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TOF spectra of direct recoils
Surface Science 146 (1984) 43% 456 North-Holland, Amsterdam
438
TOF SPECTRA OF DIRECT RECOILS I. Chemisorption of O,, H,O, and CH,OH J. Albert SCHULTZ, Moshe H. MINT2 J. Wayne RABALAIS Department
Received
of Chemistt7;,
16 March
University
1984; accepted
of Houston.
on magnesium
*, Thomas R. SCHULER
Houston.
for publication
Texas
77004.
and
USA
26 June 1984
Time-of-flight (TOF) analysis of directly recoiled surface atoms (neutrals and ions) produced by pulsed Ar” ion irradiation has been used to monitor chemisorption of 0,. H,O, and CH,OH on polycrystalline magnesium. The intensities of the H, C, 0, and Mg direct recoils are obtained as a function of gas exposure dose. The reaction with O2 is fast, with saturation occurring at - 10 L (1 L = low6 Torr s), and following either a one- or a two-site adsorption model while the H,O and CH,OH reaction are slower, with saturation occurring at - 20 L, and following a nucleation and growth expression. The hydrogen direct recoil intensitiy has been calibrated to the amount of hydrogen on a methoxylated surface. This calibration allows determination of H coverages during the 0, and H,O reactions and shows a surface stoichiometry consistent with Mg(OH), for the water reaction. The analysis indicates that the oxygen forms an oxide at sites that are within the outermost Mg layer while the H,O and CH,OH form hydroxide and methoxide layers which are above the outermost Mg layer. Scattering and direct recoil cross-sections and shadow cones are calculated by means of a program which uses the Molibe approximation to the interaction potential for single binary encounters.
1. Introduction It has been known for some time that atoms surface by means of single collision encounter the surface at a grazing angle [1,2]. The energy recoiling from a primary ion of energy E, and
can be directly recoiled from a with a primary ion incident on E, of a target atom of mass M2 mass M1 is
4A
E, = E, (1 + A)2 cos2+,
(1)
where A = M,/M, and + is the recoil angle (angle between the direction of incidence of the primary ion and the recoiling target atom). Particles detected at a forward scattering angle comprise both the directly recoiled secondary * On sabbatical
leave from the Nuclear
Research
Center-Negev,
Beer Sheva. Israel.
0039-6028/84/$03.~ G Elsevier Science Publishers B.V. (North-Holland Physics ~blishing Division)
atoms and the scattered primary atoms. The energy Es of primary atoms singly scattered into a laboratory scattering angle # is
E, = E,
COS+*(d2-sin2#3)"2
(1 +A)
,
’
(2)
1
where the positive sign is for A > 1 and both signs apply for A < 1. Multiple scattering sequences can be approximated by repeated application of eq. (2). Direct recoiling of surface atoms has been applied [3,4] to structure determinations of adsorbates on single crystalline surfaces by analyzing the energy of the recoiled ions with an electrostatic sector. The angle of incidence and azimuth were independently varied in order to bring surface atoms into positions in which they would either shadow the adsorbate atom from the primary beam or block the recoiled adsorbate ion on its way to the detector. By recording the intensity of the recoiled ions as a function of crystal azimuthal angle at various elevation angles, it is possible to observe shadowing and blocking of the adsorbate atoms and hence infer surface structure from the data. This technique has been used for oxygen adsorbates [3,4]. Directly recoiled hydrogen ions have been observed [5,6] by electrostatic sector analysis, however very little quantitative information can be obtained because the fraction of hydrogen recoiled from most materials as ions is very low [7] and is dependent upon the surface condition and coverage (a disadvantage to quantification shared also by SIMS [SJ). Also, the energy of the recoiled hydrogen ion is often comparable to the energies of cascade sputtered secondary ions which places the recoiled hydrogen peak on a high background of secondary ions of various masses. Another approach to surface hydrogen detection, which was originally suggested by Chen et al. [9], has been used in this work. This approach uses a pulsed ion beam directed onto a surface at a grazing angle of incidence. The scattered primaries and directly recoiled surface atoms travel toward the detector in pulses. Time-of-flight (TOF) analysis of these pulsed atoms provides their energy distribution. By judicious choice of M,, E, and +, a TOF spectrum can be obtained in which the direct recoil peaks are separated from each other as well as from the scattered primary peak. With sufficiently energetic primary ions, the directly recoiled atoms will have kinetic energies greater than several hundred eV; thus, the detector, which in this case is a channel&on, will be sensitive to neutrals as well as ions. The sensitivity of a channeltron to neutrals is proportional to the velocity of the atom [lO,ll]. Hydrogen ions at 100 eV can be detected with about 20% efficiency [lo] (this efficiency increases rapidly with energy) while heavier ions require higher energies [ll]. The detection efficiency for atoms is presumably higher than for ions at the same energy due to the extra electron. Previous experiments have shown [12] that low energy ion scattering is
440
J.A.
Schultz
et al. /
TOF specrrcr of direct recorls. I
mainly sensitive to the outermost atomic layer of a surface. Direct recoil peaks in which the recoiled atoms have energies given by eq. 1 also come mostly from the outermost atomic layer [4]. The detector signal I, due to neutrals and ions i scattered or recoiled into the solid anxgle of the detector is given by I, = I+en,u,(l
- a,),
(3)
where I+ is the primary
ion current
incident
on the sample, c is an instrumen-
tal sensitivity factor including the angular acceptance of the detector and the detection efficiency of atom i, n, is the number density of atoms i in the surface layer, u, is the scattering or recoiling cross-section for atom i, and (Y, is a “masking factor” with a range 0 < OL,< 1 representing the degree to which atom i is shadowed
or blocked
by all other atoms in the surface.
Little use of direct recoil detection has been made until recently. Detection of hydrogen by this method was first demonstrated by Luitjens et al. [13] on a copper surface contaminated by residual gases. We have observed [6] the hydroxylation of a bombardment damaged CsBr surface exposed to residual vacuum contaminants. This was accomplished by monitoring (i) the increase in H and 0 direct recoil peaks and (ii) the concomitant decrease during the course of the reaction in both the Cs and Br direct recoil peaks as well as the primary Ar scattering from Cs and Br. We have recently shown [14] that a methoxylated magnesium surface can be used as a standard for a saturated hydrogen other
outermost
reactions
present study of 0, investigation
atomic layer, from which absolute
can be determined.
These
and H,O reactions
has been the observation
observations
hydrogen
with Mg. An unexpected of bulk impurity
coverages
have motivated hydrogen
finding of this migration
the surface during the reaction of Mg with pure 0, [15]. The purpose of this paper is to demonstrate the use of the direct technique
to monitor
adsorbate
coverage
and structure
during
for the to
recoil
reaction
of a
surface with hydrogen containing molecules. Magnesium has been studied partly as a prelude to studying magnesium alloys which are important as hydrogen storage materials and partly to take advantage of the known chemistry of Mg with 0, [16], H,O [17], and CH,OH [14]. Polycrystalline Mg was used due to the limitation of our vacuum chamber which allows measurements at only three fixed scattering angles. The anisotropies single crystalline sample are thus deliberately avoided.
2. Experimental
which
occur
from
methods
2.1. Instrumentation The instrumental
requirements
flight analysis are described
for low energy ion scattering
in several publications
with time-of-
[12,13] and a block diagram
a
J.A.
Schultz
et al. /
TOF specfra
of direct
recoils.
I
441
of our spectrometer system is shown in fig. 1. The source is an ion beam which is pulsed, velocity selected, energy analyzed (mainly to eliminate unpulsed neutrals from the beam), and directed onto the sample with x-y deflectors. The trigger output of the pulse generator used to form the ion pulse goes to the start of a time-to-amplitude converter (TAC) after being delayed by slightly less than the time necessary for the ion pulse to travel from the pulse plates to the target. The TAC is stopped when a scattered primary or recoiled secondary prticle arrives at the detector (channeltron electron multiplier). A voltage proportional to the time between start and stop signals is produced by the TAC and fed into a multichannel analyzer (MCA) operated in the pulse height analysis (PHA) mode so that the channel number of the MCA is proportional to the particle time of flight. By repeating this sequence, a histogram of the distribution of particle flight times is generated. The operating conditions used for these experiments are as follows: (1) primary beam, 3 keV Art, 150 ns pulse width, 600 pA current, 20 kHz pulse rate, (2) 25 ‘, 30’ and 52O scattering and recoiling angles, 87.5 ’ angle of incidence from surface normal, 1.5O incident beam divergence half angle, 1.5
TIMING
MCAIPHA
_
ELECTRONICS
PULSE GENERATOR
TAC STOP
START-iDELAY
t
ELECTROSTATIC
Fig. 1. Block diagram sector analysis (ESA), D = energy analyzer,
of the spectrometer system used in the time-of-flight (TOF), electrostatic and Auger experiments. A = ion source, B = Wien filter, C = pulse plates, E = lenses and deflectors,
tor, I 1, I 2, I 2 = electron
multiplier
detectors.
F = sample, G = bellows, H = electrostatic
deflec-
mm X 8.0 mm beam spot size on sample, (3) 0.5’ acceptance half angle of detector, 65, 110, and 170 cm flight paths, ca. 3 kHz count rate for scattered primaries and recoiled secondaries, and (4) 2 X 10 “’ Torr base pressure of UHV chamber. A cylindrical mirror electron analyzer with on-axis electron beam was used for monitoring surface composition by Auger electron spectroscopy (AES).
