HYDROGEN IONS EMISSION UNDER FAST
CHARGED PARTICLES : THE BEGINNING OF THE
DESORPTION PROCESS
E. Da Silveira, M. Blain, E. Schweikert
To cite this version:
E. Da Silveira, M. Blain, E. Schweikert. HYDROGEN IONS EMISSION UNDER FAST
CHARGED PARTICLES : THE BEGINNING OF THE DESORPTION PROCESS. Journal
de Physique Colloques, 1989, 50 (C2), pp.C2-79-C2-84. <10.1051/jphyscol:1989215>. <jpa00229412>
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JOURNAL DE PHYSIQUE
Colloque C2, supplement au n02, Tome 50, fgvrier 1989
HYDROGEN IONS EMISSION UNDER FAST CHARGED PARTICLES : THE BEGINNING OF THE
DESORPTION PROCESS
E.F.
M.G.
DA S I L V E I R A ( ~ ) ,
BLAIN and E.A.
SCHWEIKERT
Center for Chemistry Characterization and Analysis Texas A and M
University, College Station, TX 77843-3144, U.S.A.
Resum6 - L' 6mission des ions H-, H+ et H: par des
surfaces
exposees au bombardment de fragments de fission du '"cf
a gt&
analysge. Des taux de desorption relatifs on 6tg mesurgs, ainsi
que la distribution de leurenergie cinetique initiale. I1 a it6
observ6 que le taux de desorption des ions H- est 5 peu
pres
le msme que celui des ions H+ mais que leur Gnergie
cinetique
initiale moyenne est clairenient inferieur 5 celles des ions
H+
et H f . I1 est proposg que le mgcanisme de la desorption est un
processus d'ionisation ou d'excitation electronique, suivid"une
dissociation du systgme. Un modgle
de
desorption
genre
dissociation d'une molecule H, est presentg. Des rgsultatsobtenus
par l'impact d'electrons sont Ggalement considergs dans
la
discussion.
P
samples
Abstract - The emission of H-, H+ and H$ ions by solid
expco-d to the bombardment of "'~f
fission fragments has
been
analyzed. Relative desorption yields have been measured, as well
as the initial kinetic energy. It was found that the H-desorption
yield is close to the H+ y i e l d b u t its average initial
kinetic
It
is
energy is clearly smaller than that of the H+ and H$.
ionization/
proposed that the basic desorption mechanism is an
electronic excitation process followed by dissociation of
the
presented.
system. A H, - dissociation desorption model is
Processes such Auger efect and ionization induced by
secondary
electrons may also participate. Results are
compared
with
with electron impact data.
1
-
INTRODUCTION
Several mechanisms have been proposed to describe the desorption ofneutraland
charged species from solids bombarded by fast charged particles [I]. Thereason
for the nlultitude of models is probably due to the fact that
many
basic
processes participate in the desorption of particles during ion or
electron
bombardment. It has been observed that the hydrogen ions desorb with a kinetic
energy which is higher than that of other desorbed ions [2,3,43.
This fact may be taken as evidence of a relatively fast emission
process
involving a possibly simpler sequence of events leading to desorption.
In this paper, we present new PDMS results which relate to the emission of H'
and H- ions from a metal surface. These results, incorporated with
related
literature data on ion and electron impacts for producing hydrogen ionstprovide
the framework for an ionization and dissociation desorption
mode 1.
The
possibility of the involvement of secondary electrons, generated during
the
interaction of thc fast chal-qcd particle with the solid, is also di-scussed.
(1) Permanent address: Department of Physics, Pontificia Universidade CatGlica,
C.P. 38071, Rio de ~aneiro,Brazil.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989215
JOURNAL DE PHYSIQUE
2
- EXPERIMENTAL
The experln~entalPDMS set-up is standard and has been described elsewhere [ 5 1 .
Only two points require special comments. A
high-transmission
grid,
electrically connected to the sample, has been introduced between the
salnple
and the entrance of the time-of-flight (TOF) drift tube. Therefore, a
region
of roughly zero electric field was created between the sample and first grid.
This geometry increases the TOF difference of ions emitted with
different
initial velocities and provides an almost symmetrical situation
for
the
desorption of positive and negative ions. The other improvement was to ground
the entrance surface of the stop detector, in order to guarantee the
same
detection efficiency for H+ and H-.
3
-
RESULTS AND DISCUSSION
Figure 1 shows the TOF spectra of H+ and H- desorbed from and aluminum
foil.
