4—11.
WORK FUNCTION AND POTENTIAL‘BARRIER 83
Values of the work function for certain materials are listed
in Table 1 at the end of this book.
They range from 1.8 to 6.3
volts per e.s.u., for various outgassed materials. However, the
value which is
found for any given material depends greatly on
the condition of the surface.
It is different for rough or smooth
surfaces and is greatly affected by
presence of gases.
It is
smaller for surfaces exposed to such electro-positive gases as are
'
readin adsorbed.
Thus, larger photoelectric currents may be
obtained from surfaces exposed to these gases.
The effect of
easily adsorbed electro-negative gases is just the reverse.
so—called “
fatigue
”
observed in some photoeleètric cells is due 'to
changes in the gas impurities on the surface; a clean metal in a
high vacuum shows no fatigue. Further, the addition of a very
thin layer (perhaps only one atom thick) of other materials over a
surface can greatly increase the photoelectric current.
Alkali
metals and alkaline
very effective, especially when
'
treated with certain gases, such as hydrogen Or
In these
_
cases
the surface layers are hydrides or pure metals on oxides,
for example, potassium hydride or cesium on cesium oxide on a
silver base.
'
'
Experiments have shown that the contact potential difference
between two surfaces in a vacuum, separated* from each other, is
_
eqùal*to the difference of their work functions. Thuè,*
_
_
"
K=q> — 44".
,
-
(4—16)
-
Let us assume that- there : are f free electrons moving _ among
,
the atoms of a'm6tal.1 ‘
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