On the dominance of an indirect mechanism for photon stimulated ion desorption from SrTiO3(100)H2O

Surface Science 178 (19%) 897-906 North-Hofland, Amsterdam

ON THE DOMINANCE OF AN INDIRECT MECHANISM FOR PHOTON ST~~~~~ IUN ~~~~UN FROM SrTiU3(1#)-H,U I.W. OWEN *, N.B. BROOKES, D.R. WARBURTON

C.H. RICHARDSON

**,

Chemistry Depariment, Manchester University, Manchester Ml3 9PL, UK

and G. THORNTON

***

Chemistv Department, Manchester University, Munches& Mi3 9PL, UK and Science and Engineering Research Cow& Doresbui): Labornto~, Warrington WA 4 $A D, UK Received

18 March

1986: accepted

for publication

6 June 19%

The adsorption of R&I on fractured SrTiOs(100) has been studied using photoemission and photon stimulated ion desorption (PSID). The photoemission results are, by comparison with data obtained on an Ar+ bombarded/annealed surface, consistent with dissociative Ha0 adsorption. PSXD spectra recorded above the SrL, and TiK edges indicate that the dominant mechanism for ion desorption is X-ray induced electron stimulated desorption.

1. Introduction Since the determination of a direct mechanism for electron and photon stimulated ion desorption (PSID) by Knotek and Feibelman (KF) [l], PSID studies of numerous surface systems have been carried out to determine the general validity of the RF model [2]. This was constructed in terms of an ionic model where core electron ionisation of a surface anion, or cation bonded to * Present address: Vacuum Generators Ltd., Men&s UK. ** Present address: Science and Engineering Research ton WA4 4AD, UK. ** * To whom correspondence should be addressed.

