8/9/2019 Spektroskopia fotoelektron

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Photoelectron and

RelatedSpectroscopies

8.1 Photoelectron spectroscopy

Photoelectron spectroscopy involves the ejection of electrons fromatoms or molecules following bombardment by monochromaticphotons. The ejected electrons are called photoelectrons and werementioned, in the contet of the photoelectric e!ect, in Section 1.". Thee!ect was observed originally on surfaces of easily ioni#able metals,such as the al$ali metals. %ombardment of the surface with photons of tunable fre&uency does not produce any photoelectrons until thethreshold fre&uency is reached 'see (igure 1."). *t this fre&uency, n t,

the photon energy is just suf+cient to overcome the wor$ function ( of the metal, so that hnt ( -81/

*t higher fre&uencies the ecess energy of the photons is convertedinto $inetic energy of the photoelectrons

1 v"hn ( 0m-8"/e"

 where mand v are their mass and velocity.

e

or$ functions of al$ali metal surfaces are only a few electronvolts1 sothat the energy of near ultraviolet radiation is suf+cient to produceioni#ation.Photoelectron spectroscopy is a simple etension of the photoelectrice!ect involving the use of higher2energy incident photons and appliedto the study not only of solid surfaces but also of samples in the gasphase. 3&uations '8.1) and '8.") still apply but, for gas2phasemeasurements in particular, the wor$ function is usually replaced by the

ioni#ation energy 4 5" so that 3&uation '8.") becomes1hn 4 0mv" -86/

e"

 

1 71 711e 9:.;8< $= mol8>:<.<;cm." This is often referred to as the ioni#ation potential, but since 3&uation '8.6) shows

that 4 hasdimensions of energy, the term ioni#ation energy is to bepreferred.

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(igure 8.1 Processes occurring in 'a) ultraviolet photoelectron spectroscopy

'?PS), 'b) @2ray photoelectron spectroscopy '@PS) and 'c) *uger electron

spectroscopy '*3S)

3ven though 3instein developed the theory of the photoelectric e!ect in19>: photoelectron spectroscopy, as we now $now it, was notdeveloped until the early 19:>s, particularly by Siegbahn, Turner andPrice.(or an atom or molecule in the gas phase (igure 8.1 schematicallydivides the orbitals Aatomic orbitals '*Bs) or molecular orbitals 'CBs)Dinto core orbitals and valence orbitals. 3ach orbital is ta$en to be non2degenerate and can accommodate two electrons with antiparallel spins.

 The orbital energy, always negative, is measured relative to a #ero of energy corresponding to removal of an electron in that orbital to in+nity.

 The valence, or outer2shell, electrons have higher orbital energies thanthe core, or inner2shell, electrons. * monochromatic source of soft 'low2

energy) @2rays may be used to remove core electrons and the techni&ueis often referred to as @2ray photoelectron spectroscopy, sometimes, ashere, abbreviated to @PS. (ar2ultraviolet radiation has suf+cient energyto remove only valence electrons and such a source is used inultraviolet photoelectron spectroscopy, or ?PS.*lthough the division into @PS and ?PS is conceptually arti+cial it isoften a practically useful one because of the di!erent eperimentaltechni&ues used.*cronyms abound in photoelectron and related spectroscopies but weshall use only @PS, ?PS and, in Sections 8." and 8.6, *3S '*ugerelectron spectroscopy), @R( '@2ray Euorescence) and 3@*(S 'etended@2ray absorption +ne structure). 4n addition, 3SF* is worth mentioning,brieEy. 4t stands for Gelectron spectroscopy for chemical analysisH inwhich electron spectroscopy refers to the various branches of spectroscopy which involve the ejection of an electron from an atom or

molecule. Iowever, because 3SF* was an acronym introduced bywor$ers in the +eld of @PS it is most often used to refer to @PS ratherthan to electron spectroscopy in general.

8.1 PIBTB3J3FTRBK SP3FTRBSFBPL "91

(igures 8.1'a) and 8.1'b) illustrate the processes involved in ?PS and @PS.%oth result inthe ejection of a photoelectron following interaction of the atom or molecule C

which isioni#ed to produce the singly charged

C0.

C 0hn MC00e -8;/

8.1.1 3perimental methods

(igure 8." illustrates, in symbolic fashion, the general arrangement of the components of an ultraviolet or @2ray photoelectron spectrometer'but see (igure 8.< for actual spectrometer designs). hen the samplein the target chamber is bombarded with photons, photoelectrons areejected in all directions. Some pass through the eit slit and into theelectron energy analyser which separates the electrons according totheir $inetic energy, rather in the way that ions are separated in a massspectrometer. The electrons pass through the eit slit of the analyseronto an electron detector and the spectrum recorded is the number of 

electrons per unit time 'often counts s

71

) as a function of eitherioni#ation energy or $inetic energy of the photoelectrons 'care issometimes needed to deduce which energy is plotted they increase inopposite directions for a particular spectrum). Jow pressure must be

maintained in the analyser 'ca 1>7<

 Torr) and in the target chamber 'ca

1>7"

 Torr).