ELECTRON SPECTROSCOPY - Analytical Chemistry (ACS

ELECTRON SPECTROSCOPY David M. Hercules, Department of Chemistry, University of Georgia, Athens, Ga. 3 0 6 0 1

LOOKING at the history of spectro scopic techniques currently used by chemists, one is struck by a com mon denominator in their develop ment. The techniques usually have been devised by physicists to study some fundamental property of mat ter, and subsequently it has been realized that information of value to chemists can be obtained from them. Development for practical use by chemists has followed. In frared and ultraviolet spectropho tometry, nmr, mass spectrometry, and electron spin resonance serve as typical examples. A new technique, electron spectroscopy, appears to be following a similar pattern and of fers the potential of becoming a widely used and highly valuable chemical tool. Electron spectroscopy is the tech nique of measuring the binding energies of electrons in molecules, by determining the energies of elec trons ejected by the interactions of a molecule with a monoenergetic beam of X-rays or photons. I t is convenient to distinguish between electron spectrometry in which the ionizing source is a high-energy X-

ray beam and that in which the source is a beam of photons from— e.g., a helium discharge. By com mon usage the former is known as ESCA or electron spectroscopy for chemical analysis and the latter as photoelectron spectroscopy or PES. Both parts have applications to chemical analysis but the instru mentation, information obtained, and interpretive procedures differ somewhat. A third related area of development is Auger spectroscopy, a subject discussed recently in these pages (1). Historically, Siegbahn and co workers utilized a double focusing iron-free magnetic spectrometer for high-resolution energy analysis of photoejected electrons. The basic theory of this instrument was de scribed in 1946 (β) and a descrip tion of the double focusing instru ment reported in 1956 (3). The value of electron spectroscopy lies in the observation of chemical shifts initially reported for copper in 1957 (4), but the general utility was ap preciated only as recently as 1964 {6-7). Utilization of electron spec troscopy for ionization potential

ANALYTICAL CHEMISTRY presents two articles on electron spectroscopy. Instrumentation and techniques have developed along two lines. In one case, Siegbahn and coworkers in Sweden developed an improved beta-ray spectrometer to measure core electrons. For applications in chemistry, he used a high-energy excitation source, X-rays, and called his technique Electron Spectroscopy for Chemical Analysis (ESCA). In the other case, Turner and his colleagues in England developed instrumentation to study the ionization potentials of valence electrons. Turner used a beam of photons and low energy and called his technique Photoelectron Spectroscopy (PES). These terms are now commonly used interchangeably as they basically measure the same things. They are complementary techniques that have great potential in both structural and analytical chemistry. For solid samples, the techniques are especially valuable in studying surfaces. David M. Hercules in his article which begins on this page discusses electron spectroscopy in general, but principally the ESCA techniques. The article by D. Betteridge and A. D. Baker of the University of Swansea, which begins on page 4 3 A, covers some of the analytical possibilities of photoelectron spectroscopy. 20 A ·

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

measurements was first reported by Turner in 1962 (8) with a descrip tion of instrumentation following in 1963 (9). Recently, Turner has re ported instrumentation and studies using high resolution electron spec troscopy to determine the vibra tional states of molecular ions (10). The basic processes common to all electron spectroscopic techniques are illustrated in block diagram form, shown in Figure 1. Ionizing radiation impinges on a sample causing ejection of an electron. The electron goes into an electron monochromator, and the energy of the photoejected electron is determined. After energy resolution by the monochromator, a signal propor tional to electron intensity is read out on the detector. The kind of information obtained depends upon the energy of the ion izing radiation and resolution of the monochromator. For example, if high-energy radiation such as Xrays or γ-rays is used, core electrons can be ejected and their binding energies determined. Conversely, if low-energy excitation is used, such as vacuum ultraviolet radiation, valence shell electrons will be ejected permitting determination of ionization potentials. I t is this qualitative flexibility coupled with a potential for quantitative mea surement that makes electron spec troscopy potentially attractive as a chemical technique. Qualitative in formation may be obtained to per mit distinction of various electrons within a molecule on the basis of their binding energies, as well as changes in their chemical environ ment, yielding structural informa tion similar to infrared or nmr. The quantitative aspects of electron spectroscopy should be comparable to X-ray fluorescence since similar excitation and detection techniques are used. THEORY OF ELECTRON SPECTROSCOPY

This section will give a brief in troduction to the theory of electron spectroscopy as it relates to chemi cal applications. There are basi cally two types of processes respon sible for emission of electrons from

REPORT FOR ANALYTICAL CHEMISTS Table I. Fundamental Pro cesses in Electron Spectroscopy Electron Ejection (1) (2)

(1) (2)

Photoionization A + hn-*A+* + eQ Electron Bombardment A + β!θ - * A+* + β!θ + β2θ Electronic Relaxation X-Ray Emission A+*^-A++hyt Auger Emission

atoms: electron ejection and elec tronic relaxation. These are sum marized in Table I. Electron exci tation may be induced by either photoionization or electron bom bardment. The ionizing radiation used can vary from γ-rays to ultra violet photons requiring only that it be in excess of the binding energy of the electron. There are two funda mental processes for electron relax ation in atoms, X-ray emission (flu orescence) and Auger electron emis sion. As will be discussed further, these are competing processes.

Figure 1.

Block diagram of essential elements of electron spectrometer

Photoionization Processes

Figure 2 shows an energy level diagram for an electrical insulator which will serve for discussion of excitation processes for core elec trons. Photoionization processes are indicated for both Is and 2p electrons. The free electron level is that energy level where the electron has become free of its normal atomic and molecular forces. Pho toionization leaves a positive hole in the atom, and the nature of the hole states is indicated by the term sym bols on the right-hand side of the diagram. Thus photoejection of a Is electron leaves a 2Si/ % state or a Κ hole. The relationship between photoelectron ejection and X-ray absorption can also be seen from Figure 2. The low- and high-en ergy limits of the Lx X-ray absorp tion transition are shown for 2s electrons. The upper energy limit for this transition is promotion of a 2s electron to the free electron level, exactly the same energy required for photoejection. This leads to the conclusion that any information contained in binding energy studies

Figure 2. solids

Primary processes for X-ray absorption and electron photoejection from

of photoelectrons also is intrinsi cally contained in X-ray absorption data, although it is far more diffi cult to extract for the latter. I t should be pointed out that pho toexcitation is not equally probable for all electrons within a given atom or for the same electron among dif ferent atoms. Generally, excitation

of an electron is inversely propor tional to r 2 where r is the orbital radius. This means that for light atoms the probability of excitation of a Is electron will be approxi mately 20 times greater than for a 2s electron. However, as the atomic number increases this becomes less important for core electrons because


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21 A

Report for Analytical Chemists

of orbital radius contraction with increasing nuclear charge. Another important point is that excitation of a given electron (say a Is electron) varies as Zz where Ζ is the nuclear charge. One would thus expect that the most intense photoelectron lines would arise from Is electrons in ele ments of high atomic number. Al though core electrons are excited most easily, excitation of outer elec trons still occurs with reasonable probability. Electronic Relaxation

