Laboratory experiments in which recombined CO, CO2, D2O, OH, N2, H2, and O2 molecules desorb from surfaces in excited internal and translational states are briefly reviewed. Unequilibrated distributions predominate from the principally catalytic metal substrates so far investigated. Mean kinetic energies have been observed up to approx. 3x, and in some cases less than, wall-thermal; the velocity distributions generally vary with emission angle, with non-Lambertian particle fluxes. The excitation state populations are found to depend on surface impurities, in an as yet unexplained way

Barrett, J. L.

Kofsky, I. L.

English

NASA Technical Reports Server

SURFACE-CATALYZED RECOMBINATION INTO EXCITED ELECTRONIC, 
VIBRATIONAL, ROTATIONAL, AND KINETIC ENERGY STATES: A REVIEW 
I. L. Kofsky and J. L. Barrett 
PhotoMetrics, Ine. ,  Woburn, MA 
Abstract. 
OH, N2, H2, 
and trans la t 
Laboratory experiments in which recombined CO, C02, D20, 
and 02 molecules desorb from surfaces in excited internal 
ional states are briefly reviewed. Unequilibrated distrib- 
utions predominate from the principally catalytic-metal substrates so 
far investigated. 
and in some cases less than, wall-thermal; the velocity distributions 
generally vary with emission angle, with non-Lambertian particle fluxes. 
The excitation state populations are found to depend on surface 
impurities, in an as yet unexplained way. 
Mean kinetic energies have been observed up to -3x, 
Introduction 
Several small-molecular species have been found to desorb after forma- 
tion on laboratory surfaces with internal and kinetic energy distributions 
(and angular fluxes) markedly different from those that would result from 
their equilibration at the substrate's temperature. The observations 
for neutral molecules other than NO2 and NO, which are reviewed separately 
in the accompanying paper by Barrett and Kofsky [these Proceedings] 
(with further data on NO* in Kofsky and Barrett [these Proceedings]), 
are listed in Table 1. 
Discussion 
This lack of equilibration, and a comparably common non-Boltzmann 
(statistical) distribution of desorption quantum states, are an at 
least qualitatively expected outcome of the dynamics of the surface 
reaction complex from which the molecular fragment originates [Polanyi, 
1972; Tully, 1980; Kori and Halpern, 1983; Ceyer et al., 19831. Indeed, 
our literature survey identified only one measurement of complete rota- 
tional and vibrational thermalization of a recombination product, OH on 
polycrystalline Pt [Talley et al., 19811 (item [14] in Table l), curi- 
ously that very species initially considered responsible for spacecraft 
glow [by several groups]. This result was interpreted [Talley et al., 
19811 as due to the rate of desorption of hydroxyl excited in Langmuir- 
Hinshelwood (among species present on the surface) reactions of 0 with H 
being small compared with the rate of collisional equilibration of the 
adsorbed radical with its substrate. We briefly touch on the more sophis- 
ticated theoretical arguments after reviewing the database of Table 1. 
These quantum state-specific measurements were made using passive 
spectrometry, tuned laser and (to a lesser extent) electron beam induced 
fluorescence, and for the velocity distributions, time-of-flight and 
most recently laser Doppler methods [Cavanagh and King, 19841. (Some 
qualitative reports of experiment results that were later verified have 
been omitted, as have gas-phase reactions of precursors formed on sur- 
149 
https://ntrs.nasa.gov/search.jsp?R=19860003785 2020-03-20T15:57:58+00:00Z
faces.) 
the surface crystal structure and its contamination (at the sub-monolayer 
level) could be characterized in situ by low-energy electron diffraction, 
Auger electron spectroscopy, and similar surface-science technologies. 
In the experiments indicated one reactant (or an impurity) was initially 
incorporated into the target surface, and in some others the atomic 
species was made to permeate through the sample from its back side; this 
of course does not mean that reactants from the beam also do not become 
adsorbed on, and in some experiments dissociated on, the substrate. (Mole- 
