J. Phys. B: Atom. Molec. Phys., Vol. 8; No. 3, 1975. Printed in Great Britain. 0 1975
Formation of negative helium ions by
electron capture from ground-state helium atoms
R A Baragiola and E R Salvatellii
Centro Atomic0 Bariloche, Instituto de Fisica 'Dr Jose A Balseiro', Comision Nacional de
Energia Atomica, Universidad Nacional de Cuyo, San Carlos de Bariloche, Argentina
Received 25 February 1974. in final form 23 August 1974
Abstract. Helium negative ions were produced in collisions between ground state helium
atoms and vapours of magnesium and lead in the energy range 20-40 keV. The results
suggest the existence of a doublet state of He-.
1. Introduction
The existence of helium negative ions was first studied by Wu (1936) who showed by
variational calculations that the lithium-like ( ls2 ~ s ) ~ S ,ground
,,
state configuration
had a negative binding energy. Following the discovery of He- by Hiby (Hiby 1939)
in a mass spectrometer, this three-electron system has been widely investigated. Holerien
and Midtal(l955,1967) and Holoien and Geltman (1967)have shown that the (Is 2s ~ P ) ~ P
state of this ion is the only one which is metastable against both direct autoionization
and radiation via the Coulomb interaction, and hence could account for the experimental observations. Of the three fine structure components J = i, and $, the J = $
sublevel is expected to be the longest lived.
Recent experimental studies of the decay of the He- ion (Nicholas et a1 1968, Blau
et a1 1970, Simpson et a1 1971) indicate the existence of two components with greatly
different lifetimes. The lifetimes of (345 +90) ps (Blau et al 1970) of the longest-lived
component is consistent with the best theoretical estimate (Estberg and LaBahn 1970)
of the lifetime of the J = $ sublevel.
The lifetime of about 10 ps of the shorter-lived component is believed to correspond
to the average lifetime of the J = $ and J = sublevels. No theoretical estimate of
these lifetimes is available at present.
The binding energy of He- (presumably that of its longest-lived state) has been
determined both by photodetachment (Brehm et a1 1967) and by electric field quenching
(Demkov and Drukarev 1964, Oparin et a1 1970) and found to be -0.08 eV with respect
to the 2 3S state of the atom.
The formation of He- by impact of He' on different targets has also been investigated. Donnally and Thoeming (1967) have proposed that this process occurs mainly
via the two-step mechanism
2
He+ + X
+
He(2 'S)
He(2 3 S ) + X + He-(4P)
t Major, Fuerza Aerea Argentina
382
Formation of H e - f r o m He( 1 ‘ S )
383
as double electron pickup (He’ + He-) should have a very low cross section since for
most of the targets studied, this process has to occur through electron capture involving
inner shells of the target particle.
Electron capture by fast He atoms on gases was first investigated by Fogel er a1
(1960). These workers found that the apparent cross section o t 1 - on noble gases
depended on the target thickness of the mercury neutralizer, an effect which they attributed to the presence of He (2 3S) in their beam. Gilbody er a1 (1968) developed an
attenuation technique to measure the fraction f of metastable atoms present in the He
beams and found that this fraction could be reduced to less than 1 % by chargeequilibrating a beam of He ions in He gas (Gilbody er a1 1971). By controlling the fraction
f of metastable beams produced, using different neutralizers and different target thicknesses, these workers could determine cross sections oml- for electron capture from
metastable He in H,, He, Kr and N, (Gilbody er a1 1970, Dunn and Gilbody 1971).
Cross sections for electron capture from ground-state He atoms in those gases were
found to be relatively insignificant.
In this paper, measurements are reported on electron capture cross section by He
atoms from magnesium and lead vapour targets in the energy range 20-40 keV. These
targets were chosen since they have similar ionization potentials but different electronic
configurations, so the effects of factors, like spin multiplicity could show up more easily.
Preliminary accounts of this work were presented previously (Baragiola et al 1971,
Baragiola 1973).
2. Experimental approach
A schematic diagram of the apparatus used is shown in figure 1. A monoenergetic,
mass-analysed beam of He+ is partially converted to a neutral He beam by passage
through the neutralization chamber N, to which spectroscopically pure He gas is
admitted, and removing the remaining charged particles by a voltage applied to plates P.
The beam enters the target cell oven T which contains the metal vapour target whose
purity was checked to be better than 99.7 %. The different charged components emerging
from T are analysed by the magnet M and detected with the movable, charge-independent,
detector D (Meckbach and Nemirovsky 1967). The background pressure in the vacuum
chamber outside the target was - 5 x
Torr for lead and - 5 x
Torr for
magnesium in the oven. The background gas target thickness inside the oven at operating
temperatures was inferred to be, from measurements of neutralization and assuming
this gas to be mainly nitrogen, a few times 10’’ atoms cm-,. If Z j is the current of
Figure 1. Schematic diagram of the apparatus. N, neutralization cell; P, deflecting plates;
T, target oven ; M, charge analysing magnet; D, movable detector; DP, trapped diffusion
pump.
