A method for producing an atomic oxygen beam is provided by the present invention. First, a material 10' is provided which dissociates molecular oxygen and dissolves atomic oxygen into its bulk. Next, molecular oxygen is exposed to entrance surface 11' of material 10'. Next, material 10' is heated by heater 17' to facilitate the permeation of atomic oxygen through material 10' to the UHV side 12'. UHV side 12' is interfaced with an ultra-high vacuum (UHV) environment provided by UHV pump 15'. The atomic oxygen on the UHV side 12' is excited to a non-binding state by exciter 14' thus producing the release of atomic oxygen to form an atomic oxygen beam 35'

Outlaw, Ronald A.

NASA Technical Reports Server

United States Patent 1191 [i l l  Patent Number: 4,828,817 
outlaw [ a ]  Date of Patent: May 9, 1989 
METHOD FOR PRODUCING AN ATOMIC 
OXYGEN BEAM 
Inventor: Ronald A. Outlaw, Newport News, 
Va. 
Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Appl. NO.: 113,955 
Filed: Oct. 29, 1987 
Int. Cl.4 .............................................. BOlD 53/32 
U.S. Cl. ............................... 423/579; 204/157.41; 
204A57.44; 204A57.45; 250/251 
Field of Search ...................... 204A57.44, 157.41, 
204A57.5; 250/251; 423/579 
References Cited 
U.S. PATENT DOCUMENTS 
3,143,647 
3,341,352 
3,359,705 
3,876,957 
3,969,481 
4,261,863 
4,488,951 
4,542,010 
Gunther et al. . 
Ehlers . 
Mullhaupt . 
Thatcher . 
Murray et al. . 
Kent et al. . 
Nolan et al. . 
Roman et al. . 
4,585,945 4/1986 Bruel et al. . 
4,599,157 711986 suzuki et al. . 
4,602,987 7/1986 Bonaventura et al. . 
4,686,022 8/1987 Rempt ............................ 204A57.41 
4617029 io/i986 e t d .  . 
FOREIGN PATENT DOCUMENTS 
Primary Examiner-John Doll 
Assistant Examiner-Wayne A. Langel 
Attorney, Agent, or Firm-George F. Helfrich; John R. 
Manning; Charles E. B. Glenn 
t571 ABSTRACT 
A method for producing an atomic oxygen beam is 
provided by the present invention. First, a material 1 0  
is provided which dissociates molecular oxygen and 
dissolves atomic oxygen into its bulk. Next, molecular 
oxygen is exposed to entrance surface 11' of material 
10. Next, material 1 0  is heated by heater 17' to facili- 
tate the permeation of atomic oxygen through material 
1 0  to the UHV side 12'. UHV side 12' is interfaced with 
an ultra-high vacuum (UHV) environment provided by 
UHV pump 15'. The atomic oxygen on the UHV side 
12' is excited to a non-binding state by exciter 14' thus 
producing the release of atomic oxygen to form an 
atomic oxygen beam 35'. 
10 Claims, 2 Drawing Sheets 
https://ntrs.nasa.gov/search.jsp?R=20080012347 2019-08-30T03:58:07+00:00Z
U.S. Patent May 9,1989 Sheet 1 of 2 4,828,817 
US. Patent May 9,1989 Sheet 2 of 2 4,828,817 
1 
4,828,817 
2 
SUMMARY OF THE INVENTION 
According to the present invention, the foregoing 
and additional objects are obtained by the following 
5 method for generating a pure atomic oxygen beam. An 
ultra high vacuum 0environment is interfaced 
with a containing atomic oxygen. This material 
contains atomic oxygen after being heated and exposed 
to molecular oxygen. The molecular oxygen dissociates 
then dissolves in the bulk of the material. The material 
TECHNICAL FIELD OF THE INVENTION is heated to a temperature which facilitates transport of 
the atomic oxygen to the ultra-high vacuum interface, 
This invention relates generally to the separation of where it on the surface in the atomic state The 
oxygen molecules into atoms and more Particularly to 15 surface atomic oxygen is excited to a non-bonding state 
the Production of a high Purity, neutral hYPerfhemal and released to the gas phase by either an electron beam 
atomic oxygen beam. or a photon beam. The material temperature is main- 
tained low enough to prevent surface oxygen from 
recombining to molecular oxygen. The resulting oxy- BACKGROUND OF THE INVENTION 
The production of a laboratory atomic oxygen beam 20 gen beam can be further purified by removal of positive 
is useful in a variety of applications. For example, re- or negative oxygen ions by using grids at the appropri- 
searchers need to simulate the flux and energy of atomic ate potential. Accordingly, an essentially pure beam of 
oxygen encountered by space vehicles in the upper atomic oxygen is produced in the terrestrial laboratory 
atmosphere. The high chemical reactivity of this atomic at sufficient flux and energy levels to simulate condi- 
oxygen flux causes substantial degradation of organic 25 tions in a low Earth orbit. 
