_
70
.
,
4.
PHOTOELECTRÏC EMISSION
-
is also true When many wave—lengths of light are involved, provided
the relative distribution of their energies remains unaltered as the
'
intensity is changed. Apparent departures occur in photoelectric
cells because of negative surface charges which form on the inside
of the enclosing bulb at varying rates. The effect of these charges
is reduced or avoided when the sensitive surface is mounted on a
separate support away
from the walls or when the photo—active
'
material covers all of the inner walls of the bulb; except, of course,
the clear spaces for the light to enter and for the lead wires.
The number of electrons emitted each second by a given sur;
face varies in a complex manner with the color or wave-length )\
’ _
of the incident light, as illustrated in gure 4—5. Here, the
”
yield
”
or
S, represents the photoelectric currents
reduced
to values which they would have if the light were of constant
intensity iii all parts
of the spectrum, i.e.,
,
'
S = i/I.
(4—2)
The various curves shown here should not be compared in numerical value; their general shape alone has signicance. One may
note that there are large photoelectric currents (per unit of light
intensity) at certain faVored wave-lengths. This so-called spectral
selectivity or preference for certain spectral regions is different for
different sensitive surfaces.
Compare the curves for potassium
_
(K) and cesium (Cs). It is also different for a given material sub-,
to different treatments during the preparation of the,cell.‘
;” ,!
Thus, as shown in the gure, 8. cesium surface on cesium oxide on a
”_”
'
silver base (Cs on CSO on Ag) has a" totally different spectral
:
j distribution curve than et pure cesium surface. In like fashion,
Potassium.
potassium hydride surfaces differ markedly from
each
A potassium hydride surface may be made Which 13
stable than an untreated potassium layer
_
types of cells are preferable for use in the ultra—'
V101€t>0thersmthe infra—red and others in the region of maximuni
;éî=—;!ÏeYeS€nSltlVïtY(about5555angstromS)
‘
-'
“this photoelectric
70
.
,
4.
PHOTOELECTRÏC EMISSION
-
is also true When many wave—lengths of light are involved, provided
the relative distribution of their energies remains unaltered as the
'
intensity is changed. Apparent departures occur in photoelectric
cells because of negative surface charges which form on the inside
of the enclosing bulb at varying rates. The effect of these charges
is reduced or avoided when the sensitive surface is mounted on a
separate support away
from the walls or when the photo—active
'
material covers all of the inner walls of the bulb; except, of course,
the clear spaces for the light to enter and for the lead wires.
The number of electrons emitted each second by a given sur;
face varies in a complex manner with the color or wave-length )\
’ _
of the incident light, as illustrated in gure 4—5. Here, the
”
yield
”
or
S, represents the photoelectric currents
reduced
to values which they would have if the light were of constant
intensity iii all parts
of the spectrum, i.e.,
,
'
S = i/I.
(4—2)
The various curves shown here should not be compared in numerical value; their general shape alone has signicance. One may
note that there are large photoelectric currents (per unit of light
intensity) at certain faVored wave-lengths. This so-called spectral
selectivity or preference for certain spectral regions is different for
different sensitive surfaces.
Compare the curves for potassium
_
(K) and cesium (Cs). It is also different for a given material sub-,
to different treatments during the preparation of the,cell.‘
;” ,!
Thus, as shown in the gure, 8. cesium surface on cesium oxide on a
”_”
'
silver base (Cs on CSO on Ag) has a" totally different spectral
:
j distribution curve than et pure cesium surface. In like fashion,
Potassium.
potassium hydride surfaces differ markedly from
each
A potassium hydride surface may be made Which 13
stable than an untreated potassium layer
_
types of cells are preferable for use in the ultra—'
V101€t>0thersmthe infra—red and others in the region of maximuni
;éî=—;!ÏeYeS€nSltlVïtY(about5555angstromS)
‘
-'
“this photoelectric