Experimental results are reviewed of the ratio, R sq., of double to single ionization of He by proton, antiproton, electron and positron impact in the energy range from 0.15 to about 10 MeV/amu. At high velocities (greater than 1 to 2 MeV/amu) values of R sq. caused by electron impact merge with those for the proton with the antiproton, electron values being up to a factor of 2 greater than that for the p, positron. At these velocities the single ionization cross sections caused by impact of any of these four particles are indistinguishable

Jacobsen, Finn M.

English

NASA Technical Reports Server

N90-18979
DOUBLE IONIZATION OF HELIL_4 BY PARTICLE IMPACT.
Finn M. Jacobsen
Institute of Physics
Arhus University
DK-8000 _rhus, Denmark.
abstract
I_!_.this communication we review experimental results of the
ratio, R "-) , of double to single ionization of He by proton, an-
tiproton, electron and positron impact in the energy range from 0 ½_
to about i0 MeV/amu. At high velocities (>1-2 Mev/amu) values of R (
caused by electrcn impact merge with those for the antiproton while
the positron results merge with those for the proton with the _, e-
values being up to a factor of 2 greater than that for the p, e . At
these velocities the single ionization cross sections caused by impact
of any of these four particles are indistirguishable.
Double ionization by charged
particle impact is a fundamental col-
lision channel in which two electrons
are removed from the target atom. Ex-
perimentally, this ocllision channel
has been studied for a variaty of
target atoms for different
projectiles I-10. Since it was disoo-
vered that the cross section for double
4+
ionization, o , of He by electron, e ,
impact exceeded that for the proton, p,
by a factor of 2 at a velocity of 1-2
MeV/a_u I,6 m%/ch effort has been devoted
to the study of this collision process.
The question arose whether this
difference in o÷÷ was due to a charge
or a mass effect. A later experiment
with antiprotons _- 7, p--+,÷ on He
that the difference in o for p and e-
was mainly a charge effect. In the
latter experiment it was found that o÷÷
for _ merge with that for e- at a
velocity of 1-2 MeV/amu. Recently, this
p+icture was confirmed in a positron,
e÷, experiment 8 where it was shown that
o for this projectile merge with that
for p at around 1 MeV/amu.
In simple terms, we may consider
three types of collisions which can
cause double ionization of He. The
first is the so-called shake off me-
chanism, SO, in which the projectile
ionizes one electron and as a result of
electron - electron correlation in the
initial state the second electron is
icnized. Secondly, the projectile may
collide with cne of the electrcns which
thereafter collides with the second one
resulting in ion/zation of both
electrons. This _two s_tep process we
label TS-I, where I indicates a single
projectile interaction. Finally, the
direct process in which the projectile
hits and ionizes both electrons, TS-
II. Individually, the cross sections of
projectile charge, q, as q , q and +q4
and as such give no hint that o
depends on the projectile charge.
However, as was first pointed out by
McGuire an interference in the final
state between the direct channel (TS-
II) and the shake-off process could
lead to a term in o ÷÷ proportional to
3
q . A similar effect can also occur due
to interference between TS-I and TS-II.
Rther than measuring the c_91ues
of o÷ait is the ratio, R'-', of
double _ single ionization that is
experimentally detenn/ned. At high
131
https://ntrs.nasa.gov/search.jsp?R=19900009663 2020-03-19T22:58:04+00:00Z
[] [] [] []
§
V oo ce- I I
. _ ,,r _ "_
Diff.pump
[] [] [] [] [] []
; ................. - = =......... - = _
Fig. 1 The positrcn beam_ for collislonStudles. The left inse_ _ _ _: i
shows the source - moderator ccnfiguraticn and the right one details
of the scattering cell. " !
