A toroidal angle-resolving electron spectrometer for surface studies

Applications

A TOROIDAL ANGLE-RESOLVING FOR SURFACE STUDIES R.C.G.

LECKEY

ELECTRON

27 August

Science 22/23 (1985) 196-205 North-Holland. Amsterdam

SPECTROMETER

and J.D. RILEY

Physics Department and Research Centre for Electron Bundoora, Victoria .?083, Australia Received

of Surface

1984; accepted

for publication

Spectroscopy,

6 November

La Trobe University,

1984

An electron spectrometer with toroidal geometry has been developed which enables angularly resolved spectra to be obtained simultaneously at a large number of angles. This reduces data acquisition times by a factor of 100 and makes accessible many experiments on surfaces and interfaces which involve reactive materials or which require long experiment times. Photoelectron spectra of gases are presented which demonstrate the analyzer’s energy and angular resolution showing it to be more than adequate for surface studies.

1. Introduction The desirability of multidetection in most forms of charged particle spectroscopy is now well accepted. Various spectrometers employing multidetection techniques in terms of particle energy and/or emission angle [1,5] have appeared in the literature and a critical assessment of many of these schemes has been published by Smith and Kevan [6]. We describe here a toroidal geometry energy analyzer which is currently operated so as to acquire data at many polar emission angles (0) simultaneously although it could, in principle, also multidetect in terms of particle energy (E). Brief details of the construction of the analyzer will be presented and related to both analytical and numerical design studies. Gas phase ultraviolet photoelectron spectroscopy data will then be presented as a demonstration of the capabilities of the instrument in terms of energy and angular resolution and of the linearity of detection efficiency as a function of emission angle. 2. Design Once it is agreed (electrons) is desirable,

that some it becomes

form of parallel detection of particles important to determine the relative merit

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B.V.

R.C.G. Leckey, J.D. Riley / Toroidal angle-resolving electron spectrometer

197

of systems which can multidetect in terms of E, 8 or p or some combination of all three. Here 8 and C,D refer to polar and azimuthal angles of emission of an electron from some solid surface. Display type analyzers such as those constructed by Eastman et al. [l] and by Rieger et al. [2] which multidetect in both 8 and p are spectacularly successful in rapidly providing data for band structure studies (for example). Some question remains, however, about the quantitative accuracy of such instruments because of uncertainties associated with the use of grids and with the normalization procedure associated with the videcon tube used, although for very many purposes the data obtained by these instruments are entirely adequate. For ultimate quantitative accuracy, we consider that it is desirable to avoid the problems alluded to above but this presently implies the use of an instrument multidetecting in either E or one angle only (but see ref. [S]). Smith and Kevan [6] consider that the E option is to be preferred; we disagree for the following reason. Multidetection in E, in all published designs, enables at best a fraction of the energy distribution equal to about one-tenth of the analyzer pass energy to be acquired at one time; multidetection in 0 as in the present design, enables data for all angles 0” < 8 < 210” to be acquired simultaneously. The gain in acquisition rate of the 8 detection system is consequently about 10 x greater than that for the E detection system. Since most spectrometers of this type are computer controlled, it is a simple matter to re-arrange the data from the angle distribution form N(B) as acquired, to the more normal energy distribution form N(E), for display purposes. The analyzer presented here has a number of features in common with the design of Englehardt et al. [3], although independently conceived (fig. 1). Briefly, for an electron (or ion) source situated on the axis, particles are focused at the entrance of a toroidal section by a three element zoom lens which also adjusts energy to match the pass energy of the sector. Those particles refocused by the sector field are selected by a ring slit in the conical focal plane electrode and are subsequently accelerated to a (demagnified) final focus on the top surface of a chevron channelplate. Particles emitted from the source for a wide range of angles out of the plane of fig. 1 are thus brought to a circular ring focus on the detector, the position of arrival on this ring being a measure of the emission angle, 0. The perspective view shown in fig. 2 of the sector field only should clarify this description. We give elsewhere [7] a detailed analytical treatment of the focusing properties of a toroidal field used in the above manner. For present purposes it is sufficient to state that the sector angle, @, for point-to-point focusing in this field is given by tan-‘(@)

= 0

with

p = [2’,‘,“+1~‘]1’2,

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Leckey, J.D. Riley I Toroidal angle-resolving electron spectrometer

1st slii

I

/

, Input Lens

/‘

v

.j HW~ZOQ Plate

-~ Channel Plate

Detector Plate

Base Plate Fig. 1. Cross section ing circle 40 mm.

of prototype

analyzer.