The relative yield ratio, Y(H+)/Y(H-), was found to be about2.Moreimportantly
it is clear that the H- ions desorb with an average initial kinetic
energy
which is smaller than that of the I?+. The shape of the Hpeak
suggests
contributiorls from two different mechanisms,oneofwhichexhibits an exponential
behavior at higher energies. The sha e of the HZ peak (not represented in the
figure) is very similar to that of He peak.
TIME (ns)
T i m e - o r - i l i g h t s p e c t r a o f H+ a ~ t dH- s l e s o r h e d i o n s
c c . r r e s p n n d i n g t o 9 kv s a m p l e b i a s . T h e r e a r e
no
( o r r e c t i o r l s
due t o o p t i e s t r a n s m i s s i o n e f f i c i e n c y
u r I: 11t , r ~xl c i t y e f f e c t s i n t i l e d e s o r p t i o n .
FIGURE 1
The ciesorpkio~~
u f tlydrocjc~:arid other Iiylit particles ca11 bc arla.l.yzed wi[l~i~l
the context of the fol.:lowiny scenari.0: A fast charged particle (ionor electron)
impinges on a sample and starts slowing down by collision with the electrons
oE atorris or molecules existing or1 the surface. Ionization and/or
elec-tronic
excitation of these atoms and molecules, possibly causing desorptiort,
wil.1
result. Due primarily to the same mechanis~m,the passage of the
projecti1.e
inside the sample will genera-te &-rays, secondary
electrons,
x-rays,
bremsstrahlung and phonon vibrations. A fraction of tliese particles/radiation
will reach the sample surfe .e causing additional ionization/excitation there
andlagain, desorption may cccur. The desorption event may be induced by such
Processes as molecular dissociation, Coulomb explosion, Auger clc-excitatiori ,
an& electron capture. Tlla desorbed species can eventually participate in gas
phase reac.ti.onswith other ions/rieutrals, undergo unirnolecular dissociation ,
or change charge s t a tc: by collisi.on wit.11 free e1.e~
trolls.
According to this plcture, charged particles with the same velocity (e.g. 100
MeV fission fragments, 1 MeV protons, or 500 eV electrons) should, in a first
approximation, play a similar role in the ionization process during
their
first interaction with the surface. If so, different charge statesor different
stopping powers will affect mainly the desorption yields but
not
energy
distributions of the desorbed particles. Therefore, comparison
of
data
resulting from desorption stimulated by heavy ions and electrons should
be
enlightening.
In Figure 2, we have plotted the initial kinetic energy distributions
H+
ions as desorbed from an aluminum substrate by two very differentprojectiles,
2 5 2 ~fission
f
fragments and 500 eV electrons. The PDMS data (continuous line)
were obtained by Macfarlane, et al. [4]. The hydrogen conies from
surface
aluminum-aluminum oxide layer. Similar data
were
contamination on a 7 0
observed for a thick Au target. On the other hand, the 500 eV electron impact
data (open circles) was reported by Ding, et al. [ 6 ] using water
adsorbed
on aluminum as a target. The same group has also obtained essentially
the
same H+ kinetic energy distribution from a tungsten sample using a
150 eV
electron beam.
- 1 0 0 MeV 2 5 2 ~ f f
0
5 0 0 e V electron
Substrate: A l
- 0.5
W
0.0
0
2
4
6
8
10
12
1 4 16 1 8 2 0
I n i t i a l k i n e t i c e n e r g y d i s t r i b u t i o n of d e s o r b e d H+
i o n s f r o m a n a.Luminurn s u r f a c e .
Continuous
line
corresponds t o
Cf f i s s i o n f r a m e n t s
( r e f . 04)
p r o j e c t i l e s and o p e n c i r c l e s c o r r e s p o n d t o e l e c t r o n
i m p a c t e x p e r i m e n t ( r e f . 06) F o r b o t h c a s e s / n o t e t h a t
t h e maximum d e s o r p t i o n e n e r g y i s a b o u t 18 eV.
FIGURE 2
These measurements show that the initial kinetic energy distributio~lof
IT+
desorbed from a surface seems to be ind.ependent of the nature
of
the
of
projectile and of the substrate for these prolectile Velocities. It is
interest to note that the maximum energy observed is about
and
18 eV
corresponds to the potencial energy of two-atoms separated by about
0.8
separated from a unitary positive charge center. This observation
supports
the H+ desorption model presented below.
4 - THE Hz DISSOCIATION DESORPTION MODEL
Fast ions or electrons which collide with a hydrogen molecule, H,,n~nyproduce
ionization and/or electronic excitation. One possible
result
is
the
dissociation of the molecule. The dissociation patways can beanalyzedthrough
the potential energy diagram presented in Figure 3. The curves corresponding
to the dissociation H,
H+ + H- and Hf
H+ + H a are
not
represented.
