electron spectroscopy vii-1...

Electron spectroscopy VII-1

VII. Electron spectroscopy

References:

- D. C. Harris och M. D. Bertolucci, Symmetry and Spectroscopy, Oxford UniversityPress, 1978.

- J. M. Hollas, Modern spectroscopy, wiley, Chichester, 1987.

- M. Karplus, R. N. Porter, Atoms & Molecules, Benjamin,1970.

- E. F. H. Brittain, W. O. George, C. H. J. Wells, Academic Press,1970.

- G. Herzberg, Spectra of Diatomic Molecules, Van Nostrand, 1950.

- G. Herzberg, Electronic Spectra and Electronic Structure of Polyatomic Molecules,Van Nostrand, 1966.

- K. P. Huber och G. Herzberg, Constants of Diatomic Molecules, Van Nostrand, 1979.

- CRC Handbook of Spectroscopy.

- Handbook of chemistry and physics, 55th Edition, R. C. Weast, Ed., CRC Press, 1974.

- T. Hase, Spektrometriset taulukot, Otakustantamo, 1984.

VII-2 Molecular spectroscopy

VII.1. Detection of electrons

The term electron spectroscopy refers to methods where the sample is ionised and theemitted electrons are observed. The most common type is photoelectron spectroscopy, inparticular X-ray photoelectron spectroscopy (XPS) but also the UV photoelectron spec-troscopy (UPS) is widely used. In most XPS spectra also Auger precesses are seen. Eventhe electron enrgy loss spectroscopy (EELS) can give valuable information. The instru-ments for photoelectron spectroscopy are often equipped to show also other kinds of spectrasuch as secondary ion mass spectra (SIMS). These, however, are outside the scope of thischapter. A summary of some the surface analytical spectroscopies where the incidentradiation is photons, electrons, or ions is given below.

Incident radiationemission hν e− jon

hν SEXAFSe− UPS AES INS

XPS LEEDion ESD SIMS

ISSRBS

SEXAFS Surface extended X-ray Absorption Fine-structure SpectroscopyUPS Ultra-violet Photoelectron SpectroscopyXPS X-ray Photoelectron Spectroscopy (a.k.a. ESCA, Electron Spec-

troscopy for Chemical Analysis)AES Auger Electron Spectroscopy

LEED Low Energy Electron DiffractionESD Electron Stimulated DesorptionINS Ion Neutralization Spectroscopy

SIMS Secondary Ion Mass SpectroscopyISS Ion Scattering Spectroscopy

RBS Rutherford Back-scattering Spectroscopy


VII.2. Photoelectron spectroscopy

Photoelectron spectroscopy (PES) is a method where the molecule is ionised byirradiating it with such photons that an electron is released. When using moderate photonenergies in the far ultraviolet region, only the weakly bond valence electrons can be releasedwhile a harder rays in the X-ray region will also release electrons from the innermost coreorbitals.

When UV light is used the method is called UV photoelectron spectroscopy (UPS)and when X-rays are used it is called X-ray photoelectron spectroscopy (XPS orESCA (Electron Spectroscopy for Chemical Analysis). The principle of these metods areshown schematically in Fig. VII.1.

1σg+

1σu+

2σg+

2σu+

3σg+

1πu

1πg

3σu+

1σg+

1σu+

2σg+

2σu+

3σg+

1πu

1πg

3σu+

e-

e-

εa) b)

Fig. VII.1. (a) UPS; (b) XPS.

Koopmans’ theorem states that the ionisation energy Ip of a molecule is related to theorbital energy ǫ of the affected molecular orbital as

Ip = −ǫ. (V II.1)

The photoelectric phenomenon means that an incident photon of the energy hν will releasean electron from the sample whereby the energy Ip is consumed and the rest of the photonenergy is converted to kinetic energy of the electron,

hν = Ip +1

2mv2. (V II.2)

(The work function is considered as a part of Ip for simplicity). When the frequency of

the incident photon is known and the kinetic energy of the released electron is measured,


one will obtain the ionisation potential Ip. The spectrum shows the electron flow (theintensity) as a function of the ionisation potential. Sometimes the spectrum is plottedusing the kinetic energy which, of course, is the primary quantity.

