A solid state model is proposed which hopefully removes some of the objections to excited atoms being sources for light emanating from surfaces. Glow features are discussed in terms of excited oxygen atoms impinged on the surface, although other species could be treated similarly. Band formation, excited lifetime shortening and glow color are discussed in terms of this model. The model's inability to explain glow emanating above surfaces indicates a necessity for other mechanisms to satisfy this requirements. Several ways of testing the model are described

Peters, P. N.

English

NASA Technical Reports Server

A MODEL FOR EXPLAINING SOME FEATURES OF 
Palmer N. Peters 
Space Science Laboratory, Marshall Space 
SHUTTLE GLOW 
Flight Center 
Abstract. A solid state model is proposed which hopefully removes 
some of the objections to excited atoms being sources for light emanating 
from surfaces. Glow features are discussed in terms of excited oxygen 
atoms impinged on the surface, although other species could be treated 
similarly. Band formation, excited lifetime shortening and glow color 
are discussed in terms of this model. 
glow emanating above surfaces indicates a necessity for other mechanisms 
to satisfy this requirement. 
described. 
The model's inability to explain 
Several ways of testing the model are 
Introduction 
The predominant species striking forward facing shuttle surfaces in 
low-Earth orbits is atomic oxygen. The glow is reddish in color and 
excited oxygen has transitions that provide red emission. The glow is 
ram and altitude dependent and so is the flux of atomic oxygen to the 
surfaces. However, the emission from free excited oxygen atoms is 
observed in the form of sharply defined line spectra and the lifetimes of 
the excited states can be shown to be very long. Thus, it seems 
improbable that excited atoms, especially oxygen atoms, contribute to the 
observed glow, which produces a continuous band spectrum. Also, 
reflected oxygen atoms would de-excite at far greater distances from the 
surfaces than is observed, and if atoms were adsorbed they should be 
nonradiatively de-excited on the surface very quickly. This view of 
emission spectra from excited atoms is what should be expected, since 
atoms in a solid that are excited sufficiently to provide atomic spectra 
by means of an arc, or other sources, are already vaporized and the 
spectra originate from sharply defined energy levels associated with free 
atoms. 
In the case of orbiting surfaces a unique situation exists which is 
not common to our experiences. Oxygen atoms and other species are 
incident to forward facing surfaces with sufficient energies to 
potentially excite such atomic states and enable the atoms to become part 
of a solid state at the same time. S o l i d  state theories rely upon the 
fact that discrete energy levels of free atoms spread into energy bands 
upon assembly into solids. 
The model to be described relies upon spreading of the energy levels 
of the atomic orbitals to provide the broadband emission and other 
features. 
169 
https://ntrs.nasa.gov/search.jsp?R=19860003788 2020-03-20T15:58:03+00:00Z
The Model 
The relative velocity, in excess of 7 km/s, between forward facing 
orbiting surfaces and ambient particles provides a collisional 
interaction energy of 5 eV for oxygen atoms. 
in excess of the 1.9 eV difference bEtween the ,1D and 3P states which 
leads to red emission lines at 6300 A and 6364 A (Figure 1). If losses 
during the collision were sufficiently low, then the 4.2 eV difference 
between the 3P normal state and the 1s state might be provided whizh 
leads to emissions in the green at 5577 d and ultraviolet at 2972 A. 
States requiring excitation energies greater than 5 eV are neglected for 
s imp1 ici ty . 
Also shown (Figure 1) is an approximation of the spreading that might 
be expected as discrete energy levels of a free atom are converted into 
energy bands of a solid. No attempt has been made to calculate densities 
of states for the poorly characterized case of an atom striking typical 
orbital surfaces, but the general trend can be noted by observing what 
occurs as progressively larger numbers of atoms are involved. Results 
for band calculations starting with six hydrogen atoms in a line show 
splitting of levels into six lines forming bands dependent upon lattice 
constant [Kittel, 19651. Kittel also gives results for noble metals as a 
function of internuclear separation. 
If specular reflection is assumed for an oxygen atom striking a 
surface with a speed of 8 km/s, it is easily determined that the atom 
would spend approximately 5 x 10-14 s within an atomic diameter on the 
surface. This is sufficient time for the atom to communicate at the 
speed of light with a large number of lattice atoms (within a hemisphere 
of radius 
producing many states within the bands. If it is necessary for the atom 
to become part of the lattice, it can be seen by comparison to 
self-diffusion coefficients that 5 eV minus 2 eV would leave sufficient 
energy to exceed most barriers to incorporation of the atom into a 
lattice (activation energies for self-diffusion are typically less than 
the 60 kcal/(g-atom) available from a kinetic energy of 3 eV [Askill, 
19711 1. 
structure is reasonably represented (Figure 1). Also, the peak intensity 
should be in the general vicinity of the discrete lines, though shifted, 
and have an asymmetrical distribution. A spectrum from STS 41-D [Mende 
et al., 19851 exhibits characteristics of this nature, with the peak 
shifted toward a longer wavelength and with a bandwidth in excess of 1.5 
eV. 
This energy is considerably 
cm), which satisfies at least one requirement for 
