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Ion fractions of scattered Ne and Ar and directly recoiled H, O, and Mg from Mg surfaces
500
Section
Nuclear
V. Scattering
Depnrtment
of Chemistry,
and Methods
in Physics Research B14 (1986) 500-506 North-Holland, Amsterdam
ion phenomena
ION FRACTIONS OF SCATIXRED FROM Mg SURFACES J. Albert SCHULTZ,
Instruments
Ne AND Ar AND DIRECTLY
Calvin R. BLAKLEY
RECOILED
*, Moshe H. MINTZ ** and J. Wayne
University of Houston, UniversityPark, Houston,
H, 0, AND Mg
RABALAIS
Texas 77004, USA
Ion fractions of Ne+ and Ar+ in the scattered flux and of H+, O+, and Mg+ in the directly recoiled flux have been determined during Ne+, Ar+, and Ar 2+ bombardment of clean, oxidized and hydroxylated magnesium surfaces. The VUV photons emitted during bombardment of clean Mg have been analyzed. The extraordinarly large ion fractions of Ne+ and O+, the sensitivity of the ion fractions to adsorbates, and the emitted photons are analyzed in terms of an ion-surface charge-exchange mechanism. The direct recoil measurements are capable of detecting the chemisorbed oxygen-oxide phase transformation on the magnesium surface.
1. Introduction When keV ion beams are scattered from surfaces, both neutrals and ions are observed in the scattered and recoiled flux. The ion fraction Y, defined as the ratio of the number of ions to the total number of particles collected at a given solid angle, has been shown to vary from about 0 to 50% for noble gas ions [l-6] and to be very high > - 90% for alkali ions [7,8]. These ion fractions are known to be dependent upon a number of parameters, including the ion energy (or velocity), the scattering angle and trajectory, orientation of the target surface, the electronic structure of the solid surface, ion, and corresponding neutralized ion, and adsorbate coverage of the surface. For ion energies in the keV range, the binary elastic collision model provides a good description [9] of ion/surface collision dynamics and is used here for interpreting the TOF spectra. The expressions for calculating the energies of atoms undergoing quasisingle scattering (SS) or quasimultiple scattering (MS) collisions and for atoms being directly recoiled (DR) are well known [9]. In this work time-of-flight (TOF) analysis [l-3,10,11] of forward scattered particles is used to study the ion fractions of scattered primary atoms and surface atoms recoiled as a result of a two-body collision with the primary particles. Primary ions of Ne+, Ar+, and Ar2+ are scattered from clean, oxidized and hydroxylated polycrystalline magnesium surfaces. In addition to the forward scattered primaries, surface atoms (H, 0, Mg) are directly recoiled with sufficient energy to be detected even though most of them are neutral. By judicious choice of the primary energy, mass, and the * Vestec, Inc. 2524 Sunset Blvd., Houston, ** On sabbatical leave from Negev, Beer-Sheva, Israel. 0168-583X/86/$03.50 (North-Holland
the Nuclear
0 Elsevier Physics
Publishing
Science
TX 77005, USA. Research Center,
Publishers
Division)
B.V.
scattering and recoiling angles, it is possible to simultaneously measure and resolve the energy distributions of the SS, MS, and DR particles. This has proven valuable as a technique for measuring surface hydrogen ]lO,ll], both as an impurity [ll-141 and as a result of reaction with hydrogen containing molecules [15-171. Progress has been made toward using these direct recoils to quantitatively measure the surface concentrations of light impu~ties such as H, C, N, and 0 ]14,16,18].
2. Experimental The instrumental requirements for low energy ion scattering with TOF analysis and the measurement of scattered and recoiled ion fractions have been described in previous publications [lO,ll]. The ion beam line uses a Wien filter, an electrostatic sector for elimination of neutrals, and an electronic chopper for primary ion pulse formation. Spectra are collected as a histogram of the distribution of particle flight times using a time-toamplitude converter. Spectra of neutrals + ions and neutrals only were collected in alternating 20 s intervals for periods of ten minutes. The operating conditions used for these experiments are as follows: 1) primary beam - 3 keV Ne+ and Ar+, 5-6 keV Ar*+, 100 ns pulse width, 0.5 nA/cm’ ion current, 50 kHz pulse rate; 2) 30’ and 50” scattering angles, 75” incident angle from surface normal and 85 cm flight path; 3) - 3 kHz count rate for scattered and recoiled particles; 4) 2 X lo-” Torr base pressure. The lower limits for the ion fraction measurements are 1 &-0.5%. Larger ion fractions can be measured with a reproducibility of & 10%. Measurements were made with biases on the channel electron multiplier cones which were sufficient to repel any directly recoiled ion of like charge, i.e.- 3500 V for positive ion fractions
J.A. Schultz et al. / Ion fractions
ofscatteredand recorledparticles from
and + 2300 V for negative ion fractions. These biases are sufficient to accelerate ions of opposite polarity to energies at which the detection efficiency is near unity. The lowest energy scattered primary particles studied here are 1520 eV; at this energy and above, the neutral detection effeciency is > 0.9. The energies, in eV, of the direct recoils produced by 3 keV Ar+(Ne+) at 30” are H(DR) = 214(408), O(DR) = 1836(2222), and Mg(DR) = 2109(2231) and by Ne+ at 50” are H(DR) = 225, O(DR) = 1246, and Mg(DR) = 1255. At these DR energies the neutral detection efficiency of the multiplier is less than unity, e.g. in the case of H(DR) at 214 and 408 eV, the detection efficiency [19] is 0.5 and 0.8, respectively. The ion fractions reported herein are uncorrected for the detection efficiencies of neutrals and ions. Since we are looking for trends in the ion fractions as a function of energy and adsorbate coverage, this does not hinder interpretation of the results. The
Mg
sample
was
cut
from
99.5%
purity
rod,
polished, and cleaned by 3 keV ArC bombardment using a separate sputter gun. Oxidized and hydroxylated Mg surfaces were prepared by exposure to 0, and Hz0 as described elsewhere [17].
