Electron Scattering Experimental Procedures

In this chapter preliminary experimental investigations are introduced to show the performance of the experimental setup.

3.1 Data Collection

The program used to collect data, written in C language, displays a menu of six options. The options are;

1) Collect Data, 2) Display Data, 3) Plot Data, 4) Save/Load Data, 5) Set Up, 6) End.

1. Option 1) is selected to run the program in the collecting mode. When chosen the following prompts (shown in bold letters) will be offered with responses in brackets:

• Number of Scans (0 to 10000, sets number of repeated runs, then press EnterJ).

• Starting Channel (0 to maximum number of scanning channel set in the

parameters, maximum of 4096 channels, EnterJ).

• Finishing Channel (0 to maximum number o f scanning channel set in the

parameters, EnterJ).

• Step Size (number 1: one scan per one channel, number 2: one scan per two

channels and so on. 1 is preferred, EnterJ).

• Time per Channel (l-65535ps) (sets time per channel to count signals, maximum

is preferred, EnterJ).

• Are these Options Correct (Y/N)? (Y for yes then program runs or N for no).

The following two options appear if there are data stored in the Buffer.

• Add to Current Data (Y/N)? (Y for yes or N for no).

• Current Data Not Saved.

Do You Want to Save it (Y/N)? (If Y a prompt to the main menu will occur, then

choose option 4), if N program runs to collect data).

When the number o f scans is finished the PC peeps, press button Q a prompt to the main menu will occur. Choose option 4) to save data.

2. Options 2) and 3) are used to view and plot the data values respectively.

3. Option 4) is used to save collected data on a floppy or hard disk or to load saved data from a disk.

4. Option 5) sets the parameters of the program. When chosen a menu of 3 sub­ options opens: 1) Program Param eters, 2) Comp Ram p P aram eters and 3) End. These options will be discussed in the following subsection 3.1.1.

5. Last option 6) exits the program and returns user to the Windows platform.

3.1.1 Program Param eters

Option 5) is chosen from the main menu to set the parameters of the program. The submenu o f option 5) consists of two set-up sub-options, as shown above, and a third option to exit it.

a) Sub-option 1) Program Param eters sets the range of the channels and the number of ramp outputs and signal inputs. When selected it prompts to the following options:

• Select Maximum Num ber of Scanning Channels Required

1) 512 2) 1024 3) 2048 4) 4096 (sets ramp steps per scan, 3 is selected). • Select N um ber of Ram p O utputs Required

1) 1 O utput 2) 2 O utputs 3) 3 Outputs (number 1 is selected). • Select Num ber of C ounter Inputs Required

1) 1 Input 2) 2 Inputs (number 1 is selected for analyser detector only. Number 2 is for both analyser and reference detector, which is not included). Selected options are redisplayed with the following message

• A re these options correct (Y/N)? (Hit Y for yes or N for no).

b) Sub-option 2) Comp Ram p Param eters, where Comp stands for compensatory, is chosen when the electron spectrometer is used in the enhanced resonance structure mode (see below). It is selected with 2 or 3 ramp outputs. One ramp to drive the bias voltage of the analyser in the energy loss mode and two ramps to provide the compensating voltages for the mid-elements L I5 and L21. The program will ask for the coefficients o f the polynomial fit to the voltages of the above elements, base ramp energy (incident beam energy) and the energy per chaimel.

Once the parameters are decided the program runs. While scanning, the data are added at each channel and stored in the buffer and at the end o f each single scan the program starts another scan. The data collection takes at least 2 hours, depending on the scanning range and number of scans as well time per channel, to accumulate a good spectrum.

In the enhanced resonance structure mode the scattered electron signal is detected as a function of the incident electron energy to determine the enhanced resonance structure o f the target gas. To run the spectrometer in this mode the energy of the electron beam is varied by changing the bias voltage, i.e. the last (LI 6) and first (L20) elements of pre and post interaction region lens stack. As a result the middle elements, L I5 and L21, must be varied otherwise the electron beam is defocused. The voltages on the elements LI 5 and L21 are plotted against the voltages of elements LI 6 and L20 respectively while maximising the current on the outer hemisphere (OH2) of the analyser. The data are fitted using a polynomial function, usually of second order, and the coefficients are used in the program.

3.2 Energy Loss Mode and Transmission Performance of the spectrometer

To detect elastically, inelastically and/or super-elastically scattered electrons it is necessary to apply a ramp voltage between the laboratory ground and the bias voltage of the analyser detector. When the ramp voltage is set to zero (before ramping) the electron primary beam is tuned and focused and elastically scattered electrons are detected. The applied ramp starts at negative voltage and increases to a positive voltage that is symmetric with respect to zero for a full range o f -1.0V to +1.0V. The start point and range of the ramp are determined by the starting and finishing chaimels of the scan, normally -0.3 to +0.7V in this work. In the negative/positive ramp range, -/+AV is applied and electrons with energy gain/loss (super-elastic/inelastic) of eAV are transmitted and focused while elastically scattered electrons and electrons at different energies are de-focused. Thus a scattered electron spectrum is built up at each angle. Energy per channel is obtained by dividing the ramp voltage range by the number of channels.