2.2. Sample
preparation
und gas dosing
A magnesium sample, cut from 99.5% purity rod (Alfa), was cleaned with the 3 keV Art beam used for pulsing. The sputtering was done with the beam in the continuous mode and rastered so that the sputtered area was about twice the analyzed area providing ion current densities of - 1.5 PA/cm>. The sample was sputtered for - 24 h to remove the oxide and carbide coating formed during bakeout. Surface cleanliness was verified by the absence of H, C, and 0 direct recoils in the TOF spectra, by the absence of C and 0 AES signals, and by the appearance of the characteristic metallic Mg AES structure. Oxygen gas (99.9995%) contained in a sealed glass bulb and distilled water and methanol, both of which were degassed by repeated freeze-thaw cycles, were introduced into the chamber through separate Varian variable leak valves. Gas exposures were made by rapidly opening a valve to achieve a desired chamber pressure for a specific time interval after which the valve was rapidly closed. Exposures are expressed in Langmuirs and are uncorrected for the nude ionization gauge sensitivity. Each point in the intensity versus dose curves was obtained in less than 2 min after gas exposure. The residual chamber gas before and after exposure and the gas purity during exposure were monitored by means of an in situ mass spectrometer. The contamination rates of surface oxygen and hydrogen from residual gas reactions with clean and oxidized magnesium were monitored. It was concluded that during the 30 min or less required for acquisition of an intensity versus dose curve, less than 5% of the measured signal intensities come from reactions with gases other than the desired ones.
3. Results 3.1. TOF spectru from
0, and H,O
reactions
with Mg
TOF spectra of scattered primaries and directly recoiled surface atoms are shown in figs. 2 and 3 for clean Mg and for Mg exposed to saturation doses of H,O and 0,. The peaks are assigned by two methods. Firstly, the scattered and recoiled particle TOF’s calculable from energies predicted by eqs. (1) and (2) are compared to the experimental TOF distributions. Secondly, mass
J. A. Schuftr et crl, / TOF specira of direcr recoils. I
443
analysis of the scattered and recoiled ions is obtained by measuring ion TOF’s through the electrostatic analyzer (ESA) which is set to pass a specific ion energy. Such a TOF-ESA analysis has been described previously [6,9]. In this manner it is possible to unambiguously assign the masses of scattered and recoiled atoms. The peaks labelled H(DR), O(DR), and Mg(DR) are from directly recoiled surface hydrogen, oxygen, and magnesium and those labelled Ar/Mg and Ar/O or Ar/H,O are from Ar scattered from either a clean magnesium surface or one reacted with Oz or H,O. The peak marked P is due to ultraviolet photons produced when the ion pulse strikes the surface 1181. This photon peak provides both a convenient time marker for the ion/surface impact and an upper limit to the temporal width of the ion pulse. One can immediately see that the widths of the direct recoil peaks are only partially due to the instrumental resolution (ion pulse width, angular divergence of primary beam, angular acceptance of detector). The tailing to higher and lower TOF (energies) is partly the result of multiple collision sequencies. This is particularly obvious when figs. 2A (9 = 25O ) and 2B (+ = 52’ ) are compared. At + = 25’ the energy differences between single and multiple collision events are small so that the singly and multiply scattered particles appear at essentially the same TOF. The separation of these events is much larger at 52” so that spectral broadening is observed. The shoulder on the short TOF side of H(DR) in fig. 2B corresponds to the TOF of a typical multiple sequence. An example of such a sequence is Ar+ striking H which then recoils at 9 = O” into either 0 or Mg from which it scatters at 52’. Such “surface recoils” have been observed [5,9] for H on difference surfaces. The amount of multiple sequences versus direct events is a complicated function of the type and energy of the primary ion and the surface geometry itself [19]. Hence, the peaks labelled “direct recoil” have contributions from such multiple sequences. The widths of the scattering peaks are similarly influenced by multiple scattering sequences. The maximum angle at which Ar can undergo single scattering (critical angle) from Mg, 0, and H is 37.5 “, 23.6 O, and 1.4*, respectively. Thus the 52O spectrum can only consist of multiple scattering events while the peaks in the 25” spectrum can contain single scattering contributions only for the Ar/Mg combination. An important feature is the constancy of the Ar scattering intensity as a function of H,O or 0, exposure. The shape of the scattering peak changes somewhat during chemisorption, however its intensity obtained by integrating the area remains unchanged, This has been verified by monitoring the primary beam intensity (and profile) before, during, and after several dosing experiments. This consistency is due to the similar scattering cross-sections for Ar scattering from Mg and 0 at 25O. This proportionality of the Ar scattering intensity to the primary ion current allows us to normalize the areas of the direct recoil peaks to the Ar scattering peak rather than the primary ion
current density. This not only results in a more rapid experimental procedure, but it also eliminates any dependence upon slight changes in primary ion current density during acquisition of intensity versus gas exposure data. Ion fractions of the direct recoils have been determined and will be reported elsewhere [20]. These ion fractions depend on a number of factors including the surface coverage of hydrogen and oxygen. For a fully “ hydroxylated” surface, direct recoil positive ion fractions are approximately 30% for Mg, 5% for 0, and 1% for H, and decrease at lower coverages. A ca. 30% negative ion fraction is measured for O(DR). Differing sensitivities of the channeltron multiplier for a neutral or an ionized atom will be most important for the Mg(DR) and O(DR), and will have little effect on the signal intensities of H(DR) because these are mainly neutrals at all covergaes. The channeltron sensitivities [lO,ll] for the direct recoils are approximately 0.8 for both O(DR)= 2000 eV and Mg(DR)= 2310 eV and 0.5 for H(DR)= 235 eV. The H(DR) signals measured at 25’ should be increased, therefore, by a factor of 1.6 in order to compare relative intensities among the three direct recoil peaks. 3.2. Comparison
of
TOF and XPS spectra for O,, H,O, and CH,OH
reactions
with Mg
In order to calibrate the direct recoil intensities to atomic concentrations we have performed scattering and XPS studies on the methoxylation of magnesium [14). The salient points of this study are: (1) The initial reaction up to 4 L methanol exposure to clean magnesium yields a surface hydroxyl and no surface carbon. (2) Methoxylation occurs after 4 L methanol exposure with saturation of all signals occuring at ca. 12 L. (3) Methoxide carbon is the only form of carbon on the surface. (4) Angle rsolved XPS yields an estimate of the ratio of surface concentrations C/Mg = 1 (within 40% uncertainty). We have used the CH,OH/Mg system to put known amounts of carbon, hydrogen, and oxygen onto the magnesium surface. Since the methyl groups are protruding above the surface 1141 and each one has three hydrogen atoms, we use this saturated CH~OH/Mg surface to approximate a saturated hydrogen concentration in the outermost atomic layer. The H(DR) peak from this surface dominates the direct recoil events and its intensity can be compared to that of H(DR) from the H,O and 0, saturated surfaces in order to obtain an estimate of the hydrogen concentration on those surfaces. The TOF spectra at 30’ for saturation CH,OH and H,O exposures are shown in fig. 4 and the corresponding peak intensities are listed in table 1. Each spectrum in fig. 4 was obtained in < 15 s (hence, the lower signal-to-noise ratio) in order to emphasize how quickly spectra with adequate statistics can be acquired. The C(DR) from the CH,OH exposure is more intense than O(DR), in agreement with previous studies [14] that show that CH,OH bonds to metal
445
J.A. Schultz et al. / TOF spectra of direct recoils. I
TIME OF FLIGHT @set)
TIME OF FLIGHT (~sec) Fig. 2. (A) TOF of clean peak
labelled
directly scattering
spectra
of scattered
Mg and Mg exposed Ar represents
recoiled angle.
hydrogen,
Ar scattering oxygen,
P = photon
and sputtered
to 15 L of H,O.
neutrals
Scattering
from
and ions for 3 keV Ar+ bombardment angle = 25 ’
Mg and 0;
and magnesium
atoms,
H(DR),
respectively.
;
flight path = 110 cm; the
O(DR),
and Mg(DR)
are
(B) Same as (A) but 52O
pulse.
15L o*CLEAN ----
0
TIME OF FLIGHT
Fig. 3. TOF clean
spectra
(psec)
of scattered
Mg and Mg exposed
IO
and sputtered
to 15 L of 0,.
neutrals
Scattering
plus ions for 3 keV Ar+ bombardment
angle = 25 o
;
flight path = 110 cm.
of
J.A.
446
surfaces surface.
studied
3.3. Exposure
Schultz
through
ef al. /
TOF
the 0 atom
spe~tru of drrect rrcoils.
with
the methyl
group
I
oriented
above
the
dose versus direct recoil intensity
Fig. 5a shows the variation of the direct recoil peak intensities, normalized to the (Ar/Mg + Ax-/O) scattering peak, as a function of clean Mg exposures
--.-rd 4 Fig. 4. TOF
spectra
Mg and Mg exposed
flight
path = 65 cm. Insert:
I 6
TIME OF FLIGHT (psec)
of scattered
clean
_____/’
and sputtered
to saturation Possible
neutrals
plus ions for 3 keV Arf
doses of CH,OH,
models
0,
and H,O.
for chemisorption
of H,O
bombardment
Scattering
of
angle = 30 o
and CH,OH
;
on a Mg
surface.