Figure 3 summarizes the elec tronic relaxation processes for an excited ion assuming a primary Κ vacancy—i.e., a 2Si/2-hole state. To the right of the diagram are shown the hole states which will result if an electron transition oc curs from a higher orbital to fill the primary Is vacancy. The nor mal Ka and Κβ /3-ray emission lines are shown as transitions from the 2p and 3d orbitals to the Is orbital, respectively. These repre sent normal X-ray fluorescence if photoionization by X-rays has been used or X-ray emission for elec tron excitation. The other major process for elec tronic relaxation of an excited ion is Auger electron emission as indi

Figure 3. 22 A ·

cated in Figure 3. Auger emission is a radiationless process in which an electron from a higher orbital undergoes a transition to fill the hole in a lower orbital and simul taneously a second electron is ejected from the atom. For ex ample, in Figure 3 as indicated for KLL Auger electron emission, one of the 2p electrons undergoes a transition to fill the primary Is va cancy while another 2p electron is ejected. The terminology KLL in dicates that the primary vacancy occurred in the Κ shell, an L elec tron underwent a transition to fill the primary vacancy, and a second L electron was ejected. The pro cess for KLM Auger electron emis sion is also shown in Figure 3. Here a 2p electron undergoes a transition to fill the Κ vacancy while a 3p electron is ejected. Au ger electron emission is an impor tant process in light atoms, and Auger electrons are encountered in electron spectroscopy. Many times they serve as a good internal stan dard since the energy of photoejected electrons depends on the en ergy of the exciting photon, while Auger electrons are independent of excitation energy. Figure 4 indicates the potential value of Auger electron studies par

Electronic relaxation processes for atom containing primary l s vacancy ANALYTICAL CHEMISTRY, VOL* 42, NO. 1, JANUARY 1970

ticularly in the case of light atoms. As can be seen, the probability of Auger electron emission is high in light atoms, virtually unity for atomic numbers below 11. Like wise X-ray fluorescence emission is at virtually zero probability for the same atoms. This explains why X-ray fluorescence has not been a useful technique for the study of light elements. Although X-ray fluorescence yield increases and subsequently Auger electron yield decreases with increasing atomic number, the 50% probability point occurs approximately with atomic number 33—i.e., arsenic. This means that for virtually the first four rows of the periodic table, Auger electron emission will pre dominate. Relationship to Existing Techniques

The discussions of photoexcita tion and electronic relaxation place electron spectroscopy in its proper perspective in relation to tech niques involving X-ray absorption and fluorescence. Consider the simulated spectra shown in Figure 5. The X-ray absorption spectrum is shown for the Κ and L absorp tion edges. Because photoej ection of an electron occurs at the upper energy limit of the absorption band, one would expect to see spacings between the Κ and L photoelectron lines comparable to the spacings of the X-ray absorption spectrum. Likewise, because X-ray fluores cence involves transitions from higher orbitals to a hole in the core level, spacing of X-ray fluorescence lines should be comparable to those in the photoelectron spectrum. For example, one would expect the spacing between the L2 and Ls pho toelectron lines to be the same as the spacing between the Km and Κβι X-ray fluorescence lines. From the standpoint of binding energy studies one would expect in formation obtained from X-ray fluorescence and electron spectros copy to be of comparable utility. Likewise, from the standpoint of chemical shift studies, one would expect the data obtained from an X-ray absorption spectrum and a photoelectron spectrum to be simi lar. However, electron spectros copy offers an advantage over


either of these techniques for the particular kind of information sought. In general, line widths are narrower for photoelectron lines than for X-ray fluorescence, and therefore, the spacing may be determined more precisely. Although chemical shift data arc contained in the X-ray absorption spectrum, the shift of an absorption edge can be measured with less precision and convenience than the location of a narrow line in an emission spectrum. Measurement of Binding Energies

Figure 4. Probability of Auger electron emission and X-ray fluorescence as function of atomic number

Figure 5. Simulated X-ray fluorescence, X-ray absorption, and photoelectron spectra for same elements

How the theoretical considerations relate to electron spectroscopic measurements can be seen from Figure 6. The energy levels of the sample are indicated assuming it to be an electrical insulator. The spectrometer is assumed to be metallic, and its levels are given on the right-hand side of the diagram. The energy of the incident photon is shown on the far left-hand side of the figure. For photoejection of a core electron in the sample, the energy of the incident photon will be distributed among four processes. First, the binding energy, Eb, of the core electron relative to the Fermi level of the material in which it is found. This is the parameter which electron spectroscopy ultimately measures. Second, the work function, s, of the material, which is the energy necessary to raise the electron from the Fermi level to the free electron level. Third, the recoil energy, Er, which results from the conservation of momentum in the photoejection process. Fourth, the kinetic energy, Ts, of the electron in space after it has left the sample. Recoil energy arises because of the necessity of appropriately distributing kinetic energy in the photoejection process. By application of the law of conservation of momentum it 'has been shown (7) that recoil energy is generally unimportant for electron ejection from light atoms. If we use aluminum K« as the exciting radiation, recoil is unimportant in atoms heavier than lithium. For copper, Ka recoil is unimportant in atoms heavier than sodium.


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dependent on atomic number, photoelectron lines will generally be narrower than X-ray fluorescence lines involving comparable levels. For example, if one is measuring the electron binding energy in cop per using aluminum Ka excitation, the intrinsic width of the aluminum radiation and the intrinsic width of one copper level will determine the line width. Studying comparable levels by X-ray fluorescence re quires the width of 2 copper levels 2 eV each. The net effect will be that the photoelectron line can have widths of about 1.5 eV, whereas the comparable X-ray line will have widths on the order of 3 to 4 eV. INSTRUMENTATION

Figure 6.

Energy considerations in measurement of electron binding energies

hv = E6 = Φ, = Φ,Ρ = T. =

energy of exciting photon binding energy of photoejected electron work function of sample work function of spectrometer kinetic energy of photoejected electrons in the vicinity of the sample T., = kinetic energy of the photoejected electron in the vi cinity of the spectrometer

Er = recoil energy

As the free electron having en ergy, T„ traverses the space be tween the sample and the spectrom eter, it will encounter a potential gradient due to the difference in work functions between the sample and the spectrometer. Therefore, it will be accelerated (or deceler ated) to the kinetic energy Tsp as it reaches the free-electron level of the spectrometer material. Adding the work function of the spectrom eter brings the electron to the Fermi level of the spectrometer. I t is clear then that the kinetic en ergy measured by the spectrometer is Tsp and that the binding energy of the electron in the sample can be computed according to Et = 24 A

·

hv — sp — Tsp — Er

(1)

If one wishes to measure the abso lute binding energy of an electron, it is important to know accurately the work function of the spectrome ter. But if only relative measure ments are desired (as will be the case for most chemical work), it is only necessary that the spectrome ter work function remain constant. This has been shown to be true (7). According to Equation 1, since the spectrometer work function is constant and the recoil energy is negligible, the uncertainty in the measured binding energy will arise from either an uncertainty in the measured photon energy or the binding energy of the core electron itself. Because the natural line width of X-ray lines is inversely