Road, Council,

Hastings, Daresbury

~~~-~~~~~~~~~~~.~~ 0 Elsevier Science Publishers B-V. (North-Holland Physics Publishing Division)

East Sussex TN34 IYQ, Laboratory,

Warring-

the anion led, via Auger decay. to a “Coulomb explosion”, the positive ion of the surface anion being repelled into the vacuum. Anion desorption following the inter-atomic Auger decay of cation core holes is of particular interest in relation to the use of PSID in surface extended X-ray absorption fine structure (SEXAFS) studies [2.3]. The PSID yield of an adsorbate remains the only means of recording SEXAFS data at substrate absorption edges. However, the surface specific nature of such measurements does rely on the dominance of a direct mechanism for ion desorption. In this regard, previous PSID studies have been encouraging in that an Auger decay mechanism has also been found to be appropriate for relatively covalent surface systems at substrate absorption edges [2]. However, at adsorbate edges of covalently bound molecular adsorbates multielectron effects have been observed [4]. In such cases the mechanism is still direct in the sense that ion desorption is local to the initial core ionisation event. The most likely indirect mechanism of PSID involves absorption in the hull\ with transfer of energy to a surface complex via the outgoing photocurrent. The resulting ion yield had been thought to be negligible, although such a mechanism has recently been found to dominate the ion yield from condensed gases [5]. The dominance of X-ray induced electron stimulated desorption (XESD), as this indirect mechanism has been termed, in the case of molecular systems has been ascribed to their lower electron stimulated desorption (ESD) threshold [5]. This arises because ion desorption can result from electron stimulated ionisation of valence electrons [6]. Low-energy secondary electrons, which form a substantial part of the photocurrent, can therefore contribute to the ion yield. Hence, the implications of this work on condensed gases for the future prospects of PSID SEXAFS are limited. In more ionic systems, where ESD requires core ionisation, the relatively high ESD threshold will exclude the secondary electron contribution to the ion yield [5]. Nevertheless. a dominating XESD mechanism has been evidenced in ion yield data from rare earth oxides [7]. The results of the present work indicate that a dominance of XESD can be observed from a highly ionic substrate, limiting the possibilities of PSID SEXAFS even further.

2. Experimental PSID measurements were carried out using the double-crystal monochromator [8] (1.75 G hv G 11 keV) on station 6.3 at the SRS, Daresbury Laboratory. Photoemission measurements were performed using the grazing incidence monochromator (20 < hv < 200 eV) [9] on station 6.1, and a double-pass CMA (Physical Electronics Inc.). Ion yield measurements employed the time structure of the SRS in single-bunch mode (0.3 ns pulse width, 320 ns repetition period) in conjunction with a time-of-flight (TOF) mass spectrometer. This

I. W. Owen et al. / PSID from SrTiO,(lOO)-H,O

hu

899

MONO

Orbit Clock

Fig. 1. A schematic operating voltages

diagram of the TOF mass spectrometer and associated electronics. are indicated. TDC = time to digital converter, histogram. histogramming memory, mono = monochromator.

Typical mem. =

consists of a 1 cm diameter drift tube of length 3 cm, - 2 cm from the sample, a matched pair of multichannel electron multiplier plates, and a 50 Q coaxial anode, the output of which is connected to a chamber feedthrough via a coaxial cable. The detector is mounted on a goniometer such that it can be rotated in the horizontal plane about the vertically mounted sample. A schematic diagram of the detector along with the associated electronics is shown in fig. 1. Total electron yield (TEY) data were obtained concurrently with ion yield measurements by monitoring the sample drain current. Normalisation of both the TEY and ion yield data to incident photon flux was accomplished using the electron yield from a copper coated tungsten mesh placed between the monochromator and the experimental chamber. To prevent charging, the SrTiO, single-crystal samples (Commercial Crystals Inc.) were partially reduced by heating (1100 K) in vacuum for 10 h prior to the experiment. This produced grey n-type semiconductors (- 1017 cme3). Samples were fractured parallel to the (100) planes in situ at chamber pressures of < 8 X lOPi mbar yielding stepped, unreconstructed (100) faces. For comparison, a SrTiO,(lOO) face prepared by argon-ion bombardment and annealing in oxygen was studied by photoemission. This sample preparation yielded a clean, unreconstructed surface, as measured by Auger and LEED, the photoemission spectra being similar to those of the fractured surface.

