Methoxylation of magnesium studied by direct recoil spectroscopy, SIMS and XPS: Calibration of relative surface hydrogen concentration

315

Surface Science 154 (1985) 315-330 North-Holland, Amsterdam

METHOXYLATION OF MAGNESIUM STUDIED BY DIRECT RECOIL SPECTROSCOPY, SIMS AND XPS: CALIBRATION OF RELATIVE SURFACE HYDROGEN CONCENTRATION J. Albert SCHULTZ, J. Wayne RABALAIS Department Received

Salvatore

CONTARINI,

Yang-Sun

JO and

of Chemistry, University of Housron, University Park, Houston, Texas 77004, USA

29 May 1984; accepted

for publication

15 November

1984

Chemiso~tion of methanol on polyc~stalline magnesium is studied by time-of-flight analysis of directly recoiled (DR) surface atoms, angular resolved X-ray photoelectron spectroscopy (XF’S), and secondary ion mass spectrometry (SIMS). The combined measurements show that decomposition occurs to form - 0.14 monolayers of surface hydroxide at methanol exposures -=z4 L. High exposures result in molecular chemisorption to form a single methoxide overlayer. The DR results reveal that the C and H of the methyl group are the outermost atoms on the saturated surface. Analysis of DR intensities allows calibration of the relative signals and comparison with calculated recoil cross sections. The metho~de/M~ system provides a standard for surface hydrogen concentrations by the direct recoil technique.

1. introduction

Determination of atomic concentrations is basic to any modern surface study. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been developed [l] into viable tools for determining relative and absolute surface concentrations for all elements except hydrogen and helium. Much of the information on the ubiquitous role of hydrogen in surface chemistry has been obtained either from surface vibrational spectroscopy [2] or through mass spectrometry [3,4] of a fraction of the surface hydrogen desorbed by ions, electrons, photons, or by heating. Several years ago it was suggested [5] that surface hydrogen could be detected by time-of-flight (TOF) energy analysis of H atoms which are recoiled into a forward scattering angle as a result of a direct collision from an ion beam of several keV kinetic energy. The energy E, of a target atom of mass jM2 recoiling from a primary ion of energy E, and mass M, can be calculated from classical mechanics [5J as, E, = E. [

4A (1 + AJ2

cos2

(1)

+*

1

0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

316

J.A. Schultz et 01. / Methoxylution

ofmagnesium

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 such a forward angle comprise both directly recoiled (DR) surface atoms and scattered primary atoms. The high recoil energy that can be imparted to hydrogen by judicious choice of EC,, M, and r#~allows detection of neutral as well as ionized hydrogen atoms with near unity efficiency by a channeltron electron multiplier [6] (e.g., for E,, = 10 keV, M, = 40, and $ = 30”, E, = 714 eV). The detection efficiency decreases with E,, being - 20% for 100 eV hydrogen [6]. Since hydrogen is the lightest element, its velocity is usually greater than other directly recoiled atoms or scattered primaries allowing easy resolution in a TOF measurement. This technique has recently been demonstrated for hydrocarbon contaminants on a Cu surface [7], for the reaction of H,O with CsBr [8], and for surface hydrogen segregation from bulk contaminants in Mg induced by reaction with 0, [9]. Work with hydrogen on single crystal Pt suggests that structural information about H binding sites may be obtained [lo]. While qualitative identification of surface hydrogen recoils is straightforward, quantitative analysis requires relating the flux FTr and experimental geometry f($) as well as the following three factors: (1) the concentration of surface atoms C,, (2) the degree to which these atoms are either “shadowed” from the incoming primary ion or “blocked” on their outward trajectory by other surface atoms as represented by the masking factor (Y,(E,, E,, +) with range 0 < (Y,< 1, and (3) the differential recoil cross section a,( E,,, E,, +). This can be expressed as 6 = F,: f(G)

C, a,(-&,,

E,, +) [L - a,(&,,

E,, +)I.