3. Results TOF spectra of neutrals plus positive ions for 3 keV Ar+ scattering from clean Mg, MgO, and Mg(OH), have been published [19]. Fig. 1A shows the spectra of neutrals plus negative ions, neutrals only, and (by subtraction) negative ions only from an 0, saturated Mg surface. The only significant negative ion is O-, with an ion fraction Yo_= 40% and independent of oxygen coverage. Negative ion fractions for H(DR), Mg(DR),
3 TOF
(rrsec)
6
2
O2
(?
501
Mg surfaces
Exposure 0.,5
(L) I.?
2;o 40 I ).O
J@+ 16 12
8 4
o Surface
coverage
(DR orb. units)
Fig. 2. Ion fraction of directly recoiled Mg+, Y,,+. versus oxygen direct recoil, O(DR), intensity. The O(DR) intensity is proportional to the surface oxygen concentration [17]. Exposure doses in langmuir (L) of 0, are shown on the upper abscissa; 1 L corresponds to a coverage of 0 = 0.7-0.8 mono-
layer.
and Ar(SS) are -C 1%. Since Ar- does not exist, we use the subtraction of neutral spectra from the neutral + ion spectra in the region of Ar scattering to determine the lower limit of Y = 1 + 0.5% mentioned previously. The results for scattered and recoiled positive ion fractions are listed in table 1. For 3 keV Ar+ bombardment, Y,+, Y,+, and Y,, + are small with no discernible trends as a function of 0, exposure, while Y,,+ is a strongly varying function of 0, exposure. This oxygen enhancement of positive metal ion emission has been observed and is well documented in SIMS [20]. Although the direct recoils are a special type of secondary
4 TOF (j~sec)
5
Fig. I. TGF spectra at 30’ for (A) 3 keV Ar+ scattering from 10 L 0, exposed Mg (solid line - neutrals + negative ions, dotted line neutrals only], (B) 6 keV Ar’+ scattering from 0.9 L 0, exposed Mg resulting in - 0.5 monolayers (solid line - neutrals + positive ions, dotted line - neutrals only). V. SCATTERING
502
J.A. Schultz et al. / Ion fractions
Table 1 Percent H’, O+, and Mgf ions surviving Mg and adsorbate covered Mg. Mg surface
Primary
ion energy
Clean Mg 0.5 L 0, 1.0 L 0, 2.0 L 0, 4.0 L 0, 10.0 L 0,
Ar+ (3) (30’)
Clean Mg 0.3 L 0, 0.9 L 0, 10.0 L 0,
Ar’+
Clean Mg 10.0 L 0,
A?+
Clean Mg 10 L 0, 6LH,O 30LH,O
Ne+ (3) (30”)
Clean Mg 10 LO, 30 L H,O
Nef
of scattered and recorled particles from
direct recoil events and Ne+ and Ar+ ions surviving
yH+
2.5 1.8 0.8 0.9
YW’
r,,*
_ _ 0.9 3.3 2.4 0.2
(6) (30’) 26 22 26 (5) (30°)
0.5
35 40 5
<1
h) hl
Y,*
events for bombardment
2,
0.46 0.38
11 11 12 22
1.9 3.5 2.7 5.0
0.33 0.28
5.0 20
1.2 4.5
0.36 0.30
hl h) h)
h) h)
45 26 43 7
0.37 0.31
61 41 6
0.30 0.36
a) Yr+ = Y,,+ or YNe+ where appropriate. r,(A) = the calculated distance of closest approach of the projectile (clean surface) or 0 atom (oxidized surface). b, Mg(DR) and O(DR) are not sufficiently well resoived from Ne(S) in order to get separate ion fractions.
ion, the oxygen induced enhancement of the positive ion yield prevails. In our previous paper [17] on Mg exposed to O,, it was shown that the O(DR) intensities are proportional to the surface oxygen concentration. Plotting Yt+- for 3 keV Ar+ bombardment of Mg versus O(DR) intensity yields the graph shown in fig. 2. The corresponding exposure doses in langmuir (L) of 0, are shown on the upper abscissa. The relatively moderate slope below - 1 L exposure (corresponding to a surface coverage of 0 = 0.7-0.8 monolayer} increases sharply by about a factor of 10 at higher exposures. This sharp break in the slope corresponds with the initiation of a surface phase transformation from chemisorbed oxygen to the precipitation of oxide islands. Such a transformation was observed by Namba et al. [21] as a discontinuity in the work function change versus 6 = 0.8 which corresponded to - 2 L exposure; on our polycrystalline surface it occurs at - I L exposure. Fig. 1B shows a typical positive ion direct recoil spectrum produced by 6 keV Ar*+. The Ar*+ data in table 1 show that Y,* is high (= 25%) at 6 keV but drops to only 05% at 5 keV. The YMe+ value from clean Mg using both 5 and 6 keV Ar2+ is significantly higher than the value using 3 keV Ar+, while on the 0, of the saturated surface Yr++ is nearly independent primary ion charge state or energy. This result reflects
*
Q,(A)
0.3 0.2 1.1 I.7 2.0 2.0
h)
(3) (SO“)
scattering
1.8 4.2 5.1 12 12 18
hl 12 12 5
Mg sur$xces
of
J,
and the Mg atom
the existence of Mg’ in the oxidized surface and the relative inefficiency of this surface (compared to the clean surface) in neutralization of recoiled ions. Typical direct recoil and scattering spectra for 3 keV Ne+ on clean and adsorbate covered Mg at B = 50” and 30° are shown in figs. 3 and 4, respectively. Note the logarithmic scale of fig. 4. The peak labelled P is a result of vacuum ultraviolet photons emitted as result of the ion-surface collision, as previously observed from CsBr [12], Mg [13,17], La [22], and Yb [23] surfaces. The relative photon intensity is greatly reduced upon oxidation or hydroxylation of the clean surface. The spectral region 30-200 nm was scanned in a 0.2 m monochromator using a similar electron multipler for detector with the systems Ne+ and Ar+ bombardment of Mg. No emission was observed with Ar+ while for Ne+, only t,3P -+‘S resonance radiation from neutral Ne at 735.9 A and 743.7 A was observed [24]. The most remarkable feature of the TOF spectra is the extraordinarily large Ne+ ion fraction (Y,,+= 61% (SO”) and 45% (30’)) surviving the collision with clean Mg. This Y,,+ value is very sensitive to adsorbate coverage as indicated by the large decreases (table 1) upon oxidation or hydroxylation. The Mg(DR) peak is partially resolved at 50’ and exhibits a small ion fraction. A small H(DR) impurity peak is observed from clean Mg. This H(DR) peak increases upon 0, exposure
fromMg
J.A. Schultz et 01. / Ion /ractmns of stuttered and recorledpcrrticles
neutral
503
surfaces
. . . . ..e..