900

I. W. Owen ef ul. /

PSID from

SrTiC$(lOO)-H@

Exposure to H,O was carried out using the vapour of doubly distilled water, dissolved atmospheric gases having been removed by several freeze/thaw cycles. For the fractured surfaces, the sample temperature remained at 300 K for H,O exposure and subsequent measurements. Because H20 does not react with Art bombarded/annealed SrTiO,(lOO) at 300 K. H,O exposure and photoemission measurements were performed with a sample temperature of - 150 K. In the photoemission experiments, the CMA axis was at 90” to the incident photon beam. To increase the surface sensitivity of the measurements. an angle of incidence of 23” with respect to the surface normal was employed. The geometry used in the PSID/TEY measurements is that shown in fig. 1. with photons incident at 45” to the surface normal, the ion detector axis lying along the surface normal.

3. Results and discussion 3.1. Characterisation

of H,O adsorption on fractured

SrTi03(100)

A photoemission study of H,O adsorption on fractured SrTiO,(lOO) has not previously been reported. However, the interaction of H,O with the (100) [lo-121 and (111) [13] faces of SrTiO, prepared by Ar+ bombardment/annealing has been extensively investigated. The well annealed surfaces are found to be relatively inert [ll-131. However, introduction of surface defects via argon ion bombardment renders the (100) and (111) surfaces very reactive to H,O, although the mode of adsorption, molecular or dissociative, remains the subject of debate [lo-131. This arises from the difficulty in distinguishing H,O from OH using photoemission or low-energy electron loss spectroscopy. the techniques of choice in such work. In this work, the adsorption mode is identified using photoemission by comparing the results for H,O adsorption on SrTiO,(lOO) faces prepared by fracturing and by Ar+ bombardment/annealing. The principal difference between these two types of surface is that the fractured surface contains an appreciable density of step sites. These sites, like the defects created by Art bombardment, promote reaction with H,O. the sticking coefficient approaching unity. In contrast. for H,O to react with a “planar” (100) surface, prepared by Art bombardment/annealing, requires a substrate temperature lower than - 200 K. A comparison of photoemission spectra for the stepped and planar (100) surfaces before and after exposure to Hz0 is shown in fig. 2. The data indicate that the mode of adsorption is different for the two surfaces, being molecular in the case of the planar surface, dissociative on the stepped surface. Whereas the adsorbate induced structure in the planar surface spectrum “fingerprints”

SrTiO, K?U~ hv=l&OeV

-8

-12 INITIM.

ENERGY ieV)

Fig. 2. ‘I;rPSspectraof cleanand EzO dosed (a) pranar SrT~O~~~~) and fb) stepped SrTiO#55). Ekposvre to HZ0 was carried out with the pfanar surface at 155 K and the stepped surface at 355 K.

INITHL

-4 ENERGY (&‘I

EWI

Fig. 3. ups &fference spectra (H&t dosed clean) For {a) 15 L H@ on stepped SrTQ (;100) adsorbed at 300 K and (b) 5.5 L Hz0 on planar SrTiO, (100) adsorbed at 155 K, compared with a gas phase photoelectron spectrum of H,O from ref. 1151, aligned at the lb2 energy, 1 L = I+;32X 10e6 mbar s.

m&2cular H&J a feature ~es~~d~~~ to 14 emission is absent in the stepped surface spectrum, jnd~~~ng the presence of adsorbed hydroxyl species. The relative positiotxs of the adsorb~te-induced 02s features are consistent with this interpretation, lying at lower initial energy in the planar surface spectrum, as expected for the more covalent, H,O species. The difference spectra obtained from the two sets of photoemission data are shown in fig. 3, The separation between the OH, and OH, peaks is 3.6 eV, 1.4 eV larger than for gas phase OH [14]. The most str~~tfo~~d explanation of this bonding shift involves the greater s~b~~sat~on csf the OH, orbital via bonding to the substrate with the OH axis inclined towards the surface normal. The bonding shifts observed when comparing the spectrum of gas phase H,O [IS] with the planar surface difference spectrum are more. difficult to interpret because H,O orbital energies are sensitive to the molecular bond angle. However, it is clear that H,O is chemisorbed on planar SrTiO,(lOO) at 150 JS and not simply condensed.

902

I. U’. Owen et d. / PSID

fmm

SrTiO,(l(N-II20

This photoemission work has important implications for the PSID study of H,O adsorption on fractured SrTiO,(lOO). Dissociative adsorption will result in the formation of surface bonds which are highly polarised. Desorption of H ’ or OH+ will therefore require core-electron excitation. 3.2. Ion desorption at the Sr L, and Ti K absorption

