Interaction of oxygen atoms with solid surfaces at eV energie

Boring, J. W.

Humphris, R. R.

Magnuson, G. D.

English

Design and construction of stainless steel atomic oxygen beam system for solid surface bombardmen

NASA Technical Reports Server

THE INTERACTION OF OXYGEN ATOMS 
WITH SOLID SURFACES AT eV ENERGIES 
F i  r s t  Annua I S t a t u s  Repor t  
I June, 1967 t o  31 May, 1968 
NASA Gran t  NGR-47-005-077 
Submit ted by:  
G. D. Magnuson 
J.  W .  B o r i n g  
8. R. Humphris 
Submit ted t o :  
N a t i o n a l  Aeronaut ics  and Space A d m i n i s t r a t i o n  
Washington, D. C. 20546 
Department of  Aerospace E n g i n e e r i n g  and Eng ineer ing  Phys i cs  
RESEARCH LABORATORIES FOR THE ENGINEERING SCIENCES 
SCGOOL OF ENG!NEER!NG AND APPL! ED S C !  ENCE 
UNIVERSITY OF VIRGINIA 
CHARLOTTESV I LLE, VI RGI N I A 
Repor t  No. EP-4011-102-68U 
June 1968 Copy No. 1 
https://ntrs.nasa.gov/search.jsp?R=19680022052 2020-03-12T10:12:58+00:00Z
I . I NTRODUCT I ON 
 his t-nnnt-+ ' " V U '  I pn\lnt-c " U I U I d  t h e  f i r s t  year  sf work performed nnrler NASA 
Grant NGR-47-005-077 dur ing  the p e r i o d  June I, 1967 through May 31, 1968. 
I n  o rde r  t o  understand i n  d e t a i l  t he  behavior  o f  space veh ic les  
i n  r a r e f i e d  atmospheres i t  i s  necessary t o  have s p e c i f i c  knowledge con- 
ce rn ing  t h e  i n t e r a c t i o n  o f  t h e  atoms and molecules o f  t h e  atmosphere 
w i t h  sur faces  of t h e  veh ic le ,  p a r t i c u l a r l y  f o r  p a r t i c l e  energ ies  cor re -  
sponding t o  space v e h i c l e  o r b i t a l  and escape v e l o c i t i e s .  The goal of 
t h e  present  research program i s  t o  i n v e s t i g a t e  t h e  mass spectrum r e s u l t i n g  
when oxygen atoms, moving w i t h  s a t e l l i t e  v e l o c i t i e s ,  s t r i k e  a s o l i d  sur face.  
I n  a d d i t i o n  t o  adding t o  t h e  small amount o f  informa ion  p r e s e n t l y  a v a i l a b l e  
concern ing p a r t i c l e - s u r f a c e  i n t e r a c t i o n s  a t  energ ies corresponding t o  
s a t e l l i t e  v e l o c i t i e s ,  t h e  r e s u l t s  w i l l  be o f  p a r t i c u  a r  va lue  i n  p r o v i d i n g  
i n fo rma t ion  t h a t  w i l l  a i d  i n  the  i n t e r p r e t a t i o n  o f  mass spec t romet r i c  
s t u d i e s  o f  t h e  neu t ra l  c o n s t i t u e n t s  o f  t h e  e a r t h ' s  upper atmosphere. 
though t h e  p r i n c i p a l  concern of t h e  research repor ted  here i s  w i t h  monatomic 
oxygen t h e  technique descr ibed i n  t h i s  r e p o r t  can be used for  s t u d i e s  o f  
o t h e r  components o f  t h e  atmosphere and i n t e r s t e l l a r  space. 
I 
A l -  
I n  t h e  s tudy o f  t he  i n t e r a c t i o n s  o f  neu t ra l  a tomic oxygen w i t h  
s o l i d  sur faces  one must consider t h e  p o s s i b i l i t y  t h a t  t h e  oxygen atoms 
spend a r e l a t i v e l y  long t ime  on t h e  sur face  and as a consequence t h e  
cha rac te r  o f  t h e  p a r t i c l e s  leav ing t h e  sur face  does n o t  depend t o  an 
apprec iab le  e x t e n t  on t h e  energy o f  t h e  i n c i d e n t  atoms. I n  t h i s  event  
one cou ld  s tudy t h e  i n t e r a c t i o n  o f  0 atoms w i t h  sur faces  us ing  t h e  more 
convent iona l  thermal techniques. However, t h e  few measurements t h a t  
have been made on p a r t i c l e - s u r f a c e  i n t e r a c t i o n s  i n  the  1-10 eV range 
i n d i c a t e  t h a t  t h e  molecules leav ing t h e  sur face  a re  f a r  from f u l l y  
accommodated t o  t h e  temperature of t h e  sur face  and hence have probably  
I 
su f fe red  o n l y  a few c o l l i s i o n s  w i t h  su r face  atoms be fo re  l eav ing  t h e  
surface. There are, un fo r tuna te l y ,  no such measurements f o r  oxygen 
atoms, b u t  one should probably be h e s i t a n t  about app ly ing  r e s u l t s  
ob ta ined from s t u d i e s  of  t h e  i n t e r a c t i o n  o f  thermal oxygen atoms w i t h  
surfaces t o  s i t u a t i o n s  where the  r e l a t i v e  v e l o c i t y  between atom and 
sur face  i s  i n  t h e  range o f  s a t e l l i t e  v e l o c i t i e s ,  p a r t i c u l a r l y  s i n c e  
t h e  k i n e t i c  energy o f  these atoms i s  c l o s e  t o  t h e  d i s s o c i a t i o n  energy 
f o r  t h e  O2 molecule. 
2 
SECTION I I 
BACKGROUND 
The p r i n c i p a l  o b j e c t i v e  of  t h e  research repo r ted  here is an 