384
R A Baragiola and E R Salvatelli
particles of charge j reaching the detector, then, at low values of the target thickness
in the interaction cell T,
n
11F,- = - ot1- n + an2+ . . . .
zjI j
Electron capture cross section values were obtained from the initial slope of the curves
Fl
-m.
3. Results and discussion
A typical plot of F , -(n)for 40 keV He on Mg in the case of an incident beam formed by
neutralization in a thick He target is shown in figure 2 . The effect of the neutralizer
gas pressure is shown in figure 3, for 40 keV He atom on Mg. An analysis of this data
can be made using a two-state approximation for the incident beam.
IO+
<-
IT (atoms cm-'I
Figure 2. F , . against II for 40 keV on Mg ( 0 )and P b (0).
The incident neutral beam was
prepared by neutralization in a thick He target.
L
I
I
2
I
I
I
I
I
4
6
Neutralizer pressure (arbitrary units)
Figure 3. Apparent electron capture cross section for 40 keV He on Mg against He
neutralizer pressure.
Formation of He- from He( 1
'3
385
Blair et al(1973) have provided experimental evidence which indicates that the metastable population of the beam is predominantly 2 3S and remains so irrespective of the
neutralizer condition. The metastable fraction f of He beams obtained by neutralizing
He' on He gas has been determined by Gilbody et al (1971). At 40 keV they found
f , = 0.075+0.005 and ,f, < 0.01 where by s and t we denote atoms formed in single
collision and charge-equilibrating conditions in the neutralizer, respectively. As
d l - i s ) = (1-fs)ool- +fsoo*1
&(t)
= (1 - f t ) o o 1 -
+fta0*1-
where O* indicates He (2 3S)atoms, then using the quoted values of,f, and f , and our
measured values of ot1 and o t l -it) presented in figure 3, we arrive at
ool- = ( 2 . 5 i 0 . 2 ) ~
10-'*cm2/atom
oo*l-= (3.6k0.9) x
cm2/atom
which implies that the measured cross sections o t I - ( t ) are essentially equal to the cross
section oO1- for electron capture from ground-state helium atoms.
'v o o l - for He on Mg and P b are plotted in figure 4. The
The results for
error bars are statistical uncertainties. Systematic errors arise mainly from the uncertainties in the values of the vapour pressure reported in the literature and are less than
20 % (R Hultgren, private communication). The results are not corrected for the decay
of the He- ions in the -30cm path between the collision chamber and the detector.
In the case of collisions with the lead target, He-(4P) could be formed by
He(1S)+Pb(3P)-+ He-(4P)+Pb+(2P)
?
?
Energy ikeV)
Figure 4. Electron capture cross sections for He o n Mg ( 0 )and Pb
corrected for the decay of He- (see text).
(m). Results are not
386
R A Baragiola and E R Salvatelli
with an energy defect AE = 27.3 eV through the transfer of the two outer electrons of
Pb to He and one of the 1s electrons of He to Pb.
In the case of
He('S)+Mg('S)
-+
He-
He-(4P) cannot be formed if the target is left in the Mg+('S) state. This would involve
a spin-flip of atomic electrons and therefore a breakdown of Wigner's spin conservation
rule (Wigner 1927). The duration of the collision, of the order of 10-l6s, makes it
extremely improbable that spin-dependent forces could cause the transition.
Spin would be conserved if, after the collision, the target is in a quartet state of Mg+
or in a triplet state of M g 2 + ,which would involve an electron promotion from the 2p
orbital of magnesium. However, those electrons correlate to the 2p orbitals of the
united atom limit (silicon) of the quasi-molecule formed in the collision. No curve
crossing effects can then exist and the transition would be adiabatic in this energy range
(Barat and Lichten 1972). For these reasons we believe that our experimental results
are consistent with the existence of a bound doublet state of He-. If the (IS' 2 ~ ) is~ s
not bound (Wu 1936, Branscomb 1962), then the lowest doublet state metastable against
autoionization is the He- (1s 2p')'p' state.
It would, however, still be able to undergo radiative decay to the lower lying (Kuyatt
et a / 1965) autoionizing (1s 2s 2p)'Po state. An estimate of this lifetime cannot be
given, although the lower bound of 5 x lO-'s is obtained if we assume a maximum
electron capture cross section of
cm2/atom at 40 keV, the observed cross section
being less, due to the decay of He- in flight to the detector.
The existence of He- ('P) is strongly supported by experimental observations on
the production of He- in single collisions of He' ('S) ions on He (1 'S) (Lockwood
er a / 1963) and on H, ('X) (Papkow and Stieger 1966) as these processes can only lead
to negative helium ions in a doublet state.
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