0
2
4 Time
of
Flight
6
6
(MS~C)
Fig. 4. TOF spectra for 3 keV Nef scattering at 30’ from clean Mg and 10 L 0, exposed Mg. C(DR) represents < 0.03 monolayer carbon impurity which built up after 30 min in vacuum. Note logarithmic intensity scale. Curve identifications as in fig. 2.
Ne2+ where the H(DR) energies are near 500 V. In the 30” spectrum, O(DR) is resolved from the scattering peak and a large Yo+ is observed. A small C(DR) impurity peak is observed also from the oxidized surface.
4. Discussion Time
of
Flight
(psrc)
Fig. 3. TOF spectra for 3 keV Net scattering at 50” from (/ Y) clean magnesium, (B) 10 L 0, exposed Mg, and (C) 30 L H, 0 exposed Mg (solid line - neutrals+posltive ions, dotted line neutrals only, dashed line - positive ions only).
as a result of oxygen induced hydrogen impurity surface segregation [13]. H(DR) is very intense on the hydroxylated surface because the stoichiometry is close to Mg(OH), [17]. Since the outermost layer for such a surface is the hydroxyl group which shadows the Mg atoms [17], the Ne interacts dominately with hydrogen and oxygen atoms; this results in near obliteration of Mg(DR) and shifting of the Ne scattering peak to longer flight times (fig. 3C) due to the multiple scattering sequences. The value of Y,+ is < I%, even for 6 keV
From the data of table 1, we make three general observations that are important to understanding the detailed mechanism of ion-surface charge exchange: 1) YAr+ is small and increases with adsorbate coverage, while Y,,+ is very large and decreases dramatically with adsorbate coverage. 2) The Ne+ bombardment induced photon emission decreases as a function of adsorbate coverage. 3) Yo+ is large for 3 keV Ne+ and > 6 keV Ar2+, but very small for 3 keV Ar+ and 5 keV Ar2+. Ion-surface charge exchange transitions can be treated [5,25-271 by dividing the particle trajectories into three segements: 1) The incoming trajectory of the primary ion where resonant and Auger neutralization (281 can occur. 2) The violent collision where the close encounter results in formation of a quasi-diatomic molecule in which ionization, neutralization, and excitation of projectile or target atoms can occur. 3) The V. SCATTERING
J.A. Schultz et al. / ion fractions of scattered and recoiled particles from Mg surfaces
504
ti
104W Internuclear
_
1 ’4r
Distance
Ne*+O ---+
Fig. 5. United atom (UA)-separated atom (SA) correlation diagrams for many electrons in the field of two differently charged nuclei. Diahatic MO’s connect AO’s of the separated atoms (right side) with those of the corresponding united atom (left side), maintaining the same value for the difference (n - 1) in both the UA and SA limits and including “swapping”. Experimental binding energies are used in the UA and SA limits and the valence levels of the UA and target SA are shown as broadened bands as they exist in the solid state. MO’s with M = 0, 1, 2, (a, n, 6) are denoted by solid, dashed, and dotted lines, respectively. The AO’s of neon in the SA limit are denoted by asterisks.
outgoing trajectory where resonant and Auger charge exchange are again possible. Neutralization along the inward trajectory has been shown [3] to be highly efficient. The Y values and photon intensities are determined by the degree of ionization and excitation in the close encounter and the survival probability of these ions and excited species as they leave the collision site [3]. During the close encounter the momentum component of the projectile normal to the surface is converted into repulsive potential energy V( r,,) at the distance of closest approach r,,. It has been shown [29] that the inelastic energy loss depends to a markedly greater extent on r,, rather than on the incident ion energy. We have calculated r,, and V( ro) by
means of a scattering program [22] which uses the Molitre approximation to the interaction potential. The calculated r, values are listed in table 1 for bombardment of clean Mg. The V( ro) values are > - 300 eV for all cases, hence there is sufficient potential energy available for ionization and excitation transitions. During this close encounter a quasimolecular is formed in which the velocity of inner-shell electrons is much larger than the relative speed of the colliding atoms. In the quasi molecular orbitals formed from the constituent atomic orbitals, electrons can be promoted at diabatic crossings to MO’s derived from higher principle quantum number AO’s of the separated atoms. Electrons promoted from inner-shells in this manner can be trapped in higher orbitals as the atoms separate, producing autoionizing and highly excited well-defined discrete states whose lifetimes are longer than the collision times. Lichten [30] has shown that for gas phase Ne+-Ne and Ar+-Ar collisions, a sharp increase in the inelastic energy loss, number of ejected electrons, and excited atoms occurs at collision energies for which there is significant overlap of the L-shells of the colliding pair. The sum of the radii of maximum radial charge density for the L-shells is 0.59 A for the Ne-Mg pair and 0.42 A for the Ar-Mg pair [23]. The r, values listed in table 1 are well within these critical internuclear distances for Ne-Mg collisions and hence excited species are expected. For Ar-Mg collisions the r. values are outside this