cules such as N2 and NO have zero activation energy for dissociation on 
some transition metal crystal planes.) A l l  but one set of the measure- 
ments (those of Harteck's group, [3], [ll], [l2], and [13] in Table 1) 
have been done within the last 6 years. The discipline and diagnostics 
have not yet developed to a stage at which all the internal quantum 
states of a desorbate as functions of its translational energy and angle 
of emission have been measured simultaneously. 
The list refers to heterogeneous recombination involving species 
incident (or diffusing) at mean velocities (1 km/sec and thus kinetic 
energies <1/2 eV. 
dopants, in processes that might be referred to as selective chemical 
sputtering into molecular states, or alternatively as reactive scattering. 
It does not include non-associative molecule scattering [Asscher et al., 
19851 or desorption [Cavanagh and King, 19841 experiments, many of which 
also provide evidence for nonequilibration and non-Lambertian 
(cos1 {angle 0 to the normal)) effluent flux distributions. 
it specifically omits physical sputtering, which refers to neutral or 
ionized incident particles with directed kinetic energy greater than the 
few tens eV threshold for dislodging atoms and complexes from the surface, 
some of which are ionized or in electronically excited states (reviewed 
in the context of spacecraft glows by Kofsky [19841). 
In this regard, most of the existing information about the ablative 
reactions with surface materials of 8 km/sec relative velocity atoms and 
molecules, such as those at orbital altitudes, comes from shuttle and 
other spacecraft exposure experiments/tests rather than the laboratory 
or theory. While the incident 5-eV oxygen atoms are well known to be 
associated with the mass loss and texturing of various types of exposed 
surfaces [Visentine et al., 19851, no connection between the rates of 
erosion and glow emission has been identified. 
by this kinetic energy in exciting spacecraft radiations is by no means 
established--the signal/noise of the measurements is insufficient to 
rule out the possibility that the glow off forward surfaces is due merely 
to the ram enhancement of densities of incident atoms and molecules. The 
mechanism might also involve 0 atom-associated (and perhaps off-surface 
induced plasma-associated) roughening of the surface to increase its 
concentration of sites where physisorption, chemisorption, and excitative- 
desorption can take place, and/or avoidance of "catalyst poisoning"-- 
buildup of reaction-inhibiting species on the lattice--by impact of 
the moderately energetic neutral (and charged) particles. In that 
sense, and in cases that the incident particles accommodate on the surface 
to participate in Langmuir-Hinshelwood reactions, the low reactant 
energy database of Table 1 is relevant to the spacecraft glows issue. 
Most of the recombination experiments in Table 1 were done on at least 
partially characterized surf aces of "catalytic" transition metals, in 
large part toward understanding the role of these (essentially, reducing) 
About half were performed in ultrahigh vacuum systems in which 
Some of these combine (as mentioned) with surface 
In addition 
Further, the role played 
150 
substrates, rather than on materials of the type exposed on orbiting 
spacecraft--generally, insulators. On this limited subset, recombina- 
tion with nonequilibrium and nonstatistical electronic, vibrational, 
and kinetic energy distributions (themselves desorpt 
of the product molecule is the rule rath 
lationally cold as well as hot desorbate 
as rotationally cold streams from non-associative d 
and King, 19813. Although forward focusing from the macroscopically 
smooth laboratory surfaces (due to the parallel. orientation of the 
surface barrier equipotentials) is common, flux distributions flatter 
than cosb The angular 
distribution of the radiating species (and potentially, the not-yet 
measured polarization of rotational-band emission from the desorbate) 
is a further diagnostic of the surface interaction that could be appli- 
cable in investigations of spacecraft glows. 
erature glass [Pravilov and Smirnova, 19811, the comparably exoergic 