(2)

Theoretical estimates of u, can be used to relate 4 to C, and (Y,. Our approach in this paper, however, is to demonstrate a system for which C, and (Y, are known, allowing calibration of the recoil flux F; and thus an experimental determination of u,. Surface methoxide complexes provide a suitable system for accomplishing this. In contrast to methanol reactions with transition metals, such as Ni(ll1) [ll], in which decomposition occurs at or below room temperature, Mg will be shown to belong to a class of metals such as Al[12], Cu[13,14] and Li[15,16] which are known to form stable methoxide surface complexes. The geometry of methoxide has been inferred to be normal to the surface on Ni(ll1) [17] at 1OO’C by vibrational spectroscopy, on Li(100) [15,16] by ab initio cluster models, and on Cu(100) [18] by near edge EXAFS. We are not aware of any studies of methanol on clean magnesium, although stable alkoxides have been identified on MgO [19,20]. The purpose of this paper is threefold: first to demonstrate the existence of the methoxide complex on magnesium, second to use the direct recoil technique to give information on the mechanism of the reaction and the geometry

J.A. Schultz ef af. / Meihoxylatt~n of magnesium

317

of the complex, and finally to discuss the suitability of a surface methoxide complex as a standard system for calibrating H, C, and 0 direct recoil intensities to absolute surface coverages of these atoms. In the process, the potential of the direct recoil technique for rapidly and simultaneously obtaining the concentration of impurity atoms in the outermost surface region will be demonstrated. The paper is organized as follows. The experimental methods are briefly described in section 2. In section 3 the results are presented as XPS and SIMS characterization of the methoxide layer, XPS and angular resolved XPS determination of the methoxide layer thickness, DR characterization of the methoxide layer, and finally the behavior of the XPS and DR signals as a function of methanol exposure. The discussion of section 4 summarizes the results on the nature of the methoxide layer and its suitability for calibration of surface hydrogen concentration. The sensitivity and acquisition time limitations of the direct recoil technique are also considered. The conclusions are presented in section 5.

2. Experimental methods The instrumental requirements for ion scattering and recoiling with TOF analysis and detection of ions and neutrals have been described elsewhere [5] and a block diagram of our spectrometer system is shown in fig. 1. The source is a pulsed, velocity selected, and energy analyzed ion beam and the detector is a channeltron electron multiplier. A time-to-amplitude converter is used to produce a voltage pulse of height proportional to particle flight times. This pulse is fed to a pulse-height-analyzer which generates a histogram of the distribution of particle flight times. The experimental parameters are as follows: (1) Primary beam 3 keV Ar+, 75 ns pulse width, 4 nA average current, 100 kHz pulse rate. (2) 67 cm flight path from sample to detector. (3) 2 x lo-‘* Torr base pressure. The angle of primary beam incidence was ‘75” from the surface normal and the recoil angle was 30”. Each spectrum was accumulated in - 15 s. The XPS and angular resolved spectra were obtained on a PHI Model 550 Electron Spectrometer using a cylindrical mirror analyzer (CMA) and Mg and Al Ka X-ray sources. Angular resolved spectra were obtained in 12’ wide sectors defined by a rotating drum in the CMA. AES spectra were acquired in both instruments. SIMS spectra were obtained by means of a TOF secondary ion mass spectrometer [Zl] as described by Poschenrieder [22]. Polycrystalline magnesium (99.5%, Alfa) was cleaned in both spectrometers by 3 keV Ar+ bombardment incident at 60” from the surface normal. This procedure roughened the Mg surface with striations along the direction of bombardment [23]. 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

318

J.A. Schultz et al. / Methoxylation TIMING

ofmagnesium

ELECTRONICS

MCAIPHA

i-

A

3 ELECTROSTATIC

Fig. 1. Block diagram of the spectrometer system used in the time-of-flight, electrostatic sector analysis, and Auger experiments. A = ion source, B = Wien filter, C = pulse plates, D = energy analyzer, E = lenses and deflector, I,, I,, I,, = electron multiplier detectors.

signals, and by the appearance of the characteristic metallic AES structure. Methanol contained in a glass vial was degassed by repeated freeze-thaw cycles and introduced into the chamber through a bakeable variable leak valve. The gas purity was monitored by means of an in situ mass spectrometer and the same methanol bulb was used on each spectrometer. Gas exposures in the ion pumped PHI chamber were consistently 1.4 times larger than in the turbomolecular pumped scattering chamber, as determined by AES measurements with CMA analyzers in both chambers. Exposures in the PHI chamber have been corrected by this factor, which has been shown to be accurate to within + 15%. All experiments were conducted at room temperature.