Table
1
Peak heights ‘) of direct recoil events at 4, = 30 o for clean Mg and Mg exposed of O,,
H,O,
to saturation
Species
H(DR)
C(DR)
O(DR)
Mg(DR)
Clean Mg
(b) 1.1
(b) (b)
(b) 3.2
10
8.8
(b)
5.1
14.4
5.8
(c)
0, Hz0 CH,OH ‘) Peak heights multiplier ” Values
are normalized
sensitivity.
less than 0.005.
‘) Unresolved
doses
and CH,OH
peaks.
to the Mg(DR)
intensity
from clean
5.8 4.3 (b) Mg and are uncorrected
for
J.A.
Schultz et al. / TOF spectra of direct recoils. 1
44-l
of 0,. The appearance of a surface hydrogen peak concomittant with the 0, exposure has been shown [15] to be a result of oxygen induced hydrogen impurity segregation and not a result of contamination residual gas (e.g. II,, H,O) reactions. A similar effect has recently been noticed [21] for Pd coated Nb samples which had been pretreated with hydrogen; subsequent 0, exposure significantly diminished their bulk hydrogen concentrations. The O(DR) and H(DR) increase and the Mg(DR) decreases smoothly as a function of 0, exposure. Saturation of these signals occurs at about 10 L 0, exposure, in agreement with LEED and electron spectroscopic results (161, demonstrating oxide formation at this exposure, The 0, saturated surface was then exposed to --
2
4
6
8
- 0.4 0
I 1
I
EXPOSURE
I 2
I
I 3
I 4
DOSE (L)
Fig. 5. (A) Direct recoil peak intensities, normalized to the (Ar/Mg +Ar/O) scattering peak, as a function of clean Mg exposures to 0,. (B) Plots of one- and two-site models (eqs. (4) and (5)) for 0, adsorption on Mg.
J.A. Schultz et al. / TOF spectra of direct recoils. I
448
successive doses of H,O until saturation occurred (- 3 L H,O). The resulting direct recoil peaks appeared at identical TOF’s to those of fig. 2A in which clean Mg was exposed to 15 L H,O. The direct recoil intensities versus dose curves are shown in fig. 6A. These curves exhibit a ten fold additional accumulation
of hydrogen
shows the variation
and a one fold increase
in the oxygen level. Fig. 7a
in direct recoil peak intensities
as a function
of clean Mg
exposures to H,O. The reaction with H,O proceeds relatively slowly until ca. 6 L, at which point the H signal increases much faster than the 0 signal, with both signals going rapidly to saturation Three These
types of adsorption
are a simplified
1 I
I
one-site
2 EXPOSURE I I
between
15 and 20 L exposure.
models were used in treating model
the exposure
the adsorption
rate
data. to be
4
3 DOSE , ,
assuming
(L) ,
,
B 5-
EXPOSURE
DOSE * CL*,
Fig. 6. (A) Direct recoil peak intensities, normalized to the (Ar/Mg + Ar/O) scattering peak, as a function of oxidized Mg exposures to H,O. (B) Plots of the nucleation and growth model (eq. (6)) for H,O adsorption on oxidized Mg.
J.A. Schultz et al. / TOF spectra of direct recoils. I
proportional
to (1 - O),
- 0) = k,L,
-ln(l
a two-site B/(1
-
(4)
model, assuming
63) =
the rate to be proportional
to (1 - @)*,
k,L,
(51
and a two-dimensional
nucleation
and growth model 1221:
-8)=k,,,L2.
-In(l
449
(6)
0.05
0.04
0.03
0.02
0.01
5
10 EXPOSURE
15
2c
DOSE
25
(1)
2-
z E
1
EXPOSURE
DOSE’
Fig. 7. (A) Direct recoil peak intensities, function of clean Mg exposures to H,O. H,O adsorption on clean Mg.
(L*)
normalized to the (Ar/Mg+ Ar/O) scattering peak, as a (B) Plots of the nucleation and growth model (eq. (6)) for
450
The surface coverage 6 is determined by dividing the direct recoil peak intensity at an intermediate gas exposure by the corresponding intensity at saturation. The 0, exposure data fits a two-site model (fig. 5B) over the entire exposure range. This agreement may be fortuitous because of blocking factors CX,which may change during the reaction (section 4.2). Two one-site models both below and above 1.5 L exposure also provide a satisfactory fit, indicating that there may be two different constant masking factors in each region. This is supported by a phase change which occurs at ca. 2 L 0, dose, as determined by LEED and other techniques, during oxidation of Mg single crystals [16]. For the H,O exposures to either oxidized or clean Mg, neither the one- or two-site models provided a satisfactory fit to the data. The nudeation and growth expression yields a good fit to the data for both O(DR) and H(DR) as shown in figs. 6B and 7B. 3.4. Direct recoil cross-sections The scattering and recoiling cross-sections a, for our specific systems were computed according to a program [23] which uses the Moliere approximation 1241 to the interaction potential and ten-point Gauss-Mehler quadrature [25] for evaluation of the scattering integral. The direct recoil cross-sections 0,’ were obtained by using the expressions for conservation of energy and momentum to determine the scattering angle x, for projectile j corresponding to the specific recoil angle +r for target atom i (30° for table 1) and then calculating the scattering cross-section u,’ at this angle x,. The equality a,‘(~,) = a,‘(+,) holds. The scattering angles x j corresponding to the 30 ’ recoil angle are listed in table 2 along with the calculated cross-section and critical scattering angle for each mass pair. Direct recoil cross-sections u,’ for C, 0, and Mg (DR) are comparable while that of H(DR) is - 5 times higher for recoils resulting from single collision events. The calculated scattering cross-sections us are plotted in fig. 8 as a function of scattering angle xAr. These curves illustrate why eI; > 0 6, = e& = u{. At a fixed angle (p, (30’ in this case), u<“s result from scattering events in which the scattering angles xAr decrease as the mass i of Table 2 to a & = 30 o recoil angle, calculated cross-sections Ar scattering angles xAr corresponding angle, and critical scattering angle xLr for Ar collisions with H, C, 0, and Mg Ar --j X X H C 0 Mg
Scattering angk (deg) 1.3 17.0 23.4 36.6
x Ar
Crosssection
Critical scattering angle, xi,
(A21
(deg)
0.16 0.036 0.028 0.029
1.44 17.45 23.60 37.48
at this
the target atom decreases. The positions of these xAr’s are indicated in fig. 8 where it is observed that u,~ for i = Mg, 0, and C are similar while ah is larger due to the extremely small angle xAr for that event. The cross-sections can change significantly if multiple collision sequences contribute to the recoils; figs. 2A and 2B indicate that the contributions from such multiple sequences are indeed significant at these forward scattering angles. The effect of multiple sequences can be observed from fig. 8. For example, if an oxygen recoil at +o = 30” results from a sequence of Ar scattering through two small xAr angles and recoiling 0 on the second collision
0
5
10
15
SCATTERING
20 ANGLE
25
30
35
(DEG.)
Fig. 8. Calculated scattering cross-sections u& for 3 keV Ar+ scattering from Mg, 0, C, and H as a function of scattering angle xAr. x,c indicates the critical or maximum anxgle through which single scattering of Ar from atom i can occur, and x, indicates the scattering angle correspondingly to 30 o recoil of atom i.
452
J. A. Schultz
et al. /
TOF
spertm oj direcr
recoils. I
rather than a single xAr = 23.6O collisionthe new recoil cross-section will be larger than that of the single collision. Scattering through sequences with three or more smaller angles will result in even higher apparent u’ ‘s; similar arguments hold for C and Mg (DR). A simulation which accounts for multiple scattering sequences is clearly required in order to accurately predict the direct recoil intensities.
4. Discussion 4. I. Surface
roughness, shadowing, and blocking effects
Surface roughness is an important factor in determining intensities since particles can be scattered or recoiled into forward angles only if the macroscopic ridges and valleys are oriented so as not to block the trajectories. For example, recoils can only be observed from the bottom of a crevice when the crevice is oriented so that the primary beam can get in and the direct recoil can get out. Considerable surface roughness exists on sputter cleaned Mg even after annealing [16]. The surface reactions monitored here are thus only those occurring on portions of the roughened surface “ visible” to the ion beam. When primary projectiles are deflected by target atoms on a surface, a region exists behind each target atom within which the projectile cannot penetrate. This region is called a shadowy cone and atoms located inside the cone of another target atom cannot contribute to the scattering process. Typical shadow cones for 3 keV Ar+ scattering from H, C, 0, and Mg as calculated from the Moliere approximation to the Thomas-Fermi interatomic potential are plotted in fig. 9A. Particles either scattered or recoiled from a surface can also be deflected by neighboring surface atoms. Hence, blocking cones exist about the neighboring atoms which will tend to limit atom ejection at specific angles. The sizes of the blocking cones are similar to those of the shadow cones of fig. 9A. Examples of the effects of shadowing, blocking, and crystallite orientation are shown in figs. 9B and 9C for 3 keV Art incident on the Mg(OOO1) basal plane and the MgO(100) plane at a + = 12.5’ angle. From fig. 9B it can be seen that nearest neighbor Mg atoms block each other so that Mg(DR) cannot occur along the [lOOO] direction but must occur along directions such as [ 11001 where the interatomic distances are - 5 A. Also, any Mg crystallite from which direct recoils are observed must be macroscopically nearly perpendicular to the surface normal. If the incident beam angle is decreased much below 12.5 ‘, every surface atom will be concealed by the shadow cone of its neighboring surface atom and the scattering intensiy from single collision events drops to zero. For the MgO(100) surfaces of fig. 9C, the situation is similar in that scattering and recoiling can only occur along directions such as [210]. Nearest
J.A. Schultz et al. / TOF spectra of direct recoik
i
453
neighbor shadowing is more severe than on the more open Mg(OOO1) surface. Considering these two as well as other possible crystalline faces on the polycrystalline Mg surface, it is evident that at low incident and recoiling angles the observed direct recoils must come from either high index (open structure) crystallite faces or from surface defects (cone tips, step edges, etc.).