ANALYTICAL CHEMISTRY, VOL. 4 2 , N O . 1 , J A N U A R Y 1 9 7 0

The block diagram shown in Fig ure 1 indicates the tasks that must be accomplished by electron spec trometers. They must produce in tense ionizing radiation, irradiate the sample to photoeject the elec tron in such a geometry that the electrons can be introduced into the monochromator, carry our energy resolution on the photoejected elec trons, detect the electrons and then read out the signal in such a way to give a relative intensity as a function of electron energy. A number of possible instrumental systems can be generated depend ing upon selection of sources, monochromators and detectors. Those components which are used or have potential use in electron spectros copy are listed in Table II. Sources

As indicated in Table II, a va riety of possible sources can be used. The ideal source provides an intense, steady level of radiation in such a way that it can be focused conveniently on the sample. The ultimate precision of measurement is determined by the stability of source intensity. Essentially there are four types of sources that pro vide the required energy: X-rays, ultraviolet photons, electrons, and •y-rays. Although γ-rays have not been used to date they are a possi ble ionizing source for photoejecting the core electrons of very heavy atoms like the transuranium ele ments. X-ray tubes used in electron


Table II. Components for Photoelectron Spectrometers Sources Electron gun

Monochromators

•y-Rays

Magnetic

X-rays

Electrostatic

Retarding Field

Cu Κα 8048 eV Cr Κα 5415 eV ΑΙ Κα 1487 eV

Detectors

Mg Κα 1254 eV

GM-Tube

Ultraviolet He resonance 21.3 eV (584 Â)

Electron multiplier Photographic

Kr resonance 10.3 eV (1201 Λ) Hg resonance 4.89 eV (2537 A)

Readout Rate meter

Degaussing

Scaler

Helmholtz coils

Computerized system

Paramagnetic shielding

spectroscopy are of conventional design and consist of a heated cath ode at high negative potential and a water-cooled anode maintained at ground potential. Cooling of Xray tubes is essential because of the high power dissipated. Oper ating conditions of 10 to 15 kV at 20 to 50 mA are not uncommon. Rotating anode sources have been used to minimize effects of local ized heating on the anode. It is also desirable to design X-ray tubes in such a way that the anode is not in direct line with the cathode but that the electron beam is focused by electrostatic deflection. This minimizes deposition of tungsten from the filament on the anode, a problem that can be particularly acute when using X-rays from light elements. A variety of excitation energies is available from X-ray sources. Some typical examples for the Ka lines of Cu, Cr, Al, and Mg are given Table II. These elements give a range of energies that will photoeject the core electrons of most elements of interest. The X-ray tube and sample com partments are usually separated by a thin window through which the exciting X-rays pass. The separa tion of source and sample compart ments is essential to avoid scat tered electrons from the X-ray source entering the sample com partment. The use of a thin filter

can eliminate white radiation from the source as well as to separate X-ray lines in certain cases. Al ternatively, one could use a bent crystal monochromator to select truly monochromatic X-rays al though this would drastically re duce the source intensity. The use of a bent crystal monochromator does appear attractive, however, from the standpoint of further re ducing electron line widths. Ultraviolet sources are also con venient for electron spectroscopy, particularly in studies of low-en ergy photoionization processes. Such a source usually consists of a gas flowing through a thin tube at about 1-Torr pressure and excita tion either by electrical or micro wave discharge. Excitation ener gies available are considerably lower than those from X-ray sources, as indicated in Table II. Such sources are particularly use ful for exciting valence shell elec trons, reducing the background caused by higher energy sources. Also the line widths of uv photons are much smaller than those of Xrays and therefore, uv-excited elec trons have narrower bandwidths. Electron gun sources have been used for a variety of studies. These direct a narrow beam of electrons onto a sample, collecting the scattered electrons with a Far aday cage. Electron gun sources have been used to excite Auger

spectra in solid samples. Typical beam currents are 1 μΑ. The particular advantage of elec tron gun sources is that they pro vide a conveniently variable source of excitation which can be con trolled in both energy and inten sity. The beam is also readily focused. Although it is difficult to excite discrete photoelectron lines in solid materials using electron sources, Auger electron emission does produce discrete lines. Elec tron excitation of gases also gives discrete lines. There are two major disadvantages to electron gun sources. First, significant scatter of the incident electrons results in high backgrounds, particularly when investigating solid samples. Second, many materials decom pose under electron bombardment which are stable under X-ray or uv excitation. Sample

As has been mentioned earlier, the physical form of the sample is not important if differential pump ing techniques are available, gas, solid, and liquid samples having been run. However, because it is more convenient to run solid and liquid samples, a practical restric tion of low vapor pressure is often placed on the sample material. To run samples having high vapor pressure at room temperature, cry ogenic probes have been used. Use of such probes permits a wide range of materials to be investigated. Another criterion for the sample is that it must not decompose under high vacuum. This would restrict one from studying say inorganic carbonates that easily evolve C0 2 . A significant problem does exist when attempting to measure pho toelectron spectra of solutions where freezing the solution may have significant effect on the result. This represents a technological problem still to be overcome. For most samples, photoejected electrons will not emerge from a depth greater than 100 Â. This means that electron spectroscopy is basically a surface technique and one must be careful to ensure that the surface is representative of the bulk of the material, if bulk prop-


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erties are to be measured. Small samples are required for electron spectra, 10~e g being adequate, and spectra possibly being obtained on as little as 10~8 g. A potential problem exists in charging of the sample if it is not in good electrical contact with ground. To date ade quate currents have been observed, even in insulators, to replace the electrons photoejected. Good elec trical contact must be maintained between the spectrometer and the sample to ensure the interpretation given in Figure 6 is correct. Selecting the energy of the ex citing radiation near to the binding energy of the photoelectron mini mizes the energy resolution re quired by the monochromator. However, at the same time it intro duces the problem of getting the electrons out a finite depth in the sample, and the selection of exci tation energy in any practical case will have to be a compromise be tween the two. Compensation for Earth's Magnetic Field

Because electron paths are influ enced by magnetic fields, it is nec essary to have the electron trajec tory determined only by the field of the monochromator. This means that the earth's magnetic field in the vicinity of an electron spec trometer must be reduced to effec-

tively zero. T h e means for com pensating earth field varies with the electron energy and t h e t y p e of monochromator used, b u t usually is done by one or a combination of two techniques: a Helmholtz coil system or paramagnetic shielding material. Because it is necessary to resolve electron energies t o 0.1%, t h e r e sidual magnetic field and fluctua tions in t h e field must be held to a comparable value. Ideally one should reduce t h e earth's magnetic field along t h e electron trajectory to below 10 4 G. This means t h a t not only should t h e residual field be compensated to this value b u t the gradient along t h e trajectory should be a t this level, as well. Compensation for t h e vertical com ponent of the earth's field is far more crucial t h a n for t h e horizon tal field. T h e most widely used technique of field compensation is a series of Helmholtz coils, t h e basic construc tion of which h a s been described by Lee-Whiting (11). Siegbahn a n d coworkers (7) have traditionally used two circular vertical coils and two sets of two square horizontal coils. F o r their location they have found this arrangement t o be sat isfactory. When using a magnetic spec trometer in an u r b a n area, difficul ties with t h e simple Helmholtz sys

