Topics in..
.
Chemical lnstrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers o j ~ m JOURNAL s by calling allention to new developments i n the t h e m , design, m availability o j chemical laboratory instrumentation, or by presenting vaejul insights and ezplanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentalia a n d instrumenla1 techniques. The editor invites correspondence from prospective contributors.
LXX. Electron Spectroscopy Instrumentation (Concluded)
The purpose of this section is to summarize and to compare the main design features of electron spectrometer systems which are currently commercially available in the United States. The essential components of an electron spectrometer system are shown in the schematic diagram of Figure. 1. A comparison of the functions of the components used to make UP the six commercial instruments is given in Table 2. The performance specifications for the spectrometers are listed in Table I.* The information cited is based upon the manufacturer's literature and upon writ*Table 1 is repeated below from page A219, April 1973 Journal of Chemical Education.
-
-
Claude A . Lucchesi and J o s e p h E. Lester, Department of Chemistry, Northwestern University, Evanston, Illinois 60201 SPECTROMETERS
length with high power output at an energy of at least 900 eV so that one would he able to excite the 1s shells of the second row elements. This energy also is sufficient to excite a t least one inner shell of all the heavier elements. The simplest compramise between power and monochromaticity is the use of a characteristic x-ray emission line of one of the lighter elements. The Ka doublet of magnesium or aluminum has been chosen by the instrument builders because these lines have reasonably narrow composite linewidths of 0.75 eV and 0.95 eV, respectively (18).As pointed out previously these linewidths are adequate for most chemical studies. The KO lines of the heavier elements have considerably larger halfwidths (for example, Cu Ka FWHM 2.6 eV, Mo Kol FWHM 6 eV) which makes, them unattractive for chemical shift measurements. The heavy element Km lines can be useful for the production of Auger electrons since higher power is available when heavy metal targets are used and since the photon linewidth is not related to the Auger linewidth. Of course, the linewidth of any x-ray source can be improved at the expense of intensity by suitable monochromatization. The basic x-ray source consists of a heated filament at -5 to -20 kV and an Al or Mg water-cooled anode at ground potential. The geometry of the target varies from instrument to instrument. The AEI ESl00 (Figure 4) and McPherson ESCA 36 use a planar anode a t a small take-off angle to the target direction. This geometry results in a high intensity line source when viewed from the sample position. In the AEI ES200 the filament is a t ground potential and the anode is at a high positive potential. This arrangement reduces scattering af electrons at the anode. In the
ten and verbal consultations with company scientists and engineers. The tables only contain data for those features and accessories actually deliverable to users as of January, 1973. No value judgments of the spectrometer systems are intended since valid judgments can be made only after extended testing of each instrument under the intended use conditions of the prospective purchaser. Furthermore, new developments are occurring so rapidly that certain comparisons may not be valid for long. Additional detailed information about a given instrument may be obtained by contacting the manufacturers listed at the end of this article. Photographs of commercial spectrometers are shown in Figures 8 through 13. Source. The ideal source would he a monochromatic source of variable waveTable 1. System Performance Specification+ Sensitivity Parameters
Peak Intensity FWHM Element (caunts/sec) (eV) AEI ES-200 DuPont 650 Hewlett-Packard ESCA-5950A McPherson ESCA-36 Vacuum Generators, Ltd. ESCA-2 Varian VIEE-15
Au Ag C Au Ag C Au Ag C Au Ag C Au Ag C Au Ag C
4f 3d 1s 4f 3d 1s 4P 3d 1s 4f 3d 1s 4f 3d 1s 4f 3d 1s
140,000 25,000 40,000 300,000 100,000 60,000 120,000 26,000 12,000 75,000 18,700 14,500 30,000 nad 10,000 1,000,000 24,000 11,700
power^
(watts)
SIB
SIN
1.30 0.90 1.0 1.20 1.15 1.15 0.8 0.87 0.8 1.35 0.88 1.0 2.36
500 500 500 350 360 350 400 (Al) 750 (Al) 800 (Al) 750 760 1000 500 (Al)
11 12 70 7 8 10 120 33 200 14 19 53 na
350 500 500 (20 sec/eV) 2500 (20secleV) 1250 (20 sec/eV) 1250 (20 seelev)
1.83 1.6 0.88 1.0
500 (Al) 2600 2400 1000
na 7 13 17
1500 350 270 470 (22 seelev) na na na na na
Peak Pasition Sensi- Resolutivity tionb Aecu- Reproduc-racy ibility (cpsI"/.) (eV) (ev) (eV) 1300 230 390 2600 1200 250 120 700 180 140 -
-
8800 220 110
0.86 0.9 0.57 0.88 1.0 0.9 -
0.1 0.1 0.1 0.04 0.1 0.1
0.05 0.05 0.03 0.015
-
0.05 0.03
Data obtained from manufacturers; Best reported system resolution; X-ray power with magnesium target except where indicated to he AL;d Not available; e Known ta be cleaned by Ar+ bombardment; 1 For silicon 2p doublet. (Continued onpageA270) Volume 50, Number 5, May 1973 / A269
Chemical lnstrumentcrtion
Figure 4. Schematic diagram ES100 Spectrometer.