Ar -
C,O,Mg
r I
DISTA TOP VIEW Mg (0001) 0
::E & TOP VIEW MgO (100) * 0 *o/P
l
&PP 0
0 -3.1 2Ac.
$I&
0
do
l
l
0
o
.
0
SIDE VIEW [t loo]
SIDE VIEW [210]
SIDE VIEW [lOOO]
SIDE VIEW [loo]
Fig. 9. (A) Examples of the shadow cones behind a target atom for 3 keV Ar+ bombardment of H, C, 0, and Mg. The target atom is at the origin 0 and r is the radius of the cone at a distance 1 A behind the atom. (B, C) Schematic representations of(B) a Mg(OOO1)and (C) a MgO(100) surface and the shadowing and blocking cones for a 3 keV Ar + ion incident at 12.~5~ from the surface plane.
454
J.A.
Schultz
et al. /
TOF spectru
of drrect recoils.
I
4.2. Surface concentrations from direct recoil intensities
For a given atom i on different surfaces, the factors u,, I+ and F, in eq. (3) can be cancelled to yield direct recoil intensity ratios such as (7)
where (x) denotes the quantities corresponding to the metal with a saturation dose of adsorbate x. The atomic densities of Mg in both the metal and the oxide are equivalent (both - 1.2 X 10” atoms/cmj) and the surface Mg concentration along crystal faces is nearly equivalent for both the metal and oxide. Existing evidence [16] on oxidation of single crystal Mg supports the view that surface oxide forms in the 2-10 L exposure region. Since nMg = nM,Wb)= nMg (H,O), the ratios of eq. (7) can be used to calculate the degree to which the Mg atoms are masked by the adsorbate atoms. We calculate from the direct recoil intensities that a Mg(02) = 0.29 and (Y~~(H~O) = 0.46. This indicates that for both adsorbates Mg(DR) is blocked more on the adsorbate covered surfaces than the metal surface, as expected from the shadow cones of fig. 9. The interesting point, however, is that Mg is blocked 32% more in the H,O covered surface than in the 0, covered surface. Also from fig. 4 it is seen that O(DR) from the water reaction is twice as intense as from the 0, reaction. These two observations suggest the hydroxyl moiety as the outermost surface group. For the hydroxylated surface we have obtained an estimate of two surface hydrogens for each magnesium atom in the following manner. Using the model that the saturated CH,OH surface contains three hydrogen atoms for each carbon atom and that this hydrogen is in the outermost layer, we can estimate the hydrogen coverages in the other surfaces. For hydrogen in the outermost layer, the masking factors should be approximately equal so that a,(CH,OH) = ‘~n(H,0) = (~“(0~) and the experimental intensity ratios provide direct estimates (eq. (7)) of the coverages. The result is n,(H,O) = 0.6n,(CH,OH) and nn(0,)=0.06nr,(CH,0H). For the CH,OH and H,O saturated Mg surfaces, the H(DR) results (outermost monolayer sensitivity) indicate a ratio of nn(CH,OH)/nn(H,O)= 3/2 and the XPS results [14] indicate ratios of nc(CH,OH)/n,s(CH,OH) = 1. These data are consistent with a model such as shown in the fig. 4 insert. The CH,OH/Mg surface contains methoxide atop Mg atoms with either 0 or OH at between-Mg sites. Any hydroxyl hydrogen in the between Mg sites is severely blocked by the neighboring methoxide. The H,O/Mg exposed surface is proposed to comprise hydroxyls at both on top and between sites. The hydroxyl hydrogen in both sites would be unblocked and contribute to the H(DR) signal. Such a model of the surface hydroxyl was previously proposed and confirmed by IR and NMR measurements [26]. We have performed angle
J.A. Schultz er al. / TOF spectra of direct recoils. I
resolved XPS studies (unpublished)
which are also consistent
455
with an hydroxyl
rich outer layer atop an oxide.
5. Summary The data obtained here indicate that surface oxide forms within 10 L 0, exposure with the oxygen assuming sites which are within the outermost Mg layer. Reactions with H,O and CH,OH are complete at ca. 20 L exposure with hydroxide and methoxide species assuming positions above the surface layer. Intensity and shadow cone analysis suggest a structural model in which the hydroxyl and methoxyl moieties are bonded through the oxygen end with the hydrogens outermost. The hydroxylated surface contains 2 H atoms for every surface Mg consistent with a stoichiometry Mg(OH),. TOF analysis of direct recoils from a surface is a viable tool for monitoring surface adsorbate concentrations, in particular, surface hydrogen. This analysis is accomplished using non-destructive primary ion doses (< 10” ions/cm*). The analysis is extremely surface sensitive and the direct recoil intensities are determined by both the number and geometry of the adsorbate atoms. The analysis is rapid compared to photoelectron spectroscopy [14]. A reduction of two orders of magnitude in the 15 s acquisition time for the data in fig. 4 can be anticipated by using a more intense ion source and multistop time-to-amplitude converters.
Acknowledgments This material is based upon work supported by the National Science Foundation under Grant No. CHE-8209298. We would like to thank Ms. Yang-Sun Jo for performing the cross-section calculations and Dr. G.C. Nelson for a copy of the program for calculating the Moliere potential and cross-sections.
References [l] ES. Mashkova, V.A. Molchanov and U. Shoska, Dokl. Akad. Nauk SSSR 161 (1965) 813. [2] W.F. van der Weg and D.J. Bierman, Physica 38 (1968) 406. [3] S. Prigge, H. Niehus and E. Bauer, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, pp. 1381-1384. [4] R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 112 (1981) 133. [5] R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 104 (1981) 384. [6] J.A. Schultz, R. Kumar and J.W. Rabalais, Chem. Phys. Letters 100 (1983) 214. [7] R.S. Bhattcharya, W. Eckstein and H. Verbeek, Surface Sci. 93 (1980) 563.
456
J.A.
Schultz et al. /
TOF spectra of direct recorls. I
[S] K. Wittmaack, Surface Sci. 89 (1979) 668. [9] Y.S. Chew G.L. Miller, D.A.H. Robinson, G.H. Wheatley and T.M. Buck, Surface Sci. 62 (1977) 133. [lo] D.H. Crandall, J.A. Ray and C. Cisneros, Rev. Sci. Instr. 46 (1975) 562. [ll] C.N. Burrous, A.J. Lieber and V.T. Zaviantseff, Rev. Sci. Instr. 38 (1967) 1477. [12] T.M. Buck, Y.S. Chen, G.H. Wheatley and W.F. van der Weg, Surface Sci. 47 (1975) 244. 1131 S.B. Luitjens, A.J. Algra. E.P.Th.M. Suurmeijer and A.L. Boers, Appl. Phys. 21 (1980) 205. [14] J.A. Schultz and J.W. Rabalais, Chem. Phys. Letters 108 (1984) 328; J.A. Schultz, S. Contarini and J.W. Rabalais, Surface Sci., submitted. [15] M.H. Mintz, J.A. Schultz and J.W. Rabalais, Phys. Rev. Letters 51 (1983) 1676. [16] H. Namba, J. Darville and J.M. Gilles, Surface Sci. 108 (1981) 446. [17] A.G. Akimov, I.L. Rozenfeld and V.G. Dagurov, Izv. Akad. Nauk. SSSR, Ser. Khim. (1979) 628. [18] K. Jensen and E. Veje, Z. Physik 269 (1974) 293. [19] A.L. Beers, Surface Sci. 63 (1977) 475. [20] J.A. Schultz, C. Blakely, M.H. Mintz and J.W. Rabalais, unpublished. 1211 M. Strongin, private communications. 1221 For nucleation and growth kinetics, see for example: (a) J.W. Christian, The Theory of Transformations in Metals and Alloys, Part I. Equilibrium and General Kinetic Theory (Pergamon, 1975); (b) C.H. Bamford and C.F.H. Tipper, Eds., Comprehensive Chemical Kinetics, Vol. 2, The Theory of Kinetics (Elsevier, Amsterdam, 1969). [23] H.H. Brongersma and T.M. Buck, Surface Sci. 53 (1975) 649. [24] I.M. Torrens, Interatomic Potentials (Academic Press, New York, 1972). [25] Z. Kopal, Numerical Analysis, 2nd ed. (Wiley, New York, 1961) p. 367. [26] P.J. Anderson, R.F. Horlock and J.F. Oliver, Trans. Faraday Sot. 61 (1965) 2754: R.K. Webster, T.L. Jones and P.J. Anderson. Proc. Brit. Ceram. Sot. 5 (1965) 153.
