Vacuum System

Two oil diffusion pumps, Edwards E04 and E06, were replaced by a single ( 6 inch) turbo-molecular pump, Leybold 600 C. The turbo pump is capable of pumping 600 l.s '\ at a speed of 36000 rpm, more than the 450 l.s’^ of the two Edwards diffusion pumps. This allows us to reach a background pressure of < 2x10'^ Torr compared to 7x10'^ Torr with the diffusion pumps.

An additional advantage of the turbo pump is that it can be left running for long periods while the diffusion pump system requires a periodic check of the liquid nitrogen coolant that condenses the oil vapour to prevent it from reaching the experimental chamber. So with the turbo pump there are no fears from oil vapour that may creep and deposit on the electrostatic lenses; charged droplets of oil produce electric fields that will deflect the electron beam.

The turbo pump is backed with an Edwards ED330 rotary pump via a 1 inch flexible stainless steel pipe. A magnetic valve and a sorption trap, containing alumina pellets, were introduced between the turbo pump and the rotary pump with the sorption trap directly after the rotary pump. The sorption trap prevents rotary pump oil contaminating the high vacuum region.

The magnetic valve is connected to a single line on/off switch inside the N20 turbo pump controller unit. The N20 unit is a frequency converter required for the operation and control o f the turbo pump. The N20 unit automatically shuts down the turbo pump and the magnetic valve in case o f any failure. An external on/off switch is added to manually switch off the magnetic valve, if needed, while the rotaiy and the turbo pumps are still running.

Two Pirani gauges are mounted on either side o f the magnetic valve to monitor the pressure in the roughing line. The first Pirani gauge, between the rotary pump and the magnetic valve, trips on (switches on) the N20 unit when the pressure drops below 0.05 torr. The second Pirani gauge, between the magnetic valve and the turbo pump, trips off the N20 unit (and the magnetic valve) if the pressure in the roughing line rises above 0.05 torr.

^ NV: N eedle V alve ( 3 ) V: Valve

MV; M agnetic V alve PG: Pirani G auge IG: Ionization G auge PT ; Pressure Transducer PBP: Prim ary B ase Plate SBP. Secondary B ase Plate STF: Sorption Trap Filter S.S: Stainless Steel...

Figure 2.1a A sketch o f the experim ental chamber.

Calorim eter

Dom e Cover

Hem ispherical lalyser Inner and outer

mumetal shields H ypoderm ic

Needle Cone Lens » Screen

s s /

I

J* . Rotating ^ Analyser

Support R otary Feed

Table Through

W indow

Pinion

Turbo Pump _Tilted

M irror 650 I s W ater V2 VI Flexible Backup Pump Gas Line Pump S.S Pipe

Mumetal Boxes

' Ë ,

Mumetal

Figure 2.1b A set o f photos showing the setup o f the electron spectrom eter. Shot (a) show the details o f the m onochrom ator and the analyser, turbo-pum p fans, hypoderm ic needle gasline and the interaction region (IR). Shots (b) and (c) shows the double m um etal shielding stages, m um etal boxes and lid. Shots (d) and (e) show an overall view with most o f the outer com ponents from below and from above respectively.

The rotary pump is controlled separately from the N20 unit. While pushing the start button on the N20 unit will start the turbo pump and open the magnetic valve. A time delay, up to 5 minutes, is set before the start of the turbo pump. The time delay procedure is crucial because the rotary pump will not compete with the roughing speed o f the turbo pump. Moreover the turbo pump runs easily and accelerates faster with a low-pressure in the experimental chamber.

The pressure in the experimental chamber is measured by Edwards, type IG3, ionization gauge, mounted to the primary base plate. The pressure in the tank drops to 2x10'^ Torr in two days while the chamber is baked by 4x150 Watts, 24 V, halogen bulbs. In order to obtain a good low pressure, the components and the inside o f the experimental chamber are cleaned thoroughly by acetone, mainly to ensure there are no grease and oil deposits. Once the pressure drops below 1 10'^ torr the electronics that control the electrostatic lens potentials, the channeltron power supply and the data acquisition system are tripped on as long as the ionisation gauge is on. If the pressure reading on the ionisation gauge exceeds 1.5 times the full scale the above system is tripped off to protect the channeltron and the electron gun filament.