3. Results 3. I. XPS and SIMS

characterization

of adsorbate

The 0 Is, C Is, and Mg 2s XPS spectra exposed to saturation doses of CH,OH

for clean and oxidized magnesium are shown in figs. 2A and 2B,

J. A. Schultz et al. / Meihoxylarion

BINDING

ENERGY

of magnesium

319

(eV)

Fig. 2. Mg Ku XPS spectra showing the 0 Is, C Is, and Mg 2s regions for clean (A) and oxidized (B) magnesium exposed to a saturation dose of CH,OH. a = clean Mg, b = clean Mg+ 10 L CH,OH, c = oxidized Mg, d = oxidized Mg+ 10 L CH,OH.

BINDING

ENERGY

eV

Fig. 3. XPS valence band spectrum of magnesium methoxide. The numbered fines represent and adsorbed methanol (Nos. I, 2,3,4,S, 7) taken positions for methoxide (Nos. 1,2,.5,7) ref. [12]. See text for details.

peak from

320

J.A. Schultz et al. / Methoxylrtion

ojmagnt~ium

respectively. Upon saturation of clean Mg with CH,OH, symmetrical 0 Is and C 1s peaks are observed at 532.7 eV and 287.2 eV binding energy, respectively, and the Mg 2s peak is significantly (20%) attenuated. C Is and 0 1s binding energies are at the position expected [12,24] for methoxyl carbon and oxygen and provide an atomic ratio of O(ls)/C(ls) = 1.17. Whereas the 0 1s binding energies of methoxyl and hydroxyl species are overlapping and would be indistinguishable in our spectra, the C 1s binding energies clearly rule out adsorbed CO (ca. 285.5 eV) [25] and carbon (ca. 282 eV) [26]. The valence band spectrum of clean Mg exposed to 50 L CH,OH is shown in fig. 3. The upper and lower curves represent 4 and 24 h acquisition times with pass energies of 50 and 25 eV, respectively. The region below 8 eV has been deconvoluted into two peaks which correspond to the Mg 3s valence band at ca. 2 eV (determined from measurement of clean Mg) and the 0 2p nonbonding MO of the adsorbate at 5.8 eV. Comparison is made to the He II spectra of condensed methanol and methoxide on Al [12] (not shown here) by means of the numbered lines. Condensation of methanol on Al at 110 K yields bands at the numbered positions; upon warming to room temperature, the bands at positions 2 and 4 (dotted lines) disappear. The disappearance of bands 2 and 4, which represent ionization of the COH “angle determining orbital” and the hydroxylic u orbital, respectively, have been associated with formation of surface methoxide [12-171. The He II spectrum of condensed methanol on Al [12] (also obtained with a CMA) exhibits the following intensity ratios: peak 4/peak 3 = 0.55 and peak S/peak 3 = 0.71. With X-ray excitation bands 4 and 5 should be even more intense than in the He II case because the orbitals associated with 4 and 5 have significant C and 0 2s character which has a higher cross section than the orbitals associated with 3 which are dominantly C and 0 2p and H 1s character [15,16]. The absence of peaks at positions 2 and 4 indicates that the adsorbate is methoxide rather than adsorbed methanol. The spectrum is also in good agreement with that of lithium methoxide surface complexes [15,16]. We conclude that there is insignificant adsorption of methanol on Mg at room temperature; methoxide is the dominant species. As will be shown in section 3.4, during the initial methanol exposure there is a small amount of decomposition which produces ca. 0.1 monolayer of 0 and H; such a surface hydroxide will have photoemission near the positions 2 and 4 [27], hence we expect there to be minor intensity in these regions. The peaks labeled 5, 6, and 7 correspond to ionization of the C 2s oxide 0 2s and methoxide 0 2s, respectively. The oxide 0 2s (peak 6) is not observed upon initial exposure; this peak may result from the 4 h exposure to the residual gases after the initial 50 L methanol exposure. Another possible source of oxide is the long continuous X-ray dose required to acquire the valence band spectrum which may partially decompose the methoxide. No carbidic-type carbon was present on either the 4 or 24 h irradiated surfaces. The oxidized magnesium surface was prepared by exposure of clean Mg to