i n v e s t i g a t i o n  o f  t h e  d i s t r i b u t i o n  of  masses t h a t  r e s u l t  when oxygen 
atoms s t r i k e  a s o l i d  sur face .  The i n t e r e s t  i s  concent ra ted  on 
oxygen atoms having a k i n e t i c  energy i n  t h e  range 4-10 eV, corresponding 
t o  v e l o c i t i e s  i n  t h e  s a t e l l i t e  range. 
p a r t  
grea 
ve r y  
d i c a t i o n  
energ ies  
s u r f  aces 
t h a t  a t  
Several recen t  s tud ies  [1],[2] have been made of  t h e  recombinat ion 
o f  atoms w i t h  su r face  atoms t o  form var ious  species of  molecules. 
measurements have been conducted under c o n d i t i o n s  where t h e  i n c i d e n t  
p a r t i c l e s  have thermal energies. I f  one i s  t o  use t h e  r e s u l t s  of  such 
s t u d i e s  f o r  a p p l i c a t i o n  t o  p r a c t i c a l  s i t u a t i o n s  where t h e  atoms s t r i k i n g  
t h e  su r face  have v e l o c i t i e s  i n  t h e  s a t e l l i t e  range, then i t  must be dem- 
o n s t r a t e d  t h a t  t h e  r e s u l t s  are e s s e n t i a l l y  independent of t h e  i n c i d e n t  
These 
c l e ' s  energy i n  t h e  range from thermal up t o  severa l  eV. 
Even though t h e  development o f  t h e  space program has generated 
i n t e r e s t  i n p a r t  i c I e-surf  ace i n t e r a c t  ions i n t h e  eV energy range , 
l i t t l e  exper imental  i n fo rma t ion  i s  c u r r e n t l y  a v a i l a b l e .  Th is  i s  
due t o  t h e  r a t h e r  awkward experimental techniques t h a t  must be used t o  
per fo rm measurements a t  these energies.  However, w i t h i n  t h e  pas t  two 
years severa l  measurements of t h e  momentum t r a n s f e r  t o  sur faces  by 
atoms, molecules, and ions i n  t h e  energy range 1-200 eV g i v e  some in -  
o f  t h e  charac ter  of  t h e  p a r t i c l e - s u r f a c e  i n t e r a c t i o n  a t  these 
A study of t h e  dependence o f  normal momentum t r a n s f e r  t o  
by argon and he1 i um atoms [3] w i t h  energ ies  up t o  4 eV i n d i c a t e  
he h ighe r  energ ies  i nves t i ga ted  t h e  momentum of  t h e  atoms 
leav ing  t h e  sur face  i s  roughly 30 percent  o f  t h e  i n c i d e n t  momentum. Work 
a t  t h e  U n i v e r s i t y  o f  V i r g i n i a  [4] i n d i c a t e s  t h a t  f o r  IO eV N2 molecules 
t h e  momentum o f  t h e  p a r t i c l e s  l eav ing  t h e  su r face  i s  about 15 percent  of  
t h e  i n c i d e n t  momeniurn. S i i i i i  iar- riieasur-einerits using ~ 2 +  i ~ i i s  iii the  
3 
energy range 5-100 eV show t h a t  t h e  r e f l e c t e d  momentum i s  30-100 percent  
o f  t h a t  of t h e  i n c i d e n t  ions.  Whi le  these s t u d i e s  were c a r r i e d  o u t  
under c o n d i t i o n s  where t h e  charac ter  o f  t h e  sur faces was n o t  w e l l  con- 
t r o l l e d ,  they a l l  i n d i c a t e  t h a t  when atoms, molecules, o r  ions moving 
w i t h  energ ies o f  severa l  eV s t r i k e  a s o l i d  sur face  a la rge  number o f  t h e  
p a r t i c l e s  leav ing  t h e  sur face have v e l o c i t i e s  comparable t o  t h e  v e l o c i t y  
of t h e  i n c i d e n t  p a r t i c l e s .  This  suggests t h a t  a t  these energ ies  many o f  
t h e  i n c i d e n t  p a r t i c l e s  c o l l i d e  w i t h  o n l y  a few o f  t h e  sur face  atoms and 
t h a t  they do n o t  spend an apprec iab le amount of t ime  on t h e  sur face.  
Al though no s i m i l a r  s tud ies  have been c a r r i e d  o u t  f o r  monatomic oxygen, 
i n  l i g h t  o f  t h e  r e s u l t s  f o r  o the r  p a r t i c l e s  one should n o t  a n t i c i p a t e  
t h a t  0 atoms s t r i k i n g  a sur face a t  s a t e l l i t e  v e l o c i t i e s  w i l l  accommodate 
s u f f i c i e n t l y  t o  the  temperature o f  t h e  sur face  t o  cause processes such 
as adso rp t i on  and recombination t o  proceed a t  a r a t e  t h a t  i s  independent 
o f  t h e  energy o f  t h e  0 atoms. Cur ren t  t h e o r i e s  on t h e  thermal accommodation 
c o e f f i c i e n t  [SI a I s 0  i n d i c a t e  t h a t  a t  h igh  energ ies ( i n  t h e  eV range) one 
expects  t h e  p a r t i c l e s  s t r i k i n g  t h e  su r face  t o  exper ience o n l y  a few col-  
l i s i o n s  w i t h  sur face  atoms and t h a t  t h e  energy o f  t h e  r e f l e c t e d  p a r t i c l e s  
w i l l  be comparable t o  the  i nc iden t  energy. 
4 

