
in Magnetic Fields," Phys. Rev. A 87, 2432-40 (1988).
8. H. F. Krause, "A Novel High-Resolution Experimental Approach for Studying La_er
Photodetachment in a Magnetic Field," Fourth Int. Symp. Prod. and Neutralization
of Negative Ion Beams (Brookhaven, N.Y., 27-31 Oct. 1986); and unpublished results
(Oak Ridge Nat. Lab., 1989).
9. G.A. Arnold and D.J. Coleman, "Surface Mediated Radical Recombination Lumi-
nescence: O + NO ÷ Ni," J. Chem. Phys. 88, 7147-56 (1988).
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Experimental investigations of low-energy (4 to 40 eV) collisions of O(-)(P2) ions and O(P3) atoms with surfaces

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