critical range for 3 keV ions and barely within it for 5 keV ions. Furthermore, by constructing diabatic correlation diagrams [30] which connected the separated atoms and the corresponding united atom with MO energy levels, it is shown in fig. 5 that for such asymmetric atomic collisions the inner shell excitation goes dominantly to the lighter lower atomic number atom. Hence, for Ne-Mg collisions, excitation energy can be channeled into Ne through its 2p level which correlates with the highly promoted 4f u MO. Autoionizing and excited states produced in this encounter are responsible for the large value of Y,,+ and the large photon yield. Note that the 4fo-3sa crossing is the first and last orbital crossing to occur on the inward and outward trajectories, respectively. Promotion from the occupied 4fo(2p AO) to the unoccupied 3so(3s AO) orbital is a direct excitation path leading to the (. . .2p53s’) ‘.3P+(. .2p6) ‘S transitions observed in the optical spectrum [24]. For Ar-Mg collisions, excitation energy is channeled dominantly into Mg through its 2p level which correlates with the highly promoted 4fa MO; the resulting excited Mg atoms have a high probability of being quenched by their neighboring surface atoms. For scattering from the adsorbate covered surfaces, Y,,+ decreases because most collisions are with lighter atoms and result in 0 2p excitation, whereas the small Y,,+ increases due to the inefficiency of the reacted surfaces in neutralization of the Ar+. Fig. 5
J.A. Schultz et al. / Ion fractions of scattered and recoiled particles from Mg surfaces
shows that for Ne collisions with 0, the 0 2p correlates with the highly promoted 4fa orbital. resulting in excited or ionized oxygen atoms. The formation of O+ by 3 keV Ne+ and 6 keV Ar2+ is consistent with this model. Oxygen can be excited by means of L-shell overlap at ‘a values less than 0.67 A and 0.79 A for Ar and Ne, respectively. The ‘a values of table 1 are well within these distances, hence excited and auto-ionizing states of 0 can be expected. This is the same mechanism [31] as that for bombardment induced F+ emission from LiF, where the threshold for F+ ejection is - 800 eV and - 1400 eV for Ne+ and Ar +, respectively. The observation of such large Y,,+ and Yo+ values indicates that the survival probabilities of these ions leaving the surface are remarkably high. This result suggests that the atoms are excited to long lived autoionizing states which do not decay until the ion-surface distance is large enough for reneutralization to be minimal. Such a mechanism has been proposed and is consistent with results on other surfaces [3,32-351.
5. Conclusions Scattering and direct recoil ion fractions are largely determined by the electronic structures of both the surface and specific ions (atoms). The models for resonant and Auger charge exchange on the inward and outward trajectories and electron promotions during the close atomic encounters provide a qualitative intrepretation for the behavior of the ion fractions and photon intensities as a function of the nature and energy of the primary ion and adsorbate coverage of the surface. For moderate primary ion energies (< 10 keV) and low ion currents (< 3 pA/cm2), the only photon emisson that we have observed from NeC bombardment of clean Mg in the VUV region is resonance radiation from excited neutral Ne. We have demonstrated that TOF-DR measurements can provide chemical information associated with the chemisorbed oxygen to oxide transformation in addition to the compositional information. This material is based upon work supported by the National Science Foundation under grant No. DMR8304658. The authors are grateful to R. Kumar and J.N. Chen for preliminary results of the bombardment induced photon emission data.
References Ill T.M. Buck, Y.S. Chen, G.H. Wheatley and W.F. van der PI
Weg, Surf. Sci. 47 (1975) 244. T.M. Buck, G.H. Wheatley and L.K. Verheij, Surf. Sci. 90 (1979) 635.
505
[31 S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Boers, Surf. Sci. 99 (1980) 631 and 652. [41 R. Kumar, M.H. Mintz, J.A. Schultz and J.W. Rabalais, Surf. Sci. 130 (1983) L311. [51 S.L. Nizhnaya, E.S. Parilis, and V.K. Verleger, Radiat. Eff. 40 (1979) 23. (61 P.J. Schneider, W. Eckstein, and H. Verbeek, Nucl. Instr. and Meth. 194 (1982) 387. 171 A.L. Boers, Nucl. Instr. and Meth. B2 (1984) 253. PI A.J. Algra, E.V. Loenen, E.P.Th.M. Suurmeijer and A.L. Boers, Radiat. Eff. 60 (1982) 173. 191 D.P. Smith, J. Appl. Phys. 38 (1967) 340. PO1 Y.S. Chen, G.L. Miller, D.A.H. Robinson, G.H. Wheatley, and T.M. Buck, Surf. Sci. 62 (1977) 133. 1111 S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Boers, J. Phys. El3 (1980) 665. WI J.A. Schultz, R. Kumar, and J.W. Rabalais, Chem. Phys. Lett. 100 (1983) 214. 1131 M.H. Mintz, J.A. Schultz, and J.W. Rabalais, Phys. Rev. Lett. 51 (1983) 1676. u41 P.J. Schneider, W. Eckstein, and H. Verbeek, Nucl. Instr. and Meth. 218 (1983) 713. Chem. Phys. Lett. 108 1151 J.A. Schultz and J.W. Rabalais, (1984) 328. Y-S. Jo, and J.W. Rabalais, [I61 J.A. Schultz, S. Contarini, Surf. Sci. 154 (1985) 315. I171 J.A. Schultz, M.H. Mintz, T.R. Schuler, and J.W. Rabalais, Surf. Sci. 146 (1984) 438. [I81 J.A. Schultz, Y-S. Jo, and J.W. Rabalais, Nucl. Instr. and Meth. BlO/ll (1985) 713. Rev. Sci. I191 C.N. Burrous, A.J. Leiber, and V.T. Zaviantseff, Instr. 