Gelius has shown that the intensity of an ESCA transition in a molecule (IMO) can beestimated on the basis of the MO-LCA coefficient of the affected atom (cAO) and thatatom’s inherent intensity in ESCA (IAO),

IMO =∑

c2AO · IAO. (V II.3)

The valence orbitals that are probed in UPS are characteristic for the chemical envi-

ronment. Koopmans’ theorem suggests that one can from the UPS deduce how easilyavailable the electrons in the molecule are and how the molecular orbitals are changed,e.g., in a chemical reaction. A more detailed analysis reveals finestructure in the UPSspectrum. From this it is possible to retrieve information of the ion’s vibrational levelsand therefore of the bond strengts etc. The UPS gives a highly characteristic fingerprintof the substances that are studied. It can be used as an analytical method for contamina-tions on surfaces. Fig. VII.2. shows the UPS spectrum of formaldehyde as an example. Inthis figure the assignments refer to the molecular orbitals from which electrons have beenextracted. A more correct notation would use the spectral terms of the final electronicstates. The finestructure of the spectral bands indicates the vibrational levels in the finalelectronic state.

2b1

1b2

5a1

1b1

4a1

H2C=O

Fig. VII.2. UPS spectrum of formaldehyde.

In the XPS a hole is created in one of the core orbitals. The innermost molecular orbitalshave not completely lost their atomic character (which is the case with the valencemolecular orbitals). Therefore one can see which elements the molecule consists of. Infact, the spectral bands are often assigned by using the symbols for the atomic orbitals,which is strictly taken incorrect. One should use the symbols for molecular orbitals alsofor the core orbitals. Still more correct would be to use the spectral terms of the finalelectronic states. A combined XPS and UPS spectrum is shown in fig. VII.3.


BINDING ENERGY (eV)

CO

UN

TIN

G R

AT

E (

c/20

0s)

OXYGEN

570 560 550 540 530 60 50 40 30 20 10

0

1000

2000

3000

4000

5000

O1s

C B A

(2Σ)(4Σ)σ2s

(2Σ)(4Σ)σ2s

(2Σ)(4Σ)σ2p

(2Π)(4Π)π2p

π2p

Fig. VII.3. XPS and UPS spectra of the oxygen molecule.

XPS (or ESCA) is a method for elemental analysis. However, the molecular core orbitalsare not pure atomic orbitals. The chemical environment is observed in the subtle details.Thus the oxidation state of the tergeted atoms determines the exact position of the spectralbands. There are chemical shifts of a few electron volts in the exact band positions. Thechemical shifts in the different states of a chromium atom are shown in fig. VII.4.

The XPS is particularly useful in studies of solid surfaces and adsorption. The X-rayspenetrate deep into the material, of course, and ionize atoms also there. However, thereleased electrons never reach to the surface of the sample and to the detector becausethey are absorbed by other atoms in the material. Only electrons originating at maximally


0

10

20

30

40

50

Cr Cr2O3 CrO3

570 575 580 585

Intensity (arbitrary units)

Ionization energy (eV)

Cr 2p3/2

Fig. VII.4. Chemical shifts in the XPS spectrum of chromium.

a few nanometer’s depth will be observed. This is shown schematically in fig. VII.5. Theeffect of the chemical surrounding would be much more prominent in the UPS spectrum,of course, but the spectral bands of solid samples in the UPS tend to be quite obscure.

hν

e-

hν e-

Fig. VII.5. The ESCA spectrum gives information only of the surface layer of the sample.


VII.3. Auger spectroscopy

Ionization in one of the innermost core molecular orbitals results in a very highly excitedion. A number of spontaneous relaxation processes may ensue. In the Auger process thehole in the core orbital is filled by dropping down a valence electron. This creates a newhole in the valence shell. At the same time a large amount of energy is released becausethe energy of the core orbital is much lower than that of the donating valence orbital. Thisenergy is used to release another valence electron. The principle of the process is shownschematically in fig. VII.6.