edges

An Ht ion yield could not be detected from the fractured SrTiO,(lOO) surface prior to Hz0 exposure, at a photon energy (4985 eV) above the Ti K edge. This is at variance with expectations based on earlier ESD work [16] which found surface hydrogen to be present on SrTiO,( 111). It seems likely that the H+ yield observed in the ESD work arose from reaction with residual H,O in the vacuum system. Following exposure of the fractured SrTiO,(lOO) surface to H20. the variation of Hi and OH+ yields above the Ti K edge (4966 eV) shown in fig. 4 was obtained. The corresponding TEY spectrum is also shown in fig. 4, along with the fluorescence yield obtained from the “prompt” TOF signal. A dominating XESD contribution to the ion yield spectra is evidenced by the similarity of the TEY, fluorescence, and ion yield spectra. This suggests that the three types of spectra arise from ionisation of titanium atoms having the same local environment. Since the TEY and fluorescence spectra are dominated by a bulk atom contribution, this indicates the dominance of a secondary, non-local mechanism of PSID. The near-edge X-ray absorption

-

Fluorescence

Yield

Total

Ywld

Electmn

OHtYield

PHOTON ENERGY WI

Fig. 4. The H+ and OH+ PSID yields from stepped SrTiO,(lOO) following exposure total electron and fluorescence yields are shown for comparison.

to H,O.

The

I. W. Owen et ul. / PSID from SrTiO,(lOO)-

H,O

903

fine structure (NEXAFS) should be particularly sensitive to the difference between surface and bulk co-ordination. The pre-edge structure (labelled A in fig. 4) arises, in the case of absorption by bulk, octahedrally coordinated Ti atoms, from dipole-disallowed 1s + 3d transitions. The intensity of the corresponding feature in H+ and OH+ ion yield spectra arising from direct PSID should be considerably enhanced. This arises from the fact that the site symmetry of surface R-H and Ti-OH species which would give rise to the ion yields must lack a centre of inversion. The mixing of Ti 3d and 4p orbitals which this allows, effectively relaxes the dipole-selection rule [17]. Further evidence of a dominating XESD mechanism at the Ti K edge can be found by analysis of relative edge-jumps, being essentially the same in the TEY and ion yield spectra. The TEY and H+ yield relative edge-jumps at the Sr L, edge (2216 eV) are also found to be the same, again indicating the dominance of XESD. These conclusions are reached by first considering the absolute edge-jump, that is the difference in count rate above and below the absorption edge which is given by [5] h I = qNZg, where q is the quantum yield, N the number of atoms samples, 1, the incident photon intensity, and (Y the detection efficiency. The relative edgejump for an elemental solid is then given by AI -F

qNa =

Ll”lNd%l ) nl

where 1 is the count rate below the edge, the sum being over all absorption edges to lower energy. For a measurement of ion (e.g., H+) desorption dominated by a local mechanism,

nl

and for TEY

AI (-1Z

qTEY

= TEY

c

%l(TEY)

nl

For the relative edge jumps in PSID and TEY to be the same requires that the relative quantum yields for PSID and TEY above and below the edge are equal. For a compound ABC with surface species of the type A-H, etc., at an A absorption edge,

AI (-1Z

qH+NA-H

H+=

~q.,cH;,N,_,~q,,o;,N~-~~q~~~H~~N~-~ nl

and similarly

for TEY. In the case of SrTiO,,

the background

due to oxygen K

edge (C) absorption can be neglected at the Sr L, and TiK edges. because it lies - 1700 eV to lower energy of Sr Lt. At the Ti K edge, the background will be dominated by Sr L edge absorption, so that for direct PSID the equality in electron, H‘&, and OH+ yield relative edge-jumps requires that 4n+%-n

4 TEY1% q TEY

N Sr

=

qn*N,,_,

qOH =

qOH-”

+

M N

%-ON Sr-OH

In the absence of accidental equality, it therefore requires there to be the same number of Sr-I-I as Ti-H species, the same number of Q-OH as Ti-OH species, and the ratios of the quantum yields to be equal. The ratios of surface species are very unlikely to be the same. Indeed, if the ESD results of Knotek [16] for an H,O dosed SrTiO, surface are transferable to the stepped (100) surface, Sr-H and Ti-OH are by far the majority species. As for the quantum yield ratios, that for TEY is expected to be different from the ion yieid value. The latter should be determined largely by the relative rates of inierutonzic Auger decay following Sr L and Ti K hole production, as opposed to the total Auger decay rates. The ratio of Sr L to Ti K interatomic Auger rates might be expected to be considerably different from the ratio of total Auger decay rates since there is much less mixing of strontium orbitals