76
4.
PHOTOELECTRÏC EMISSION
‘
positive
ions bombard the cathode so vigorously as to dest‘rOy
”
its sensitive surface in a short time.
4—7;
The Energy of the Photo-electrons.—The kinetic energy
of the ejected electrons may be measured by the use of magnetic
and electric elds, as in the case of thermionic electrons described
'
in chapter 2. The more usual method, however, is to apply a
suiciently negative potential to the collecting electrode to just
reduce the photo-current
to zero.
The critical retarding potential
(Vo of gure 4—7) is proportional to the energy
of the fastest
-
cell, Which Will thus stop the most energetic electrons, must be
correctedby an amount K to give the true energy Vo; The contact
.
potential,2 K, is the inherent potential dierence which exists
\
between the two kinds of material forming the anode and cathode. *
_
It is positive in commercial photo—cells but may be positive or
j
negative according to the materials used for the electrodes. The
‘
critical
potential, Vo, is thus equal to V + K.
Then,
705 = (V+ K)e = %mvzmax,
(4—6)
where 6 and m are the charge and mass of the electron and an… their
,
greatest velocity. The values of V
,
Vo, and K are indicated in
,
gure,
4—7.
The horizontal part of the Curve would extend left‘to
_
the current axis, instead of decreasing as it does, if K were zero,
i.e., if both anode and cathode were made of identical materials.
The decrease of current at the lower voltages may be explained
'a's due to the existence in the photo-current of electrons of all
“energies from zero up to a denite maximum amount; velocities
.—
Zero up to approximately 108 centimeters per.second. Thus,
*
,
'VÔl'Ëàges immediately to the left of O’ Stop increasingly energetic
«
”€leCffOnS—ancl Vo VOlts repel even the fastest ones.
'
Fmmthe
the curve to the left of O’ in gure4—7,
possible to deduce the number of electrons
Çliêrent velocities. The velocity distriâution is as
f®H°WS
€ 5 are very
in
current, j
also very
the
limits;
ofelectrns
“sesœaœmParatwely
'ïÿroäâËk value; about'044 of
thatofthemaX1mum
Asr'expîlaièd
'
on the Sommerfeld
4.
PHOTOELECTRÏC EMISSION
‘
positive
ions bombard the cathode so vigorously as to dest‘rOy
”
its sensitive surface in a short time.
4—7;
The Energy of the Photo-electrons.—The kinetic energy
of the ejected electrons may be measured by the use of magnetic
and electric elds, as in the case of thermionic electrons described
'
in chapter 2. The more usual method, however, is to apply a
suiciently negative potential to the collecting electrode to just
reduce the photo-current
to zero.
The critical retarding potential
(Vo of gure 4—7) is proportional to the energy
of the fastest
-
cell, Which Will thus stop the most energetic electrons, must be
correctedby an amount K to give the true energy Vo; The contact
.
potential,2 K, is the inherent potential dierence which exists
\
between the two kinds of material forming the anode and cathode. *
_
It is positive in commercial photo—cells but may be positive or
j
negative according to the materials used for the electrodes. The
‘
critical
potential, Vo, is thus equal to V + K.
Then,
705 = (V+ K)e = %mvzmax,
(4—6)
where 6 and m are the charge and mass of the electron and an… their
,
greatest velocity. The values of V
,
Vo, and K are indicated in
,
gure,
4—7.
The horizontal part of the Curve would extend left‘to
_
the current axis, instead of decreasing as it does, if K were zero,
i.e., if both anode and cathode were made of identical materials.
The decrease of current at the lower voltages may be explained
'a's due to the existence in the photo-current of electrons of all
“energies from zero up to a denite maximum amount; velocities
.—
Zero up to approximately 108 centimeters per.second. Thus,
*
,
'VÔl'Ëàges immediately to the left of O’ Stop increasingly energetic
«
”€leCffOnS—ancl Vo VOlts repel even the fastest ones.
'
Fmmthe
the curve to the left of O’ in gure4—7,
possible to deduce the number of electrons
Çliêrent velocities. The velocity distriâution is as
f®H°WS
€ 5 are very
in
current, j
also very
the
limits;
ofelectrns
“sesœaœmParatwely
'ïÿroäâËk value; about'044 of
thatofthemaX1mum
Asr'expîlaièd
'
on the Sommerfeld