Radius

and where c = a/b; a is the cylindrical, Similarly, the voltages to be applied a pass energy of eV, are given by v I,?

It

of toroid

sectors

b the spherical to the toroidal

SO mm, radius

of generat-

radius of the toroid. electrodes to achieve

1

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is clear that the sector field cannot completely be treated in isolation; fringe field corrections and the electron-optical characteristics of the input and output lenses must also be considered. We have consequently also performed detailed numerical calculations for the complete spectrometer shown in fig. 1. These calculations involved a relaxation-type solution to Laplace’s equation with appropriate boundary conditions followed by a set of trajectory calculations for electrons of varying energy and/or emission angle.

R.C.G.

Lackey, J.D. Riley

/ Toroidal angle-resolving

pdsition Fig. 2. Isometric projection detector ring.

electron spectrometer

sensitive

199

detector

of toroid sectors showing possible electron trajectories

focused on

An example of this procedure is given in fig. 3. Because the final focus is known to be of ring shape, it is possible to determine the arrival position of each analyzed electron (and hence its polar emission angle) in a quasi one-dimensional manner (fig. 4). We have chosen to do this using the miniature RC transmission line technique described by [B] and to determine the arrival position using the rise time method [9]. This has the advantage of using readily available NIMKAMAC-type instrumentation thereby minimizing interfacing problems with the control computer (LeCroy 3500).

3. Results The operation of a spectrometer multidetecting in polar angle is most stringently monitored via an angle-resolved gas phase ultraviolet photoemission (UPS) study. Although many features of the operation of such a spectrometer could perhaps more easily be determined by an electron

200

R.C.G.

Leckey,

J.D. Riley

1 Toroidal angle-resolving

SPCULT.

electron spectrometer

306

Fig. 3. Typical electron trajectories in full analyzer showing the primary focus of input lens, the second focus of toroidal sectors and the final focus at detector plate.

R.C.G.

Leckey,

J.D. Riley

/ Toroidal angle-resolving

f--:=

electron spectrometer

201

Resistive Layer Ceramic Conducting Layer

Resistive Strip

Conductors Charge Sensitive Amplifiers Single Channel Amplifier Delay

stop

start

Time to Digital Converter

LeCroy 3500

Fig. 4. Detector

plate showing

resistive

strips

and following

electronics.

Only one strip is used.

impact study of the LEED type or by reference to published band structure data from a single crystal, the gas phase UPS experiment using an inert gas is unique in that it enables both energy resolving power and the linearity of angular sensitivity to be monitored in a natural and precise way. Before discussing the gas phase data, however, it is instructive to consider the linearity of the spectrometer in terms of the angle, 0. Fig. 5 represents the angular distribution, N(B), obtained from a 0.35 mm diameter tungsten wire sample whenever the spectrometer is closed by a metal shutter, except for a series of 0.5 mm slits spaced at 30” intervals in terms of 8. The spectrum shown illustrates the angular resolution of the system under these conditions (2.3 channels per degree; using 0.3 nsec resolution on the time to 10 eV pass energy) and the linearity with digital converter; 9 eV electrons; angle of the resistive strip detector.

R.C.G.

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Leckey, J.D. Riley / Toroidal angle-resolving

electron spectrometer

d 0

i_

300

+--

300 CHANNEL

~-+-

300

-4

z50

NUnBER

(Angle) Fig. 5. Angular spectrum for 9 eV electrons and 10 eV pass energy from 0.35 mm metal pin and 0.5 mm slits at 30” in a baffle mounted inside the inner can. Widths shown on peaks are theoretical base widths calculated from the geometry of the pin and slits.