J.A. Schuttz et al. / ~eth~xylut~on

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321

10 L 0,; this is known [28] to form an oxide layer of between two and four atomic layers thick on M&0001). On polycrystalline Mg we have shown [29], using direct recoils, that this exposure results in an outermost surface layer containing both oxygen and magnesium for which the O(DR) signals saturate at 2-5 L. The XPS spectra of fig. 2B exhibit a single 0 Is line at 530.7 eV corresponding to an oxide 1231 and a Mg 2s line which is slightly broadened but not at all reduced in peak area compared to clean Mg. The Mg 2s and 0 Is peak areas, corrected for cross section [26] differences, yield a ratio Mg/O = 1.7. Assuming a model of a homogeneously distributed oxide layer and considering the widely different electron escape depths [29] for these two lines [X(Mg 2s) = 22 A, X(0 Is) = 14 A], these data are consistent with an oxide which is about three atomic layers thick. This is reasonable compared to the oxidation studies [23,28] on Mg single crystals. The XPS results after exposure of this oxidized surface to 10 L CH,OH are shown in fig. 2B. In contrast to the invariance of the Mg 2s intensity upon 0, exposure, the CH,OH exposure attenuates the Mg 2s intensity of the oxidized surface by about 20% as seen before on the clean Mg surface. The C Is peak is at 287.2 eV which is consistent with a methoxyl carbon. The oxide 0 1s peak is also attenuated and a methoxy (or hydroxyl) 0 1s peak appears at 532.7 eV. Some of the oxide 0 1s attenuation may be due to the conversion of some surface oxide to hydroxide during initial CH,OH exposure. This is supported by the fact that the area of the 532.7 eV oxygen peak determined by deconvolution yields an atomic ratio of 0(532.7)/C 1s = 1.25. Both the O(532.7 ev)/C 1s ratio as well as the absolute intensity of the C 1s peak are equal within If: 5% for CH,OH exposures to either clean or oxidized Mg surfaces. This indicates that the same amount of methoxide forms on both surfaces. The SIMS spectrum of clean Mg exposed to 10 L methanol is shown in fig. 4. Ions characteristic of a methoxide surface layer, such as CH,O+ (31 amu) and cluster ions containing CH30’ at 5.5, 79, and 95 amu, are observed. The O+ and OH+ peaks, which are characteristic of a hydroxylated surface, are small compared to the CH: peak. On a Mg surface containing approximately equal amounts of hydroxide and methoxide (prepared by dosing with H,O to half saturation followed by CH,OH to full saturation), the CH_q , OH+, H20f, and H,O+ ion intensities were all comparable. Thus, the small OH’/CH: ratio obtained upon exposure to methanol is suggestive of a surface with a small hydroxide and large methoxide coverage, in agreement with the XPS results. We have demonstrated elsewhere 1313 that the O+ and OH+ peaks are accurate indicators, at all coverages, of the surface oxygen (or hydroxyl) concentration on Mg. However, it is unclear in the present study how much of the OH+ might originate from recombination of methoxyl oxygen with hydrogen. Also unclear is the origin of the 24MgH+ peak, which is partially responsible for the intensity at 25 amu (Mg isotopic abundance: 24Mg (78.7%),

1

40

SECONDARY

50

60

ION MASS

70

80

90

omu

Fig. 4. Time-of-flight secondary ion mass spectrum of magnesium Ar + bombardment. Intensity in logarithmic units.

25Mg(10.1%), 26Mg(11.2%); this may be from during decomposition of methanol to methoxide.

methoxide produced by 3 keV

magnesium

hydride

formed

3.2. Methoxide film thickness The significant attenuation of the Mg 2s peak upon CH,OH saturation of both clean and oxidized Mg point to a layer of methoxide on top of the surface. This will be corroborated by direct recoil spectra in section 3.3. The ratio of XPS intensities from a clean metal 1: and an adsorbate covered metal I, can be expressed as 1321,

where X, is the mean free path of metal photoelectrons in the solid, 8 is the electron take off angle measured with respect to the surface normal, Q? is the adsorbate coverage, and d is the adsorbate film thickness. Assuming cf, = 1 and using the experimental Mg 2s intensities before and after methoxylation,

J.A. Schultz et al. / Methoxylation

ofmagnesium

323

we obtain estimates of the methoxide layer thickness on clean Mg and oxidized Mg to be d = 3.3 and 3.7 A, respectively. This is approximately the linear dimension of a methoxide group. The methoxide film thickness has been determined independently by using angular resolved XPS. The ratio of XPS intensities from the adsorbate covered metal I, and the adsorbate itself Z, can be expressed as [32], I ,=K1-exp(‘-d/A,cos8) exp( -d/X,

Zm

cos 0)

’

(4)

where K is a spectrometer constant. For this experiment we used Al Ka X-rays in order to ionize the Mg Is electrons which have a very short escape depth, h,,(ls) = 5.5 A. Since carbon is on top of the methoxide film and since Ac(ls) = 23 A, the escape depth angular dependence of C 1s electrons can be considered constant and it is a good approximation to consider only the angular dependence of the Mg 1s electrons. Eq. (4) simplifies to, ln?