ε

1σg+

1σu+

2σg+

2σu+

3σg+

1πu

1πg

3σu+

a)ε

1σg+

1σu+

2σg+

2σu+

3σg+

1πu

1πg

3σu+

e-

b)ε

1σg+

1σu+

2σg+

2σu+

3σg+

1πu

1πg

3σu+

e-

c)

Fig. VII.6. Auger spectroscopy: a) the ground state; b) ionisation in X-ray photoelectronspectroscopy; c) the Auger process.

A pure Auger spectrum is shown in fig. VII.7. It is a spectrum of lithium fluoride in gasphase. The spectral bands have been assigned by using quantum chemical calculations.However, the assignments are not shown here. The Auger process is customarily describedby indicating in which shell the original ionisation occured and where the final two holesreside in electronic structure of the doubly ionised molecule. In LiF the initial X-rayionisation extracted an electron from the K shell. Afterwards the hole was filled by anelectron from the L shell and finally one further electron was ejected from the L shell.


KLL Auger spectrumof LiF molecule

Fig. VII.7. The Auger spectrum of Lithium fluoride.


VII.4. Electron energy loss spectroscopy

The Electron Energy Loss Spectroscopy (EELS) measures those electrons in X-rayphotoelectron spectroscopy that suffer of an inelastic collision within the material beforeit reaches the detector. The electron donates some of its kinetic energy to the scatteringcenter and therefore shows in the X-ray spectrum a lower kinetic energy than it originallyhad, and consequently a seemingly higher binding energy than the primary electrons.Those electrons will generate weak and broad bands in the ESCA spectrum. A schematicexample is shown in fig. VII.8.

Electron energy loss (eV)

Cou

ntin

g ra

te

Zero loss peak

Plasmon resonance

Atom core loss peak

Fine structure

Fig. VII.8. An electron energy loss spectrum schematically.

Electrons that suffer more than one inelastic collision are many but their final kineticenergy varies and they just increase the background level in the spectrum.

The electrons may interact and exchange energy via a number of different energy levelsystems in the sample. Thus one can have, e.g., phonon resonanses. A phonon is thequantum of the extremely slow low energy vibrations of the whole crystal lattice in solidsamples. Most often they make the primary X-ray photoelectron bands broader. Theplasmon resonances show as broad and quite weak bands next to the primary band. A


plasmon is the quantum of plasma oscillations, in solid conductors specifically the oscil-lations of all the electrons in the conduction band of the material. Further electron energyloss bands may arise from inter- and intra-band transitions, secondary ionisations from thecore orbitals or Cerenkov radiation.

E.g., if one observes exceptionally meny electrons with an energy shifted by 285 eV fromthe strong primary band one can assume that there is a high concentration of carbon in thesample and that the electrons collide with carbon atoms and ionise them as the ionisationenergy of carbon lies exactly at this energy.

Inelastic scattering will alter the propagatin direction of the electrons. Accurate measure-ments at different angles may give information of the dispersion properties of the material.


VII.5. The ESCA spectrometer

The principle of an ESCA spectrometer is shown i fig. VII.9. It comprises an X-raytube as a source, the source is in the upper left corner in the figure; the sample, which is inthe lower part of the figure; and a detector, that measures the electron flow. The detectordepicted in the figure determines the energy of the electrons by bending their path in amagnetic field. The principle of an UPS spectrometer does not differ very much from thatof an XPS spectrometer, other than that the source is a UV lamp, of course.

Ultravakuum,10-9 Pa

Ein

= hν

Ei

e-

Ek

Fig. VII.9. An ESCA spectrometer schematically.

In the UV photoelectron spectroscopy a suitable UV source is used. It is most oftenbased on the Helium I emission at 24.5874 eV or Helium II emission at 54.403 eV (2372.3eller 5250.5 kJ/mol). In X-ray photoelectron spectrometer the source is an X-ray tube.The anode material of the X-ray tube determines the wavelength of the emitted radiation.A few examples are given in the table below.1 The principle of an X-ray tube is shownschematically in fig. VII.10.