of
the AEi
Du Pant and Varian instmments a n annular target surrounding the sample (which in these cases is a disc and a cylinder, respectively) is used. With this geometry these manufacturers achieve a larger x-ray target area and higher photon flux a t the sample surface. This is one reason for the considerably higher counting rates exhihited by these instruments. In all five of the above instruments the x-ray target and the sample are separated by a thin aluminum or beryllium window. This window serves to exclude electrons scattered from the x-ray target from the sample chamber. In the ease of an aluminum window and an aluminum target, the window effectively filters the KB line and the background radiation without significantly attenuating the Ka doublet. Hawever, the aluminum window, when used with other x-ray targets, will produce undesired photoelectron lines excited by the Al Kaa-rays from the aluminum window. The vacuum requirements of the x-ray source are not stringbnt-a pressure of less than 10-5 tom is adequate if hydrocarbon vapors whieh produce target contamination are excluded. Another target contaminant is tungsten from the filament; thus, tube geometries in whieh the anode is not in a direct line with the filament may extend the o ~ e r a t i n e time between shutduwn. tor target rleanlnz. 'The lleu4ctt-I'ackard 595OA rpectmmeter (shown ~rhrmntir.allyin P ~ g u r r pnrduces an x-ray source with an effectiue line width of less than 0.5 eV by dispersing the Al K a j , ~x-rays on a Rowland circle via a bent crystal diffraction "grating." The Ka line is spatially dispersed across the target so that the photoelectrons ejected from a level in the sample have different kinetic energies depending on the posi~~~~
tion across the sample from whieh they originated. In the lens system and energy analyzer this dispersion results in all photoelectrons from one level being focused on the same line a t the exit plane of the analyzer. (The theory of this system is discussed in Ref. I, Appendix 10 for a magnetic energy analyzer and in Ref. 19 for an electrostatic energy analyzer.) In addition to compensating for the intrinsic source linewidth, the dispersed source also significantly reduces the hackground in the spectrum since the Bremsstrahlung photoelectrons and the Kaa,. satellite lines are eliminated. For studies of many ehemical systems this feature of the dispersed source may be more important than the reduced linewidth sinee one should have lower detection limits with the improved signal-to-background. It can he advantageous to have a source of small linewidth if an element exists in several oxidation states in a sample and i t is desired to extract the binding energies of each of these oxidation states. (AEI has announced as an accessory for their spectrometers a monochromatized x-ray source. Preliminary performance data for an ES200 equipped with this source are as fallows for a gold sample: IP = 60,000 cps a t FWHM of 0.8 eV and a t a n x-ray power af 1500 watts. The S I B for this sample was 75. The system resolution is reported to be 0.5 eV.) If only outer shell excitation is desired, then the ultraviolet resonance lines of helium a t 21.2 or 40.8 eV and linewidths of less than 0.01 eV may he used. Recently, other sources such as monochromatized cyclotron radiation ( E 70 eV). Tracing the path of electrons through the spectrometer, one sees that photoelectrons. ejected from the sample are retarded and enter a law resolution "pre-filter" (low resolution parallel plate analyzer) which deflects the electrons of a chosen energy range through a n aperture toward the lowpass filter. Those electrons of energies less than El are reflected and imaged on an aperture in the center of the analyzer tube. Thase of higher energies strike the mirror
Figure 8. Photograph of AEI ESlOO Electron Spectrometer.