METHOD FOR PRODUCING AN ATOMIC 
OXYGEN BEAM 
ORIGIN OF THE INVENTION 
Government and may be 
The invention described herein was made by an em- 
by Or for the government for govern- 
ployee Of the 
tured and 
thereon or therefor. 
mental purposes without the payment of any 10 into atomic oxygen at the surface. The atomic oxygen 
materials, thereby resulting in reduced material life- 
times. Also, such an atomic oxygen beam may be used 
in the calibration of mass spectrometers and other de- 
tection systems which are utilized in determining the 
densities of the gases within the flight envelope of 
spacecraft in low Earth orbit. Other applications in- 
clude studying the kinetic properties of atomic oxygen 
as it collides with other molecules, determining drag 
coefficients, and studying basic interaction between 
atomic oxygen and surfaces. 
Currently, numerous methods are used to generate 
atomic oxygen beams. For example, radio frequencies 
or microwaves excite an oxygen bearing gas to create a 
gas discharge which includes atomic oxygen and other 
species. Second, oxygen molecules are heated in the 
vicinity of a surface until oxygen atoms are thermally 
dissociated. Third, electrons are impacted against an 
oxygen bearing gas to create oxygen ions, which are 
mass separated and neutralized to atomic oxygen by a 
charge transfer cell. All prior methods share a combina- 
tion of the following problems: low flux density, low 
mean energy, wide eergy distribution, numerous excited 
states of oxygen and accompanying gases, and synergis- 
tic reactions from different species. More significantly, 
these methods result in a high contamination back- 
ground density of molecular oxygen or other gas prod- 
ucts which distort the true interaction between the 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a pictoral view of a specific embodiment of 
FIG. 2 is pictoral view of another specific embodi- 
the present invention: and 
ment of the present invention. 
30 
DETAILED DESCRIPTION OF THE 
INVENTION 
Referring to FIG. 1, a material 10 is shown which has 
a high oxygenic solubility and diffusivity and which 
dissociates moleuular oxygen (02)  and dissolves atomic 
oxygen (0) into its bulk. Examples of such a material 
include silver, zirconium, titanium, and hafnium. 
Material 10 may be a source of atomic oxygen as the 
result of one of two processes. In the first specific em- 
bodiment of the present invention, the material 10 is 
pre-charged with oxygen. Material 10 is a polycrystal- 
line wire located within chamber 26. The wire is bent 
45 into a hairpin shape to provide minimal temperature 
variation along the straight portion 22 of material 10. 
The wire is cleaned and then resistively heated to ap- 
proximately 500" C., as measured by thermocouple 24. 
Next, material 10 is charged with 0 2 ,  which is intro- 
50 duced into chamber 26 at a pressure ranging from 0.1 to 
loo0 Torr, for approximately one hour. Next, chamber 
26 is maintained at a temDerature of amroximatelv 200" 
35 
40 
C. at a pressure of 1 x 10-8  TO^ for'ipproximatily 12 
atomic Oxygen and the sample material. For example, if hours. During this bake-out period, the temperature of 
the background pressure is 1 X Torr, the entire 55 material 10 in chamber 26 is maintained below 20" C. to 
surface of the sample is covered in one second by gases -e any loss of oxygen. Isolation valve 30 remains 
such as co and molecular oxygen, thereby Preventing a closed during this bake-out period. As chamber 26 
clear distinction between the effects of these reactive COOIS, material 10, and grids 20 and 21 are degassed. 