Assunling that this system is adiabatic, the Frank-Condon principle states that
an electronic transition will occur vertically in the
potential
energy
diagram, i.e. the transition occurs in the rangeofthe internuclear distances,
R, corresponding to the vibrational ground state of H,. These distances
are
indicated by the hatched region in Figure 3. If'the molecule
dissociates
after transition, the total kinetic energy of the fragments can be determined
from the potencial energy curves. For example, a single ionization producing
-+
+
C2-82
JOURNAL DE PHYSIQUE
Hf in the 2CUt dissociative state releases about 15 eV of kinetic energy.
A
double ionization process (Coulomb explosion) produces about 19 eV. It
is
important to note that if the 8, dissociates as a gas phdse n~nlecule,
each
fragment(Ho or H+) will have half of the total kinetic energy of the system.
the
However, if one of the protons is considered fixed, e.g. if one end of
molecule is fixed on a surface, the other particle will acquire all of
the
kinetic energy available due to conservation of energy. This will also betrue
for thedissociation (M + H) + M + H, in which the fragment M is much heavier
thanthe hydrogen.
Potential energy diagrams of
the Hz, H& and H? systems.
FIGURE 3
Assuming Chat, a s a first ayprcximation, tl?c source of d c s o ~teG hydrogen is a
hydrogen molecule on the surface (or a hydrogen atom bound to a
potential
this
point charge), t ! ~ e t 1~' ) ~ . desorption time can be easily evilluated using
model. Observing that the dwell time of a fast projectile(10-'7sec/fi) and that
the ionization process is completed by the same period, the desorption
time
is given by the Coulomb explosion of the two hydrogen ions. The acceleration
stage of the explosion (defined as the time it takes the fragments to
reach
90% of their final velocity) lasts roughly lo-'' seconds and corresponds to R
varying from * 1
to * 5 A.
Some refinements can be incorporated in this binary collision model.
For
instance, a cubic projectile charge state dependence appears in thedesorption
yield if an electron exchange process between target and projectiles
is
considered ( 7 ) . This dependence has been indeed verified experimentaly for l l f
desorption induced by heavy projectiles (8). Also charge exchange process may
the
occur in the energy distribution of the framents. Figure 4 illustrates
a
energy diagram for the fIo + e- collision, with production of H+ or H- as
result. Not that very low energy electrons (less than 10 ev)cannot'psrtfcipate
in this process and that electrons of higher energy (15 eV and greater) will
generate principally H+. Consequently, for the production of H- there is
a
window from about 9 to 14 eV for electrons to most efficiently produced H-.
Potencial energy levels of the HO,
H+ and H- systems. Energy levels of
the H- system higher than 0.76 eV
(electron afinity) are relative to
auto-ionizing states.
FIGURE 4
5
-
CONCLUSIONS
The shape of the initial kinetic energy distribution of desorbed
hydrogen
ions has been observed to be fairly insensitive to the mass of
projectiles
(ions or electrons) as well as the type of substrate (metal
or
organic
material). This fact is evidence that the H+ desorption mechanism is
mainly
an electronic excitation process followed by a Coulomb repulsion interaction.
Therefore, under these circunstances, temperature should not be an
adequate
Hphysical quantity to parametrize the energy distribution. Most desorbed
ions probably comes from the dissociation of unstable neutral or
negative charged molecules. The measurement of relatively high H- suggests that it
is
formed through an electron capture process by high energy (5-15 eV) Ho atoms
produced in
- like dissociations. The effect of secondary electrons in the
desorption process is enhanced due to their production yield (ranging from 1
to 150 electrons per fast projectile) but, on the other hand, is reduced duetc
their low energy (most of them have just a few eV's of kinetic energy). These
electrons probably can only participate in the desorption of low energy Hor
negative hydrocarbon ions. From electron impact measurements, on can estimate
that Q 20 eV secondary electrons contribute mainly to the production of H-ions
'H
for
while Q 100 eV secondary electrons have the highest cross section
production.
HZ
For future experiments, it is suggested to measure the desorbed ion
initial
such
as
velocity distribution dependence on certaia projectile parameters
charge, velocity and mass in order to better understand
the
desorption
mechanism.
6
-
ACKNOWLEDGEMENT
Work supported by NSFGrantINT-8602288. EFS wishes to acknowledge CNPq
FAPERJ for supporting his fellowship.
and
JOURNAL DE PHYSIQUE
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