impact velocities it is well known that
the single ionization cross secticns of
into an annihilati_ target of
al_aini_n and then detected by a 125 mm
x i00 mm Nal detector. The gas cellhelitnn are _stincglishable for 7ei f
e+ , D and p with same velocities _ contained a pair of parallel plate
and are well described within _ Born electrodes 40 mm long and separated by :
approximation. Below, a brief 20 ram, which were electrically biased -
description of the exper_i___._ to _de an extraction f_i_@!_dfor _
proceduces in the determination of R i_._ Of the eiectrodes c_ta_ a
is given. This is followed by a review I0 .......mm aperture _ed_-_-_th a-_ _"
and discussion of the experimental tr_ssion grid. Some of the ions -
results, produced by __fmpAbt_ ab!e |
....... hhroug ZYEght _--
Fig. 1 showns the exper_tal _ where they were further accele4_ "
setup used in the positron measure- rated by a fac_or_ 4,5 Q (Q being
ments. The e÷ beam with an intensity of their charge state) and focussed onto
10 4 sec-i and an energy sp.reg_, of 2 - 3 ....__ of a ceratrcn detector. Just =
eV is obtained from a 2 mCi Na _ prior to impact _ontbe__ ions
and an annealed tungst_ __ .... _ad_tional accelerated 3.9 Q keV. _
moderator. After acceleration to the This impac_ energy resulted in _desired energy the beam is transported detection efficiency for He + and
to the gas cell by an axial magnetic i_. - ..... z
field of 50 +gauss" At the end of the =
gas cell the e are further accelerated
132
lO I I 1 I I
lO00eV e* on He
e_
0
U
He*
He 4+
xlO3
j .{,_
200 400 600 800 1000
TOF {nsec]
Fig. 2 shows a time of flight spec-
trum obtained for positrons colli-
dingwiththeHe target at an im-
pact energy of 1 keV.
The extraction field for the ions
was such that the total flight times
were independent of their position of
creation in the viewed portic_of the
gas cell. The ratio R' -" were
determined by the Time Of Flight
technique, TOF, in which the ceratrcn
signal was used as a start in an
inverted TOF coincidence setup with the
stop signal supplied by the NaI
detector. An example of a TOF spectrum
is shc_ in Fig. 2. As observed, a tail
appears cn the single ionization peak
due to delayed arrival of some of He +
caused by resonant charge transfer
reations in the gas. It was possible to
account for all the single ionization
events by including the tall when
integrating over these events s .
Basically, the expeI_i,_ntal pro-
cedures follcwed in the e , _ and p
measurenents were the same as that used
for the e+ with differences being: i)
the use of thin degrader foils to
change the inpact energy in the case of
the 13 and in addition applying a TOF
measurement for a more _c_ate
determination of the _ energies "_'v , 2)
the use of a pulsed deflection system
to provide a timing signal in the e
case and 3)applying a bunched beam
delivered from a tandem accelerator in
the p studies. Furthermore, for the
three latter particles the experiments
were performed in a mgnetic field free
region. The effect of the magnetic
field present in the e+ case on the
detection efficiency of the He ion were
investigated and found unimportant for
_saction fields greater than i00
V/era . For more detailed information on
the _exper+imental techniques employed in
the e , e , D and p studies the reader
is referred to the original papers.
Fig. 3 displays experimental re-
sults. The solid lines represent values
for e-, _ and p with the latter results
being average v_._es as measured
several groups _' , . As observed the e
results merge with that of the p data
and as such ccmfirm _ results
obtained by Andersen et.al. _that the
large difference between the e and the
p data is caused by a charge rather
than a mass effect.
.o
v
\ \ Helium torget
P" \ _P • positrons
\\ .....R(p ) R(p-)
\
(p) _.-:-(_
e-
/
_.__, , , _,,,I _L _ _,_J
01 0.2 05 t 2 5 10 20
E[ MeV/amu]
Fig. 3 shows the ratio of double
to single ionization of He for
protrons, antiprotcns, electrcns
and posi_ as a function of
impact energy.
133
ORIGINAL PAGE IS
OF POOR QUALITY
At lower energies the values of R
for e- and e+ falls below that for
and p, respectively. This is probably
mainly due to the much lower kinetic
energy of the light particles resulting
in fewer available final states for
these projectiles compared 50 that of
the much heavier _ and p. A similar
effect is observed when single
ionizaticn cross sections for e+ and e
are compared to that for the protnn. To
see whether this mass effect is similar
for e- _ e÷ it has been suggest_ to
write R (e÷) as:
R(2)(e ÷) = R(2)(p)(R(2)(e -)/R(2)(_))
and the result of this relation is
shown as t_he dashed line in Fig. 3 and
fits fairly well the positron results
at impact energies in excess of 0.5
MeV/amu. This may indicate that the
deviation between the e" and _ results
(and oo_ly that for the e÷
and p) at energies between 0.5 and 2-5
MeV/amu is due to kinematic effects. At
lower +energies the results for the p
and e are influenced by electron
capture resulting in the formation of H
and Ps, respectively. In the e÷
experiment it was not possible to
deduce the significance of double
ionization of the He atom with Ps in
the final state.