SECTION I l l  
EXPERl MENTAL APPARATUS 
The atomic oxygen beam f o r  these exper iments i s  produced by - 
forminn Y 0 !ens i:: a: ion s ~ i i ~ c e ,  a c c e l e r a t i n g  t h e  ions e l e c f r o s t a t i c a i i y  
t o  t h e  des i red  energy, and then removing t h e  excess e l e c t r o n  by c o l l i s i o n a l  
detachment, r e s u l t i n g  i n  a fas t ,  neu t ra l  atomic oxygen beam. 
E f f o r t s  du r ing  t h i s  i n i t i a l  year  have been devoted t o  t h e  des ign 
and c o n s t r u c t i o n  o f  t h e  a l  I s t a i n l e s s  s tee l  atomic oxygen beam system, and 
t o  the  procurement o f  t h e  r e q u i s i t e  apparatus and ins t rumenta t ion .  
A schematic o f  t he  beam system i s  shown i n  F ig .  I .  The system 
c o n s i s t s  o f  an ion  source, mass a n a l y s i s  chamber, n e u t r a l i z a t i o n  chamber, 
and an u l t r a - h i g h  vacuum t a r g e t  chamber. These components w i l l  be de- 
sc r i bed  i n  more d e t a i l  i n  the f o l l o w i n g  paragraphs. 
The ion  source i s  a magnet ica l l y  conf ined, o s c i l l e t i n g  e l e c t r o n  
bombardment source capable o f  genera t ing  in tense beams o f  p o s i t i v e  or  
negat ive  ions [ 7 ] .  
0 2  and C02 as source gases, both o f  which g i v e  about t h e  same t o t a l  
c u r r e n t  o f  0- ions. 
f i l a m e n t  l i f e t i m e  o f  on l y  about 6 t o  8 hours. Since t h i s  i s  incon- 
v e n i e n t l y  s h o r t  we are  p resen t l y  us ing  C02 which r e s u l t s  i n  a q u i t e  
s a t i s f a c t o r y  f i l a m e n t  l i f e t i m e  o f  about 50 hours. 
For t h e  genera t ion  o f  0- i on  beams we have used both 
Using O2 however, as the  source gas r e s u l t s  i n  a 
The negat ive ions are e x t r a c t e d  from t h e  source and focused by 
a t h r e e  element e i n z e l  lens system. Immediately downstream o f  t h e  
e i n z e l  lens are  mounted fou r  p a i r s  o f  v e r t i c a l  and h o r i z o n t a l  s t e e r i n g  
p l a t e s  t o  d i r e c t  t h e  beam into t h e  ent rance s l i t  of t h e  magnet. 
source chamber i s  pumped by a 4 i n .  o i l  d i f f u s i o n  pump w i t h  a r a t e d  
pumping speed o f  750 R/sec. 
loca ted  a water cooled b a f f l e ,  l i q u i d  n i t r o g e n  t r a p ,  and a 4 i n .  gate 
The 
Between t h e  pump and t h e  chamber a r e  
<,-I . ,-  TI-..- AL .-I.-.-. * I  ^-e^ v a l v e .  IIIu> i i ie puiiip;iiy =peed a f  fhs S O U ~ C B  chamber i s   educed t o  
6 
about 170 R/sec. The background pressure i n  
5 x T o r r  (no  source gas) and normal l y  r 
I x T o r r  w i t h  t h e  source i n opera t ion .  
t h e  source chamber i s  about 
ses t o  a pressure of  about 
I n  t h e  magnet s e c t i o n  the ions a r e  de lec ted  through an angle of  
z n O  L- 
JW ) U T  se\ection. i i i e  aver-aye r-adius  O$ t i le  ioii pai-i-1 i s  23.5 
The entrance and e x i t  s l i t s  of t h e  magnet a r e  o f  equal s ize ,  0.635 cm 