gases and atomic oxygen. After this cooling is complete, the pressure in chamber 
Accordingly, it is a primary object of this invention 60 26 becomes UHV (less than 1 x 10-9 Torr) Isolation 
to generate an atomic oxygen beam which closely simu- valve 30 is then opened to mass spectrometer detection 
lates low Earth orbit conditions. device 16. The pressure of chambers 26 and 28 is then 
It is a further object of this invention to accomplish lowered to approximately 5 x 10- 12 Torr. At this time 
the above object without the contamination which is the wire is resistively heated to a temperature sufficient 
characteristic of the prior art. 65 to promote atomic oxygen diffusion to the surface, and 
Other objects and advantages of this invention will subjected to electron or photon bombardment to pro- 
become apparent in the specification and drawings duce the atomic oxygen beam, as described below. If 
which follow. material 10 is silver, a sufficient temperature is 500" C. 
4,828,817 
3 4 
Alternatively, in the second embodiment of the pres- charged grid 21 sweeps aside the negatively charged 0 
ent invention material 10 may be a source of atomic atoms. 
oxygen by the following process. Referring now to Enclosure 25 surrounds material 10, exciter 14, and 
FIG. 2, material 10 is a thin membrane. Material 10 cylindrical grids 20 and 21. Since atomic oxygen is 
may be of a thickness of approximately a quarter of a 5 extremely reactive to metals, enclosure 25 should be 
millimeter. Molecular oxygen at a pressure ranging composed of a material which has a small recombina- 
between 0.1 and lo00 Torr is introduced to the entrance tion coefficient for 0 to 0 2 .  Preferably, enclosure 25 is 
surface 11' of material 10. This second embodiment comprised of silicon dioxide (SiOz), which has a recom- 
allows for a continuous flux of oxygen atoms to be bination coefficient of approximately 
produced. 10 Accordingly, the present invention produces an 
Material 10 is heated between approximately 400" e. atomic oxygen beam 35 or 35' whish is essentially pure 
and approximately 800" C. by a heater 17' while being and contains neutral atomic oxygen with an energy 
exposd to 0 2 .  This heating allows for control of the level ranging from approximately 1 eV to 10 eV. This 
amount of atomic oxygen passing through material 10'. atomic oxygen beam 35 has a flux of up to 1015~m-~s-. 
This heating by heater 17' may be accomplished by an 15 Also, the low background pressure created by the ultra- 
electrical resistance heater located outside the vacuum high vacuum 15 allows for less reaction products be- 
system. Also, the upstream pressure of 0 2  against en- tween the atomic oxygen beam 35 or 35' and any reac- 
trance surface 11' in the second embodiment of the tive gases. Thus, the present invention produces an 
present invention aids in this control of atomic oxygen. atomic oxygen beam 35 which simulates low Earth 
trance surface 11' of material 10'. Next, the 02 dissoci- We claim: 
ates into 0 and dissolves into the bulk of material 10. 1. A method of producing an atomic oxygen beam 
Next, the atomic oxygen exits the bulk of material 10 comprising: 
and becomes an adsorbed atomic layer at UHV side 12'. providing a material having two sides, which material 
The temperature of UHV side 12' should be less than 25 dissociates molecular oxygen and dissolves atomic 
500" C. when material 10' is silver. When the 0 atoms oxygen into its bulk: 
arrive at UHV side 12', surface diffusion occurs at tem- exposing one side of said material to molecular oxy- 
gen; 
sions and subsequent recombination into desorbed 0 2  providing an ultra high vacuum environment of a 
molecules. To prevent recombination, the 0 atoms must 30 pressure of less than 1 x 10-9Torr on the other side 
remain in the adsorbed state by maintaining UHV side of said material; 
12' below 500" C. An ultra-high vacuum pump 15' is heating said material to a sufficient temperature to 
located on the UHV side 12' of material 10. This vac- promote atomic oxygen permeation to the other 
uum pump 15' provides a clean, non-contaminating side of said material; 
environment on the UHV side 12' with a pressure of 1 35 exciting atomic oxygen to a non-binding state which 
to 5 x 10- 12 Torr. results in its release from the other side of said 
Next, the 0 atoms are desorbed from UHV side 12' or material as an excited atomic oxygen beam. 