Figure 7. Block diagram of electron spectrometer system 26 A ·

ANALYTICAL CHEMISTRY. VOL. 42, NO. 1, JANUARY 1970

t e m are encountered. We have used three sets of three independent Helmholtz coils along with a feed back system as indicated in Figure 7. T h e vertical coils are 12 ft square with t h e center coil located approximately a t t h e center of t h e horizontal plane of t h e spectrometer and the upper and lower coils a p proximately 4.5 ft above and below the spectrometer. T h e horizontal coils are ca. 9 ft square and a r e placed a t ca. 3-ft intervals. A set of nine independent power supplies is used so t h a t t h e current through each coil m a y be varied indepen dently. This was necessary because of the extreme gradients encoun tered in buildings containing struc t u r a l steel. Even by use of this system, t o minimize t h e field gra dient along t h e electron trajectory, a small additional correction coil was necessary. Despite t h e elaborate Helmholtz coil system used, large fluctuations in field were observed in t h e 2—10 mG region depending upon the time of t h e d a y and t h e d a y of t h e week. Fluctuations correlated with traffic along city streets as well as a sub way station located some 600 m away. Field perturbations due t o traffic and subway trains, could be compensated by use of a feedback loop as indicated in Figure 7. T h e feedback network used a magne tometer probe placed a t t h e plane a t t h e spectrometer where t h e spec trometer contributed no net field and feedback t o t h e center coil of the vertical Helmholtz system. T h e use of paramagnetic m a t e rial such as μ,-metal or netic and conetic shielding is not permissible with magnetic spectrometers. B e cause these materials shield by cut ting t h e force lines of t h e earth's field, they also would perturb t h e spectrometer field and t h e required resolution would not be possible. However, if an electrostatic mono chromator is used, it is possible to shield t h e instrument with p a r a magnetic materials or by combin ing a less highly controlled H e l m holtz coil system with p a r a m a g netic materials. I t is this particu lar consideration which most strongly suggests t h a t electron spectrometers built for use in chem ical laboratories should utilize elec-


trostatic r a t h e r monochromators.

than

magnetic

Monochromators

Essentially three types of mono chromators h a v e been used in elec tron spectroscopy—retarding field, magnetic, and electrostatic. The latter two simply use different means of focusing an electron along a trajectory, while the retarding field spectrometer is quite different in principle. T h e principles of each are outlined in Figure 8. Retarding field spectrometers h a v e been reported b y A l - J o b o u r y and T u r n e r (9) and by F r o s t et al. {12). T h e T u r n e r instrument used a cylindrical geometry for the re t a r d i n g electrode while the Frost instrument used spherical elec trodes. T h e distance from the source to collector in the cylindrical spectrometer was a b o u t 4 cm and the spherical one about 3 in. T h e basic idea of a retarding field spec trometer is to h a v e two grids, G1 and G a in Figure 8A between a source of electrons, S, and a collec tor, C. T h e grids are metallic screens and provide approximately 7 0 % transmission. An increasing potential difference is applied between the grids to re t a r d electrons between t h e source and collector. W h e n the potential difference is large enough, electrons of a given energy, e2, will be re t a r d e d and t h e signal a t the col lector will be reduced. T h e collec tor signal, q, is amplified b y a charge-sensitive amplifier, differen tiated, and read out on a recorder, as t h e potential difference between the grids is scanned. T h e obvious d i s a d v a n t a g e in a retarding field spectrometer is t h a t the initial sig nal is the sum of the signals from all electrons, which makes distinc tion between low energy electrons difficult. Background from scat tered electrons is particularly bothersome. R e t a r d i n g field spec trometers a p p e a r to be particularly good for low-energy electrons a t low background, such as for u v ex citation of gaseous samples. T h e y h a v e large effective solid angles. M a g n e t i c monochromators used in electron spectroscopy are largely of the double focusing v a r i e t y a d a p t e d from the design of Sieg-

Figure 8.

Schematic diagrams of electron spectrometers

A. Retarding field spectrometer S = source C = collector Gi,G2 = grids ΔΕ = voltage source

ei,e2 = electrons of different energies q = charge from cup E0 = voltage from charge-sensitive amplifier

B. Double focusing magnetic spectrometer C. Spherical electrostatic spectrometer and retarding field

b a h n et al. {23). These are ironfree instruments generally m a n u factured from brass or aluminum, a 30-cm radius being adequate for most chemical work. Double fo cusing is provided by an inhomogeneous magnetic field produced by a set of four cylindrical coils placed a b o u t t h e electron trajectory as shown in Figure 8B. Also as shown, t h e critical angle between the source and the detector is w\/2 or 254°56'. As originally derived, t h e instrument used two concentric solenoids for coils, b u t a coil sys t e m was devised t o permit better access t o t h e spectrometer sample chamber for chemical work ( 7 ) . Siegbahn's scribed an

group has also de electrostatic mono

chromator (7), but the most recent design is one due to Helmer and Weichert (IS), which is a modifi cation of the spherical electron an alyzer described by Percel (H). This t y p e of monochromator con sists of two concentric spheres with a potential difference between t h e m as shown in Figure 8C. T h e de sign of Helmer and Weichert u t i lizes a retarding field to reduce t h e initial electron energy from ca. 1000 V to 100 V. This permits a smaller monochromator t o be used since the energy resolution is smaller and also provides an in crease in luminosity which more t h a n compensates for t h e decrease in brightness caused by t h e reduc tion of electron kinetic energy.


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This is the monochromator design used by Varian in their commercial instrument. Again, a great advantage of electrostatic monochromators over magnetic monochromators is that a minimal Helmholtz system is required for the former.

citing X-ray line. In this situation one encounters a drastic reduction in intensity, and a tracking method must be used to count the electron tracks on the emulsion of the photograph. An instrument to accomplish this has been described (7).

Detectors

Basically the detector used in electron spectroscopy must count electrons. To date four types have been used. In the Siegbahn-type instruments, GM counters, electron multipliers, and photographic plates have been used while in the Turner-type instrument, charge cups have been used. The charge cup detector is dictated by the nature of the retarding field spectrometer (9, 12), where the electrons passing through the grids accumulate charge on the cup. This type of detector has been shown in Figure 8A. The GM-type detector originally used by Siegbahn et al. was a proportional flow counter using a Formvar window and a postacceleration system for the detection of electrons below 2 keV (7). Electron multipliers have been used, both continuous-channel and the dynodc types. The continuous channel multiplier counts electrons with high efficiency to very low energies and, being small, can be placed readily in the focal plane of the spectrometer. If necessary, several channel detectors can be put over the exit slit to improve sensitivity. Our instrument uses a dynode-type electron multiplier which is connected to a charge sensitive amplifier and then to a counter, as indicated in Figure 7. This type of multiplier is quite sensitive, has low background, and is convenient to use. Photographic detection has been used (7) with electron spectrography. I t offers the advantage of integrating over fluctuations in the exciting radiation and is capable of detecting very low electron signals. However, for most scanning work relating to chemistry electron multiplier detectors are more convenient. One place where photographic detection is particularly useful, is when an X-ray monochrometer is used to isolate the ex28 A ·