438
TOF SPECTRA OF DIRECT RECOILS I. Chemisorption of O,, H,O, and CH,OH J. Albert SCHULTZ, Moshe H. MINT2 J. Wayne RABALAIS Department
Received
of Chemistt7;,
16 March
University
1984; accepted
of Houston.
on magnesium
*, Thomas R. SCHULER
Houston.
for publication
Texas
77004.
and
USA
26 June 1984
Time-of-flight (TOF) analysis of directly recoiled surface atoms (neutrals and ions) produced by pulsed Ar” ion irradiation has been used to monitor chemisorption of 0,. H,O, and CH,OH on polycrystalline magnesium. The intensities of the H, C, 0, and Mg direct recoils are obtained as a function of gas exposure dose. The reaction with O2 is fast, with saturation occurring at - 10 L (1 L = low6 Torr s), and following either a one- or a two-site adsorption model while the H,O and CH,OH reaction are slower, with saturation occurring at - 20 L, and following a nucleation and growth expression. The hydrogen direct recoil intensitiy has been calibrated to the amount of hydrogen on a methoxylated surface. This calibration allows determination of H coverages during the 0, and H,O reactions and shows a surface stoichiometry consistent with Mg(OH), for the water reaction. The analysis indicates that the oxygen forms an oxide at sites that are within the outermost Mg layer while the H,O and CH,OH form hydroxide and methoxide layers which are above the outermost Mg layer. Scattering and direct recoil cross-sections and shadow cones are calculated by means of a program which uses the Molibe approximation to the interaction potential for single binary encounters.
1. Introduction It has been known for some time that atoms surface by means of single collision encounter the surface at a grazing angle [1,2]. The energy recoiling from a primary ion of energy E, and
can be directly recoiled from a with a primary ion incident on E, of a target atom of mass M2 mass M1 is
4A
E, = E, (1 + A)2 cos2+,
(1)
where A = M,/M, and + is the recoil angle (angle between the direction of incidence of the primary ion and the recoiling target atom). Particles detected at a forward scattering angle comprise both the directly recoiled secondary * On sabbatical
leave from the Nuclear
Research
Center-Negev,
Beer Sheva. Israel.
0039-6028/84/$03.~ G Elsevier Science Publishers B.V. (North-Holland Physics ~blishing Division)
atoms and the scattered primary atoms. The energy Es of primary atoms singly scattered into a laboratory scattering angle # is
E, = E,
COS+*(d2-sin2#3)"2
(1 +A)
,
’
(2)
1
where the positive sign is for A > 1 and both signs apply for A < 1. Multiple scattering sequences can be approximated by repeated application of eq. (2). Direct recoiling of surface atoms has been applied [3,4] to structure determinations of adsorbates on single crystalline surfaces by analyzing the energy of the recoiled ions with an electrostatic sector. The angle of incidence and azimuth were independently varied in order to bring surface atoms into positions in which they would either shadow the adsorbate atom from the primary beam or block the recoiled adsorbate ion on its way to the detector. By recording the intensity of the recoiled ions as a function of crystal azimuthal angle at various elevation angles, it is possible to observe shadowing and blocking of the adsorbate atoms and hence infer surface structure from the data. This technique has been used for oxygen adsorbates [3,4]. Directly recoiled hydrogen ions have been observed [5,6] by electrostatic sector analysis, however very little quantitative information can be obtained because the fraction of hydrogen recoiled from most materials as ions is very low [7] and is dependent upon the surface condition and coverage (a disadvantage to quantification shared also by SIMS [SJ). Also, the energy of the recoiled hydrogen ion is often comparable to the energies of cascade sputtered secondary ions which places the recoiled hydrogen peak on a high background of secondary ions of various masses. Another approach to surface hydrogen detection, which was originally suggested by Chen et al. [9], has been used in this work. This approach uses a pulsed ion beam directed onto a surface at a grazing angle of incidence. The scattered primaries and directly recoiled surface atoms travel toward the detector in pulses. Time-of-flight (TOF) analysis of these pulsed atoms provides their energy distribution. By judicious choice of M,, E, and +, a TOF spectrum can be obtained in which the direct recoil peaks are separated from each other as well as from the scattered primary peak. With sufficiently energetic primary ions, the directly recoiled atoms will have kinetic energies greater than several hundred eV; thus, the detector, which in this case is a channel&on, will be sensitive to neutrals as well as ions. The sensitivity of a channeltron to neutrals is proportional to the velocity of the atom [lO,ll]. Hydrogen ions at 100 eV can be detected with about 20% efficiency [lo] (this efficiency increases rapidly with energy) while heavier ions require higher energies [ll]. The detection efficiency for atoms is presumably higher than for ions at the same energy due to the extra electron. Previous experiments have shown [12] that low energy ion scattering is
440
J.A.
Schultz
et al. /
TOF specrrcr of direct recorls. I
mainly sensitive to the outermost atomic layer of a surface. Direct recoil peaks in which the recoiled atoms have energies given by eq. 1 also come mostly from the outermost atomic layer [4]. The detector signal I, due to neutrals and ions i scattered or recoiled into the solid anxgle of the detector is given by I, = I+en,u,(l
- a,),
(3)
where I+ is the primary
ion current
incident
on the sample, c is an instrumen-
tal sensitivity factor including the angular acceptance of the detector and the detection efficiency of atom i, n, is the number density of atoms i in the surface layer, u, is the scattering or recoiling cross-section for atom i, and (Y, is a “masking factor” with a range 0 < OL,< 1 representing the degree to which atom i is shadowed
or blocked
by all other atoms in the surface.
Little use of direct recoil detection has been made until recently. Detection of hydrogen by this method was first demonstrated by Luitjens et al. [13] on a copper surface contaminated by residual gases. We have observed [6] the hydroxylation of a bombardment damaged CsBr surface exposed to residual vacuum contaminants. This was accomplished by monitoring (i) the increase in H and 0 direct recoil peaks and (ii) the concomitant decrease during the course of the reaction in both the Cs and Br direct recoil peaks as well as the primary Ar scattering from Cs and Br. We have recently shown [14] that a methoxylated magnesium surface can be used as a standard for a saturated hydrogen other
outermost
reactions
present study of 0, investigation
atomic layer, from which absolute
can be determined.
These
and H,O reactions
has been the observation
observations
hydrogen
with Mg. An unexpected of bulk impurity
coverages
have motivated hydrogen
finding of this migration
the surface during the reaction of Mg with pure 0, [15]. The purpose of this paper is to demonstrate the use of the direct technique
to monitor
adsorbate
coverage
and structure
during
for the to
recoil
reaction
of a
surface with hydrogen containing molecules. Magnesium has been studied partly as a prelude to studying magnesium alloys which are important as hydrogen storage materials and partly to take advantage of the known chemistry of Mg with 0, [16], H,O [17], and CH,OH [14]. Polycrystalline Mg was used due to the limitation of our vacuum chamber which allows measurements at only three fixed scattering angles. The anisotropies single crystalline sample are thus deliberately avoided.
2. Experimental
which
occur
from
methods
2.1. Instrumentation The instrumental
requirements
flight analysis are described
for low energy ion scattering
in several publications
with time-of-
[12,13] and a block diagram
a
J.A.
Schultz
et al. /
TOF specfra
of direct
recoils.
I
441
of our spectrometer system is shown in fig. 1. The source is an ion beam which is pulsed, velocity selected, energy analyzed (mainly to eliminate unpulsed neutrals from the beam), and directed onto the sample with x-y deflectors. The trigger output of the pulse generator used to form the ion pulse goes to the start of a time-to-amplitude converter (TAC) after being delayed by slightly less than the time necessary for the ion pulse to travel from the pulse plates to the target. The TAC is stopped when a scattered primary or recoiled secondary prticle arrives at the detector (channeltron electron multiplier). A voltage proportional to the time between start and stop signals is produced by the TAC and fed into a multichannel analyzer (MCA) operated in the pulse height analysis (PHA) mode so that the channel number of the MCA is proportional to the particle time of flight. By repeating this sequence, a histogram of the distribution of particle flight times is generated. The operating conditions used for these experiments are as follows: (1) primary beam, 3 keV Art, 150 ns pulse width, 600 pA current, 20 kHz pulse rate, (2) 25 ‘, 30’ and 52O scattering and recoiling angles, 87.5 ’ angle of incidence from surface normal, 1.5O incident beam divergence half angle, 1.5
TIMING
MCAIPHA
_
ELECTRONICS
PULSE GENERATOR
TAC STOP
START-iDELAY
t
ELECTROSTATIC
Fig. 1. Block diagram sector analysis (ESA), D = energy analyzer,
of the spectrometer system used in the time-of-flight (TOF), electrostatic and Auger experiments. A = ion source, B = Wien filter, C = pulse plates, E = lenses and deflectors,
tor, I 1, I 2, I 2 = electron
multiplier
detectors.
F = sample, G = bellows, H = electrostatic
deflec-
mm X 8.0 mm beam spot size on sample, (3) 0.5’ acceptance half angle of detector, 65, 110, and 170 cm flight paths, ca. 3 kHz count rate for scattered primaries and recoiled secondaries, and (4) 2 X 10 “’ Torr base pressure of UHV chamber. A cylindrical mirror electron analyzer with on-axis electron beam was used for monitoring surface composition by Auger electron spectroscopy (AES).