The system is protected by a series of electrical trips. If the cooling water pressure drops, mains power fails and/or air leaks the whole system is switched off.

2.4 The Secondary Base Plate (SBP)

Since the mumetal shield cannot be strained, as this will destroy its properties, a large fiat smooth circular aluminium secondary base plate (diameter 620 mm) was manufactured to hold the monochromator and the analyser. The secondary base plate is bolted to the primary base plate while lifted by eight aluminium pillars that pass through eight holes in the mumetal. Where necessary ports were cut through for pumping. In the centre a 95 mm port was cut to allow the cylinder o f the interaction region to be firmly and directly fitted to the primary base plate. The monochromator

that consists of the electron gun, hemispherical energy selector and

accelerating/decelerating lenses is bolted to the secondary base plate while the analyser rotates on the secondary base plate, see figures 2.1a and 2. lb.

2.5 The M echanical Design of the Analyser

The non-magnetic stainless steel cylinder o f the interaction region, 330 mm long of diameter 55.8 mm and wall thickness of 4.3 mm, acts as the pivot for the phosphor-bronze bearing of the analyser rotating table and provides the location for the hypodermic needle gas source.

The rotating table that holds the analyser and its components is bolted to the phosphor-bronze bearing that is free to rotate about the cylinder. The outer edge o f the rotating table, 268 mm from the interaction region cylinder, is supported by two bearings. To be able to rotate the analyser, a large gear wheel is bolted to the base of the phosphor-bronze bearing and chained to a manually driven small pinion. The pinion is mounted on the axis of the Vacuum Generator rotary feedthrough (VG model RD3). The external part of the rotary feedthrough is marked with an angular scale and the ratio of rotation between the pinion and the gear wheel is 8:1. A third gear wheel, mounted in the same plane of the other two, can be adjusted to set a proper tension on the chain. All gears and the chain are made from non-magnetic stainless steel and are located between the primary and secondary base plates.

The secondary base plate and the gearing system allow the analyser to freely rotate about the interaction region cylinder from - 5 to 140 degrees.

2.6 The Gas Line

The main gas under study is water vapour. Water exists in the liquid state under normal temperature and pressure with gases (mainly air, which consists mainly N2, O2 and others) dissolved in it. Water vapour was obtained from a 50 ml ampoule containing de-ionised distilled water. The easy and effective way to remove dissolved gases from water is to put the ampoule under low pressure. The metal neck o f the ampoule was connected to the gas line which is pumped down using an Edwards ED75 rotary pump (see figure 2.6a).

Under low pressure the water in the ampoule boils at room temperature and the residual gases bubble and escape, hence the dissolved gases are pumped out. The ampoule is left under vacuum for -2 0 minutes, after the water stops bubbling, to maximise the purity of the water and to flush the gas line. The pressure in the gas line is monitored using an INFICON Pressure Transducer (type CM120-G100A), which is

calibrated against the Pirani gauge as shown in figure 2.6b, in conjunction with a digital voltmeter (DVM) to check for leaks into the gas line. After a day of pumping the DVM reads 0.026 V (2.5 10'^ Torr) indicating optimum vacuum has been obtained. Valve V4 is closed to separate the ampoule and the needle valves (NV 1,2,3) from the gas line rotary pump.

\ alv e V4 S u p p o r t e r / Ga.s line •fo mm.' o Readings Fitting 1 0Û y=4.31 x"^"' exp( x >-1.3 5 0,1 0.05 0.10 0.15 0.20 0.25 0

Pressure Transducer Readings (V) Figure 2.6b Calibration with w ater vapour o f the pressure transducer (very sensitive at high pressures) against the Pirani gauge that is sensitive at low pressures. Throughout the experim ent w ater vapour pressure in the gas line reads

Figure 2.6a A photo o f the gas line setup 0.045 V that is equivalent to 0.3 Torr. Water vapour is introduced into the experimental interaction region through needle valve NV2. Water vapour effuses from a capillary tube (hypodermic needle, 10 mm long and 0.5 mm internal diameter). The capillary tube is made from demagnetised stainless steel and soldered to a 6 mm diameter flange, which is bolted to the interaction region cylinder. The end of the capillary tube is only 3 mm from the centre of the interaction region. To ensure that the background gas pressure does not build up inside the cylinder, 2/3 of the cylinder wall was cut away. With water vapour effusing from the hypodermic needle the background pressure in the experimental chamber increases from 2x10'^ to 2x10'^ Torr as measured using the ionisation gauge (Edwards, type IG3), while water vapour pressure in the gas line reads 0.3 Torr.