38 (1967) 1477; W.E. Nersen Jr, W.C. Turner, and W.F. Cummins, Rev. Sci. Instr. 50 (1979) 1227; D.H. Crandall, J.A. Ray, and C. Cisneros, Rev. Sci. Instr. 46 (1975) 562. in: Inelastic Ion-Surface Collisions, eds., PO1 K. Wittmaack, N. Tolk, J.C. TuIIy, W. Heiland, and C.W. White (Academic Press, New York, 1977) p. 153. H. Namba, J. Darville, and J.M. Gilles, Surf. Sci. 108 PI (1981) 446. WI R. Kumar, M.H. Mintz, and J.W. Rabalais, Surf. Sci. 147 (1984) 15. Langmuir 1 ~231 R. Kumar, J.N. Chen, and J.W. Rabalais, (1985) 294. 1241 J.W. Rabalais, R. Kumar, and J.N. Chen., to be published. P51 L.M. Kishinevsky, E.S. Parilis, and V.K. Verleger, Radiat. Eff. 29 (1976) 215. WI L.K. Verhey, B. Poelsema, and A.L. Boers, Radiat. Eff. 27 (1975) 47. 1271 L.K. Verhey, B. Poelsema, and A.L. Boers, Nucl. Instr. and Meth. 132 (1976) 565. WI H.D. Hagstrum, Phys. Rev. 96 (1954) 336; H.D. Hastrum, in: Electron and Ion Spectroscopy of Solids, eds., L. Fiermans, J. Vennik, and W. Dekeyser (Plenum, New York, 1978) p. 273. v91 A.L. Boers, Nucl. Instr. and Meth. B4 (1984) 98. I301 W. Lichten, J. Phys. Chem. 84 (1980) 2102; M. Barat and W. Lichten, Phys. Rev. A6 (1972) 211. (311 J.A. Schultz, P.T. Murray, R. Kumar, H.K. Hu, and J.W. Rabalais, F+ Ejection from LiF Surfaces by Ion
V. SCATTERING
506
J.A. Schultz et al. / Ion fractions of scattered and recoiled particles from Mg surfaces
Bombardment, in: Desorption Induced by Electronic Transitions: DIET I, eds. N.H. Tolk, M.M. Traum, J.C. Tully, and T.E. Madey, Springer Series in Chem. Phys. 24 (1983) 191. [32] W. Eckstein, V.A. Molchanov and H. Verbeek, Nucl. Instr. and Meth. 149 (1978) 599.
[33] S.B. Luitjens, A.J. Alga, and A.L. Beers, Surf. Sci. 80 (1979) 566. [34] L.K. Verheij, B. Poelsema, and A.L. Boers, Radiat. Eff. 31 (1976) 23. [35] W. Heiland and E. Taglauer, Nucl. Instr. and Meth. 132 (1976) 535.
Section
Nuclear
V. Scattering
Depnrtment
of Chemistry,
and Methods
in Physics Research B14 (1986) 500-506 North-Holland, Amsterdam
ion phenomena
ION FRACTIONS OF SCATIXRED FROM Mg SURFACES J. Albert SCHULTZ,
Instruments
Ne AND Ar AND DIRECTLY
Calvin R. BLAKLEY
RECOILED
*, Moshe H. MINTZ ** and J. Wayne
University of Houston, UniversityPark, Houston,
H, 0, AND Mg
RABALAIS
Texas 77004, USA
Ion fractions of Ne+ and Ar+ in the scattered flux and of H+, O+, and Mg+ in the directly recoiled flux have been determined during Ne+, Ar+, and Ar 2+ bombardment of clean, oxidized and hydroxylated magnesium surfaces. The VUV photons emitted during bombardment of clean Mg have been analyzed. The extraordinarly large ion fractions of Ne+ and O+, the sensitivity of the ion fractions to adsorbates, and the emitted photons are analyzed in terms of an ion-surface charge-exchange mechanism. The direct recoil measurements are capable of detecting the chemisorbed oxygen-oxide phase transformation on the magnesium surface.
1. Introduction When keV ion beams are scattered from surfaces, both neutrals and ions are observed in the scattered and recoiled flux. The ion fraction Y, defined as the ratio of the number of ions to the total number of particles collected at a given solid angle, has been shown to vary from about 0 to 50% for noble gas ions [l-6] and to be very high > - 90% for alkali ions [7,8]. These ion fractions are known to be dependent upon a number of parameters, including the ion energy (or velocity), the scattering angle and trajectory, orientation of the target surface, the electronic structure of the solid surface, ion, and corresponding neutralized ion, and adsorbate coverage of the surface. For ion energies in the keV range, the binary elastic collision model provides a good description [9] of ion/surface collision dynamics and is used here for interpreting the TOF spectra. The expressions for calculating the energies of atoms undergoing quasisingle scattering (SS) or quasimultiple scattering (MS) collisions and for atoms being directly recoiled (DR) are well known [9]. In this work time-of-flight (TOF) analysis [l-3,10,11] of forward scattered particles is used to study the ion fractions of scattered primary atoms and surface atoms recoiled as a result of a two-body collision with the primary particles. Primary ions of Ne+, Ar+, and Ar2+ are scattered from clean, oxidized and hydroxylated polycrystalline magnesium surfaces. In addition to the forward scattered primaries, surface atoms (H, 0, Mg) are directly recoiled with sufficient energy to be detected even though most of them are neutral. By judicious choice of the primary energy, mass, and the * Vestec, Inc. 2524 Sunset Blvd., Houston, ** On sabbatical leave from Negev, Beer-Sheva, Israel. 0168-583X/86/$03.50 (North-Holland
the Nuclear
0 Elsevier Physics
Publishing
Science
TX 77005, USA. Research Center,
Publishers
Division)
B.V.
scattering and recoiling angles, it is possible to simultaneously measure and resolve the energy distributions of the SS, MS, and DR particles. This has proven valuable as a technique for measuring surface hydrogen ]lO,ll], both as an impurity [ll-141 and as a result of reaction with hydrogen containing molecules [15-171. Progress has been made toward using these direct recoils to quantitatively measure the surface concentrations of light impu~ties such as H, C, N, and 0 ]14,16,18].