2.2. Sample
preparation
und gas dosing
A magnesium sample, cut from 99.5% purity rod (Alfa), was cleaned with the 3 keV Art beam used for pulsing. The sputtering was done with the beam in the continuous mode and rastered so that the sputtered area was about twice the analyzed area providing ion current densities of - 1.5 PA/cm>. The sample was sputtered for - 24 h to remove the oxide and carbide coating formed during bakeout. Surface cleanliness was verified by the absence of H, C, and 0 direct recoils in the TOF spectra, by the absence of C and 0 AES signals, and by the appearance of the characteristic metallic Mg AES structure. Oxygen gas (99.9995%) contained in a sealed glass bulb and distilled water and methanol, both of which were degassed by repeated freeze-thaw cycles, were introduced into the chamber through separate Varian variable leak valves. Gas exposures were made by rapidly opening a valve to achieve a desired chamber pressure for a specific time interval after which the valve was rapidly closed. Exposures are expressed in Langmuirs and are uncorrected for the nude ionization gauge sensitivity. Each point in the intensity versus dose curves was obtained in less than 2 min after gas exposure. The residual chamber gas before and after exposure and the gas purity during exposure were monitored by means of an in situ mass spectrometer. The contamination rates of surface oxygen and hydrogen from residual gas reactions with clean and oxidized magnesium were monitored. It was concluded that during the 30 min or less required for acquisition of an intensity versus dose curve, less than 5% of the measured signal intensities come from reactions with gases other than the desired ones.
3. Results 3.1. TOF spectru from
0, and H,O
reactions
with Mg
TOF spectra of scattered primaries and directly recoiled surface atoms are shown in figs. 2 and 3 for clean Mg and for Mg exposed to saturation doses of H,O and 0,. The peaks are assigned by two methods. Firstly, the scattered and recoiled particle TOF’s calculable from energies predicted by eqs. (1) and (2) are compared to the experimental TOF distributions. Secondly, mass
J. A. Schuftr et crl, / TOF specira of direcr recoils. I
443
analysis of the scattered and recoiled ions is obtained by measuring ion TOF’s through the electrostatic analyzer (ESA) which is set to pass a specific ion energy. Such a TOF-ESA analysis has been described previously [6,9]. In this manner it is possible to unambiguously assign the masses of scattered and recoiled atoms. The peaks labelled H(DR), O(DR), and Mg(DR) are from directly recoiled surface hydrogen, oxygen, and magnesium and those labelled Ar/Mg and Ar/O or Ar/H,O are from Ar scattered from either a clean magnesium surface or one reacted with Oz or H,O. The peak marked P is due to ultraviolet photons produced when the ion pulse strikes the surface 1181. This photon peak provides both a convenient time marker for the ion/surface impact and an upper limit to the temporal width of the ion pulse. One can immediately see that the widths of the direct recoil peaks are only partially due to the instrumental resolution (ion pulse width, angular divergence of primary beam, angular acceptance of detector). The tailing to higher and lower TOF (energies) is partly the result of multiple collision sequencies. This is particularly obvious when figs. 2A (9 = 25O ) and 2B (+ = 52’ ) are compared. At + = 25’ the energy differences between single and multiple collision events are small so that the singly and multiply scattered particles appear at essentially the same TOF. The separation of these events is much larger at 52” so that spectral broadening is observed. The shoulder on the short TOF side of H(DR) in fig. 2B corresponds to the TOF of a typical multiple sequence. An example of such a sequence is Ar+ striking H which then recoils at 9 = O” into either 0 or Mg from which it scatters at 52’. Such “surface recoils” have been observed [5,9] for H on difference surfaces. The amount of multiple sequences versus direct events is a complicated function of the type and energy of the primary ion and the surface geometry itself [19]. Hence, the peaks labelled “direct recoil” have contributions from such multiple sequences. The widths of the scattering peaks are similarly influenced by multiple scattering sequences. The maximum angle at which Ar can undergo single scattering (critical angle) from Mg, 0, and H is 37.5 “, 23.6 O, and 1.4*, respectively. Thus the 52O spectrum can only consist of multiple scattering events while the peaks in the 25” spectrum can contain single scattering contributions only for the Ar/Mg combination. An important feature is the constancy of the Ar scattering intensity as a function of H,O or 0, exposure. The shape of the scattering peak changes somewhat during chemisorption, however its intensity obtained by integrating the area remains unchanged, This has been verified by monitoring the primary beam intensity (and profile) before, during, and after several dosing experiments. This consistency is due to the similar scattering cross-sections for Ar scattering from Mg and 0 at 25O. This proportionality of the Ar scattering intensity to the primary ion current allows us to normalize the areas of the direct recoil peaks to the Ar scattering peak rather than the primary ion
current density. This not only results in a more rapid experimental procedure, but it also eliminates any dependence upon slight changes in primary ion current density during acquisition of intensity versus gas exposure data. Ion fractions of the direct recoils have been determined and will be reported elsewhere [20]. These ion fractions depend on a number of factors including the surface coverage of hydrogen and oxygen. For a fully “ hydroxylated” surface, direct recoil positive ion fractions are approximately 30% for Mg, 5% for 0, and 1% for H, and decrease at lower coverages. A ca. 30% negative ion fraction is measured for O(DR). Differing sensitivities of the channeltron multiplier for a neutral or an ionized atom will be most important for the Mg(DR) and O(DR), and will have little effect on the signal intensities of H(DR) because these are mainly neutrals at all covergaes. The channeltron sensitivities [lO,ll] for the direct recoils are approximately 0.8 for both O(DR)= 2000 eV and Mg(DR)= 2310 eV and 0.5 for H(DR)= 235 eV. The H(DR) signals measured at 25’ should be increased, therefore, by a factor of 1.6 in order to compare relative intensities among the three direct recoil peaks. 3.2. Comparison
of
TOF and XPS spectra for O,, H,O, and CH,OH
reactions
with Mg
In order to calibrate the direct recoil intensities to atomic concentrations we have performed scattering and XPS studies on the methoxylation of magnesium [14). The salient points of this study are: (1) The initial reaction up to 4 L methanol exposure to clean magnesium yields a surface hydroxyl and no surface carbon. (2) Methoxylation occurs after 4 L methanol exposure with saturation of all signals occuring at ca. 12 L. (3) Methoxide carbon is the only form of carbon on the surface. (4) Angle rsolved XPS yields an estimate of the ratio of surface concentrations C/Mg = 1 (within 40% uncertainty). We have used the CH,OH/Mg system to put known amounts of carbon, hydrogen, and oxygen onto the magnesium surface. Since the methyl groups are protruding above the surface 1141 and each one has three hydrogen atoms, we use this saturated CH~OH/Mg surface to approximate a saturated hydrogen concentration in the outermost atomic layer. The H(DR) peak from this surface dominates the direct recoil events and its intensity can be compared to that of H(DR) from the H,O and 0, saturated surfaces in order to obtain an estimate of the hydrogen concentration on those surfaces. The TOF spectra at 30’ for saturation CH,OH and H,O exposures are shown in fig. 4 and the corresponding peak intensities are listed in table 1. Each spectrum in fig. 4 was obtained in < 15 s (hence, the lower signal-to-noise ratio) in order to emphasize how quickly spectra with adequate statistics can be acquired. The C(DR) from the CH,OH exposure is more intense than O(DR), in agreement with previous studies [14] that show that CH,OH bonds to metal
445
J.A. Schultz et al. / TOF spectra of direct recoils. I
TIME OF FLIGHT @set)
TIME OF FLIGHT (~sec) Fig. 2. (A) TOF of clean peak
labelled
directly scattering
spectra
of scattered
Mg and Mg exposed Ar represents
recoiled angle.
hydrogen,
Ar scattering oxygen,
P = photon
and sputtered
to 15 L of H,O.
neutrals
Scattering
from
and ions for 3 keV Ar+ bombardment angle = 25 ’
Mg and 0;
and magnesium
atoms,
H(DR),
respectively.
;
flight path = 110 cm; the
O(DR),
and Mg(DR)
are
(B) Same as (A) but 52O
pulse.
15L o*CLEAN ----
0
TIME OF FLIGHT
Fig. 3. TOF clean
spectra
(psec)
of scattered
Mg and Mg exposed
IO
and sputtered
to 15 L of 0,.
neutrals
Scattering
plus ions for 3 keV Ar+ bombardment
angle = 25 o
;
flight path = 110 cm.
of
J.A.
446
surfaces surface.
studied
3.3. Exposure
Schultz
through
ef al. /
TOF
the 0 atom
spe~tru of drrect rrcoils.
with
the methyl
group
I
oriented
above
the
dose versus direct recoil intensity
Fig. 5a shows the variation of the direct recoil peak intensities, normalized to the (Ar/Mg + Ax-/O) scattering peak, as a function of clean Mg exposures
--.-rd 4 Fig. 4. TOF
spectra
Mg and Mg exposed
flight
path = 65 cm. Insert:
I 6
TIME OF FLIGHT (psec)
of scattered
clean
_____/’
and sputtered
to saturation Possible
neutrals
plus ions for 3 keV Arf
doses of CH,OH,
models
0,
and H,O.
for chemisorption
of H,O
bombardment
Scattering
of
angle = 30 o
and CH,OH
;
on a Mg
surface.
