dotunal of Electron Spectroscopy and Related Phenomena, 19 (1980) 179-182 0 Elsevler Sclentlfic Pubhshmg Company, Amsterdam - Pnnted m The Netherlands

Prelimmary communication

AZIMUTHAL ANGULAR-DEPENDENT XPS STUDIES USING THE AEI (KRATOS) INSTRUMENT

STEPHEN EVANS

Edward Davies Chemrcal Laboratones, Unwersrty College of Wales, Aberystwyth, Dyfed, SY23 1NE (Gt Bntaan)

(R.ece~ved 27 November 1979)

An mcreasmg mterest rs now developmg m measurements of the azimuthal angular-dependence of photoelectron mtensrties [l-4] wdh the reahsatlon that structural mformalzon dlfflcult or unposslble to obtam by other tech- nrques can be delnved from such data m a relatively strarghtforward manner. Hitherto, these experrments have mostly been conducted usmg specrally- constructed mstrumentatlon mcorporatmg sophrsticated speclmen-mampu- lators: although standard commercial XPS instruments often permit the polar angle to be varred by rotation of the probe, the mqonty make no provrsron for any m s&u vanation of the azunuthal angle. Moreover, the space avtiable m many mstruments - mcludmg the mdely-used AEI (Kra- tos) ES senes - ls too limited to allow the mtroductron of standard manlpu- lators. This communication bnefly describes a prototype device which enables azrmuthal angular-dependence measurements to be made usmg a standard Kratos probe, without affecting m any way the capabrhty of the mstrument for normal XPS work.

Thlrs device, shown m Fig 1, consists prmclpally of a new probe trp mter- changeable wrth the standard Kratos tips. Its body 1s machmed from a cop- per rod 29mm long and 8 mm m diameter to the dunensrons grven m the Figure. The sample -adrsc12mmmdrameteranduptolmmthlck-rs mounted concentrrcally on a 7-mm pmlon attached to a l-mm shaft sup- ported by two mmrature ball-races pressed mto a 4-mm hole drilled through the tip. A smnlar hole, normal to the first and parallel to the axis of rotation of the probe, carrres a 4-mm pmlon, agam supported by two ball-races, to engage wrth the 7-mm pmion carrymg the sample. Rot&on of the spmdle attached to the smaller pinion thus permits contmuous variation of the azr- muthal angle of 81133883031.

Control of this rotatron from outsrde the vacuum is achieved by a bellows-

180

wtlp and drive for azImutha

mgular dependent XPS studres

Fig 1 Probe tip to enable azlmuthal angular-dependence measurements to be carned out on an AEI ES 200A electron spectrometer (a) croes-sectlon through centre-lme of tip, with end elevations left and right,, (b) top view of tip, (c) end of drive shaft to engage with tlP

sealed rotary dnve mounted on the source chamber &ectly opposite the specimen preparation chamber The spmdle on the probe tip must he a httle below the probe centre-hne If optnnum count-rates are to be mamtamed as glancing electron exit 1s approached, while a certm amount of both lateral and vertical motion H permitted by the bellows assembly on which the probe ls mounted The spmdle and the tive thus cannot be assumed to be m hne, and some form of flexible couplmg 1s essential. On the prototype device, a small co&sprmg carrymg a disc perforated by a 3-mm square hole E, mounted on the drwe, and the spindle on the probe tip 1s equipped ~th a square shaft tapered to a pomt, to engage progresswely with the disc as the probe IS trans- lated mto the source chamber. The aznnuthal angle can then be vaned m situ either contmuously (by belt drive from a geared motor), or m a stepmse manner, at any desrred constant polar angle.

Some prehmmary results obtamed from a smgle crystal of a blot& mica usmg this system, with a motor drive, are shown m fig. 2 The outer curves show the raw mtenslty data, six-fold averaged m accordance with the ap- proxnnate crystallographic symmetry of the mica [5] to ehmmate spurious sources of amsotropy, while the mner curves (*‘flower patterns”) show the net amsotropy expanded by subtraction of the mmunum mtenslty [ 1, 21. Data were collected for polar angles (measured with respect to the normal to the surface) of 15O, 30”, 45O and 60”, and it IS notable #at the K 2p patterns are slgnrf~antly more complex overall than those from either the cations m tetrahedral coordmatlon (Sl) or those m octahedral coordmatlon (Mg, Fe). This 1s presumably related to the crystallographc location of the potassnun which, m contrast to the other elements mentloned, has a coordmatlon number of twelve m micas 153. Unfortunately, however, a full theoretical

181

btotrte

9 -45O

Al 2p

M9 2P

Fe 3p

St 2p

K 2P

Wg 2 Typical results obtamed from blotlte mica at a polar angle of 4S” Note the vwtual identity of the Fe and Mg data, expected because the won and magnesium both occupy octahedral sites excluswely [7 ]

treatment 1s not yet possible for such a complex sohd. Nevertheless, m prm- clple rt should be possible to use the azimuthal dlffractlon patterns for Al, Sl, Mg and Fe to determine quantltatwely the dlstnbutlon of Al and Fe between the tetrahedral and octahedral sites, as has recently been achieved from polar angular-dependence measurements [3,6,7], and mth the advantage that the samplmg depth remams constant throughout the expenment, but at present it seems that certam (remediable) lmperfectlons m the device are resultmg m unacceptably large errors m the angular scale. Speclflcally, backlash m the gears must be kept to a mmnnum, and tight-spots m their rotation elnnm- ated. otherwise, the sprmg m the drive results m no&near&y between the real and mdlcated angular scales. Free play m the races supportmg the sample should also be mmunal, because even small random vanatlons m the polar angle will result m large mtenslty changes at the high polar angles essential for the successful apphcatron of these techniques m structural surface chem- lstry 11, 23. Neither of these problems seems msoluble, however, and the device described here thus offers a neat and economical solution to the prob- lem of adding an azunuthal angulardependence capability to a wide range of exlstmg sohd-state photoelectron spectrometers

ACKNOWLEDGEMENT

The S R. C IS thanked for support, mcludmg an Advanced FellowshIp, and Miss E. Raftery for expenmental assistance.

182

REFERENCES

1 S Kono, S M Goldberg, N F T Hall and C S Fadley, Phys Rev Lett ,41 (1978) 1831

2 C S Fadley, S Kono, L G Petersson, S M Goldberg, N F T Hall and Z Hussam, Surf Scl ,89 (1979) 62

3 S Evans, E Raftery and J M Thomas, Surf Scl ,89 (1979) 64 4 D Norman, H H Farrell, M M Traum, N V Smkh, D P Woodruff, B W Holland,

C Seabury, R Purtell, R P Merrill and T N Rhodm, Surf Scl ,89 (1979), m press 6 W A Deer, R A Howle and J Zussman, m Rock-forming Minerals, Vol 3 (Sheet

Slhcates), Longman, London, 1971 6 S Evans and E Raftery, Solid State Commun , (1980), m press 7 S Evans and E Raftery, Clay Miner ,15 (1980), m press