λ [nm] E [eV] λ [nm] E [eV]

Cr 0.228894 5414.8045 Zn 0.143470 8638.906Mn 0.210114 5898.8010 Mo 0.070908 17479.372Fe 0.193538 6404.0062 Rh 0.061308 20216.12Co 0.178839 6930.3780 Pd 0.058526 21177.08Ni 0.165736 7478.2521 Ag 0.055923 22162.917Cu 0.154007 8047.8227 W 0.020894 59318.847

1 R. D. Deslattes et al., Rev. Mod. Phys., 75 (2003) 35 - 99.


High voltageFocusing cup

Tungstenfilament

Metal anode Copper tube

Cooling water

Beryllium windowX-rays

Fig. VII.10. An X-ray tube schematically.

A magnesium anode emits X-ray radiation at 0.989 nm or 1253 eV. Its natural linewidth

is 0.70 eV which is then the practical limit for the resolution of X-ray photoelectron spec-troscopy when using a non-monochromatised source. However, a suitable crystal can beused as a monochromator in order to obtain the exactly correct wavelength. E.g., the Kαband of aluminium at 0.83386 nm or 1486.7 eV can be monochromatised by using the quarzsurface 0101 to obtain a linewidth as low as 0.16 eV which is in practice the best resolutionthat can be obtained with an ESCA spectrometer. The customary, not fully optimised,adjustments in a spectrometer reduce the resolution so that the best performance one canexpect in standard spectra is maybe 0.4 - 0.6 eV. In the case of non-monochromatisedradiation one can expect a resolution of 0.9 - 1.0 eV.

There are many types of electron detectors. The most common one is based on the sameprinciple as the ion detectors in the traditional mass spectrometers, i.e., the electrons movein a strong electromagnetic field that bends their path the more the slower they propagate.The field is adjusted so that the electrons with a given velocity hit the detector. The kineticenergy can then be determined on basis of the strength of the electromagnetic field. Sucha hemispherical detector is shown schematically in fig. VII.11. Its resolution dependson the electronic lenses that focus the electrons into the entrance slit of the detector, andof the homogenity of the field in the detector. In most ESCA spectrometers the field in thedetector is kept constant and the the focusing lens system is adjusted so that the electronsentering the detector have the correct energy. The alternative method is to also adjust thefield in the detector. The latter type is often found in Auger spectronmeters because thedetector’s resolution increases and the influence of the low energy noise is reduced.

In modern spectrometers multi-channel detctors are used. In that case, the electro-magnetic field strength can be kept constant. One possibility is to use a CCD detector.

Other detector types are used rarely. In some cases a time-of-flight detector (ToF)may be used. This type of detector is quite common for detection of ions, both in ordinarymass spectroscopy and in SIMS but quite rarely used in detection of electrons.


Fig. VII.11. Hemispherical detector.

The accuracy of the spectrometer’s scale for ionisation energies of the sample can be verifiedby using known calibration samples. Some examples are given in the table below.kändakalibreringsprov. 2 In the table are given the binding energies in electron volts as measuredby using two different X-ray sources at a resolution of 0.2 eV. The angle of emission is setto 15. The radiation from the aluminium soirce is monochromatised.

Band Al kα Mg Kα

Ni FE(fit) 0.064 ± 0.004 0.062 ± 0.001Au 4f7/2 84.000 ± 0.001 84.000 ± 0.009Ag 3d5/2 368.249 ± 0.010 368.255 ± 0.005Cu 2p3/2 932.681 ± 0.002 932.670 ± 0.006

The sample is ionised and therefore the extracted electrons must be replaced in the sample.Otherwise the sample soon gains a positive surface charge that makes further ionisationdifficult and causes a tilted background in the spectrum. For this reason the typical samplematerial is metal.