Table
1
Peak heights ‘) of direct recoil events at 4, = 30 o for clean Mg and Mg exposed of O,,
H,O,
to saturation
Species
H(DR)
C(DR)
O(DR)
Mg(DR)
Clean Mg
(b) 1.1
(b) (b)
(b) 3.2
10
8.8
(b)
5.1
14.4
5.8
(c)
0, Hz0 CH,OH ‘) Peak heights multiplier ” Values
are normalized
sensitivity.
less than 0.005.
‘) Unresolved
doses
and CH,OH
peaks.
to the Mg(DR)
intensity
from clean
5.8 4.3 (b) Mg and are uncorrected
for
J.A.
Schultz et al. / TOF spectra of direct recoils. 1
44-l
of 0,. The appearance of a surface hydrogen peak concomittant with the 0, exposure has been shown [15] to be a result of oxygen induced hydrogen impurity segregation and not a result of contamination residual gas (e.g. II,, H,O) reactions. A similar effect has recently been noticed [21] for Pd coated Nb samples which had been pretreated with hydrogen; subsequent 0, exposure significantly diminished their bulk hydrogen concentrations. The O(DR) and H(DR) increase and the Mg(DR) decreases smoothly as a function of 0, exposure. Saturation of these signals occurs at about 10 L 0, exposure, in agreement with LEED and electron spectroscopic results (161, demonstrating oxide formation at this exposure, The 0, saturated surface was then exposed to --
2
4
6
8
- 0.4 0
I 1
I
EXPOSURE
I 2
I
I 3
I 4
DOSE (L)
Fig. 5. (A) Direct recoil peak intensities, normalized to the (Ar/Mg +Ar/O) scattering peak, as a function of clean Mg exposures to 0,. (B) Plots of one- and two-site models (eqs. (4) and (5)) for 0, adsorption on Mg.
J.A. Schultz et al. / TOF spectra of direct recoils. I
448
successive doses of H,O until saturation occurred (- 3 L H,O). The resulting direct recoil peaks appeared at identical TOF’s to those of fig. 2A in which clean Mg was exposed to 15 L H,O. The direct recoil intensities versus dose curves are shown in fig. 6A. These curves exhibit a ten fold additional accumulation
of hydrogen
shows the variation
and a one fold increase
in the oxygen level. Fig. 7a
in direct recoil peak intensities
as a function
of clean Mg
exposures to H,O. The reaction with H,O proceeds relatively slowly until ca. 6 L, at which point the H signal increases much faster than the 0 signal, with both signals going rapidly to saturation Three These
types of adsorption
are a simplified
1 I
I
one-site
2 EXPOSURE I I
between
15 and 20 L exposure.
models were used in treating model
the exposure
the adsorption
rate
data. to be
4
3 DOSE , ,
assuming
(L) ,
,
B 5-
EXPOSURE
DOSE * CL*,
Fig. 6. (A) Direct recoil peak intensities, normalized to the (Ar/Mg + Ar/O) scattering peak, as a function of oxidized Mg exposures to H,O. (B) Plots of the nucleation and growth model (eq. (6)) for H,O adsorption on oxidized Mg.
J.A. Schultz et al. / TOF spectra of direct recoils. I
proportional
to (1 - O),
- 0) = k,L,
-ln(l
a two-site B/(1
-
(4)
model, assuming
63) =
the rate to be proportional
to (1 - @)*,
k,L,
(51
and a two-dimensional
nucleation
and growth model 1221:
-8)=k,,,L2.
-In(l
449
(6)
0.05
0.04
0.03
0.02
0.01
5
10 EXPOSURE
15
2c
DOSE
25
(1)
2-
z E
1
EXPOSURE
DOSE’
Fig. 7. (A) Direct recoil peak intensities, function of clean Mg exposures to H,O. H,O adsorption on clean Mg.
(L*)
normalized to the (Ar/Mg+ Ar/O) scattering peak, as a (B) Plots of the nucleation and growth model (eq. (6)) for
450
The surface coverage 6 is determined by dividing the direct recoil peak intensity at an intermediate gas exposure by the corresponding intensity at saturation. The 0, exposure data fits a two-site model (fig. 5B) over the entire exposure range. This agreement may be fortuitous because of blocking factors CX,which may change during the reaction (section 4.2). Two one-site models both below and above 1.5 L exposure also provide a satisfactory fit, indicating that there may be two different constant masking factors in each region. This is supported by a phase change which occurs at ca. 2 L 0, dose, as determined by LEED and other techniques, during oxidation of Mg single crystals [16]. For the H,O exposures to either oxidized or clean Mg, neither the one- or two-site models provided a satisfactory fit to the data. The nudeation and growth expression yields a good fit to the data for both O(DR) and H(DR) as shown in figs. 6B and 7B. 3.4. Direct recoil cross-sections The scattering and recoiling cross-sections a, for our specific systems were computed according to a program [23] which uses the Moliere approximation 1241 to the interaction potential and ten-point Gauss-Mehler quadrature [25] for evaluation of the scattering integral. The direct recoil cross-sections 0,’ were obtained by using the expressions for conservation of energy and momentum to determine the scattering angle x, for projectile j corresponding to the specific recoil angle +r for target atom i (30° for table 1) and then calculating the scattering cross-section u,’ at this angle x,. The equality a,‘(~,) = a,‘(+,) holds. The scattering angles x j corresponding to the 30 ’ recoil angle are listed in table 2 along with the calculated cross-section and critical scattering angle for each mass pair. Direct recoil cross-sections u,’ for C, 0, and Mg (DR) are comparable while that of H(DR) is - 5 times higher for recoils resulting from single collision events. The calculated scattering cross-sections us are plotted in fig. 8 as a function of scattering angle xAr. These curves illustrate why eI; > 0 6, = e& = u{. At a fixed angle (p, (30’ in this case), u<“s result from scattering events in which the scattering angles xAr decrease as the mass i of Table 2 to a & = 30 o recoil angle, calculated cross-sections Ar scattering angles xAr corresponding angle, and critical scattering angle xLr for Ar collisions with H, C, 0, and Mg Ar --j X X H C 0 Mg
Scattering angk (deg) 1.3 17.0 23.4 36.6
x Ar
Crosssection
Critical scattering angle, xi,
(A21
(deg)
0.16 0.036 0.028 0.029
1.44 17.45 23.60 37.48
at this
the target atom decreases. The positions of these xAr’s are indicated in fig. 8 where it is observed that u,~ for i = Mg, 0, and C are similar while ah is larger due to the extremely small angle xAr for that event. The cross-sections can change significantly if multiple collision sequences contribute to the recoils; figs. 2A and 2B indicate that the contributions from such multiple sequences are indeed significant at these forward scattering angles. The effect of multiple sequences can be observed from fig. 8. For example, if an oxygen recoil at +o = 30” results from a sequence of Ar scattering through two small xAr angles and recoiling 0 on the second collision
0
5
10
15
SCATTERING
20 ANGLE
25
30
35
(DEG.)
Fig. 8. Calculated scattering cross-sections u& for 3 keV Ar+ scattering from Mg, 0, C, and H as a function of scattering angle xAr. x,c indicates the critical or maximum anxgle through which single scattering of Ar from atom i can occur, and x, indicates the scattering angle correspondingly to 30 o recoil of atom i.
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spertm oj direcr
recoils. I
rather than a single xAr = 23.6O collisionthe new recoil cross-section will be larger than that of the single collision. Scattering through sequences with three or more smaller angles will result in even higher apparent u’ ‘s; similar arguments hold for C and Mg (DR). A simulation which accounts for multiple scattering sequences is clearly required in order to accurately predict the direct recoil intensities.
4. Discussion 4. I. Surface
roughness, shadowing, and blocking effects
Surface roughness is an important factor in determining intensities since particles can be scattered or recoiled into forward angles only if the macroscopic ridges and valleys are oriented so as not to block the trajectories. For example, recoils can only be observed from the bottom of a crevice when the crevice is oriented so that the primary beam can get in and the direct recoil can get out. Considerable surface roughness exists on sputter cleaned Mg even after annealing [16]. The surface reactions monitored here are thus only those occurring on portions of the roughened surface “ visible” to the ion beam. When primary projectiles are deflected by target atoms on a surface, a region exists behind each target atom within which the projectile cannot penetrate. This region is called a shadowy cone and atoms located inside the cone of another target atom cannot contribute to the scattering process. Typical shadow cones for 3 keV Ar+ scattering from H, C, 0, and Mg as calculated from the Moliere approximation to the Thomas-Fermi interatomic potential are plotted in fig. 9A. Particles either scattered or recoiled from a surface can also be deflected by neighboring surface atoms. Hence, blocking cones exist about the neighboring atoms which will tend to limit atom ejection at specific angles. The sizes of the blocking cones are similar to those of the shadow cones of fig. 9A. Examples of the effects of shadowing, blocking, and crystallite orientation are shown in figs. 9B and 9C for 3 keV Art incident on the Mg(OOO1) basal plane and the MgO(100) plane at a + = 12.5’ angle. From fig. 9B it can be seen that nearest neighbor Mg atoms block each other so that Mg(DR) cannot occur along the [lOOO] direction but must occur along directions such as [ 11001 where the interatomic distances are - 5 A. Also, any Mg crystallite from which direct recoils are observed must be macroscopically nearly perpendicular to the surface normal. If the incident beam angle is decreased much below 12.5 ‘, every surface atom will be concealed by the shadow cone of its neighboring surface atom and the scattering intensiy from single collision events drops to zero. For the MgO(100) surfaces of fig. 9C, the situation is similar in that scattering and recoiling can only occur along directions such as [210]. Nearest
J.A. Schultz et al. / TOF spectra of direct recoik
i
453
neighbor shadowing is more severe than on the more open Mg(OOO1) surface. Considering these two as well as other possible crystalline faces on the polycrystalline Mg surface, it is evident that at low incident and recoiling angles the observed direct recoils must come from either high index (open structure) crystallite faces or from surface defects (cone tips, step edges, etc.).