2. Experimental The instrumental requirements for low energy ion scattering with TOF analysis and the measurement of scattered and recoiled ion fractions have been described in previous publications [lO,ll]. The ion beam line uses a Wien filter, an electrostatic sector for elimination of neutrals, and an electronic chopper for primary ion pulse formation. Spectra are collected as a histogram of the distribution of particle flight times using a time-toamplitude converter. Spectra of neutrals + ions and neutrals only were collected in alternating 20 s intervals for periods of ten minutes. The operating conditions used for these experiments are as follows: 1) primary beam - 3 keV Ne+ and Ar+, 5-6 keV Ar*+, 100 ns pulse width, 0.5 nA/cm’ ion current, 50 kHz pulse rate; 2) 30’ and 50” scattering angles, 75” incident angle from surface normal and 85 cm flight path; 3) - 3 kHz count rate for scattered and recoiled particles; 4) 2 X lo-” Torr base pressure. The lower limits for the ion fraction measurements are 1 &-0.5%. Larger ion fractions can be measured with a reproducibility of & 10%. Measurements were made with biases on the channel electron multiplier cones which were sufficient to repel any directly recoiled ion of like charge, i.e.- 3500 V for positive ion fractions
J.A. Schultz et al. / Ion fractions
ofscatteredand recorledparticles from
and + 2300 V for negative ion fractions. These biases are sufficient to accelerate ions of opposite polarity to energies at which the detection efficiency is near unity. The lowest energy scattered primary particles studied here are 1520 eV; at this energy and above, the neutral detection effeciency is > 0.9. The energies, in eV, of the direct recoils produced by 3 keV Ar+(Ne+) at 30” are H(DR) = 214(408), O(DR) = 1836(2222), and Mg(DR) = 2109(2231) and by Ne+ at 50” are H(DR) = 225, O(DR) = 1246, and Mg(DR) = 1255. At these DR energies the neutral detection efficiency of the multiplier is less than unity, e.g. in the case of H(DR) at 214 and 408 eV, the detection efficiency [19] is 0.5 and 0.8, respectively. The ion fractions reported herein are uncorrected for the detection efficiencies of neutrals and ions. Since we are looking for trends in the ion fractions as a function of energy and adsorbate coverage, this does not hinder interpretation of the results. The
Mg
sample
was
cut
from
99.5%
purity
rod,
polished, and cleaned by 3 keV ArC bombardment using a separate sputter gun. Oxidized and hydroxylated Mg surfaces were prepared by exposure to 0, and Hz0 as described elsewhere [17].
3. Results TOF spectra of neutrals plus positive ions for 3 keV Ar+ scattering from clean Mg, MgO, and Mg(OH), have been published [19]. Fig. 1A shows the spectra of neutrals plus negative ions, neutrals only, and (by subtraction) negative ions only from an 0, saturated Mg surface. The only significant negative ion is O-, with an ion fraction Yo_= 40% and independent of oxygen coverage. Negative ion fractions for H(DR), Mg(DR),
3 TOF
(rrsec)
6
2
O2
(?
501
Mg surfaces
Exposure 0.,5
(L) I.?
2;o 40 I ).O
J@+ 16 12
8 4
o Surface
coverage
(DR orb. units)
Fig. 2. Ion fraction of directly recoiled Mg+, Y,,+. versus oxygen direct recoil, O(DR), intensity. The O(DR) intensity is proportional to the surface oxygen concentration [17]. Exposure doses in langmuir (L) of 0, are shown on the upper abscissa; 1 L corresponds to a coverage of 0 = 0.7-0.8 mono-
layer.
and Ar(SS) are -C 1%. Since Ar- does not exist, we use the subtraction of neutral spectra from the neutral + ion spectra in the region of Ar scattering to determine the lower limit of Y = 1 + 0.5% mentioned previously. The results for scattered and recoiled positive ion fractions are listed in table 1. For 3 keV Ar+ bombardment, Y,+, Y,+, and Y,, + are small with no discernible trends as a function of 0, exposure, while Y,,+ is a strongly varying function of 0, exposure. This oxygen enhancement of positive metal ion emission has been observed and is well documented in SIMS [20]. Although the direct recoils are a special type of secondary
4 TOF (j~sec)
5
Fig. I. TGF spectra at 30’ for (A) 3 keV Ar+ scattering from 10 L 0, exposed Mg (solid line - neutrals + negative ions, dotted line neutrals only], (B) 6 keV Ar’+ scattering from 0.9 L 0, exposed Mg resulting in - 0.5 monolayers (solid line - neutrals + positive ions, dotted line - neutrals only). V. SCATTERING
502
J.A. Schultz et al. / Ion fractions
Table 1 Percent H’, O+, and Mgf ions surviving Mg and adsorbate covered Mg. Mg surface
Primary
ion energy
Clean Mg 0.5 L 0, 1.0 L 0, 2.0 L 0, 4.0 L 0, 10.0 L 0,
Ar+ (3) (30’)
Clean Mg 0.3 L 0, 0.9 L 0, 10.0 L 0,
Ar’+
Clean Mg 10.0 L 0,
A?+
Clean Mg 10 L 0, 6LH,O 30LH,O
Ne+ (3) (30”)
Clean Mg 10 LO, 30 L H,O
Nef
of scattered and recorled particles from
direct recoil events and Ne+ and Ar+ ions surviving
yH+
2.5 1.8 0.8 0.9
YW’
r,,*
_ _ 0.9 3.3 2.4 0.2
(6) (30’) 26 22 26 (5) (30°)
0.5
35 40 5
<1
h) hl
Y,*
events for bombardment
2,
0.46 0.38
11 11 12 22
1.9 3.5 2.7 5.0
0.33 0.28
5.0 20
1.2 4.5
0.36 0.30
hl h) h)
h) h)
45 26 43 7
0.37 0.31
61 41 6
0.30 0.36
a) Yr+ = Y,,+ or YNe+ where appropriate. r,(A) = the calculated distance of closest approach of the projectile (clean surface) or 0 atom (oxidized surface). b, Mg(DR) and O(DR) are not sufficiently well resoived from Ne(S) in order to get separate ion fractions.