Scan and Readout Systems

The scan and readout systems used in electron spectroscopy are basically of three types: continuous scan, incremental scan, and multichannel analyzers. In the continuous scan system, the field on the spectrometer is increased continuously as a function of time, and the signal from the detector is simultaneously monitored by a rate meter. The spectrometer current scan and the output from the rate meter are synchronized to allow accurate recording of spectra. Although the continuous scan method is valuable for roughly locating the positions of peaks, noise is sufficiently high that it cannot be used in high resolution work. The incremental scan method increases the current through the spectrometer in a series of small steps, counting the signal from the detector during each increment. When the increments are plotted as a function of field, a spectrum is produced. In their early work, Siegbahn et al. utilized a mechanical stepping system although a computerized stepping system such as that shown in Figure 7 is preferred. The multichannel analyzer approach in a sense combines the two, scanning a large number of increments continuously between two

limits of field. It is this approach which is probably the best suited to chemical spectrometers. Some feel for the total system associated with electron spectrometers can be obtained from Figure 7. The regulated X-ray power supply determines the ultimate precision with which electron intensities can be measured. The probe in the center of the spectrometer feeds back to the Helmholtz coil power supply system through the magnetometer to keep the field of the spectrometer constant. Commands to the computer which run the scan system are relayed through teletype. The current through the coils is read out by the digital voltmeter (DVM) and fed back to the computer. Likewise the computer programs a power supply to scan small increments over the range of the spectrum desired. This can be over a single photoelectron peak or over several. The accumulated signals are continuously displayed on a television screen and when sufficient resolution is achieved, the spectrum is printed. CHEMICAL SHIFTS

The type of information obtainable from electron spectroscopy depends upon the mode of excitation used. For electron bombardment, largely Auger spectra result. If ultraviolet radiation is used, only outer electrons are excited and if soft X-rays are used, inner shell electrons are ejected. Because of space limitations, we will discuss only those applications associated with inner electron photoejection. Auger spectra have been discussed previously {1) and ultraviolet excited spectra largely give ionization

Table III.. Chemical Shifts" of Oxidation States Element1' -2

-1 c

—

*o

-2.0

—

—

*0

Copper (Is)

—

Iodine (4s)

—

— *0

Europium (3d)

—

—

Nitrogen (Is) Sulfur (Is) Chlorine (2p)

0 — *0

Oxidation State +1 +2 +3 +4.5" — +5.1

+4 —

+5 +8.0

+6 —

+7

—

—

+4.5

—

+5.8

+7.1

-

— +9.5

+5.3

—

+6.5

—

—

—

— *0

—

—

+3.8

—

+0.7

+4.4

—

—

—

—

+9.6

—

*0

• All shifts given in electron volts. M e a s u r e d relative to indicated oxidation state ( * ) . Electrons m e a s u r e d given in p a r e n t h e s e s . 0 Arbitrary zero for m e a s u r e m e n t , e n d nitrogen in NaN3. * Middle nitrogen in NaNa. 6


—

—


potential d a t a and the structures of excited ions. T h e chemical shift d a t a for inner electrons appear to be most applicable to analytical chemistry. T a b l e I I I shows a compilation of photoelectron chemical shifts re corded to d a t e for copper (15), sul fur (16, 17), iodine (18, 19), eu ropium (18, 19), chlorine (20), and nitrogen (7). Shifts are reported relative to some indicated oxida tion state. As can be seen from T a b l e I I I , the magnitude of the chemical shift varies from one ele ment to another as well as from one oxidation state to another for the same element. For example, the chemical shift between the -)-5 and + 7 oxidation states of chlorine is 2.4 eV, whereas for iodine it is 1.2 eV. Likewise, the shift between copper 0 and + 1 is 0.7 eV, but the shift between copper -\-l and -\-2 is 3.7 eV. I n addition to the chemical shifts reported in T a b l e I I I for formal oxidation state changes, chemical shifts for different atoms in organic compounds are known. Extensive d a t a have been obtained for or ganic nitrogen and sulfur com pounds (7), and the implication of these to analytical chemistry will be discussed in the next section. Model for Interpreting Oxidation State Changes

An atomic core electron is influ enced by the attractive force of the nucleus and a repulsive force due to the outer electrons. T h e reptdsive force is a net shielding effect be tween the nucleus and the core electrons. If a change in oxida tion number occurs in t h e valence shell the shielding effect of the v a lence electrons on a core electron is reduced and the binding energy of the core electron increases. There fore, in the most simple sense shifts in photoelectron peaks with oxida tion state reflect t h e increase in binding energy of core electrons as valence electrons are removed. An increase in positive oxidation state will cause a n increase in binding energy of a core electron, and a decrease in kinetic energy when t h a t electron is photoejected. R e flecting this decrease, the magnetic or electrostatic field necessary to 30 A ·

focus the electron will be smaller; therefore, an increase in positive oxidation state will cause a shift to lower field for the photoejected electron. Because one usuallv photoejects core electrons, changing the electron density in the valence shell will have similar effects on Κ and L levels and these photoclectrons show approximately the same shift per given change in oxidation state. Also one would predict t h a t electron shifts for higher degrees of ionization, such as -\-b to -f-6 vs. -\-l to -\-2, would be larger as indi cated in Table I I I . T h e magnitude of the shift in photoelectron peak, expected on the basis of this simple model, can be computed readily (7). If a charge, q, is removed from the valence shell of an atom the potential en ergy of the inner electrons will be changed by the amount AE = q/r

(2)

where r is the radius of the orbital of the valence electron. AE is given in atomic units. For remov ing a unit charge from a distance of 1 Â (two atomic u n i t s ) , the change in energy expected would be 0.5 &v or 14 eV. Obviously this is much too large. T h e erroneous assumption in the simple electrostatic model was t h a t the valence electron is removed to an infinite distance from the core electron. This is not true in the case of ionic solids since the valence electron is moved only to some finite distance, on a counter-ion. A correction is necessary for this effect, comparable to the Madelung lattice correction. T h u s Equation 2 can be modified

AE = (J - | ) 9

(3)

Where a. is the Madelung constant, generally on the order of 1.7. If we consider now t h a t the electron has moved only a distance of 3 Â to a counter-ion, the change in energy predicted is ca. 6 eV. Although this number is still larger t h a n most observed shifts, it is approaching an appropriate value. I t has also been assumed above t h a t interactions between atomic electrons arise from the shielding effect of the


outer electrons, assuming no overlap of the atomic orbitals. T h a t this is not a valid assumption is well known. Wh en one wishes to consider electron shifts for nonionic compounds (such as organic compounds), the problem becomes even more complex. I n a system having an easily polarized electron cloud, such as 7r-molecular orbitals, a perturbation at some distance can affect the electron density around a given atom. For example, changes a t the p-position of an aromatic ring affect the electron density on the other side of the ring. Therefore, any a t t e m p t to explain photoelectron shifts for complex organic molecules based on simple electrostatics will be inadequate. There are two approaches which have been tried to correlate electron shifts for organic molecules : electronegativity and molecular orbital calculations. Although few correlations have been done for each, it appears t h a t molecular orbital calculations more -accurately predict relative binding energies, probably reflecting the better overall utility of molecular orbital calculations in describing electron densities. Correlation of Chemical Shifts with Computations