2 M. P. Seah, I. S. Gilmore and G. Beamson, Surf. Interface Anal. 26 (1998) 642.


VII.6. Band positions in ESCA

The observed electrons in the XPS originate from the core molecular orbitals that haveto a large extent retained their atomic orbital character. Therefore one can identify theelements that the molecule comprises. X-ray photoelectron spectroscopy is a method forelemental analysis. Tables over characteristic ionisation energies for the electron shellsin various atoms have been published. A summary is shown in the tables below.3 In thiscase, a Mg X-ray source has been used. Because Auger bands are often observed in theESCA spectra also the chracteristic Auger band positions are reported here. Also they arehighly characteristic for the elements. The energies in bold face are those that one shouldlocate first when identifying the elements.

The binding energies in the tables below are for pure elements. The exact positions of thebands depend to some extent on chemical shifts as illustrated in fig. VII.4. They givea hint on the oxidation state of the atoms but no detailed conclusions of the chemicalsubstances can be drawn.

Element Atom ESCA lines Auger linesnr 1s 2s 2p1/2 2p3/2 KL1L1 KL1L23 KL23L23

Li 3 56

Be 4 113

B 5 191 1082

C 6 287 993

N 7 402 875

O 8 531 23 779 764 743

F 9 686 30 645 628 599

Ne 10 863 41 14 491 468 435

Na 11 1072 64 31 332 303 264

Mg 12 90 51Al 13 119 74

Si 14 153 103 102

P 15 191 134 133

3 Perkin-Elmer, Handbook of X-ray photoelectron spectroscopy


Element Atom ESCA lines

nr 1s 2s 2p1/2 2p3/2 3s 3p1/2 3p3/2 3d3/2 3d5/2 4s 4p1/2 4p3/2

S 16 229 166 165 17

Cl 17 270 201 199 17

Ar 18 319 243 241 22

K 19 378 298 293 33 17

Ca 20 439 350 347 44 25

Sc 21 501 407 402 53 31

Ti 22 565 464 458 62 37

V 23 630 523 515 69 40

Cr 24 698 586 577 77 46 45

Mn 25 770 652 641 83 49 48

Fe 26 847 723 710 93 56 55

Co 27 927 796 781 103 63 61

Ni 28 1009 873 855 112 69 67

Cu 29 1098 954 934 124 79 77

Zn 30 1196 1045 1022 140 92 89 10

Ga 31 1144 1117 160 108 105 20

Ge 32 184 128 124 32 31

As 33 207 148 143 45 44

Se 34 232 169 163 58 57

Br 35 256 189 182 70 69

Kr 36 287 216 208 89 88 22

Rb 37 322 247 238 111 110 29 14

Sr 38 358 280 269 135 133 37 20

Y 39 395 313 301 160 158 45 25

Zr 40 431 345 331 183 181 51 29

Nb 41 470 379 364 209 206 59 35

Mo 42 508 413 396 233 230 65 38

Tc 43 544 445 425 257 253 68 39

Ru 44 587 485 463 266 282 77 45

Rh 45 629 522 498 314 309 83 49

Pd 46 673 561 534 342 337 88 54

Ag 47 718 604 573 374 368 97 58


Element Atom Auger lines

nr L3M23M23 L2M23M23 L3M23M45 L3M23M45 L3M23M45 L3M45M45 L2M45M451P 3P 1P

S 16 1103

Cl 17 1071

Ar 18 1037 1035

K 19 1005 1003

Ca 20 964 961

Sc 21 920 892

Ti 22 873 839

V 23 822 784

Cr 24 767 729

Mn 25 715 670 620

Fe 26 659 608 553

Co 27 604 597 546 541 483 468

Ni 28 548 542 482 476 410 393

Cu 29 486 479 416 408 396 337 317

Zn 30 429 422 352 343 329 265 242

Ga 31 368 361 284 275 257 189 162

