SOLUCIONES A ALGUNOS PROBLEMAS AMBIENTALES EN LA

As d e scribed in c h a p te r o n e a to m ic a n d m o le cu la r states are se p a ra te d by 100s o f m eV a n d m o le cu la r vibrations typ ica lly by 10s o f m eV. Therefore if m o le cu la r vibrational transitions a re to b e investigated sp e ctro sco p ica lly the resolution o f th e e lectron b e a m p ro d u c e d a n d the d e te c tio n o f s ca tte re d electrons should also b e o f th e o rd er o f 10s o f m eV. H ow ever th e e n erg y spread o f electrons leaving the filam e n t is -5 0 0 meV. A filter lens ty p e o f e le ctro n gun (Mason 1986) m a y re d u c e this to 100 or 200 m eV, but to low e r th e resolution further th e e lectron b e a m has to b e ‘m o n o c h ro m a te d '.

A 180° hem ispherical analyser is used to re d u c e th e energ y spread o f a b e a m o f electrons, figure 3.6.

Chapter Three Outer Hemisphere Inner Hemisphere HO

Figure 3.6 : Schematic Diagram of a 180° Hemispherical Analyser

A beam of electrons entering the hemisphere is steered through 180° by setting voltages on the inner a nd outer sections ot the hemisphere. These are c a lc u la te d using the ratio ot the sphere diameters tor the inner hemisphere (HI) and the outer hemisphere (HO):

H I_{diam e te r} H O HO diam eter diam eter H I X E lec tro n P a s s E n e rg y = X E le c tro n P a s s Energy' - 3.6) d iam eter

At the entrance and exit ot the hemispheres it is necessary to include field correction rings in order to re d uce the effects ot field distortion as the electron beam passes from an electron gun into the hemisphere. The electrostatic field p roduced by the lens stack is perpendicular to that ot the analyser causing fringing fields at the e ntrance and exit ot the hemisphere, disrupting the beam ot the electrons. To correct tor this aberration tw o rings are set to voltages either side ot the mean path energy ot the electrons around the analyser, but less than the voltages applied to the inner and outer hemisphere.

The resulting electron beam em erging from the hemisphere has a reduced energy spread. The resolution ot the beam AEj/2 (Full Width Halt Maximum) can

be determ ined using equation 3.7. (Read 1974).

A E y

W

Chapter Three

which the beam enters the hemisphere whilst a and b are param eters d e p e n d e n t upon the dimensions of the hemispheres and the size of the entrance and exit holes. For the energy selectors in the UCL electron energy loss spectrom eter the values of these param eters are 0.82 a nd 0.24 respectively (Mason 1987a)

From equation 3.7 it can be seen that in order to obtain a high resolution beam the pass energy, should be low a nd the m ean path of the electrons, R,

should be as large as possible. The reduction in size of the entra nce a nd exit apertures (fV) and ensuring that the b eam enters with angle a close to zero also help to im prove the resolution of the exit electron beam . The overall resolution of the spectrom eter is d e p e n d e n t upon both the electron m o n och ro m a to r and the electron analyser hemispheres and is ca lc u la te d using equation 3.8

(3.8) where AEm and AE^ are the resolution of the electron m onochrom ator a nd the electron analyser respectively.

The cylindrical lens elements used to form a lens stock for both the m onochrom ator and the electron analyser are m a d e from titanium, with the apertures m a d e from m olybdenum . Titanium is used due to its non-m agnetic properties and its suitability for high tem perature bake-out of the system. The internal surfaces of the hemisphere elem ents ore c o a te d with soot from on oxyacetylene torch; the apertures and internal surfaces of the lens elements ore c o a te d with 'A quadag', a graphite solution, in order to re d u ce the reflectivity of the elements.

a) M ethod for Lens element b) Arrangem ent o f 02

mounting o f lens deflectors within [j^ e ram ic

elem ents an element

Ceramic Rod — Aluminium Bench ^

Ceramic bush- ___ _

-Wire to feed-through for electrical connection

Figure 3.7 : Cross section to stiow the a) mounting of the lens elements, b) arrangement of deflectors In an element.

Chapter Three

The lens elements are m ounted on to an aluminium bench as shown in figure 3.7a. All lens elements have an external dia m e te r of 15 mm which mokes alignm ent simple. The lens elem ents sit on ceram ic rods in order to electrically insulate them from each other. Studding (diam eter 2.5 mm) passes through the centre of the bench from e a c h lens elem ent to the underside of the bench where they are fixed in p la c e with M2.5 nuts. The studding a nd nuts are isolated from the aluminium bench using ceram ic bushes. The electrical connection is m ode at this point using high tem perature PTFE insulated wire. A ccu ra te separation of the elements is achie ve d using a feeler g a ug e . Figure 3.7b shows the arrangem ent of deflectors included within some elements to allow small corrections to be m ode to the direction of the electron beam .

3 . 3 . T h e E l e c t r o n M o n o c h r o m a t o r

A schem atic diagram of the electron m onochrom ator used in these studies is shown in Figure 3.8. The basic com ponents ore on emission system, on electron gun, a 180° hemispherical analyser and the pre-interaction region optics. The m onochrom ator is m ounted to a m echanism which allows it to be rotated off the 0° axis a b ou t the central axis of the spectrom eter in the range -10° to +50°.

Electron Gun Emission System

HO