12 by exciter 14' or 14 to form atomic oxygen beam 35' 2. The method of claim 1 wherein the excited atomic 
or 35. Exciter 14' or 14 may excite the atomic oxygen to oxygen beam is exposed to a negatively charged means 
a non-binding state in one of two ways. First, exciter 14' 40 to remove positively charged atomic oxygen from the 
or 14 may direct low energy electrons with an energy beam. 
level of approximately 100 to 500 electron-Volts (eV) at 3. The method of claim 1 wherein the excited atomic 
UHV side 12' or 12. These electrons should have a oxygen beam is exposed to a positively charged means 
current density of at least 0.5 milliamps per square centi- to remove negatively charged atomic oxygen from the 
meter upon UHV side 12 and is another control of the 45 beam. 
atomic oxygen flux. Exciter 14' or 14 may be a tungsten 4. The method of claim 1 wherein the said material is 
filament. In the second specific embodiment, exciter 14' selected from the group consisting of silver, zirconium, 
is a tungsten filament which is placed parallel to mate- titanium, and hafnium. 
rial 10 and is located approximately two millimeters 5. The method of claim 1 wherein the step of exciting 
from UHV side 12'. The tungsten filament is resistively 50 the atomic oxygen to a non-binding state includes bom- 
heated to provide the desired low energy electrons. barding said material with electrons at the side of the 
This technique is referred to as electron stimulated ultra high vacuum environment. 
desorption (ESD). 6. The method of claim 1 wherein the step of exciting 
Second, exciter 14 or 14 may be photon emitter the atomic oxygen to a non-binding state includes bom- 
which directs a high flux of photons at UHV side 12' or 55 barding said material with photons at the side of the 
12 to excite the exciting 0 atoms to non-binding states. ultra high vacuum environment. 
These photons should also have an energy level suffi- 7. A method of producing an atomic oxygen beam 
cient to desorb the 0 atoms. This technique is referred comprising: 
to as photon stimulated desorption (PSD). providing a material which dissociates molecular 
oxygen and dissolves atomic oxygen into its bulk; 
side 12 in atomic oxygen beam 35 are desired 0 neu- placing said material in a chamber; 
trals. However, a small percentage of the 0 atoms are heating said material to at least approximately 500" 
positively charged atomic oxygen. To prevent these C.; 
charged particles from being present in the atomic oxy- exposing said material to molecular oxygen at a pres- 
gen beam, grid 20 is provided. Grid 20 is charged with 65 sure between approximately 0.1 to lo00 Torr; 
a negative bias and thereby sweeps aside any positively heating said chamber to approximately 200" C. at a 
charged 0 atoms. An even smaller percentage of the pressure of approximately 1 x 108 Torr while main- 
exiting 0 atoms are negatively charged. Positively taining said material at a temperature of 20' C.; 
In the second embodiment, 0 2  is adsorbed onto en- 20 orbit conditions. 
peratures above 550" C., which results in 0 atom colli- 
The majority of the 0 atoms desorbing from UHV 60 
4.828.8 17 , ,
5 
providing an ultra-high vacuum environment of less 
than 1 x 10-9 Torr to said exposed material; 
heating said exposed material to a sufficient tempera- 
ture to promote atomic oxygen diffusion to the 
surface of said material; 
exciting atomic oxygen to a nonbinding state which 
results in its release from the surface of said mate- 
rial as an atomic oxygen beam. 
10 
15 
20 
25 
30 
35 
40 
45 
6 
8. The method of claim 7 wherein the said material is 
selected from the group consisting of silver, zirconium, 
titanium, and hafnium. 
9. The method of claim 7 wherein the step of exciting 
the atomic oxygen to a non-binding state includes bom- 
barding said material with electrons at the side of the 
ultra high vacuum environment. 
10. The method of claim 7 wherein the step of excit- 
ing the atomic oxygen to a non-binding state includes 
bombarding said material with photons at the side of the 
ultra high vacuum environment. * * * * *  
50 
55 
60 
65 


Method for producing an atomic oxygen beam

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