i n  w i d t h  and 1.25 cm high.  
A f t e r  e x i t i n g  from t h e  magnet s e c t i o n  t h e  
another  t h r e e  element e i n z e l  lens and d i r e c t e d  by 
s e t  of  s t e e r i n g  p l a t e s  i n t o  the n e u t r a l i z i n g  c e l l  
n e u t r a l i z a t i o n  chamber. See Fig. I .  The n e u t r a l  
14 in .  i n  diameter and 12 i n .  long. T h i s  chamber 
ons are  focused by 
means o f  a second 
mounted i n  t h e  
z a t i o n  chamber i s  
i s  pumped by a 6 in .  
o i l  d i f f u s i o n  pump w i t h  a ra ted  pumping speed o f  1440 R/sec, w i t h  a 
water  cooled b a f f l e ,  l i q u i d  n i t r o g e n  t rap ,  and 6 in .  ga te  v a l v e  between 
t h e  pump and chamber. The pumping speed a t  t h e  chamber i s  t h u s  reduced 
t o  about 350 !?,/set. Normal background pressure i n  t h e  n e u t r a l i z a t i o n  
chamber i s  3 x T o r r .  This p ressure  r i s e s  t o  about 5 x T o r r  
o p e r a t i n g  o n l y  t h e  source, and t o  about 8 x T o r r  w i t h  t h e  source 
o p e r a t i n g  and n e u t r a l i z i n g  gas i n  t h e  n e u t r a l i z i n g  c e l l .  
The n e u t r a l i z i n g  c e l l  i s  an aluminum c y l i n d e r  o r i e n t e d  so t h a t  
t h e  beam t r a v e r s e s  a long t h e  l o n g i t u d i n a l  a x i s  o f  t h e  c y l i n d e r .  The 
c e l l  has an i n s i d e  diameter of about 3.18 cm and i s  about 7.62 cm long. 
The c e l  I conta ins  a cy1 i n d r i c a l  w i r e  g r i d  mounted c o n c e n t r i c a l  l y  w i t h  
t h e  c e l l  a x i s .  T h i s  g r i d  can be used t o  measure t h e  e l e c t r o n s  detached 
i n  t h e  c o l l i s i o n  process. An i o n  gage, mounted d i r e c t l y  on t h e  neutra- 
l i z i n g  c e l l ,  i s  used t o  moni tor  t h e  n e u t r a l i z i n g  gas pressure.  The 
pressure i n  t h e  c e l  I when evacuated i s  about 8 x Torr ,  whereas 
t h e  c e l  I pressure when neut ra l  i z i n g  t h e  0- beam i s  about 5 x t o  
T o r r .  
The t a r g e t  chamber employs o n l y  metal-to-metal seals .  I t  has 
an i n s i d e  diameter of 17 i n .  and i s  20 in .  long. The chamber i s  
7 
pumped by a NRC-206 6 i n .  Orb-Ion pump w i t h  a r a t e d  pumping speed fo r  
a i r  o f  900 R/sec. I n  a d d i t i o n  t o  t h e  Orb-Ion pump t h e  t a r g e t  chamber 
a l s o  employs a d isk ,  cooled t o  about 2Z°K by a Malaker Corpora t ion  
"Cryomi te "  cryopump, t o  condense those mo I ecu I es e n t e r i  ng from t h e  
n e u t r a l  i z a t i o n  chamber. Thus, t h e  t a r g e t  chamber i s  e n t i r e l y  f r e e  
of  o rgan ic  m a t e r i a l s  except f o r  t h a t  smal l  amount e n t e r i n g  from t h e  
n e u t r a l i z a t i o n  chamber. The u l t i m a t e  vacuum o f  t h e  t a r g e t  chamber 
has n o t  y e t  been determined bu t  should be i n  t h e  T o r r  range. 
The i n i t i a l  pump-down fo r  leak d e t e c t i o n  us ing  o n l y  the  Orb-Ion pump 