surface and are either absorbed or seattered out of the main path. A quadrupale lens aids in alignment of the beam of reflected electrons on the aperture. Those electrons passing through the aperture encounter a high-pass electron filter set a t a voltage V z , corresponding to energy E2 slightly lower than E l . Thase electrons of energy greater than E2 pass through the filter and are detected. The resolution of the analyzer is determined by the difference between EX and EZ and by the shape of the transmission function of the mirror and filter. A system resolution of better than 1 eV for energies up to 1200 eV has been obtained. The real advantage of this analyzer scheme is that the acceptance angle is large (-n/25 steradians) and that the filter transmission function improves as the acceptance angle is increased. From the specifications in Table 1 the only apparent disadvantage is a high spectrum background. An explanation of one entry in Table 2 is possibly necessary. By "scan mode" is denoted the set of instrument potentials that are varied in order to bring photoelectrans of different energies into focus on the detector. "Retarding voltage" implies that the potential(s) of the retarding lens(es) are varied in order to sean'electron energies. "Hemisphere voltages" imply that the potentials on the inner and outer spheres of the analyzer itself are varied. Detectors. The electrons that traverse the analyzer are detected in the AEI, Du Pont, McPhersan, and Vacuum Generators
Figure 9. Photograph of Du Pont 650 Electron Spectrometer, A282 /Journal of Chemical Education
Figure 10. Photograph of 5950A spectrometer system.
Hewlett-Packard
spectrometers by channel electron multipliers. In the Varian instrument the electron multiplier is a discrete dynode multiplier. In all of these instruments the electrons leaving the analyzer exit slit (or filter) are amplified by the electron multiplier with a gain of 106 t o loa. T h e resulting pulses are fed into an amplifier and pulse-heipht discriminator and then into a counter, computer or ratemeter. In the Hewlett-Paekard spectrometer the detector is a n array of channel multipliers called a Channelplate which is placed a t the exit image plane of the analyzer. Impingement of a n electron on one of the channels of the array results in a pulse of electrons exiting from t h a t channel a n d striking a phosphor screen mounted a n a glass plate in the vacuum system. T h e light pulses emitted by this screen are detected by a vidicon tube (television camera) outside the window. T h e position of
the light pulse on t h e array corresponds to a known electron kinetic energy and deteetion of a light pulse results in a count being stared in the channel of a multichannel sealer corresponding to that energy. T h e use of this parallel detector with which electrons of energies over approximately a 10 eV energy range can be simultaneously detected is necessary because of the low x-ray intensity striking the sample in this instrument. Since there are appmximately 150 channels across a n energy range of 10 eV, the contribution of the detector aperture to the observed linewidth is negligible. However, the simultaneous counting reduces the data acquisition time by a factor equal t o t h e number of channels in whieh counts may be simultaneously stored. Data Acquisition and Presentation. There are two basic data acquisition and /or presentation modes in the commercial spectrometers-analog or digital. In a n analog system t h e pulses from t h e discriminator go to a ratemeter which drives the Y-axis of a n X-Y recorder, T h e X-axis is driven by the analyzer voltage generator. Thus, a spectrum of counting rate versus kinetic energy is produced directly. The operator has control over the usual parameters: scan magnitude, scan speed, fullscale count rate and time constant. As shown in Table 2, this type of display is the normal mode of operation for t h e AEI, Du Pant and Vacuum Generators spectrometers. In the digital systems the pulses from the discriminator go to a channel in either a computer memory or a multichannel an-
F g u r e 1 1 . Photograph of McPlieisan E S C A ~ 3 6 with three analyzer assemblies n the tore-
ground.
alyeer. T h e channel is selected on the basis of the analyzer voltage. T h e contents of the memory are then recalled when the operator wants to observe the spectrum. Hewlett-Packard, McPherson and Varian have this mode as their normal mode of operation. In the Hewlett-Paekard spectrometer the storage device is a m u l t i ~ channel scaler. T h e scan amplitude, number of channels used, and dwell time per channel are the important parameters in this type of data acquisition system. In the McPherson and Varian instruments the storage device is a mini-computer which also controls the spectrometer functions. I n these three spectrometers the spectrum is displayed on a CKT screen during data acquisition so that the aperator can observe the build-up of signal in
Volume 50, Number 5, May 1973 / A283
Chemical lnstrumeflfat'on
convolution to remove the contribution of the x-ray source to the spectral peaks. ~t oresent. AEI and Varian offer fast-Fouriertransform algorithms in their software packages for this purpose. In general, the utility of a computer an an ESCA spectrometer in terms of data acquisition and manipulation is worth the additional cost.