Hartree-Fock Free Ion—Approximation. Attempts to correlate chemical shifts with numerical calculations have been tried by Siegbahn et al. (7) and F a d l e y et al. (19). Although neither permits quantitative comparison between calculated and observed shifts, both point out interesting qualitative considerations. Fadley et al. attempted t o account for shifts in core electron binding energies by two models. T h e first utilized an energy cycle to split the calculated binding energies into a free-ion contribution and a contribution from the M a d e lung correction. I t was found t h a t the Madelung correction contributed a significant portion of the binding energy shift and could be used to explain why simple calculations gave values too large. T h e second model was essentially a charge shell approximation for bonding electrons in a fixed geome-


t r y with other ions. I t was found to give the correct qualitative di rection of the chemical shift. Free-ion H a r t r e e - F o c k calcula tions were made for fluorine, chlo rine, bromine, iodine, and europium in several oxidation states. E a c h of these predicted too large a value for the observed shift but less t h a n t h a t predicted by the classical model. T h e electron oxidation shifts for fluorine were predicted between 10 and 25 eV, chlorine 10 to 15 eV, bromine 9 to 13 eV, io dine 8 to 13 eV, and europium ca. 20 eV. T h e magnitude of the Madelung correction to be applied in a given situation is complicated by several factors. First, the exact geometry must be known. Second, because photoelectrons are ejected from sights near t h e surface, the full three-dimensional Madelung correction is not completely appro priate. Therefore, an average be tween 0.5 and 1 of the maximum Madelung correction was consid ered. I t was found t h a t when 3 / 4 maximum correction was applied to K I 0 3 and KIO4, the fractional ionic characters and ionic change computed correlated well with measurements obtained from Mossbauer spectroscopy. Other prob lems, which also need to be con sidered, are local charging of the sample which can shift the refer ence level, as well as electronic re laxation effects. This study also pointed out t h a t it is not appropriate t o use a com mon ion as an internal standard when comparing chemical shifts of various salts. F o r example, in measuring shifts for K I , K I 0 3 , and KIO4, one might be tempted to use a potassium ion peak as an internal s t a n d a r d . However, the Madelung correction also can result in shifts of several eV in the location of the potassium line. Both free-ion H a r t r e e - F o c k cal culations and charge-shell approxi mations can be used to determine general trends in binding energy shifts. B o t h predict a decreasing chemical shift going down a column in the periodic table, say from chlorine to iodine. T h e y predict accurately t h a t the chemical shift varies little from one electron to another throughout the core. Go32 A ·

determine if two nitrogen atoms in a molecule will give separate photoelectron peaks. I n addition to the results shown in Figure 9 for nitrogen, Siegbahn and coworkers (7) have also ob tained reasonable correlations for sulfur compounds and inorganic chlorine compounds. Correlation with CNDO Charges

Figure 9. Correlation between calcu lated charge and binding energy for ni trogen I s electrons. After Siegbahn et a/. (7)

ing across a row of the periodic table from left to right shows an in crease in chemical shift of the same electron. Electronegativity Correlations. Siegbahn has done extensive corre lations between shifts in electron peaks and the charges calculated from resonance theory (7). T h e charge, q, on an atom m a y be de fined a s :

a = Q + Σ Is Q is the formal charge and IN is the partial ionic character summed over all bonds involving the atom. T h e partial ionic character is re lated to the differences in electro negativity between the atoms form ing a bond. F o r example, the amino nitrogen is calculated to have a charge of —0.42 while the nitro nitrogen is calculated to h a v e a charge of + 0 . 8 7 . A typical correlation for nitrogen is shown in Figure 9. Although there is not a linear relationship between binding energy and calcu lated charge, most of the com pounds fall on the line except for sodium azide (1), potassium cya nide (2), sodium nitrite (15), and valero nitrite (16). T h e correla tion is good considering the a p proximate method used for t h e computions and the wide variety of structures considered. T h u s elec tronegativity correlations provide a convenient first approximation to


Recently Hollander et al. (21) have published a correlation be tween the calculated nitrogen atom charge by C N D O methods and the measured photoelectron binding peaks for 19 nitrogen containing compounds. T h e y observed two lines in their correlation, one char acteristic of anions and the other characteristic of neutral molecules and cations. Whether the second line for cations is related to the greater positive lattice potential or an artifact in the C N D O method is yet unknown. A noteworthy fea ture is t h a t all points are well cor related and there are no large dis crepancies, such as those found in the electronegativity correlation. CHEMICAL APPLICATIONS Structure Determination

Electron spectroscopy has great potential as a technique for chemi cal structure determinations, possi bly comparable in scope to nmr and infrared. As one example, Figure 10 shows the carbon-Is spec t r u m for methyl trifluoroacetate. T h e photoelectron peaks are situ ated approximately under the car bon atoms of their origin, indicating it is possible to distinguish between t h e four types present. Figure 11 shows the photoelectron spectrum of the nitrogen-Is electrons from 2 - ( 4 nitrobenzene sulfonamido) pyridine. Again, the photoelectron peaks are located approximately under the nitrogens of origin. N o t only is qualitative information available from these spectra but the ratio of numbers of atoms of a given t y p e can be determined as indicated by the 1:1 ratios of the spectra. I n the nitrogen-Is spectrum of sodium azide, N a N 3 , two peaks are ob served in a 2:1 ratio corresponding to the two identical end nitrogens and the center nitrogen.


Figure 10. Carbon I s photoelectron spectrum for ethyl trifluoroacetate. After Siegbahn et a/. (7)

Figure 1 1 . Nitrogen I s photoelectron spectrum for 2-(4-nitrobenzene sulfonamide) pyridine. After Siegbahn et al. (7)

Figure 12. Correlation chart for nitrogen I s electron binding energies and functional groups | ( )

1 = range of observed energies = number of compounds used in correlation

Data used to establish correlations are largely due to Siegbahn et al. (7)

To obtain some idea as to the types of compounds between which electron spectroscopy will distinguish, Figure 12 shows the approximate ranges of photoelectron peaks observed for different organic functional groups containing nitrogen atoms (22). The numbers in parentheses are the number of compounds for which data were available, but because of the small number of examples for many classes of 34 A ·

compounds, these charts must be regarded as preliminary. Several features are noteworthy. For example, in Figure 12 it is apparent that electron spectroscopy will clearly distinguish between types of inorganic nitrogen compounds, such as nitrate, nitrite and cyanide. Further, compounds containing nitrogen-oxygen bonds tend to have photoelectron peaks at higher binding energies, ca. 405 eV,


than those containing nitrogennitrogen or nitrogen-carbon bonds, ca. 400 eV, and those containing charged nitrogen atoms in the middle region, ca. 402 eV. Therefore, simply knowing the location of a peak gives some idea as to the nature of the nitrogen atom present. An interesting phenomenon is observed in the correlation for nitro compounds where 19 examples were available. It appears, to a first approximation, that aromatic nitro compounds bunch toward one end of the correlation, while aliphatic compounds bunch toward the other. It is quite reasonable that as more electron spectral data become available for larger numbers of nitrogen compounds, the broad correlations on the chart would break up into subsets. The correlation for sulfur-2s electrons shown in Figure 13 is similar to that for nitrogen. Sulfur-oxygen compounds tend to bunch at higher binding energies, ca. 165 eV, whereas sulfur-sulfur or sulfur carbon compounds generally tend to show lower binding energies, ca. 163 eV. It is interesting to note that compounds containing both sulfur-sulfur and sulfur-oxygen bonds tend to pull peaks toward the middle—i.e., increasing the binding energy of sulfur-sulfur bonds and decreasing the binding energy of sulfur-oxygen bonds. Again, although only a few compounds are available for correlation, it appears that electron spectroscopy may be a convenient technique for distinguishing between sulfhydryl and disulfide bonds, an item of no small interest to biochemists. Electron spectroscopy has already been applied to the solution of one structural problem. There had been some doubt about the structures of disulfide-dioxide, as to whether they show the disulfoxide structure, A, or the thiolsulfinate structure, B, as indicated below.