,“,, e

= In K’ - x C

.

m

The angular resolved photoemission intensities treated according to eq. (5) were linear over the range 0” < 0 < 60” and yielded d = 4.5 + 1.5 A. Hence, both methods are in agreement that the layer thickness is approximately equal to the length of the methoxide group. We therefore, assign a stoichiometry of one methoxide group per surface magnesium atom, within an uncertainty of about 40%. 3.3. Direct recoil spectra TOF spectra of directly recoiled surface atoms and scattered primaries at 30” are shown in fig. 5 for clean Mg and various CH,OH exposures to clean Mg. The 5 L and 12 L exposures represent half and full saturation coverages, respectively. The assignment of the four direct recoil peaks and the scattering peak was made by using eq. (1) (and the corresponding equation for scattering [5]) to obtain recoil and scattering energies which were converted to times-offlight using the assumed mass and known path length. These assignments have been corroborated as described elsewhere [5,8] by timing the ionized recoiled and scattered particles through an electrostatic sector set to pass a specific ion energy. The masses of the ions at that energy can then be unambiguously determined. The neutral particles are then assumed to have the same energy distribution as the ions. The calculated TOF’s for H(DR), C(DR), O(DR), and Mg(DR) as well as for singly and doubly scattered Ar from Mg at 30” are listed in table 1. The Ar scattering peaks at 5.5-7.5 ps result primary from multiple scatter-

324

J.A. Schultz et al. / Methoxylation

of magnesrum

Ar (MS)

-CLEAN

12L ___ 6L ........ 3 L

IiI\ ,,

-IoL

2

4

6

DR & MS

TIME OF FLIGHT

8 (WC)

(

T I

Fig. 5. TOF spectra of direct recoils and scattered primary Ar for 3 keV Ar+ bombardment of clean polycrystalline M&A) and oxidized M&B) exposed to various doses of methanol. The peaks labeled H, C, 0, and Mg correspond to directly recoiled H, C, 0, and Mg, respectively, and those labeled Ar(MS) correspond to scattered Ar. The DR peaks are X 8. Table 1 Calculated energies and minimum TOF’s a) for directly recoiled H, C, 0, and Mg atoms from a methoxylated Mg surface as well as for Ar singly (SS) and double (DS) scattered from clean Mg using 3 keV Ar+ bombardment and 6’ = $J = 30”

Energy (eV) TOF (w)

H(DR)

C(DR)

O(DR)

Mg(DR)

Ar(SS)

Ar(DS)

214 3.29

1597 4.17

1836 4.49

2109 5.13

1705 7.36

2371 6.24

‘) The flight path is 67 cm.

J.A. Schultz et al. / Methoxylation

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325

ing from clean Mg which shifts to longer flight times as the surface is methoxylated. Although we are considering a polycrystalline surface, direct recoils are only possible either from crystallites whose local surface normal bisects the angle of specular reflection or from defect sites. This is because with an angle of incidence of 75” from this local normal, tilting of the crystallite by more than a few degrees will cause macroscopic blocking of either the incident beam or the outgoing recoil. For clean Mg the only direct recoil peak observed is Mg(DR). At half saturation, the H(DR) and Mg(DR) peaks are prominent and the C(DR) and O(DR) are resolved. At this coverage the methoxide is bound to approximately every other Mg surface atom, resulting in significant shadowing of the Mg atoms. At full coverage the Mg atoms are almost totally shadowed. The results are consistent with a model in which the methoxide group projects above the surface with the methyl group shadowing and blocking the underlying magnesium. The H(DR)/C(DR) ratio is constant at half and full coverages indicating that neither C nor H are significantly blocked. This is expected when one considers the Ar deflection angles for surface recoils at @I= 30” as listed in table 2. Primary Ar is deflected only 1.3” from its original direction for a H recoil at 30” and loses only bt7% of its primary energy. Since the velocity of H at 215 eV is 1.8 times that of Ar at 2,785 eV, the H(DR) will leave the collision region quickly while Ar continues almost undeflected to strike C which is then recoiled. Hence, the surface H to C concentration ratio available for DR is the methyl stoichiometry of 3/l. Fig. 4B shows the DR spectra of oxidized magnesium which was exposed to methanol: The H(DR) and C(DR) intensities from this methanol saturated oxide surface are identical to those of fig. 4A obtained from the methanol saturated Mg surface. This is consistent with the XPS results from both saturated surfaces.