FUTURE DEVELOPMENTS
Figure 12. Photograph of the Vacuum Genera-
tors ESCA-2.
the spectrum. When a scan is completed, the spectrum may be platted on an X-Y recorder. The contents of the scaler or memory also may be punched onto paper tape for future manipulation. AEI, Hewlett-Packard and Vacuum Generators have computer control and data acquisition as options far their spectrometers. The presence of a dedicated mini-computer allows the operator to perform some data handling operations, e.g., smoothing, satellite subtraction, or deconvolutian. Also, with the computer the spectrometer may be set up to scan a number of binding energy ranges sequentially without any intervening operator action. One also may make multiple scans of one region to enhance the signal-to-noise ratio of the spectrum. The computer may be used far data de-
A284
I Journal of Chemical Education
ESCA is a relatively new and high-cost technique which is undergoing rapid development. Nevertheless, the present instrumentation has advanced to the stage where the major impetus to continued growth must come from significant new applications of the method. Widespread use in the study of the chemistry of surfaces of solids will assure its place in chemical instrumentation. Certainly, its application to problems in catalysis and surface coatings shows great promise. One area in which much work needs to be done is sample cleaning and pretreatment leading to reproducible sample surfaces. Use of ion sputtering to obtain composition depth profiles will become important. Since ESCA is one of the very few techniques by which the second row elements (and their oxidation states) may be determined, future developments towards quantitative methods for these elements are quite probable. Developments in the instrumentation itself will probably be weighted by the applications trends. The addition of multichannel detectors to dispersive in st^-
LITERATURE CITED 18. Noreland. E.. E b t i g , B.. Ksllne, E.. and Chefal. A. R., Meirulagio. 5 (3). 60 119fi9). 19. Siegbahn. K., Hammond. D.. Fellner-Feldepg, H.. Hainett, E. F.. Scimrs. 176, 245 119721. 20. Eastman, D. E., end Cashion, d. K.. Phys Re". Left.. 27, 1520(1971). 21. Kraure.M.0.. Chem.Phvr.Lett., IofiJ,65(1971). 22. Amy, J. W.. and Baifinger. W. E.. Ahatrsetr of 14th Eastern Analytical Sympnrium. Pnper No. 6.
.""., ,a,'.
Figure 13. Photograph of the Varian VIEE-15 s p e c t r o m e t e r system.
ments without the simultaneous addition of a monochromator could increase sensitivity by u p to 100 fold. Certainly, monochromatized sources will become mare common (for the reasons outlined in the text). T h e use of electron flood guns or other means of surface charge control will spur the utilization of monochromatized sources in commercial spectrometers. Alternate photon sources will be developed; e.g., sodium or yttrium x-ray sources. Probably, the most useful developments in instrumentation will he the expansion of compute? software far data analysis and signal recognition and retrieval.
23. Disnis. W. P. and Lester, .I. E.. Anal Chrm. [in P""). 2 4 Swartz. W. E.. Watts. P. H., Watts. .I. P.. Brasch. J. W.. and Lippincoft. R. R.. A n d C h m . 44. 2001 119721. 25. Svsrthoim, N. and Siegbshn, K:. Ark M a t . h t r . Fyr. 13A. 21 (19461: Siegbahn, K.. and Svsr. fholm, N.. Nature. 157,8721L9461. 26. Purcell, E. M., Phvs R e " . 61. 81R 11939). 27. Hueho~,A. L.. and %ansky. V.. 1'h.y~. Rev., 34. 284 119291; Hughes. A. L.. and MeMillen. J. H.. Phys Re 0..34, 291 (1929). 28. Blauth.E.,Z. Ph.~.$ik.147.228(19StilI. 29. Hafner. H.. Simpson, I . A.. and Kuyaff. C. E.. S r i . In5lrum 39.31(196Rl.
h e . J. D.. R P U . sci. ~ m t r u m 43, , ,291 ( ~ 9 7 2 1 31. Golden. D. E.. and Zeeca, A , Re". Sci. Inrtrum 4%
Q.
.
32. ~ u c h i t s l .D: A , and Rideen. .I. D.. Appl Phys. Left.. ifi, 348 119701. 33. Hoimer, J. C.. and Woichoit. N. H.. Appl P k w L*t. l a 266 11968).
Note added in proof: Vacuum Generators, Ltd. have provided the following data for their new ESCA 3: "Peak intensity on the Au 4f line of 107,000 counts per sec a t FWHM of 1.3 eV (300,000 counts per sec a t 2.0 eV) with a power of 300 watts. and a best reported resolution on Ag 3d of 0.87 eV."
.
ACKNOWLEDGMENTS The authors wish to thank Howard Harrington of Hewlett-Packard, John Rendina of McPherson, William Riggs of du Pont, and Alan Wolstenholme of AEI for helpful discussions. The spectra were obtained with a n electron spectrometer obtained, in part, with funds from NSF Grant GP28134. J.E.L. acknowledges support from NSF a n d PRF far photoelectron spectroscopy research.
Volume 50,Number 5. May 1973 / A285