in the vatence band than Ti3d orbitals [IS]_ Hence, it is difficult to reconcile the observed relative edge-jumps on the basis of a direct PSID mechanism. A dominating XESD mechanism would, however, produce just such results since ion desorption arises from the outgoing photocurrent. There is less evidence for the dominance of XESD (or of direct PSID) at the Sr L, edge. The background at the Hence, assuming a direct Sr L, edge will arise largely from St L,%, absorption. PSID mechanism, for the relative edge-jumps of electron and H+ yield to be the same requires only that the Sr L, to Sr L,, quantum yield ratios are similar. The above considerations of NEXAFS and relative edge-jumps quite clearly indicate that XESD is the dominant mechanism at the Ti K edge, the situation at the Sr L, edge being less clear. Again assuming that Knotek’s [16] conclusions regarding the surface species on SrTiO,-H,O are transferable to the fractured (100) surface, then it is not surprising that XESD dominates the H + yield at the TiK edge since few surface Ti-H species should be present. However, it is surprising that the H’ yield is so high. The dominance of XESD in the OH’ yield is even more surprising since surface Ti-OH species should be present which would give rise to a direct PSID OH+ yield. This might have been expected to dominate the XESD OH’ yield because the Ti-OH bond will be strongly polarised. The ESD threshold will therefore be the Sr N,,, edge ( - 21 eV), as observed by Knotek [IQ and secondary, inelastically scattered electrons cannot contribute to XESD. This is the significant difference between this work and that on condensed gases [5].

I. W. Owen et al. / PSID from SrTi0,(100-

H,O

905

4. Conclusions The results on SrTiO,(lOO)-H,O imply that even in the case of an electronegative adsorbate on an ionic, polarising substrate, XESD can dominate at substrate absorption edges. A more detailed appreciation of the factors determining the relative yields of XESD and direct PSID requires information on the energy distribution of the photocurrent and the ESD cross section [5]. Unfortunately, this information is not available at present. The observation of a dominating XESD mechanism in the case of SrTiO,-H,O further reduces the perceived potential of PSID SEXAFS. However, because the factors which influence the XESD yield have yet to be quantified, the true limitations of the technique remain unclear. Certainly, the ion yield from XESD is strongly substrate dependent; there are now two systems where the surface specific nature of PSID has been employed in SEXAFS measurements, Mo(lOO)-0 [3] and Si(lOO)-H,O [19].

Acknowledgements This work was supported by the Science and Engineering Research Council (UK), including the provision of synchrotron radiation facilities, and studentships to IWO, NBB, and DRW. Additional support was received from the United Kingdom Atomic Energy Authority and Vacuum Science Workshop Ltd.

References [l] M.L. Knotek and P.J. Feibelman, Phys. Rev. Letters 40 (1978) 964. [2] M.L. Knotek, Rept. Progr. Phys. 47 (1984) 1499. [3] R. Jaeger, J. Feldhaus, J. Haase, J. Stahr, Z. Hussain, D. Menzel and D. Norman, Phys. Rev. Letters 45 (1980) 1870. [4] R. Jaeger, R. Treichler and J. St&r, Surface Sci. 117 (1982) 533. [5] R. Jaeger, J. Stohr and T. Kendelewicz, Phys. Rev. B28 (1983) 1145; Surface Sci. 134 (1983) 541. [6] D. Menzel and R. Gomer. J. Chem. Phys. 41 (1964) 3311; P.A. Redhead, Can. J. Phys. 42 (1964) 886. [7] J. Schmidt-May, F. Senf, J. Voss, C. Kunz, A. FIodstrdm, R. Nyholm and R. Stockbauer, Surface Sci. 163 (1985) 303; in: Desorption Induced by Electronic Transitions DIET II, Eds. W. Brennig and D. Menzel (Springer, Berlin, 1985). [8] A.A. MacDowell, D. Norman, J.B. West, J.-C. Campuzano and R.G. Jones, Nucl. Instr. Methods A246 (1986) 131. [9] M.R. Howells, D. Norman, G.P. Williams and J.B. West, J. Phys. El1 (1978) 199. [lo] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Solid State Commun. 24 (1977) 623. [ll] C. Webb and M. Lichtensteiger, Surface Sci. 107 (1981) L345. [12] R.G. Egdell and P.D. Naylor, Chem. Phys. Letters 91 (1982) 200.

[13] S. Ferrer and G.A. Somorpi, Surface Sci. 94 (1980) 41. [14] S. Katsumata and D.R. Lloyd. Chem. Phys. Letters 45 (1977) 519. [15] C.M. Truesdale. S.H. Southworth, P.H. Kobrin. D.W. Lindle, Ci. Thornton and D.A. Shirley. J. Chem. Phys. 76 (1982) 860. [16] M.L. Knotek. Surface Sci. 101 (1980) 334. [17] D.H. Maylotte, J. Wong. R.L. St. Peters. L.W. Lytlr and R.B. Greegor. Science 214 (1981) 554. [1X] N.B. Brookes, D.S.-L. Law, D.R. Warburton, T.S. Padmore and Cr. Thornton. Solid State Commun. 57 (1986) 473. [19] R. McCrath. I.T. McGovern. D.R. Warburton. (+. Thornton and D. Norman. Surface Sci. 178 (1986) 110.