Water vapour can also be fed into the tank through needle valve NVl of the side leak pipe for the measurement of the scattered electrons by background gas without the presence of the gas beam.

2.7 The Electron Lenses

The electron spectrometer consists o f three main parts, the electron gun monochromator, the interaction region and the electron analyser detector. The spectrometer is designed to make measurements of elastic and inelastic differential cross sections (DCS). The range of the incident electron energies is 3 to 20 eV and detected over a range of scattered angles from 10 to 135 degrees. Good angular resolution (< 2°) and energy resolution (50 to 60 meV) are essential to avoid overlapping between different excitation processes.

The spectrometer uses electrostatic lenses that have a similar analogy to optical lenses. They are used to focus/defocus electrons and are defined by reference, principal and focal planes. These lenses can be cylinders or apertures. Many studies

{Simpson 1964, Read et al 1974 and Brunt et al 1977) were published on the design methods of high-resolution electron spectrometers.

In the experiment all electrostatic lenses are titanium cylinders of 10 and 15 mm inner (D) and outer (H) diameters respectively. Cylinders are made of titanium because the reflection o f electrons on the inner surface o f titanium is minimal while other materials have to be sooted. The cylindrical lenses are easily fastened to carefully machined optical benches and are electrically isolated from the benches by ceramic rods. Each cylindrical lens is clamped in position using stainless steel studding insulated from the optical benches by ceramic spacers, as can be seen in figure 2.7a.

Electrostatic Lenses are usually formed o f two (double-element lens) or three (Einzel or triple-element lens) cylinders. In the double element lens the focal (fi and

fj) and mid focal (Fi and F2) lengths are functions of the voltage ratio V2/V1. In the case o f the triple element lens the parameters are functions of two independent voltage ratios V2/V1 and V3/V1. Vi, V2 and V3 are the potentials applied to the cylinders o f the electrostatic lens, listed in the direction of travel of the electron beam. A symmetric lens is a special case where Vi= V3, then the lens parameters are again a function o f only the ratio V2/V1. An asymmetric lens is the general case where

Harting and Read (1976) have published data tables, listing the parameters for different types of lenses as a function o f voltage ratios.

Figure 2.7b shows a good lens design. The length. A, o f the middle element (central cylinder) o f the triple lens is half the inner diameter (A = 0.5xD). While the

length of outer elements is at least IxD and the spacing or gap between cylinders is G = O.lxD to ensure there is no field penetration. The advantage of triple-element lenses over double-element lenses is the fine-tuning affected by adjusting the voltage of the middle element without affecting the voltage ratios of the following and preceding lenses. Ceram ic Rods Stud Ceramic Spacers PTFE Insulation D eflectors

Figure 2.7a Two

drawings o f the

optical bench that holds the cylindrical lenses. V2>V1 D e'T rajectory f Equipotential Contours Double-element Lens Reference Plane (RP) e' Direction

T

D Optical axis D = 10 mm H = 15 mm A = 4 or 5 mm G = 1 mm Triple-element Lens V3>V1 - H

Figure 2.7b Double and triple cylindrical lenses show ing the reference planes and the trajectory o f an electron ( e ) in an E-field H arting a n d R e a d { \9 1 6).

2.7.1 Imaging of an Electron Beam

Focusing of electron beam relies on the principle of deflecting electrons by an electrostatic electric field (or a magnetic field). The trajectory of the deflection depends on the strength and direction of the electrostatic electric field (E-field). Therefore a source of electrons can be imaged as the electrons pass through regions at different potentials due to deflections in the changing E-field.