Ge 32 305 297 215 205 184 113 82



nr 3s 3p1/2 3p3/2 3d3/2 3d5/2 4s 4p1/2 4p3/2 4d3/2 4d5/2 4f5/2 4f7/2

Cd 48 772 652 618 412 405 109 68 11

In 49 828 704 666 453 445 123 79 19

Sn 50 884 757 715 494 486 137 91 26 25

Sb 51 948 814 768 539 530 155 105 35 34

Te 52 1009 873 822 585 575 171 114 44 43

I 53 1071 930 874 630 619 186 123 52 50

Xe 54 1144 997 938 685 672 209 141 65 63

Cs 55 1084 997 738 724 230 170 158 77 75

Ba 56 1137 1062 795 780 254 192 179 92 90

La 57 1126 851 834 274 210 195 104 101

Ce 58 1184 900 882 290 222 207 112 108

Pr 59 950 930 305 237 218 114

Nd 60 1001 980 318 248 227 120

Pm 61 1060 1034 337 264 242 129

Sm 62 1110 1083 349 283 250 132

Eu 63 1166 1136 366 289 261 136

Gd 64 1186 380 301 270 141

Tb 65 398 317 284 150

Dy 66 412 329 293 154

Ho 67 431 345 306 161

Er 68 451 362 320 169

Tm 69 470 378 333 180

Yb 70 483 392 342 194 185

Lu 71 507 412 359 207 197

Hf 72 537 437 382 224 213 19 17

Ta 73 566 464 403 241 229 27 25

W 74 594 491 425 257 245 36 34

Re 75 628 521 449 277 263 45 43

Os 76 657 549 475 294 279 55 52

Ir 77 692 579 497 313 297 65 62

Pt 78 726 610 521 333 316 76 73

Au 79 763 643 547 354 336 89 85

Hg 80 803 681 577 379 359 104 100

Tl 81 845 721 608 406 385 122 118

Pb 82 893 762 645 435 413 143 138

Bi 83 942 807 681 467 443 164 159

Th 90 1168 968 714 677 344 335

U 92 1046 781 739 391 380

Np 93 1086 816 771 414 402

Pu 94 1121 850 802 439 427

Am 95 883 832 463 449

Cm 96 919 865 487 473

Bk 97 958 901 514 498

Cf 98 994 933 541 523



nr 5s 5p1/2 5p3/2 5d3/2 5d5/2 6s 6p1/2 6p3/2

Te 52 14

I 53 16

Xe 54 19

Cs 55 24

Ba 56 23

La 57 34 17

Ce 58 37 18

Pr 59 38 20

Nd 60 38 23

Pm 61 38 22

Sm 62 41 20

Eu 63 34 24

Gd 64 36 21

Tb 65 42 28

Dy 66 63 26

Ho 67 51 20

Er 68 61 25

Tm 69 54 32 26

Yb 70 55 33 26

Lu 71 58 34 27

Hf 72 64 37 30

Ta 73 71 45 37

W 74 77 47 37

Re 75 81 44 33

Os 76 86 60 48

Ir 77 98 65 53

Pt 78 105 69 54

Au 79 110 75 57

Hg 80 127 84 65

Tl 81 137 100 76 15 13

Pb 82 148 107 84 22 19

Bi 83 161 120 94 29 26

Th 90 290 226 179 94 87 43 26 18

U 92 325 262 197 104 96 46 29 19

Np 93 206 101 29 18

Pu 94 216 119 105 31 18

Am 95 351 216 109 31 18

Cm 96 232 113 32 18

Bk 97 246 120 34 18

Cf 98 124 35 19



nr M45N23V M5N45N45 M4N45N45

Nb 41 1088 1056

Mo 42 1068 1056

Tc 43 1047 1008

Ru 44 1025 981

Rh 45 1002 954

Pd 46 979 928

Ag 47 903 897

Cd 48 889 872

In 49 853 846

Sn 50 827 819

Sb 51 803 794

Te 52 775 765

I 53 748 737

Xe 54 724 711

Cs 55 698 684

Ba 56 671 657

La 57 632

Ce 58 594

Pr 59 555

Nd 60 519

Pm 61 481

Sm 62 440

Eu 63 402

Gd 64 362


nr N7O45O45 N6O45O45 N67O45V

Pt 78 1192

Au 79 1184

Hg 80 1176 1173

Tl 81 1169

Pb 82 1162 1159

Bi 83 1155 1151

U 92 1100 1005

Np 93 1064 970


VII.7. The intensities in ESCA

The intensity of the spectral bands in ESCA depends on many factors. The most impor-tant one is obviously the number of scattering centra, i.e., the concentration.