0 0 t t R—S—S—R (A)

0 t R—S—S—R I (B) 0

For structure A, one sulfur peak should be observed in the sulfuroxygen bond region. For B, two sulfur lines should be observed, one


Figure 13. Correlation chart for sulfur 2s electron binding energies and functional groups | ( )

] = range of observed energies = number of compounds used in correlation

Data used to establish correlations are largely due to Siegbahn et al. (7)

in the sulfur-oxygen region and one near sulfur-sulfur. The observed photoelectron spectrum of cystine S—dioxide shows two lines indicating the validity of structure B.

tion potentials for a number of fluorinated benzenes and Dewar and Worley have measured the ionization potentials of 67 com pounds (29).

Determination of Ionization Potentials.

Surface Studies

Returning for a moment to lowenergy (ultraviolet) excitation, considerable data have been reported indicating electron spectroscopy to be useful for measuring ionization potentials, particularly below the 21.21-eV. limit of the helium resonance line. Al-Jobury and Turner have reported ionization potential data (24) for 48 compounds including small gaseous molecules aliphatic, alicyclic, d é finie, and aromatic hydrocarbons, as well as a number of alchohols and amines. Also, Radwan and Turner have published ionization potentials for ozone (25), and Baker and Turner have presented ionization potential data for acetylene, diacetylene and their deuterated derivatives (26). Baker, May, and Turner (27) have determined ττ2 and 7rs ionization potentials for 21 monosubstituted benzenes and 23pdisubstituted benzenes. Clark and Frost (28) have published ioniza

Electron spectrocsopy is largely a surface technique which consti tutes both an asset and a liability. When doing structural studies, one must be certain that the surface is representative of the bulk material. On the other hand, this is a particu larly valuable asset when applied to surface chemistry. The average penetration of a photoelectron is probably not much greater than 20 to 50 Â—certainly 100 À at the outside. This has been illustrated by Siegbahn et al. (7) when they deposited different numbers of multiple layers of a-iodo stearic acid and measured the intensity of the photoelectron peak from iodine. For depths of 1, 3, and 10 double layers, they observed signals in the ratio 1: 2.1 : 3.5. Although this indicates that multiple layers do not give additive signals, it points out one of the great advantages of electron spectroscopy for studying surfaces: that a sur-

face can be studied even though it is covered by more than one layer of material. In the same study Siegbahn et al. obtained iodine spectra of a surface layer containing one iodine atom per 10 À2—i.e., less than a monolayer. Therefore, sensitivity of electron spectroscopy is such that submonolayers on surfaces may be studied. It appears, on the basis of these preliminary studies, that electron spectroscopy has great potential in surface chemistry. It should have a significant advantage over techniques such as low-energy electron defraction (LEED) because both the surface and the layer covering it can be viewed simultaneously. This should be particularly advantageous to catalyst studies, for the nature of changes occurring at active sights on the catalysts, as well as changes in the material being absorbed on the catalyst, can be determined simultaneously. To date no other instrumental technique is capable of providing this type of information on a wide variety of compounds. Quantitative Analysis

In addition to its use as a qualitative technique in structural chemistry, electron spectroscopy also shows potential as a quantitative analytical tool. Because photoelectron spectra can be obtained for virtually any element, it is not inconceivable that electron spectroscopy could become a universally applicable technique for determining elemental ratios in a wide variety of samples. It shows particular promise for obtaining elemental ratios in organic compounds with much greater ease than can be done by either mass spectrometry or wet chemical analysis. There are a number of factors which affect the recorded intensities of photoelectron lines. One of these is the photoelectron cross section given by μ = CZ4A3 + b where the first term containing Z4 is an absorption term and b repre sents a smaller scattering term. This effect tends to favor excitation of high Ζ elements since, all other factors being equal, they will have


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higher absorption coefficients for the exciting X - r a y lines. Another factor of importance in recording intensities is the kinetic energy of the electron emerging from the sample. I n general, the greater the kinetic energy, t h e more likely the electron to emerge u n perturbed by the sample. W h e n one considers a given t y p e of core electron (such as 2s electrons) of a number of different elements, this effect will tend to favor the elements of low atomic number since the binding energies of their electrons will be lowest. Another factor which will be somewhat important is the detector efficiency, although with electron multipliers this is much less a problem t h a n with the earlier G M detectors. In general, detector efficiency will show slight favoritism toward higher energy electrons again favoring elements of low atomic number. Based on the factors above, it is a p p a r e n t t h a t a system will have to be well calibrated for excitation with a given X - r a y line. This sort of calibration should not be difficult to perform, and once a spectrometer system is calibrated, recalibration should not be necessary. Using calibrations, such as those suggested, Siogbahn et al. (7) have performed q u a n t i t a t i v e analyses on a number of samples. T h e y quantitatively determined the carbonto-chlorine-to-sulfur ratios in a number of organic compounds, as well as the carbon, nitrogen, oxygen, and sulfur ratios in a variety of amino acids and insulin. W i t h the exception of oxygen analyses the results were quite good, generally t o within ± 5 % . Sicgbahn et al. (7) have also analyzed brass samples containing zinc 10—50%, copper 50-907c, tin ca. 0.1% and lead ca. 0.87c· Again, the relative precision was approximately 5 % . Because Siegbahn et al. were not primarily interested in electron spectroscopy as an analytical tool, their calibrations were somewhat crude and do not represent the ultim a t e achievable precision which could be expected from the technique. I t is not inconceivable t h a t with better calibrations precision below 1% should be obtainable.


Advantages and Disadvantages of Electron Spectroscopy as Analytical Technique

A t this point after extolling the virtues of electron spectroscopy for several pages, it might be well to consider w h a t the pros and cons of the technique are for the analytical chemist. T h a t it has much potential in both qualitative and q u a n t i t a t i v e analysis is evident and t h a t the potential applications to chemistry have not yet, to use a pun, "scratched the surface." Before anyone undertakes a program in electron spectroscopy, however, there are a number of factors which should be known, some of which have been mentioned before b u t which will be summarized here. Electron spectrometers require residual magnetic fields of ca. 10~4 G and a vacuum of better t h a n 10~5 Torr. T h e technique is good for solid, liquid, and gaseous samples. Solid samples, both amorphous and crystalline, can be placed on t a p e or p u t into pellets, similar to the use of K B R in t h e infrared. Gas samples can be studied by using differential pumping techniques, the sample c o m p a r t m e n t serving as the containers. Liquid samples, assuming they h a v e reasonable vapor pressure a t room t e m p e r a t u r e , must be frozen on a cold finger to reduce vapor pressure. Electron spectroscopy to d a t e has not been effective in studying solutions because of the high v a c u u m necessary—even using differential pumping techniques to keep the solvent out of t h e m a i n v a c u u m c h a m b e r ; a n y studies done on solutions under these conditions would be of questionable validity, with the solvent boiling from the surface of the solution. If v a c u u m p u m p oil containing carbon is used, unless precautions arc taken t o minimize diffusion of the p u m p oil into the main v a c u u m system, one obtains hydrocarbon coatings on all samples. This can be both an a d v a n t a g e and a disadv a n t a g e . T h e c a r b o n - I s photoelectron line can be used as a s t a n d a r d ; however, it can also be annoying when studying surfaces or doing q u a n t i t a t i v e analysis. T h e technique is both qualitative and q u a n t i t a t i v e . Usually photoelectron lines of adjacent elements of t h e periodic t a b l e are far a p a r t