The ramp is applied to all elements of the analyser detector except first element L20 to avoid introducing an E-field in the vicinity of the interaction region. However, since the following element L21 is ramped, this results in a slight de-tuning

of the first zoom lens of the analyser and a fall off in the transmission of the electrons with large energy loss/gain. Previous work to investigate the transmission o f the spectrometer {Mapstone PhD thesis, 1990) showed that the transmission falls off by less than 1% for incident electron energies higher than 4 eV in the ramp range of -0.3 to 0.5 eV. For low incident electron energies transmission correction is necessary, however, the lowest incident electron energy in this work is 6.0 eV.

3.3 The Scattering Experimental Techniques

In electron scattering experiments two techniques may be employed. First the ‘gas cell’ technique {Kuyatt, 1968), where the experimental chamber is filled with the target gas to a certain pressure. This technique fell from favour because of many disadvantages. To observe a significant signal the pressure has to be increased to a level that might affect the CEM, electron optics and electron beam. Another disadvantage is the wide Doppler broadening effect. The second ‘crossed beam’ technique is widely used in electron scattering experiments. A beam of incident electrons crosses a gas beam effusing from a fine hole, capillary (hypodermic) tube, an array o f capillary tubes or a supersonic nozzle. The advantage of such arrangement is the high gas number density in the interaction region nearly 1 0 0 0 : 1 with respect to the background gas density. Using a hypodermic needle significantly reduces the Doppler broadening {Read, 1975). If a small aperture is used the gas beam will have a cosine intensity distribution and the Doppler broadening is not reduced. The only disadvantage is the determination of the size of the gas beam and hence the overlap volume of interaction with the electron beam.

In both techniques the energy, including energy spread, and angular distributions of the scattered electrons by target molecules are determined as functions of the energy of the incident electron beam.

3.3.1 Doppler Broadening

Doppler broadening in the energy spread of a monochromatic electron beam arises from their interaction with thermal motion o f the target gas molecules. The

3.1

where y=ml{m+M), m is the mass of the projectile electron (5.5x10"^ amu). M is mass of target molecule in amu. k is Boltzmann’s constant (8.625x10'^ eV/K), T is temperature of target gas in Kelvin and Eo is energy of incident electron in eV.

Since w « M th e n {m+M) % Mand using masses in atomic mass units (amu), T at room temperature (290K) equation 3.1 can be approximated to

= 1 2 .3 ^ ^ (meV)

/2 V M 3.2

The maximum Doppler broadening occurs for large Eg and/or small M. For water molecules tV/=18 amu and for Eg=20 eV the addition in incident electron energy spread is 13 meV at FWHM which agrees very well with experimental observation, see figure 3.3. la for Doppler broadening in the presence of water vapour.

Using the crossed beam technique, by using a hypodermic needle, reduces the broadening in the electron beam by a factor of ‘/aAe {Read 1975). The reduction factor Ab is the full angular range (in radians) at half-maximum intensity of the gas beam

{Lucas 1972) as illustrated in figure 3.3.1b.

Using a long narrow hypodermic needle will produce a gas beam, which has a cosine ‘number densit} ’ distribution of the power n that is a function in I d, provided the gas pressure behind the hypodermic needle is fixed. Therefore Ab is a function of hypodermic needle dimensions and hence the Doppler broadening.

S

1

Channel num ber (0.9m eV /Ch)

700

F ig u re 3.3.1a A com parison o f tw o electron energy loss spectra. The FW HM is 37 and 52 meV in the absence (solid blue) and presence (dashed red) o f w ater vapour respectively. The 15 meV difference is mostly due to D oppler broadening and partly due to rotational broadening.

Hypodermic Needle

F ig u re 3 .3 .l b A sketch showing the full angular range (A b ) at half­

m axim um density o f the gas beam effusing from a hypodermic needle

Background Pressure boundary Iso-density contour oc cos"(0)

Maximum Gas density Maximum Gas density Virtually zero Gas density

3.3.2 Single Collision Condition

In electron scattering experiments it is very important to avoid multiple electron collisions with target molecules. The condition for the determination of the cross sections is that the electron must undergo a single collision. Therefore it is necessary to find the maximum gas flow at which single collisions occur. This is done by measuring the count rate of the scattered electrons, at fixed incident electron energy and scattering angle, as a function of the background gas pressure. Gas is admitted into the chamber through the hypodermic needle and scattered electron signal is recorded with the background pressure as shown in figure 3.3.2a.

I