NO + 0 surface reaction was not reported in connection with the several 
measurements of its gas-phase rate coefficient (references [6a-e] in 
Barrett and Kofsky [these Proceedings]). 
N-readily quantifiable by Auger electron or x-ray photoelectron spec- 
troscopy--plays a part in several of the internal excitations of the 
desorbed molecules; no theoretical explanation of this effect, which 
presumably involves formation of an intermediate complex, appears to 
have been advanced. Similarly, interference with the recombination- 
desorption process by other molecules introduced into the flow of the 
surface has been observed [Harteck and Reeves, 1964; references in 
Barrett and Kofsky, these Proceedings]; indeed, interference and sat- 
uration of reactive sites are ubiquitous features of surface studies. 
Presumably these species occupy, or create, sites at which adsorption/ 
excitative desorption occurs. Such an interference effect may be the 
reason for the "anomalously" rapid increase in intensity of the glow off 
Atmosphere Explorer at altitudes below 160 km (where atmospheric [02] 
is becoming comparable to [O]) reported by Yee and Abreu [1983]. 
can also occur [Cavanagh and King, 19841. 
Note that while CO and 0 have been found to recombine on room temp- 
Surface contamination by the Group V and Group VI elements S, 0, and 
Some Conclusions 
A detailed classical trajectory model exists for the more exoergic 
Rideal-Eley (one participant incident from the gas phase) recombination 
of 0 with C adsorbed at three fold sites on the Pt (111) crystal face 
[Tully, 19801. The theory predicts that of the -6 eV exothermicity, 
statistically 2 eV appears in vibrational, 1 eV in rotational, and 2 eV 
in translational energy of the CO desorbate, with forward peaking; this 
result is largely independent of the applied gas-surface interaction 
potential (which was derived empirically). Experimentally no such high 
translational energies have been observed for any species/substrate 
combination; on the other hand even though most of the laboratory results 
have been interpreted as due to L-H rather than R-E reactions, a similarly 
large fraction of the available energy in internal excitation is commonly 
reported (as Table 1 shows). As the interaction potentials and binding 
energies of upperatmospheric aeronomic species on the poorly-characte- 
rized and in many cases locally inhomogeneous and highly textured sur- 
faces of exposed spacecraft materials are much less well understood, as 
151 
is the formation of complexes on them, similar ab initio calculations 
of the channeling of energy into radiative states of desorbate molecules 
are unlikely to provide quantitative predictions of glow brightnesses. 
Existing information about quantum state-specific surface recombin- 
ations, in sum, indicates that when newly-formed molecules are desorbed 
from transition metals (and some glasses) a high probability exists 
that their internal energy is comparable to the reaction exothermicity, 
even when both participants were previously adsorbed on the surface; and 
that the degree of this excitation, and the surface reaction rate, often 
depends on the concentration of (oxidizing) surface contaminants, which 
themselves may be introduced from the incident beam. The database repre- 
sented by Table 1, comparable NO-NO2 surface reactions [Barrett and 
Kofsky, these Proceedings], non-associative desorption and scattering 
of various species (including in particular NO [Asscher et al., 19851) 
from clean model surfaces, and related surface processes are far below 
the critical mass required for development of credible predictive theory 
(as is in general the case for catalysis [Somorjai, 19811). Nonetheless 
some empirical extrapolation might be made to the largely insulating, 
rougher-surfaced materials exposed and as putative contaminants on low- 
Earth-orbiting spacecraft by applying the concepts developed to order 
the laboratory data (for example formation of complexes at desorption 
sites). 
paper on NO2 and NO recombination [Barrett and Kofsky, these Proceedings]. 
Additionally, measurements analogous to those performed in the laboratory 