There have be a number of theo-
retical studies of double ionization of
the He target since McGuire 12 _ la
suggested that the diff_ in o÷+
for p and e was due to interference
between the two different double
ionization mechanisms SO and TS-II.
Later S_rensen 7 argued that the ob-
served diff_ of R for p _ -p
could be explained by an interference
between the two two-step mechanisms TS-
I and TS-II. At impact energies greater
than 1-2 MeV/amu of interest here one
may question whether it is reasonable
to speak about two distinct processes
when ccnsidering the SO and TS-I
mechan/_ns. In both of these cases the
energy transfered by the projectile to
the "first" e- is generally low such
that dynamic correlation between this
134
e- and the other target e- should not
be ig_Dred. Double ionization by high
energy photons results in the ejection
of a fast electron and the subsequent
electronic relaxation may result in
The highionization of _) e-
energy limit of _ of He _ _otcns
is about one order of magnitulde4 greater
than that for particle impact _ •
In order to illustrate how in-
terf_ in the fin_ _state may in-
fluenc_ the values of R'-" differently
for positive and negative projectiles
we foTllow the ideas of Andersen
et.al.. In the SO and TS-I types of
collisions the projectile interacts
only with one electron through the
_p_erturbation-_e2/r, while the
e is ionized as a result of e - e
correlation. _ently, we may write
the total transition amplitude for
these p_ as
ai = -QC I (i)
where CI is a constant. In the direct
process, TS-II, where the projectile
interacts with both electron we may
write the total transition amplitude as
2
aiI = (-QC I )(-QC 2 )= Q Cii (2)
with CTI being another constant. By
ignorin_ any other processes which may
lead to double ionization, we can
÷÷
express o as
C+÷ = Z Is I + aI112
= Q2rIC112 + o'zlcx 2
- + CICII I
2 4 ........a--
= g o I + O cii - Q 2Zin t (3)
= .......
where o_ and o ii are the cross sections
for do_ble iCx_/zation as a result of
one and two projectile interactlcns and
indicates a sum_tion over the final
states. Oint is the oontribution due to
interference between these two
processes. Under the assumption that
o + (He ÷+ ) = 4o + (p) then we obtain from
Eq. 3
r _ _= _
Rint
RI = R (2}(p)_(2){_}-R(2}(He+ ÷ _3
RII = -R (2) (p)/2+R (2 )(_)/6+R (2 )(He +• )/3
= (R(2)(_)-R(2)(p))/4 (4)
By applying Eqs. 4 to the experimental
results for _, p and He+ ÷ Andersen et.
al. 7 obtained the results displayed in
Fig. 4. The dashed lipes in Fig. 4 are
obtained from theory/estimates detailed
in ref. _. As observed R I is
independent of the projectile energy in
a_reement with expectation as o T like
o--ls caused by a single projectiIe in-
teraction. RII is proportional to I/E
in rough agreement with the
interpretation that o is caused by
two successive first I_m types of
collisions between the projectile and
the target electrons. The interference
term _se is approximately proportional
to E--_-Vwhich is to be expected from
the energy dependence of o I and oil.
10
xl0 "3
5
!
cr
05
°2I
03
05
i i
He target
%%
k
k
..... theory
I I l I
1 2 5 10 20
E (MeV/amu)
Fig. 4 _ the oontributions of
the various mechani_ns involved in
double ionization of the He target,
see text for details.
{_her theoretical interpretations
of R'-" for the He target .have
advanced... Reading and Ford _° , Olson _°
and Veg_h'' have all emphasized the role
of e - e correlation in the
postulated mechanisms by which this
interaction may lead to a charge
depen_]ency of o'* andBriefly,• Reading
Fores _ have suggested a model called
interception in which they argue in the
following way. A positive projectile
outside the He atom will pull the
nearest e away from the second one and
thus reducing the probability of the
TS-I mechanism while a _negative
projectile will push the two e toward
each other. Reading and For_ 5 and
Olsen z6 have also pointed out that in
close collisions the screening of the
nucleus depends on the projectile
charge. For negative projectiles a
transient decrease in the binding
energy occurs which may result in an
÷9
enhancement of c over that for
positively charged projectiles.