Ar -
C,O,Mg
r I
DISTA TOP VIEW Mg (0001) 0
::E & TOP VIEW MgO (100) * 0 *o/P
l
&PP 0
0 -3.1 2Ac.
$I&
0
do
l
l
0
o
.
0
SIDE VIEW [t loo]
SIDE VIEW [210]
SIDE VIEW [lOOO]
SIDE VIEW [loo]
Fig. 9. (A) Examples of the shadow cones behind a target atom for 3 keV Ar+ bombardment of H, C, 0, and Mg. The target atom is at the origin 0 and r is the radius of the cone at a distance 1 A behind the atom. (B, C) Schematic representations of(B) a Mg(OOO1)and (C) a MgO(100) surface and the shadowing and blocking cones for a 3 keV Ar + ion incident at 12.~5~ from the surface plane.
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TOF spectru
of drrect recoils.
I
4.2. Surface concentrations from direct recoil intensities
For a given atom i on different surfaces, the factors u,, I+ and F, in eq. (3) can be cancelled to yield direct recoil intensity ratios such as (7)
where (x) denotes the quantities corresponding to the metal with a saturation dose of adsorbate x. The atomic densities of Mg in both the metal and the oxide are equivalent (both - 1.2 X 10” atoms/cmj) and the surface Mg concentration along crystal faces is nearly equivalent for both the metal and oxide. Existing evidence [16] on oxidation of single crystal Mg supports the view that surface oxide forms in the 2-10 L exposure region. Since nMg = nM,Wb)= nMg (H,O), the ratios of eq. (7) can be used to calculate the degree to which the Mg atoms are masked by the adsorbate atoms. We calculate from the direct recoil intensities that a Mg(02) = 0.29 and (Y~~(H~O) = 0.46. This indicates that for both adsorbates Mg(DR) is blocked more on the adsorbate covered surfaces than the metal surface, as expected from the shadow cones of fig. 9. The interesting point, however, is that Mg is blocked 32% more in the H,O covered surface than in the 0, covered surface. Also from fig. 4 it is seen that O(DR) from the water reaction is twice as intense as from the 0, reaction. These two observations suggest the hydroxyl moiety as the outermost surface group. For the hydroxylated surface we have obtained an estimate of two surface hydrogens for each magnesium atom in the following manner. Using the model that the saturated CH,OH surface contains three hydrogen atoms for each carbon atom and that this hydrogen is in the outermost layer, we can estimate the hydrogen coverages in the other surfaces. For hydrogen in the outermost layer, the masking factors should be approximately equal so that a,(CH,OH) = ‘~n(H,0) = (~“(0~) and the experimental intensity ratios provide direct estimates (eq. (7)) of the coverages. The result is n,(H,O) = 0.6n,(CH,OH) and nn(0,)=0.06nr,(CH,0H). For the CH,OH and H,O saturated Mg surfaces, the H(DR) results (outermost monolayer sensitivity) indicate a ratio of nn(CH,OH)/nn(H,O)= 3/2 and the XPS results [14] indicate ratios of nc(CH,OH)/n,s(CH,OH) = 1. These data are consistent with a model such as shown in the fig. 4 insert. The CH,OH/Mg surface contains methoxide atop Mg atoms with either 0 or OH at between-Mg sites. Any hydroxyl hydrogen in the between Mg sites is severely blocked by the neighboring methoxide. The H,O/Mg exposed surface is proposed to comprise hydroxyls at both on top and between sites. The hydroxyl hydrogen in both sites would be unblocked and contribute to the H(DR) signal. Such a model of the surface hydroxyl was previously proposed and confirmed by IR and NMR measurements [26]. We have performed angle
J.A. Schultz er al. / TOF spectra of direct recoils. I
resolved XPS studies (unpublished)
which are also consistent
455
with an hydroxyl
rich outer layer atop an oxide.
5. Summary The data obtained here indicate that surface oxide forms within 10 L 0, exposure with the oxygen assuming sites which are within the outermost Mg layer. Reactions with H,O and CH,OH are complete at ca. 20 L exposure with hydroxide and methoxide species assuming positions above the surface layer. Intensity and shadow cone analysis suggest a structural model in which the hydroxyl and methoxyl moieties are bonded through the oxygen end with the hydrogens outermost. The hydroxylated surface contains 2 H atoms for every surface Mg consistent with a stoichiometry Mg(OH),. TOF analysis of direct recoils from a surface is a viable tool for monitoring surface adsorbate concentrations, in particular, surface hydrogen. This analysis is accomplished using non-destructive primary ion doses (< 10” ions/cm*). The analysis is extremely surface sensitive and the direct recoil intensities are determined by both the number and geometry of the adsorbate atoms. The analysis is rapid compared to photoelectron spectroscopy [14]. A reduction of two orders of magnitude in the 15 s acquisition time for the data in fig. 4 can be anticipated by using a more intense ion source and multistop time-to-amplitude converters.
Acknowledgments This material is based upon work supported by the National Science Foundation under Grant No. CHE-8209298. We would like to thank Ms. Yang-Sun Jo for performing the cross-section calculations and Dr. G.C. Nelson for a copy of the program for calculating the Moliere potential and cross-sections.
References [l] ES. Mashkova, V.A. Molchanov and U. Shoska, Dokl. Akad. Nauk SSSR 161 (1965) 813. [2] W.F. van der Weg and D.J. Bierman, Physica 38 (1968) 406. [3] S. Prigge, H. Niehus and E. Bauer, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, pp. 1381-1384. [4] R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 112 (1981) 133. [5] R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 104 (1981) 384. [6] J.A. Schultz, R. Kumar and J.W. Rabalais, Chem. Phys. Letters 100 (1983) 214. [7] R.S. Bhattcharya, W. Eckstein and H. Verbeek, Surface Sci. 93 (1980) 563.
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[S] K. Wittmaack, Surface Sci. 89 (1979) 668. [9] Y.S. Chew G.L. Miller, D.A.H. Robinson, G.H. Wheatley and T.M. Buck, Surface Sci. 62 (1977) 133. [lo] D.H. Crandall, J.A. Ray and C. Cisneros, Rev. Sci. Instr. 46 (1975) 562. [ll] C.N. Burrous, A.J. Lieber and V.T. Zaviantseff, Rev. Sci. Instr. 38 (1967) 1477. [12] T.M. Buck, Y.S. Chen, G.H. Wheatley and W.F. van der Weg, Surface Sci. 47 (1975) 244. 1131 S.B. Luitjens, A.J. Algra. E.P.Th.M. Suurmeijer and A.L. Boers, Appl. Phys. 21 (1980) 205. [14] J.A. Schultz and J.W. Rabalais, Chem. Phys. Letters 108 (1984) 328; J.A. Schultz, S. Contarini and J.W. Rabalais, Surface Sci., submitted. [15] M.H. Mintz, J.A. Schultz and J.W. Rabalais, Phys. Rev. Letters 51 (1983) 1676. [16] H. Namba, J. Darville and J.M. Gilles, Surface Sci. 108 (1981) 446. [17] A.G. Akimov, I.L. Rozenfeld and V.G. Dagurov, Izv. Akad. Nauk. SSSR, Ser. Khim. (1979) 628. [18] K. Jensen and E. Veje, Z. Physik 269 (1974) 293. [19] A.L. Beers, Surface Sci. 63 (1977) 475. [20] J.A. Schultz, C. Blakely, M.H. Mintz and J.W. Rabalais, unpublished. 1211 M. Strongin, private communications. 1221 For nucleation and growth kinetics, see for example: (a) J.W. Christian, The Theory of Transformations in Metals and Alloys, Part I. Equilibrium and General Kinetic Theory (Pergamon, 1975); (b) C.H. Bamford and C.F.H. Tipper, Eds., Comprehensive Chemical Kinetics, Vol. 2, The Theory of Kinetics (Elsevier, Amsterdam, 1969). [23] H.H. Brongersma and T.M. Buck, Surface Sci. 53 (1975) 649. [24] I.M. Torrens, Interatomic Potentials (Academic Press, New York, 1972). [25] Z. Kopal, Numerical Analysis, 2nd ed. (Wiley, New York, 1961) p. 367. [26] P.J. Anderson, R.F. Horlock and J.F. Oliver, Trans. Faraday Sot. 61 (1965) 2754: R.K. Webster, T.L. Jones and P.J. Anderson. Proc. Brit. Ceram. Sot. 5 (1965) 153.