of the sensitivity of off-surface emission intensity to substrate temper- 
ature--which assesses the binding of reaction participants to the 
surface, and thus may identify precursor species-and angular distribution 
of the desorbate flux would complement those already proposed in 
understanding and scaling spacecraft glow excitation processes. 
Some further application of these ideas is in the accompanying 
References 
Asscher, M., E. Pollak, and G. A. Somorjai, A model for vibrational and 
translational accommodation of NO molecules during scattering from 
a Pt (111) surface, Surf. Sci., 149, 146-156, 1985. 
Becker, C. A . ,  J. 0. Cowin, L. Wharton, and D. J. Auerbach, C02 product 
velocity distributions for CO oxidation on platinum, J. Chem. Phys., 
67, 3394-3395, 1977. [6 in Table 11 
mutation of metastable nitrogen generated by nitrogen atom recom- 
bination on cobalt and nickel, Chem. Phys. Lett., 90, 457-460, 1982. [2] 
thermally desorbed from Ru (lll), Phys. Rev. Lett., 47, 1829-1832, 1981. 
BreKen, W., and P. McIntyre, Vibrational relaxation and electronic 
Cavanagh, R. R., and D, S .  King, Rotational and spin-state distributions: NO 
Cavanagh, R. R., and D. S .  King, Molecule-surface interztions and dynam- 
ics, J. Vac. Sci. Tech., E, 1036-1037, 1984. 
Ceyer, S. T., W. L. Guthrie, T. H. Lin, and G. A. Somorjai, D20 product 
angular and translational energy distributions from the oxidation of 
deuterium on Pt (lll), J. Chem. Phys., 78, 6982-6990, 1983. [8] 
Comsa, G., R. David, and K. D. Rendulic, Vxocity distribution of H7, HD, - -  
and D2 molecules desorbing from polycrystalline nickel surfaces, Phys. 
Rev. Lett., 38, 775-778, 1977. [91 
152 
Comsa, G., R. David, and B.-J. Schumacher, The angular  dependence of f l u x ,  
mean energy, and speed r a t i o  f o r  D2 molecules desorbing from a N i  (111) 
sur face ,  Surf.  Sci.,  85, 45-68, 1979. [ l o ]  
(A Cu) molecules, D i s c .  Faraday SOC., 37, 82-86, 1964. [ l l ]  
Phys., 80, 1-6, 1984. [13] 
of spacec ra f t  r a m  glows, Radio Sci.,  19, 578-586, 1984. 
ox ida t ion  of C on P t ,  Chem. Phys. Lett., 98, 32-36, 1983. [7]  
Harte k 
Kenner, R. D., and E. Ogryzlo, Orange chemiluminescence from N02, J. Chem. 
Kofsky, I. L., Spectroscopic  consequences of Papadopoulos' d ischarge model 
Kori, M., and B. L. Halpern, Vibra t iona lTnergy  d i s t r i b u t i o n  of CO i n  t h e  
Kori, M., and B. L. Halpern, Vibra t iona l ly  exc i ted  C02 from t h e  r eac t ion  
P., and R. R. Reeves, Formation and r eac t ion  of t h e  exc i t ed  02 54-  
of 0 atoms and adsorbed CO on platinum, Chem. Phys. L e t t . ,  - 110, 223- 
229, 1984. [4]  
Mannella, G. G., and P. Harteck, Surface-catalyzed e x c i t a t i o n s  i n  t h e  
Mannella, G. G o ,  R. R. Reeves, and P. Harteck, Surface catalyzed e x c i t a t i o n  
Polanyi,  J. C., Some concepts i n  r eac t ion  dynamics, Accounts Chem. Res., 
Pravi lov,  A. M., and L. G. Smirnova, Chemiluminescence process i n  O(3P) + 
Somorjai, 6. A . ,  Chemistry i n  Two Dimensions: Surfaces,  Cornel l ,  I t haca  
Tal ley,  L. D., W. A. Sanders, D. J. Bogan, and M. C. Lin, Dynamics of 
oxygen system, J. Chem. Phys., 34, 2177-2180, 1964. [12] 
with N and 0 atoms, J. Chem. Phys., 33, 636-637, 1961. [3]  
5, 161-168, 1972. 
60 + M system, Kinet. Catal., 22, 416-419, 1981. 151 
NY, 1981, p. 377. 
hydroxyl r a d i c a l  desorp t ion  from a po lyc rys t a l l i ne  platinum su r face ,  
J. Chem. Phys., 25, 3107-3113, 1981. [14] 
Thorman, R. P., D. Anderson, and S .  L. Bernasek, I n t e r n a l  energy of 
heterogeneous r eac t ion  products: Nitrogen atom recombination on 
i r o n ,  Physa Rev. L e t t . ,  11, 743-746, 1980. [ I ]  
Tul ly ,  J. C. ,  Dynamics of gas-surface in t e rac t ions :  Reaction of atomic . -  - 
oxygen wi th  adsorbed carbon on platinum, J. Chem. Phys., 73, 6333- 
6342, 1980. 
Visent ine,  J. T., L. J. Leger, J. F. Kuminecz, and I. K. Spiker ,  STS-8 
atomic oxygen e f f e c t s  experiment, AAIA 23rd Aerospace Sciences Meeting, 
Paper #85-0415-CP, 1985. 
ment i n t e r a c t i o n ,  Geophys. Res.  L e t t . ,  10, 126-129, 1983. 
Yee, J a  H., and V. J. Abreu, V i s ib l e  glow induced by spacecraft-environ- 
153 
m
 