25
2O
_o15
x
b
I0
i I I
He torget
,.* ,,,
ttt ..swor • Previous resultsProton resuEts
........ Reading and Ford
--- V#gh
..... Olson
•"- Ford end Reading
(Common
high -energy limit)
":_k.\
\,& "-._.'<j _
01 1 10
E (MeV)
Fig. 5 _ theoretical and ex-
perimental results for the ratio of
double to single ionization of He
by p  #act.
135
ORIGINAL PAGE IS
OF POOR QUALITY
of for p andp were compared to
theoretical predictions 15- iB and their
figure is reproduced in Fig. 5.1 The
calculation of Reading and For_I 5 is
based on the so-called forced-lmpuls_
methods, FIM, while that of Olson
results from a classical trajectory
_te _io, CTMC, study: Vegh _'
explains the diff_ in o for p
and _ due to oorrelated motion of the
target electrons during the collision.
The results obtained by FIM seems most
successful although at higher energies
it only account for 50% of the measured
effect. In a later calcutta_on of the
high. energy limit of R" --Read_ and
For_ o obta/ned excellent agreement
with experiment by including d waves in
their expansion.
In conclusion, it seems at present
not possible experimentally _ sort
cut which of the many effects in double
ionization of He that are dcminant for
the diff_ in o +" for positive and
negative projectiles. However, what is
established is the simularities of the
e" and p re_ts and co_ly
those of e- and _. Hence, f_
studies of correlation phexKinena can be
carried out usir_ any of the two sets
of projectiles.
Ref_
IL.J. Puckett and D.W. Martin, Phys.
Rev AI, 1432 (1970).2
T.D. Mark, J. Chem. Phys. 63, 3731
...................3_'(1_5); _ '_-: _ --
H.K. Haugen, L.H. Andersen, P.
Hvelplund and H. Knudsen, Phys. Rev.
A26, 1962 (1982).
4H. KnucL._.n,L.H.An_, P.
Hvelplund, G. Astner, H. Cederquist,
H. Danared, L. LilJeby and K.G.
Rensfelt, J. Phys. B 17, 3545
s (1984)"
M.B. Shah and H.B. Gilbody, J' Phys.
B 18, 899 (1984);
6L.H-?Andersen,P--Hvelp1_, H.
Knudsen, S.P. _ller, K. EI_,
K.G. Rensfelt and E. UggerbDj, Phys.
Rev. Lett. 57, 2147 (1986).
7
L.H. Andersen, P. Hvelplund, H.
Knudsen, S.P. F_ller, A.H. S_rensen,
K. Elsener, K.G. Rensfelt and E.
Uggerh_j, Phys. Rev. A 36, 3612
(1987).
8
M. Charlton, L.H. Andersen, L.
Brun-Nielsen, B.I. Deutch, P.
Hvelplund, F.M. Jaoobsen, H.
Knudsen, G. Laricchla, M.R. Poulsen
and J.O. Pedersen, J. Phys. B 21,
L545 (1988).
9M. Charlton, L. Brun-Nielsen, B.I.
Deutch, P. Hvelplund, F.M.
Jacobsen, H. Knudsen, G. Laricchia,
and M.R. Poulsen, J. Phys. B in
press.
IVL.H. Andersen, P. Hvelplund, H.
Knudsen, S.P. Y_ller, J.O.P.
Pedersen, S. Tang-Petersen, E.
Ugger_J, K. Elsener and E.
Morenzcnl, to be published.
_D. n-o,..e,G, Kruse,W. Raith_ "
G. Sinaplus, Phys. Rev. Left. 57,
13031.(1986)'_J.H i__::_:
_:(_re, Phys. Rev. u_tt. 4_,
1153 (1982).
13
J.H. McGuire and J. Burgdofer,
Phys. Rev. A 36, 4089 (1987).
14T.A. Carlson, Phys. Rev. 156, 142
(1967).
ISJ.F. Reading and A.L. Ford, J.
Phys. B 20, 3747 (1987).
16R.E. Ol_n, Phys. Rev. A 36, 1519
(1987).
17L. Vegh, Phys. Rev. A 3-7, 992
(1988).
18J.F. Reading and A.L. Ford, J.
Phys. B 21, L685 (1988).
136
_i
!
=
i
=
E
=


Double ionization of helium by particle impact

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