Table 2 Experimental recoil intensity ratios ‘) for 0 = 0.5 and 1.0, Ar deflection $I = 30” recoil angle, and calculated recoil cross sections Ar + X X

Experimental

ratios X/C

e = 0.5

e=l.o

Ar deflection angle (deg)

angles corresponding

Calculated cross sections b’

to a

Calculated ratios X/C

(AZ) H C 0

1.0

1.3

1.0

1.0

1.3 17.0 23.4

‘) Assuming a stoichiometry of 3H/lC and channeltron = 0.8. b, Calculated for 3 keV Ar+ at the indicated scattering angle for the target atom.

0.16 0.036 0.028 efficiencies

4.4 1.0 0.77 [6] of S, = 0.5 and S, = So

angle which corresponds

to a 30” recoil

3.4. DR and XPS as a function

of CH.,OH

exposure

At exposures of < 4 L CH,OH, the prominent DR peaks are H(DR), O(DR) and Mg(DR), with the latter intensity reduced from its value on the clean surface. The C(DR) appears only after higher exposures. This behavior can be observed from plots of the intensities of the four DR peaks as a function of exposure in fig. 6. All four curves were acquired simultaneously in less than 1.5 min and contamination of the surface between exposures was shown to be negligible. Shadowing of Mg by the 0 and C is a rapidly increasing function of exposure. The absence of C(DR) for exposures up to - 4 L is clear as well as the growth of H(DR) and O(DR). The rate of growth equal (fig. 4). At > 4 L of H(DR) and O(DR) up to - 4 L is approximately exposures, C(DR) becomes apparent and the rate of change of H(DR) with exposure increases and begins to track the C(DR) curve, both of which saturate at ca. 8-10 L exposure. These DR results are consistent with dissociation of the methanol to form a surface hydroxyl species at (4 L exposure followed by methoxylation at > 4 L exposure.

\I

18‘X

'X

6

Mg(DR)

X

H(DR) x i/2 .*_....-*

14 z

\ X

;*

,/ .*-*

B.‘“”

C (DR)

Methanol Exposure Fig. 6. Plots of direct recoil intensities (H(DR), C(DR), O(DR), Mg(DR)) 1s and 0 Is) as a function of methanol exposure (in L) to clean Mg.

and XPS intensities

(C

J.A. Schultz et al. / Methoxylation

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327

The XPS results support the DR results. At < 3 L exposure (not shown), a single 0 Is peak is observed at 532.7 eV which corresponds to the methoxyl/ hydroxyl position. The intensity of this 0 1s as well as the C Is peak as a function of exposure are shown in fig. 6. Preferential buildup of 0 at < 4 L exposure is observed and the shape of the C Is curve at > 4 L exposure closely follows the 0 1s curve. Also, the exposure behavior of the C 1s signal is very similar to the C(DR) behavior. Due to the absence of C 1s signal in the < 4 L region, we assign the initial 0 1s peak to a surface hydroxyl decomposition product of the methanol reaction. H(DR) is seen to increase with exposure in this region as it must to be consistent with the hydroxyl assignment. At > 4 L exposures the XPS, SIMS, and DR results are consistent with the molecular chemisorption of methoxide. 4. Discussion 4. I. Nature of the methoxide

overlayer

The combined XPS, SIMS and DR results provide unique characterization of the CH,OH/Mg reaction. At < 4 L exposure the appearance of H(DR), O(DR) and hydroxide 0 1s and the absence of C(DR) and C 1s on the Mg surface show that CH,OH is dissociating to form a surface hydroxide with loss of the methyl group. This hydroxide coverage is estimated to be - 0.14 the dominance of H(DR), C(DR) and monolayers. At higher exposures, O(DR) over Mg(!DR), the invariance of the H(DR)/C(DR) intensity ratio, the appearance of methoxyl C 1s and 0 1s lines with a stoichiometric ratio of approximately unity, the methoxide-like valence band structure, and the appearance of secondary ions characteristic of methoxide all indicate the formation of a methoxide overlayer [33]. A possible reaction sequence for the formation of surface hydroxyl and absence of any carbon at low exposure will now be considered. Our sample contains a 600 ppm (atm/atom) hydrogen impurity [34]. Enough of this impurity segregates to the surface upon oxygenation [8] to form approximately a 0.05 monolayer coverage [29] of surface hydrogen. If this hydrogen surface segregation occurs during the initial stages of the methoxylation reaction, it could allow conversion of the methyl group into methane which would desorb before a surface carbide could form. The absence of carbon in any form at low exposure and the presence at higher exposures of only methoxy carbon is quite different from the significantly carbided surface resulting from methanol reacting with SC [24]. 4.2. Sensitivity