m
 
v
-
 
0
 
k
 
a
J
a
 
m
 
0
 
U
 
h
 
U
 
d
 
U
 
0
 
4
 
a) 
3
 
r
l 
d
 
U
 
m
 
.I4
 
U
 
(d
 
U
 
v
) 
1
 
3 z 0 0 cn U (d E-r ul 0 U H m 0 A L 30 
0
2
 
@
S
 
r
l 
W
 NM
 
a
h
 
G
I
 
N
U
 
n
 
d
 
4
 
4
 
U
 
pc 
n
 
00 
U
 
001 
n
 
M
 
0
 
d
 
.rl 
U
 
d
 
a
 r) g 
'E: 
.!% 
a
 
3: I 
Cll 
.- U 0 .c U 7 rl (d V .d U m -Id U (d U m n 3 JZ ii M 0 0 4 4 1
 
30 rl u-4 0 u \ F4u - fi Y 0 0 
. aJ k 7 U (d LI aJ a El QJ U rl (d d U (d k P .d 3 U *rl U co 1 6" 2 111 w 3 a 24 E-c 9) v) 1-I al 0 a aJ U (d U UJ m m aJ 4 G 5 ..L 5 % m N U aJ k 7 U (d k aJ B U aJ V (d W k 7 m ill m E-c 9( 
pc 
0
 
m
 Bo k W a 9) 
0
 
- m .d a a a V 0 k PI a, m aJ F 5 % 2 fJ L-.l h W a U & aJ k k (d h P m U Ll 0 a a, k M C .d h m z 8 U U (d Q 5 0 U k a, W d I N 0 z 0 z .. 
cn 
w
 
z cn w €- U x w z z €- a p: 0 cn w P z 5 a H U 13 L 8 2 3 H €- H a U w p: w U 4 L4 cr: 3 cn 4 W II c4 3 
m
 
7
 
\D
 
N
U
 
N
m
b
 
m
92L 
M
 
0
 
0
 
9
 
I 
0
 
0
 
m
 - 50 rl w 0 u 0 \ F4u - z c\1 0 u 
.I 
m
 
(d
 
30" 
W
o
g
 
4
N
 
0
 
-
0
 
3
-
 
a
 
0
-
 
G
r
l
 I 
@
W
O
 
0
 
o
o
m
 
v
v
 
303030 
r
l
4
I
-
l
 
W
W
G
 
0
0
0
 
Z
Z
Z
Z
 
U
 
P
I 
m
 
m
h
 
a
4
 
$
8
 
N
 
z
 
N
 
O
 
154 


Surface-catalyzed recombination into excited electronic, vibrational, rotational, and kinetic energy states: A review

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server