A broadband spectrum can be produced, assuming that the energy band 
Model Testing 
To test the model it seems appropriate that spectra of some type 
dependent upon solid state band structure should be compared to glow 
spectra. Soft x-ray emission spectra provide information on some band 
structures [Kittel, 19651 which arise when inner K and L levels are 
ionized by electron impact and outer electrons make transitions to the 
170 
inner vacancies. Since inner levels are relatively sharp because of 
shielding by the outer electro 
spectra are mostly due 
outer electrons made t 
was found to have a fu 
which should represent a good 
conduction band. 
order 1 eV to 10 eV. Optical absorption co 
should also provide information about transitions betwfen bands dKittel, 
19651. Major absorption peaks for Cu and Au are 2000 A to 3000 A wide 
and peaked in the visible. Thus, the bandwidth for the glow is of the 
correct order compared to other spectra dependent upon solid state 
bandwidths (toward lower limit). 
To provide a higher probability for glow from the surface the lifetime 
of the excited oxygen state should be less than the time spent traveling 
one atomic diameter during reflection ((10-14 s ,  as mentioned earlier) or 
less than a period of lattice vibration ($10-13 s ) ,  if the atom is 
adsorbed. The exclusion principle AE *At% h provides a means of 
comparing linewidths to mean lifetimes [White, 19341. Taking 0.7 eV for 
the half width of the spectral peak from STS 41-D data [Mende et al., 
19851, and assuming the same uncertainty applies when AE = 0.7 eV and A t  
is the mean lifetime of the excited state, we obtain 
Bandwidths of other materials are general 
6 x 10-l5 s . h 
0.7 eV At % 
In the derivation of pressure broadening of atomic spectra the 
collision damping contribution, which broadens the lines, has a 
correction added due to alteration of the energy levels due to the close 
approach of the second atom [White, 19341. The initial and final states 
of the excited atom are not modified equally by collision since an 
excited, or outer, state is affected more than an inner tightly bound 
state. 
while spaced less than 10 A. Asymmetry and redshifting of the spectral 
lines occur. This does not conflict in general with the appearance of 
the STS 41-D spectrum and the fact that a solid would produce greatly 
modified states. 
acted as quenching agents, reducing radiative de-excitations by collision 
losses. Carbon dioxide was particularly noted for this effect. If 
carbon dioxide or other contaminants have a high coverage on the surface, 
one might speculate quenching of the excitation or de-excitation of the 
incident atoms; this would provide for differences in glow intensities 
for different materials. Those materials producing a minimum of gaseous 
products, but exposing a cleaner, lattice-type structure might be 
expected to glow with more intensity (the opposite of what might be 
expected for chemical reactions providing the glow). The data from STS 
41-D may show such a trend [Mende et al., 19851. This could be further 
tested by using a high glow intensity surface coated with a thin plastic 
film. When the film is burned away, the glow should intensify. Using a 
half coated disk rotated behind an aperture and also phase-sensitive 
The effect is a shkft toward lower frequencies for atoms emitting 
Ground-based studies of airglow [Young, 19661 found a number of gases 
171 
detection would provide a low-noise meas 
surfaces could also be used if limited so 
above the surface did not dominate. If 
acceptable gases or materials) mi 
glow. The distribution of oxygen 
surface (measurements by J.C. Gregory a 
may indicate a higher accommodation than would be predicted by hard 
sphere collision models. Mechanisms such as excitationlradiation which 
would deplete a large fraction of the particle's incident energy could 
help explain excessive accommodation coefficients where measured values 
exceed theoretical predictions. It should also be possible to measure 
incident energy thresholds for enhanced adsorption and glow if variable 
energy neutral oxygen atom sources are developed. If accommodation is 
found to be an adjustable parameter, numerous applications of this 
parameter to studies for understanding and controlling drag and torques 
in low-Earth orbits are possible. 
As a solid state model, no mechanism is provided to explain glow 
emanating above surfaces. 
Conclusions 
A hypothetical solid state model appears to explain a number of 
features for glow emanating from a surface in low-Earth orbit. A 
specific case of oxygen atoms impacting the surface with 5 eV energy may 
provide the atom excitation, energy level broadening, and shortened 
lifetimes necessary to produce an appropriate spectrum. The model does 
not provide a mechanism for glow emanating above surfaces. 
References 
Askill, J., Tracer diffusion in metals, in Handbook of Chemistry and 
Physics, edited by R. C. Weast, 52nd ed., pp. F-48-F-51, The Chemical 
Rubber Company, Cleveland, Ohio, 1971. 
Kittell, C., Introduction to Solid State Physics, 2nd ed., pp. 275-268, 
318. 310, and 319, Wilev & Sons, New York, 1965, 
Mende,-S. B:, et al.; Shuttle glow-measurements on STS 41-D, Final Report 
Skinner, H.W.B., Trans. Roy. SOC., I_ A239, 95: 1940. 
White, H. E., Introduction to Atomic Spectra, pp. 423-432, McGraw-Hill, 
Young, B. A . ,  The airglow, Scientific American, 214, 102-110, 1966. 
to Space Telescope Program Office, MSFC, Alabama, 1985. 
New York, 1934. 
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A model for explaining some features of shuttle glow

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