of the direct recoil technique

The peak area of the saturated hydrogen direct recoil signal in fig. 5 is ca. 4000 c/s. Signal backgrounds seem to occur from desorbed metastables or

from unpulsed neutrals scattered onto the sample from the beam line. The total background counts from the clean surface under the hydrogen peak is ca. 20 c/s. From the calibration of the methoxide thickness we know that the methoxide to surface magnesium stoichiometry is approximately 1 to 1 ( rfi40%). The three hydrogens of the methyl group all appear to be “visible” to the impinging Ar+, therefore, we can assign a lower limit to the detection of hydrogen of 0.03 H/surface Mg with a signal to background of 2 : 1. Detection limits for C, 0, and Mg are similar. Some improvement of sensitivity can be anticipated by using a more intense ion source which would allow better pulse definition and the reduction of neutral scattering from the ion optics onto the sample. The use of a higher intensity ion source coupled with a multiple stop time digitizer is anticipated to permit acquisition of spectra comparable to those shown in fig. 5 within 1 s. Combination of the XPS, SIMS and direct recoil techniques is powerful. Information about the number and relative location of atoms in the outermost surface layer can be obtained rapidly with the direct recoil technique, while XPS and SIMS provide chemical and depth information that is unavailable from direct recoils. 4.3. Suitability

of methoxide

as a surface hydrogen calibrant

The one to one stoichiometry of the methoxide to surface Mg determined by XPS allows an estimate of the absolute surface concentration of methyl hydrogens if the surface density of Mg is known. An experiment on single crystalline magnesium can provide this information because the C(DR) and H(DR) signals from the methoxide do not appear to be significantly blocked. The methoxide/Cu(lOO) system may also be a good candidate because of the existing scattering studies [35,363 from Cu(100) and because the methoxide geometry is available from other techniques [18]. Methoxide-Mg provides a system for measuring the relative direct recoil cross-sections of H and (within certain assumptions [37]) 0 to C. This has been discussed in detail elsewhere [37], but the main points are as follows. The ratio of fluxes from two directly recoiled surface atoms is given from eq. (2) as F _I= F,

c,e,fI c&(1

-a,) -a,).

Since 4 is related to the observed 4 a S,/s,, we obtain the ratio

(6) signal

S, and the detection

F, _ ‘Is, i”; Z$si ’

efficiency

s, by

(7)

Solving eqs. (3) and (4) for the cross section ratio yields u _ 0,c,(l _t a, - S,s,C,(l

- 5) - a;) +

(8)

J.A. Schultz et al. / Methoxylation

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329

We have included in table 2 the experimental fluxes as determined by using relative peak heights for S, at methanol exposures of 6 L (&J= 0.5) and 12 L (0 = 1). The calculated cross sections have been reported in a previous paper [37]. We have reproduced them here for comparison to these experiments.

5. Conclusions The combined XPS, SIMS, and direct recoil (DR) results show that upon exposure of Mg to CH,OH, decomposition occurs at low exposure (< 4 L) to form a hydroxide with 10-20s surface coverage followed by formation of a single methoxide overlayer at higher exposures. The results are consistent with bonding through the oxygen atom and the methyl group as the outermost atomic layer. TOF analysis of neutrals and ions directly recoiled from a surface by means of a nondestructive (lOI ions/cm’) pulsed ion beam has been demonstrated as a viable method for analysis of surface hydrogen. The system of methanol chemisorbed on polycrystalline magnesium provides a standard which allows calibration of the relative surface hydrogen concentration. Methoxylated Mg single crystals studied with a 10 keV Ar+ beam should provide an experimental system for calibration of H(DR) intensity in which absolute hydrogen coverages can be obtained as well as structural information. Recoil cross sections calculated for single binary collisions do not provide good agreement with our experimental intensities for H(DR) but are satisfactory for C(DR) and O(DR).

Acknowledgement This material is based upon work supported Foundation under Grant No. CHE-8209398.

by

the National

Science

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