Replacing the wire sample by a gas jet from a 0.35 mm aperture results in an example of the energy resolution obtainable from one toroidal spectrometer (fig. 6). This example was taken using a spectrometer with a = 40 mm (cylindrical radius), b = 50 mm spherical radius; sector angle = 127”; entrance slit 0.5 mm; focal plane slit 0.5 mm; pass energy 2.0 eV. Fig. 6a shows the energy resolution of 34 meV obtainable at a particular value of 0 (*2”) whereas fig. 6b illustrates the reduction to 56meV observed when the energy distribution is integrated over all angles 0 < 8 < 210” thereby illustrating that the apparent binding energy of the Kr 4~,,~,~,~ peaks does not vary by more than 22 meV as 0 varies. It would be desirable in an angle-resolving spectrometer for the intensity variations as a function of angle to be representative of the emitted intensity. One test of this behaviour is the monitoring of the angular distribution from an inert gas since this is well known to obey [lo]

203

R.C.G. Leckey, J.D. Riley J Toroidal angle-resolving electron spectrometer

a

0-

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I

CHRNNEL

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b

t

0

q

.056eV

-

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q Ia

1

q -

-2

i

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I

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I

I

I

I

I

I

I

I

I

I

I

I

I

I

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CHRNNEL NWlBER 20 220 Fig. 6. (a) He1 photoelectron spectra of Kr4pj,z peak at 2eV pass energy at one angle with integration over 4”. (b) He1 photoelectron spectra of K4p3,2.1,2 peak at 2eV pass energy summed over all acceptance angles.

204

g

R.C.G.

= ;

Leckey,

J.D. Riley

I Toroidal angle-resolving

electron spectromete,

[1+ PP,(cos01 ,

where 5 is the angle between the E vector of the incident photons and the direction of detection of the photoelectron and - 1 < p s 2 is the asymmetry parameter (characteristic of the angular momentum associated with each electron state). For the geometry utilized in our laboratory (chosen primarily to avoid specular reflection problems when a single crystal sample is used) the light is incident at 30” to the plane of detection (0 plane). The above formula is consequently modified to read (in terms of the polar emission angle 19): Z(0) m 1 - ;/3[; sin’ 8 - +] . This function with /? = 1.27 is compared with experimental data from Kr 4p,,, line in fig. 7. While much of the angular dependence is reflected in the experimental curve it would be difficult to extract an absolute fi value from this curve. Relative values of /3 from different gases can be obtained. Minor variations in intensity are believed to derive from an ageing channel plate and a not absolutely dust free surface of both plate and detector.

I

-180

I

-90

Angle

I

I

0

90

(rel E)

Fig. 7. Variation of intensity of Kr 4p3,s peaks with angle compared p = 1.27 as described in text.

with theoretical

/3 curve for

R.C.G.

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J.D. Riley

/ Toroidal angle-resolving

electron spectrometer

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4. Conclusion The toroidal angular-resolving relatively simple energy 230 meV optimized for gas

geometry permits the construction of a multidetection analyzer using conventional construction techniques and detection systems. The resolution in both angle k 1” and is more than adequate for solid state use and can be phase measurements.

References [l] [2] [3] (41 [5] [6] [7] [8] [9] [IO] [ll]

D.E. Eastman, J.J. DoneIon, N.C. Hien and F.J. Himpsel, Nucl. Instr. Methods 172 (1980) 327. D. Rieger, V. Saile, R.D. Schnell and W. Steinmann, Nucl. Instr. Methods 208 (1983) 777. H.A. Englehardt, W. B%ck and D. Menzel, Rev. Sci. Instr. 52 (1981) 835. R.G. Smeenk, R.M. Tromp, H.H. Kestein, A.J.H. Boerboom and F.W. Saris, Nucl. Instr. Method 195 (1982) 581. H.A. van Hoof and M.J. van der Wiel, J. Phys. El3 (1980) 409. N.V. Smith and S.D. Kevan, Nucl. Instr. Methods 195 (1982) 309. R.C.G. Leckey and J.D. Riley, Nucl. Instr. Methods B, in press. E. Mathieson, Nucl. Instr. Methods 97 (1971) 171. G.W. Frazer, E. Mathieson, K.D. Evans, D.H. Lumb and B. Steer, Nucl. Instr. Methods 180 (1981) 255. J. Cooper and R.N. Zare, J. Chem. Phys. 48 (1968) 942. J. Kreile and A. Schweig, J. Electron Spectrosc. Related Phenomena 20 (1980) 191.