Formation of negative helium ions by electron capture from ground-state helium atoms

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1 J. Phys. B: Atom. Molec. Phys., Vol. 8; No. 3, Printed in Great Britain 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 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 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, 2 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 ( ) 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 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 He+ +X + He(2 'S) He(2 3S)+X + He-(4P) t Major, Fuerza Aerea Argentina 382

2 Formation of He- from 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 ot1- 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 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 Zj 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.

3 384 R A Baragiola and E R Salvatelli particles of charge j reaching the detector, then, at low values of the target thickness n in the interaction cell T, 11 F,- = -- zj Ij - ot1 - n + an 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 Pb (0). The incident neutral beam was prepared by neutralization in a thick He target. L I I I I I I I Neutralizer pressure (arbitrary units) Figure 3. Apparent electron capture cross section for 40 kev He on Mg against He neutralizer pressure.

4 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, = 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 dl-is) = (1-fs)ool- +fsoo*1 &(t) = (1 -ft)oo1- +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 otl -it) presented in figure 3, we arrive at ool- = (2.5i0.2)~ 10-'*cm2/atom oo*l- = (3.6k0.9) x cm2/atom which implies that the measured cross sections oti -(t) are essentially equal to the cross section oo1 - for electron capture from ground-state helium atoms. The results for 'v ool- for He on Mg and Pb are plotted in figure 4. The 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 on Mg (0) and Pb (m). Results are not corrected for the decay of He- (see text).

5 386 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 Mg2+, 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 ~)~s is 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. References Baragiola R A 1973 Proc. 8th Int. Conf. Physics of Electronic and Atomic Collisions (Beograd: Institute of Physics) Abstracts pp Baragiola R A. Salvatelli E R and Lantschner G 1971 Proc. 7th Int. Con$ Physics of E/ectronic and Atomic Collisions (Amsterdam: North-Holland) Abstracts pp Barat M and Lichten W 1972 Phys. Rec. A Blair W G F, McCullough R W, Simpson F R and Gilbody H B 1973 J Phys. B: Atom. Molec. Phys Blau L M, Novick R and Weinflash D 1970 Phys. Rec. Lett Branscomb L M 1962 Aromic and Molecular Processes (New York: Academic Press) chap 4 Brehm B, Gusinov M A and Hall J L 1967 Phys. Reo. Lett Demkov Y M and Drukarev G F 1964 Zh. Eksp. Teor. Fiz (1965 Soc. Phys.-JETP ) Donnally B L and Thoeming G 1967 Phys. Reo Dunn K F and Gilbody H B 1971 Proc. 8th Int. Con$ Physics ofelectronic and Atomic Collisions (Amsterdam: North-Holland) pp Estberg G N and LaBahn R W 1970 Phys. Ret.. Lett Fogel Ya M, Ankudinov V A and Pilipenko D V 1960 Zh. Eksp. Teor. Fiz (Soc. Phys.-JETP ) Gilbody H B, Browning R and Levy G 1968 J. Phys. B: Atom. Molec. Phys Gilbody H B, Dunn K F and Browning R 1970 J. Phys. B: Atom. Molec. Phys. 3 L19-21 Gilbody H B, Dunn K F, Browning R and Latimer C J 1971 J. Phys. B: Atom. Molec. Phys Hiby J W I939 Ann. Phys., Lpz Hol~ien E and Geltman S 1967 Phys. Rec

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