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The Lens System

If S1 has the dimension D1, then by theory the imaged area of the sample has the

dimension DS with

(1)

The magnification of the lens stage is selectable (see table 2 on page 16). The magnific-

ation is changed electrically by connecting appropriate voltages to the lens electrodes.

The voltages are a function of the spectrometer voltage U0 which is nominally equal to

Ekin - Epass + workfunction. Ekin is negative for electrons and positive for ions.

The PHOIBOS system can operate in a Fixed Retarding Ratio (FRR) or in a Fixed Analyzer

Transmission (FAT) mode. In FAT mode the applied voltage to the hemispheres is

defined by equation 3 on page 18. In the FRR mode, the pass energy is given by Epass =

Ekin /R, with the retardation ratio R.

The actual size of the analyzer sampling area DS is, in principle, given by equation 1.

However, due to spherical aberration of the input lens, the image in the plane of the

entrance slit is diffused. The degree of diffusion increases for a fixed magnification with

the input lens acceptance angle. This means that the observed area in the focal planes

of the input lens system is smeared out with increasing angle, resulting in larger

sampling dimensions than given by equation 1. Thus the lens acceptance angle is select-

able by the magnification modes, keeping the spherical aberration at a well known, ac-

ceptable value.

A second reason for confining lens acceptance angles are angle-resolved measurements,

e.g. in tilt experiments or angle resolved photoemission investigations. Confining the

lens acceptance angle is also essential in ISS, as the kinetic energy in ISS depends on the

scattering angle, and thus peak broadening or double peaks appear when the lens ac-

ceptance angles are too large.

Figure 6: High Point Transmission Mode

PHOIBOS 13

SPECS PHOIBOS Series User Manual

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