Figure 2.7.1a shows the path of an electron in a double-element lens. For convenience the E-fields were replaced by the equipotential contours that are always perpendicular to the E-field. The focusing properties of this type of lens is a function of the voltage ratios, as mentioned before, plus G/D the ratio of the gap to the lens’s inner diameter. For a triple-element lens the focusing property becomes a function of A/D, the ratio of the mid element length to the inner diameter, in addition to the voltage ratios and G/D.

RP: Reference Plane PP. Principal Plane PF: Principal Foci f. Focal length F: M id-focal Length

— -I

Optical Axis PF, P P2 RP PP, PF,

F ig u re 2.7.1a A drawing showing how the PF and PP are located with respect to the RP. The RP is located through the centre o f the gap in double cylinder lens and through the centre o f the m id­ elem ent lens in the triple cylinder lens {Harting a n d Read, 1976).

Once the cardinal points of an electrostatic lens are defined the image {Q) of an electron beam source (object F) can be predicted. The location of the principal foci (focal length f\ and fi) and the principal planes (mid-focal length F\ and F i\ as illustrated in figure 2.7.1a, determine the cardinal points. Newton's equation of a thick lens can now be applied to determine the relationship between the object and the image distances

OP - 2.1

where F and Q are measured from the reference plane (RP).

The linear magnification {M) of the image, an important relation that allows us to determine the electron beam size, is defined by

M = ^ A , = . - ( g .r L .) 2 2

The divergence of the electron beam at the image is a factor that has to be counted for. The divergence relation is derived from the Helmholtz-Lagrange law, as illustrated in figure 2.7.2b

n sin(6*i) = ^ 2 sin(6^) Vf/2 2.3

where r\ and ^2 are the heights of the object and the image from the optical axis respectively, and O2 are the half angles of the rays at the object and the image

respectively and U\ and Ui are the regions of fixed potentials at the object and the image respectively. It is obvious that a very small image size results in a very big divergence.

Therefore the design of the electrostatic lenses, mainly the length of the cylinder, must count for the image size and the divergence of the beam. This is crucial in order to obtain a parallel focused electron beam with a low filling factor (50% filling factor). To facilitate such a property apertures, named as windows (W) and pupils (P), are introduced to define the object and the image of the electron beam (see figure 2.8.2a). Pupils define object and image positions and sizes while windows limit the divergence of the electron beam.

2.7.2 Aberrations

Electron beams suffer from aberrations due to non-monochromatic electron energies and non-paraxial electron trajectories. Equations 2.1 and 2.2 are applicable to paraxial electron trajectories; therefore the electron beam object and image are generally larger than that calculated using the above equations. This difference is known as aberration of the image. It is very important to count for this effect as it can lead to over 100% increased image size and a 100% filling factor.

The full aberration of an image results from a sum of many aberrations that can be divided into two main groups: Aberrations due to 1) Geometric Lens Errors and aberrations due to 2) Electron Beam properties.

D isc o f Least Confusion RP

F ig u re 2.7.2a Aberration o f an axial object (P) at th e im age ( 0 . RP is the reference plane and Ar is the radius o f spherical aberration {H arting a n d Read, 1976).

1) Aberrations due to Geometric Lens Errors: These are five types of

aberrations. Four of them (coma, astigmatism, curvature of field and distortion) are difficult to quantify as they result from the finite size of the image. Brunt and Read

(1975) showed that they contribute 30% at most to spherical aberration, which is formed by the non-paraxial electrons, as illustrated in figure 2.7.2a. Electrons

diverging from the same source (object) are focused at different points along the optical axis o f the electrostatic lens. The radius (Ar) o f the spherical aberration can be calculated using the formula, Klemperer and Barnet (1971),

Ars = MCsO^ 2.4

where M from equation 2.2 is the linear magnification, a is the maximum half angle of the electron divergence at the object and Cs is the spherical aberration coefficient which is a function of both the object and image distances.

Keeping the filling factor at less than 50% will result in a magnitude of aberration less than 3% o f the lens diameter. Filling factor (p) is the ratio of the maximum distance between the optical axis and the extrapolated incident electron trajectories to the radius o f the lens. Note that magnitude o f spherical aberration

In document Electron and photon interactions with molecules (Page 44-56)