ion, the oxygen induced enhancement of the positive ion yield prevails. In our previous paper [17] on Mg exposed to O,, it was shown that the O(DR) intensities are proportional to the surface oxygen concentration. Plotting Yt+- for 3 keV Ar+ bombardment of Mg versus O(DR) intensity yields the graph shown in fig. 2. The corresponding exposure doses in langmuir (L) of 0, are shown on the upper abscissa. The relatively moderate slope below - 1 L exposure (corresponding to a surface coverage of 0 = 0.7-0.8 monolayer} increases sharply by about a factor of 10 at higher exposures. This sharp break in the slope corresponds with the initiation of a surface phase transformation from chemisorbed oxygen to the precipitation of oxide islands. Such a transformation was observed by Namba et al. [21] as a discontinuity in the work function change versus 6 = 0.8 which corresponded to - 2 L exposure; on our polycrystalline surface it occurs at - I L exposure. Fig. 1B shows a typical positive ion direct recoil spectrum produced by 6 keV Ar*+. The Ar*+ data in table 1 show that Y,* is high (= 25%) at 6 keV but drops to only 05% at 5 keV. The YMe+ value from clean Mg using both 5 and 6 keV Ar2+ is significantly higher than the value using 3 keV Ar+, while on the 0, of the saturated surface Yr++ is nearly independent primary ion charge state or energy. This result reflects
*
Q,(A)
0.3 0.2 1.1 I.7 2.0 2.0
h)
(3) (SO“)
scattering
1.8 4.2 5.1 12 12 18
hl 12 12 5
Mg sur$xces
of
J,
and the Mg atom
the existence of Mg’ in the oxidized surface and the relative inefficiency of this surface (compared to the clean surface) in neutralization of recoiled ions. Typical direct recoil and scattering spectra for 3 keV Ne+ on clean and adsorbate covered Mg at B = 50” and 30° are shown in figs. 3 and 4, respectively. Note the logarithmic scale of fig. 4. The peak labelled P is a result of vacuum ultraviolet photons emitted as result of the ion-surface collision, as previously observed from CsBr [12], Mg [13,17], La [22], and Yb [23] surfaces. The relative photon intensity is greatly reduced upon oxidation or hydroxylation of the clean surface. The spectral region 30-200 nm was scanned in a 0.2 m monochromator using a similar electron multipler for detector with the systems Ne+ and Ar+ bombardment of Mg. No emission was observed with Ar+ while for Ne+, only t,3P -+‘S resonance radiation from neutral Ne at 735.9 A and 743.7 A was observed [24]. The most remarkable feature of the TOF spectra is the extraordinarily large Ne+ ion fraction (Y,,+= 61% (SO”) and 45% (30’)) surviving the collision with clean Mg. This Y,,+ value is very sensitive to adsorbate coverage as indicated by the large decreases (table 1) upon oxidation or hydroxylation. The Mg(DR) peak is partially resolved at 50’ and exhibits a small ion fraction. A small H(DR) impurity peak is observed from clean Mg. This H(DR) peak increases upon 0, exposure
fromMg
J.A. Schultz et 01. / Ion /ractmns of stuttered and recorledpcrrticles
neutral
503
surfaces
. . . . ..e..
0
2
4 Time
of
Flight
6
6
(MS~C)
Fig. 4. TOF spectra for 3 keV Nef scattering at 30’ from clean Mg and 10 L 0, exposed Mg. C(DR) represents < 0.03 monolayer carbon impurity which built up after 30 min in vacuum. Note logarithmic intensity scale. Curve identifications as in fig. 2.
Ne2+ where the H(DR) energies are near 500 V. In the 30” spectrum, O(DR) is resolved from the scattering peak and a large Yo+ is observed. A small C(DR) impurity peak is observed also from the oxidized surface.
4. Discussion Time
of
Flight
(psrc)
Fig. 3. TOF spectra for 3 keV Net scattering at 50” from (/ Y) clean magnesium, (B) 10 L 0, exposed Mg, and (C) 30 L H, 0 exposed Mg (solid line - neutrals+posltive ions, dotted line neutrals only, dashed line - positive ions only).
as a result of oxygen induced hydrogen impurity surface segregation [13]. H(DR) is very intense on the hydroxylated surface because the stoichiometry is close to Mg(OH), [17]. Since the outermost layer for such a surface is the hydroxyl group which shadows the Mg atoms [17], the Ne interacts dominately with hydrogen and oxygen atoms; this results in near obliteration of Mg(DR) and shifting of the Ne scattering peak to longer flight times (fig. 3C) due to the multiple scattering sequences. The value of Y,+ is < I%, even for 6 keV
From the data of table 1, we make three general observations that are important to understanding the detailed mechanism of ion-surface charge exchange: 1) YAr+ is small and increases with adsorbate coverage, while Y,,+ is very large and decreases dramatically with adsorbate coverage. 2) The Ne+ bombardment induced photon emission decreases as a function of adsorbate coverage. 3) Yo+ is large for 3 keV Ne+ and > 6 keV Ar2+, but very small for 3 keV Ar+ and 5 keV Ar2+. Ion-surface charge exchange transitions can be treated [5,25-271 by dividing the particle trajectories into three segements: 1) The incoming trajectory of the primary ion where resonant and Auger neutralization (281 can occur. 2) The violent collision where the close encounter results in formation of a quasi-diatomic molecule in which ionization, neutralization, and excitation of projectile or target atoms can occur. 3) The V. SCATTERING
J.A. Schultz et al. / ion fractions of scattered and recoiled particles from Mg surfaces
504
ti
104W Internuclear
_
1 ’4r
Distance
Ne*+O ---+
Fig. 5. United atom (UA)-separated atom (SA) correlation diagrams for many electrons in the field of two differently charged nuclei. Diahatic MO’s connect AO’s of the separated atoms (right side) with those of the corresponding united atom (left side), maintaining the same value for the difference (n - 1) in both the UA and SA limits and including “swapping”. Experimental binding energies are used in the UA and SA limits and the valence levels of the UA and target SA are shown as broadened bands as they exist in the solid state. MO’s with M = 0, 1, 2, (a, n, 6) are denoted by solid, dashed, and dotted lines, respectively. The AO’s of neon in the SA limit are denoted by asterisks.