The inherent transition probability can be approximated on basis of the symmetry prop-erties. The transition moment obeys the equation

< Ψi|µ|Ψfψe >, (V II.4)

where Ψi is the wavefunction of the neutral, non-ionised atom or molecule describing ann-electron system. The wavefunction Ψf describes the ionised system with n− 1 electronsand ψe is the wavefunction of the departing electron. A symmetry analysis indicates thatthe departing electron’s wavefunction must have a suitable shape in order the transitionmoment to be finite. The electron is observed at a very large distance from the positivecharge and therefore one can assume that the electron feels a central field. This allows theuse of atomic notations s and p for the shape of the wavefunction ψe. Therefore the termss or p channel are often used. The scattering theory leads to a similar conclusion.

The bands marked in bold face in the above tables are those that have the highest inherentintensities.

Another important factor that affects the band intensity is the electron’s free path in thematerial. The deeper the ionisation occurs the more difficult it is for the electron to reachthe surface of the sample and fly to the detector. The free path depends on the electron’skinetic energy. Therefore the intensity must be corrected for the electron’s energy, i.e.,for the energy of the X-ray source. If one uses radiation from an aluminium source (1487eV) the typical free path is 1.4 to 2.0 nm. This implies that overwhelmingly most of theobserved electrons originate from maximally 6-8 nm below the sample surface.

More formally one can express the intensity of the incident X-ray radiation at the depth zin the sample as

Iz = I0(1 −R)sin θ

sin θ′e−z/(lhν sin θ′). (V II.5)

Most of the quantities are defined in fig. VII.12. The intensity I0 is the incident flow ofX-ray photons on the sample surface. Reflection R of the X-ray radiation can be ignoredprovided that the angle θ is not very small, say under 5. The quantity lhν is the free pathof the X-ray radiation in the material.

The number of scattering centra, i.e., atoms in a unit volume is

N =nA0

cos θdz. (V II.6)


zdz

θθ’

e-

A 0, Ω 0

Fig. VII.12. Intensity in XPS.

The symbol n stands for the number of atoms per unit volume. This equation is valid ifthe illuminated part on the sample surface is larger than the aperture of the detector, A0.The probability of photoelectric emission in the solid angle Ω is

P =σ

dΩΩ0, (V II.7)

where σ is the differential cross section of the photoemission and Ω is the angle betweenthe incident electron flow and the principal axis of the detector.

The probability that a photoelectron reaches the detector without collisions with atomscan be calculated from the Lambert-Beer law as

C = e−z

le(E) cos θ . (V II.8)

Here le(E) stands for the electron’s free path between inelastic collisions in the materialwhen its kinetic energy is E. An electrons that experiences a collision looses energy andcontributes to the spectrum’s background instead of the characteristic spectral band of theionisation process.

The detector reading depends on the effectivity of the detector, D0, and on the fractionof photoelectrons with the energy E that reach the analyser and are transported to thedetector at the energy E0, F (E0/E).


Summarising these contributions one can write the observed integrated intensity of elec-trons from the sample surface to depth t as

It = IzA0Ω0D0F

(

E0

E

)

nσ

Ω

1

cos θ

∫ t

0

e−z

le(E) cos θ dz. (V II.9)

When forming the ratio of integrated intensities for two substances A and B many of theconstants in the equations cancel. The ratio is

It(A)

It(B)=F (A)σ(A)

∫ t

0n(A)e

−z

le(A) cos θ dz

F (B)σ(B)∫ t

0n(B)e

−z

le(B) cos θ dz. (V II.10)

Based on these ratios it is possible to calculate the per cent ratios of the various componentsn(A) on the surface, provided that the free paths le and the scattering cross sectons σ areknown. It is customary to compare the integrated areas of all bands of the elements in thespectrum.

