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From this table, it is clear that by increasing the angle of incidence between the target's emitting face and the impinging laser radiation, the percentage of reflected light generally increases for p-polarised light but varies considerably for s- polarised light. At the angles of incidence used in this work (approximately 80 to 85° ), the efficiency of reflection for p-polarised light is greater than that for s- polarised light. Also, by increasing the wavelength of the incident radiation (and hence decreasing the photon energy) the reflectivity efficiencies increase for both polarisation angles. However, the proportion of light reflected with p-polarised light incident on the metal is always greater at a fixed wavelength than with s- polarised light.

From this, it may be concluded that for a sample moimted in the vertical plane, s-polarised light will be absorbed by the surface more efficiently than p-polarised light. A higher percentage of absorbed light infers a higher surface temperature and hence it would be reasonable to assume that thermionic emission processes should be enhanced when the laser radiation is incident parallel to the surface.

? 1). À=620nm | 1 2). 0=60° 0° R % Rn% À/nm R,% R,% 0 8 8 . 0 88.3 500 683 33.2 2 0 8&4 87.7 540 833 59.5 40 91.2 8 6 . 2 580 90.8 74.3 60 94.2 81.5 660 94.3 84.8 70 97.2 80.4 80 98.7 82.8 85 97.8 90.1

{Table 2.5} The reflectivity of a gold surface at 1). varying angles of incidence and 2). varying wavelengths of incident radiation on a bulk metal

spectra. Thermionic emission from the target surface may become significant even if the average temperature rise of the illuminating target surface is low. This is because very high local temperatures may occur on the metal surface because of microscopic roughness {Bunkin & Prokhorov (1967)). Whenever the interaction chamber was vented to air, the target surface was checked for signs of surface degradation. In the first instance this was carried out by eye. The surface structure of the gold target was also investigated using a surface profiler known as 'ProScan 1000'.

ProScan 1000 is a non-contact 3D measuring system for automatic and rapid inspection of surfaces. It employs state-of-the-art laser-based displacement probes for z-axis measurements, achieving accuracies down to 0.01 pm. Its high precision stepper motor table is used for x and y position. General features include: a 3D isometric view of the scanned component with the relative height shown in colour bands; a plan view of the scanned component with the relative height shown in colour shades and cross-section profiles of the x- and y-axes.

Figure 2.5 shows a typical spectrum for the gold surface taken with the surface profiler. It is immediately apparent that the target face was not as smooth as the human eye perceives. Not only is there a microscopic roughness throughout its cross-section, but also an underlying feature that shows that the surface was shghtly curved. This image was taken after the sample had been subjected to many hours of intense laser radiation, and there is no evidence of surface damage by the laser that can be distinguished from the background roughness. Improvement of the smoothness of the surface would have required other methods of preparation not available for this work, e.g. electro-mechanical polishing.

82 CO T C J t-1 CL, o m " i CO !'■ l - l = 3 m 7~l <L M (U i K s-< ; <H u 1 hJ m m <L 1 - f»1 (M ; K <C Î m 1 c n : CO i (U p . , i z : ; < r ! CO ] O 1 CO Z ; m I t , m m !>■ M M J i CO INI 2 ‘ i f&, : i r d m ■ n Z : m l \ <r U i Oh £ CO : , J (M o < M f t ; X 0-; ^ ; X ■•I I K '■ S "Zi \ 1E 1 - ’.-i 1E I n U H I C O ' ' 1X &

{Figure 2.5} The surface profile, taken with 'Proscan 1000', of the emitting face of the gold target after it had been subjected to hours of intense laser radiation.

two energy analysers used are also provided, together with a discussion of their operational characteristics. The section begins with a brief guide to the terms and principles used in designing an electron optical system.

2,4,1 The Physical Properties O f An Electron Beam

Electrons travelling together in the form of a beam may be described by a number of physical terms; window, pupil position, beam angle and pencil angle', which characterise their spatial distribution and angular divergence. In this section, the terminology used to define an electron beam is described.

The spatial and angular divergence of any focused electron beam may be determined using the Helmholtz-Lagrange law. The Helmholtz-Lagrange law states that the brightness of an image cannot exceed the brightness of an object. If an image of radius rz, at a potential Vj, with pencil angle 0^ is to be produced from an object of radius r,, potential V^, and pencil angle 0 |, then the Helmholtz- Lagrange law requires that {Figure 2.6};

(Vi)’\is in 0 i = (V 2)\sin02, (2.4)

or, for paraxial rays where 0 is small and hence sin 0 ~ 0 ,

(T/i)%fi0, = 0/:)%f202. (2.5)

In order to minimise the filling factor of a lens, the angular divergence of the electron beam must be kept small. This can be achieved by placing two apertures

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