outgoing trajectory where resonant and Auger charge exchange are again possible. Neutralization along the inward trajectory has been shown [3] to be highly efficient. The Y values and photon intensities are determined by the degree of ionization and excitation in the close encounter and the survival probability of these ions and excited species as they leave the collision site [3]. During the close encounter the momentum component of the projectile normal to the surface is converted into repulsive potential energy V( r,,) at the distance of closest approach r,,. It has been shown [29] that the inelastic energy loss depends to a markedly greater extent on r,, rather than on the incident ion energy. We have calculated r,, and V( ro) by
means of a scattering program [22] which uses the Molitre approximation to the interaction potential. The calculated r, values are listed in table 1 for bombardment of clean Mg. The V( ro) values are > - 300 eV for all cases, hence there is sufficient potential energy available for ionization and excitation transitions. During this close encounter a quasimolecular is formed in which the velocity of inner-shell electrons is much larger than the relative speed of the colliding atoms. In the quasi molecular orbitals formed from the constituent atomic orbitals, electrons can be promoted at diabatic crossings to MO’s derived from higher principle quantum number AO’s of the separated atoms. Electrons promoted from inner-shells in this manner can be trapped in higher orbitals as the atoms separate, producing autoionizing and highly excited well-defined discrete states whose lifetimes are longer than the collision times. Lichten [30] has shown that for gas phase Ne+-Ne and Ar+-Ar collisions, a sharp increase in the inelastic energy loss, number of ejected electrons, and excited atoms occurs at collision energies for which there is significant overlap of the L-shells of the colliding pair. The sum of the radii of maximum radial charge density for the L-shells is 0.59 A for the Ne-Mg pair and 0.42 A for the Ar-Mg pair [23]. The r, values listed in table 1 are well within these critical internuclear distances for Ne-Mg collisions and hence excited species are expected. For Ar-Mg collisions the r. values are outside this critical range for 3 keV ions and barely within it for 5 keV ions. Furthermore, by constructing diabatic correlation diagrams [30] which connected the separated atoms and the corresponding united atom with MO energy levels, it is shown in fig. 5 that for such asymmetric atomic collisions the inner shell excitation goes dominantly to the lighter lower atomic number atom. Hence, for Ne-Mg collisions, excitation energy can be channeled into Ne through its 2p level which correlates with the highly promoted 4f u MO. Autoionizing and excited states produced in this encounter are responsible for the large value of Y,,+ and the large photon yield. Note that the 4fo-3sa crossing is the first and last orbital crossing to occur on the inward and outward trajectories, respectively. Promotion from the occupied 4fo(2p AO) to the unoccupied 3so(3s AO) orbital is a direct excitation path leading to the (. . .2p53s’) ‘.3P+(. .2p6) ‘S transitions observed in the optical spectrum [24]. For Ar-Mg collisions, excitation energy is channeled dominantly into Mg through its 2p level which correlates with the highly promoted 4fa MO; the resulting excited Mg atoms have a high probability of being quenched by their neighboring surface atoms. For scattering from the adsorbate covered surfaces, Y,,+ decreases because most collisions are with lighter atoms and result in 0 2p excitation, whereas the small Y,,+ increases due to the inefficiency of the reacted surfaces in neutralization of the Ar+. Fig. 5
J.A. Schultz et al. / Ion fractions of scattered and recoiled particles from Mg surfaces
shows that for Ne collisions with 0, the 0 2p correlates with the highly promoted 4fa orbital. resulting in excited or ionized oxygen atoms. The formation of O+ by 3 keV Ne+ and 6 keV Ar2+ is consistent with this model. Oxygen can be excited by means of L-shell overlap at ‘a values less than 0.67 A and 0.79 A for Ar and Ne, respectively. The ‘a values of table 1 are well within these distances, hence excited and auto-ionizing states of 0 can be expected. This is the same mechanism [31] as that for bombardment induced F+ emission from LiF, where the threshold for F+ ejection is - 800 eV and - 1400 eV for Ne+ and Ar +, respectively. The observation of such large Y,,+ and Yo+ values indicates that the survival probabilities of these ions leaving the surface are remarkably high. This result suggests that the atoms are excited to long lived autoionizing states which do not decay until the ion-surface distance is large enough for reneutralization to be minimal. Such a mechanism has been proposed and is consistent with results on other surfaces [3,32-351.
5. Conclusions Scattering and direct recoil ion fractions are largely determined by the electronic structures of both the surface and specific ions (atoms). The models for resonant and Auger charge exchange on the inward and outward trajectories and electron promotions during the close atomic encounters provide a qualitative intrepretation for the behavior of the ion fractions and photon intensities as a function of the nature and energy of the primary ion and adsorbate coverage of the surface. For moderate primary ion energies (< 10 keV) and low ion currents (< 3 pA/cm2), the only photon emisson that we have observed from NeC bombardment of clean Mg in the VUV region is resonance radiation from excited neutral Ne. We have demonstrated that TOF-DR measurements can provide chemical information associated with the chemisorbed oxygen to oxide transformation in addition to the compositional information. This material is based upon work supported by the National Science Foundation under grant No. DMR8304658. The authors are grateful to R. Kumar and J.N. Chen for preliminary results of the bombardment induced photon emission data.
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