The accuracy obtained in quantitative analysis of most elements is in the range of permilles. In optimal conditions one can reach an accuracy of a millionth part (ppm) butthis requires a very high concentration on the surface or an exceptionally long expositiontime. For the strongest spectral bands one may obtain relative atomic concentrations inporcent that are 90 - 95 % of the true concentrations, but in normal samples typically80 - 90 %. For weak bands whose intensity is 10 - 20 % of the intensity of the strongestbands one can expect a reliability of 60 - 80 %. The anslysis time varies between 1 and 10min. Under such standrad conditions one will be able to measure the concentrations in amulticomponent system or determine the chemical shifts. The detection limit is 0.1 - 1.0% in number of atoms. Under optimal conditions one can reach a detection limit of 100ppm but this requires a measuring time of several hours (8 - 16 h).

The dimension of the analysis spot is typically 10 - 200 µm.


VII.8. Examples of ESCA spectra

In X-ray photoelectron spectroscopy one primarily measures the kinetic energy of theelectrons. The spectrum is therefore often presented using this energy scale. However, thequantity characteristic for the elements is the binding energy of electrons to the atom ormolecule. This quantity can be determined on the basis of the kinetic energy as

Ip = Ein − Ekin − Φ. (V II.11)

Before the spectrum can be analysed, the binding energy corresponding to each band mustbe determined. Therefore the natural choise is to directly show the energies in the bindingenergy scale. A spectrum of silicon in fig. VII.13. llustrates this.

Binding energy (eV)

Cou

ntin

g ra

te

020040060080010000

2

4

6

8

10

Si 2p

Si 2s

O 1s

F 1s

O KLL

C KLL

x104

Fig. VII.13. XPS spectrum of silicon.

The samples in XPS are most often metals or very thin surface coating, e.g., oxide. Insulat-ing materials are usually quite problematic as samples because there will be an increasinglyhigh positive surface charge the longer the experiment continues and this will distort thebackground of the spectrum. Thin filmes can be placed on a metal substrate and then onemay in favorable conditions obtain a reasonably good spectrum of an insulating material.The spectrum of polypropylene in fig. VII.14. is an example of this.


Binding energy (eV)

Cou

ntin

g ra

te

020040060080010000

2

4

6

8

10 C 1s

x105

Binding energy (eV)

Cou

ntin

g ra

te

051015202530350

1

2

3

4

5x102

Fig. VII.14. XPS spectrum of polypropylene.

The photoelectron spectum is sometimes shown in the original energy scale, i.e., as a func-tion of the kinetic energy. In this case the energy scale is inverted. The high backgroundwill in that case be found at the low kinetic energies. The spectrum of gold is shown asan example if this in fig. VII.15. One must of course know the wavelength of the incidentradiation and the work function of the sample in order to be able to identify the bandswhose position still depends on the binding energy, of course.

The quantitative analysis is simplified appreciably if the variable background is removedfrom the spectrum. However, it is not always obvious where the background line must bedrawn. Every manipulation of the spectrum must be made carefully and be documentedin detail. Fig. VII.16. Show an example of manipulating the spectrum.

The most common satellite structures observed in the spectrum are plasmon structures.An example is shown in fig. VII.17.


Kinetic energy (eV)

Cou

ntin

g ra

te

1000 2000 30000

5

10

15

20

Au

4fAu

4d

Au

4pA

u 4pA

u M

NH

4

Au

3d

x103

Fig. VII.15. The spectrum of gold.

Binding energy (eV)

Cou

ntin

g ra

te

0200400600800100012000

3

6

9

12

15

Pd 4pPd 4s

Pd 3d

Pd 3pPd 3s

Pd Auger lines

x104

Fig. VII.16. Removal of background. (a) Original spectrum.


Binding energy (eV)

Cou

ntin

g ra

te

0200400600800100012000

3

6

9

12

15

Pd 4pPd 4s

Pd 3d

Pd 3p

Pd 3s

Pd Auger lines

x104

Fig. VII.16. Removal of background. (b) Manipulated spectrum.

Binding energy (eV)

Cou

ntin

g ra

te

7080901001101201300

3

6

9

12

Al 2pAl 2s

Plasmon structures from Al 2p

x103

Fig. VII.17. Plasmon structures.

electron spectroscopy vii-1...

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