and rubber gasket  sea ls  r e s u l t e d  i n  a pressure of  8 x T o r r  w i t h  
no d e t e c t a b l e  leaks. 
The mass determinat ions w i l l  be performed u s i n g  a G r a n v i l l e -  
P h i l l i p s  Spectrascan 750 quadrupole gas analyzer .  
a r a t e d  s e n s i t i v i t y  o f  about T o r r  fo r  N2 and should be adequate 
f o r  our purposes. The analyzer  has been received and w i l l  be checked 
thorough ly  and p u t  i n t o  opera t ion  w i t h i n  t h e  n e x t  two weeks. 
T h i s  ana lyzer  has 
a 
SECTION I V  
FUTURE WORK 
We have now completed the  c o n s t r u c t i o n  phase of t h e  research  
program, as planned. The f i r s t  exper iment  t o  be per formed i n  t h e  
coming year  i s  s imp ly  t o  observe t h e  mass components i n  a c u b i c a l  
accommodation chamber i n t o  which o u r  f a s t ,  n e u t r a l  a tomic  oxygen 
beam i s  admi t ted  The accommodation chamber i s  c o n s t r u c t e d  of two 
p ieces  o f  copper e a s i l y  taken a p a r t  by removal o f  seve ra l  smal l  
screws. The des red  s u r f a c e  m a t e r i a l  can t h u s  be e l e c t r o p l a t e d  on 
t h e  copper chamber q u i t e  e a s i l y .  
chamber and gas ana lyze r  i s  now proceeding, and we shou ld  be a b l e  
t o  make o u r  f i r s t  p r e l i m i n a r y  measurements w i t h i n  t h e  n e x t  few 
weeks . 
Mounting of t h e  accommodation 
9 
REFERENCES 
I .  Wood, B. J .  and H. Wise, Jr., J. Phys. Chem. - 65, 1976 (1961) .  
2. Wise, H. and B. J. Wood, T h i r d  I n t e r n a t i o n a l  Symposium on 
R a r e f i e d  Gas Dynamics, Vol .  I ,  p. 470, Academic Press 
( 1963). 
3. Abauf, N. and D. G. H, Marsden, F i f t h  i n t e r n a t i o n a l  Symposium 
on R a r e f i e d  Gas Dynamics, Vol .  I ,  p. 199, Academic Press (1967).  
4. Bor ing,  J.  W. and R. R. Humphris, S i x t h  I n t e r n a t i o n a l  Symposium 
on R a r e f i e d  Gas Dynamics, MIT, Cambridge, Massachusetts, J u l y  
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5. Mai r ,  W. N., B. W. Viney, and J.  S. C o l l i g o n ,  F i f t h  I n t e r n a t i o n a l  
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( 1962). 
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DISTRIBUTION LIST 
Na t iona l  Aeronaut ics and Space A d m i n i s t r a t i o n  
A t t n  : Techn i ca I Reports O f f  i c e r  
S c i e n t i f i c  and Technical  I n fo rma t ion  O f f i c e  
Code US 
Washington, D.C. 20546 
G. D. Magnuson 
J.  W .  Bor ing  
R. R. Humphris 
U n i v e r s i t y  L i b r a r y ,  A t t n :  Dr .  R. Fran tz  
A. R. Kuhlthau 
R. A. Lowry 
C. E. C a r l s t o n  
R. S. Synder 
M. S.  M i l l e r  
J.  Baldr ige,  J r .  
RLES F i  les 


The interaction of oxygen atoms with solid surfaces at eV energies Annual status report, 1 Jun. 1968 - 31 May 1969

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NASA Technical Reports Server

NASA Technical Reports Server