so that overlap between lines from similar electrons of nearest neighbors does not occur. For example, the nitrogen Is line is at ca. 400 eV, oarbon at 290 eV, and oxygen at 530 eV. A similar situation is found for silicon, phosphorus, and sulfur. Electron spectroscopy is useful for almost every element in the periodic table. It is a particularly valuable technique for determining one element in the presence of many others. For example, Siegbahn has observed a cobalt signal from vitamin B J 2 where cobalt is one in 180 atoms. However, electron spectroscopy under current conditions of resolution is not good for distinguishing one atom only slightly different from say 20 others. In this case the photoelectron peak from the one atom in question would appear only as a slight shoulder on a much higher peak and would probably be lost in the noise. The size of the sample required can be quite small. Photoelcctron spectra are obtainable from micrograms of material, and Siegbahn has reported spectra from samples as small as 10~8 g. I t is not inconceivable that electron spectroscopy could do a chemical analysis on a fingerprint. In addition to its sensitivity, the technique should be, with reasonable advances in calibration, capable of 1 % precision. Electron spectroscopy is generally a nondestructive technique. However, some organic compounds do decompose under bombardment from X-rays. For example, pnitroaniline has been shown to decompose as have some chlorinated anilines. When an electron gun source is used for excitation, generally sample decomposition will be greater. For the determination of trace quantities of one element in the presence of others, electron spectroscopy should be sensitive to the tenths of a per cent range. ACKNOWLEDGMENTS

The author wishes to acknowledge the valuable assistance of John Jack, William Swartz, and Anthony Waraksa in setting up our electron spectroscopy program. In

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ANALYTICAL CHEMISTRY, VOL.. 4 2 , NO. 1 , J A N U A R Y 1 9 7 0

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39 A

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addition, I would like t o acknowledge contributions from Stig H a g strom, Carl Nordling, R a g n a r N o r d berg, Anders F a h l m a n , and K a i Siegbahn. Support of our photoelectron spectroscopy research b y t h e United States Energy Commission under C o n t r a c t A T (30-1)-905 to t h e L a b o r a t o r y for Nuclear Science a t M I T is gratefully acknowledged.

·

This work was supported in part t h r o u g h funds provided by t h e United States A t o m i c Energy Commission under Con t r a c t No. A T ( 3 0 - l ) - 9 0 5 .

BIBLIOGRAPHY

(1) L. A. Harris, Anal. Chem. 40, 14, 24A (1968). (2) N . Svartholm and K. Siegbahn, Arkiv. Mat. Astron. Fys., 33A, 21 (1946). (3) K. Siegbahn and K. Edvarson, Nucl. Phys.,1, Ï37 (1956). (4) C. Nordling, E. Sokolowski, and K. Siegbahn, Arkiv. Fys., 13, 483 (1958). (5) S. Hagstrom, C. Nordling, and K. Siegbahn, Phys. Lett., 9, 235 (1964). (6) C. Nordling, S. Hagstrom, and K. Siegbahn, Z. Physik, 178, 433, 439 (1964). (7) K. Siegbahn et al, ESCA "Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy," Almquist and Wiksells, Uppsala, 1967. (8) L>. W. Turner and M. I. Al-Joboury, Λ Chem. Phys., 37, 3007 (1962). (9) M. I. Al-Joboury and D. "VV. Turner, J. Chem. Soc, 1963, 5141. (10) D. W. Turner, Proc. Roy. Soc, (London) A307, 15 (1968). (11) G. E. Lee-Whiting, "Uniform Mag netic Fields," At. Energy Can., Ltd., Chalk River Report CRT-673, Chalk River, Ont., 1957. (12) D . C. Frost, C. A. McDowell, and D. A. Vroom. Proc. Roy. Soc, (Lon don) A296, 566 (1967). (13) J. C. Helmer and N . H. Weichert, Appl. Phys. Letters, 13, 266 (1968). (14) Ε. Μ. Purcell, Phys. Rev., 54, 818 (1938). (15) C. Nordling, E . Sokolowski, and K. Siegbahn, Ark. Fys., 13, 483 (1958). (16) S. Hagstrom, C. Nordling, and K. Siegbahn, Z. Physik, 178, 439 (1964). (17) S. Hagstrom, C. Nordling, and K. Siegbahn, Phys. Lett., 9, 235 (1964). (18) C. S. Fadlev. S. Β. Μ. Hagstrom, J. M. Hollander, M. P . Klein, and D. A. Shirley, Science, 157, 1571 (1967). (19) C. S. Fadley, S. Β. Μ. Hagstrom, M. P . Klein, and D . A. Shirley, J. Chem. Phys., 48, 3779 (1968). (20) A. Fahlman, R. Carlsson, and K. Siegbahn, Ark. Kemi, 25, 301 (1966). (21) J. M. Hollander, D . N . Hendrickson, and W. J. Jolly, J. Chem. Phys., 49,3315 (1968). (22) D. M. Hercules, unpublished studies. (23) G. Atelson, K. Hamrin, A. Fahlman, C. Nordling, and B. J. Lindberg, Spectrochim. Acta, 23A, 2015 (1967). (24) M. I. Al-Joboury and D. W. Turner, J. Chem. Soc, 1964, 4434. (25) T. N . R a d w a n a n d D. W. Turner, ibid., 1966A, 85.

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(26) C. Baker and D. W. Turner, Chem Comms., 1967, 797. (27) A. D. Baker, D. P . May, and D . W. Turner, / . Chem. Soc, 1968B, 22. (28) I. D . Clark and D . C. Frost, J. Amer. Chem. Soc, 89, 244 (1967). (29) M. J. S. Dewar and S. D. Worley, J. Chem. Phys., 50, 654 (1969).

ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 1 , JANUARY 1970

D. M. Hercules graduated from J u n i a t a College with a B.S. degree in chemistry in 1954; he received his P h . D . from Massachusetts I n s t i t u t e of Technology in analytical chemistry in 1957 after studying with L. B . Rogers. After faculty association a t Lehigh University, J u n i a t a , and M I T , he became as sociate professor a t t h e University of Georgia in September 1969. Dr. Hercules' research interests include t r a c e analysis b y fluorescence and phosphorescence, relationships be tween luminescence and molecular structure, chemiluminescence, elec troluminescence, the chemistry of molecules in electronically excited states, a n d electron spectroscopy.

ELECTRON SPECTROSCOPY - Analytical Chemistry (ACS

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