.
80
4.
PHOTOELECTRÏC EMISSION
In the case of a potassium hydride surface irradiated with blue
light of wave—length 4500 angstroms (as in Section 4—3), the energy
in each photon is 4.37 micro-micro—ergs. The observed yield of
000358 coulombs per joule is equal to 1.56 X 10_21 coulombs _
per
Since the charge of each electron is 1.59 X 10*19
'
coulombs, the quantum yield y is 000977 electrons per photon.
Approximately, then, one hundred photons were absorbed for
each electron emitted; the efciency was about one per
cent of
.
quantum equivalence; an unusually favorable case. As a further
'
example: in the selective region (4360A) of a potassium surface, a
sensitivity of 00345 coulombs per
calorie was observed.
Then,
fty photons were absorbed for each electron ejected. Again, in ’
the case of a platinum surface, at 250,up.(= 2500 angstroms), a
Ü
sensitivity of 1.5><10—4 coulombs per calorie corresponds to
.
about 5600 photons per electron. Values of _y = 10“4 are common.
.
,
If the intensity of the light is increased, more electrons will be
_
-
emittedbut the ratio of the number of electrons ejected to the
number of photons absorbed remains the same for a given surface
.
.
and wave-length. The eiciency is a property of the surface.
Ï_Ï
4—10. Einstein’s Photoelectc Equation.—In 1905, Einstein
_predicted that the energy of the fastest photo-electrons would be
direCtly proportionalto the frequency of the incident light; He
stated this in the
equation,
'
.
%m02max : ]ZV '_ ÇÔ,
'
(4—10)
where 772» is
the electron, vmax is the velocity of the
fastest electron,
the frequency of the light, & is Planck’s constant
'
(= 6547 X 10Ç\2îx erg—sec.) and q5 is the Energy required for an ,
electron to escape fr0m the surface of the emitting
,j‘}
,
_(çbis analogOus to the action of gravity in preventing the escape
;
95 the atmosphere of the earth.) The magnitude 1111 of each
or photon of radiant energy depends dnectly
frequency;
the higher the frequency, the greater the
°V€rtoaneïe°tmn> S€fVeS to remove it from the surface (45 e1gS
arereq“1œd)afterWhlch the
energy is in the kmetœ
velocmeS°ftheeleth0nS>,ÿWçrè
the frequency®f
80
4.
PHOTOELECTRÏC EMISSION
In the case of a potassium hydride surface irradiated with blue
light of wave—length 4500 angstroms (as in Section 4—3), the energy
in each photon is 4.37 micro-micro—ergs. The observed yield of
000358 coulombs per joule is equal to 1.56 X 10_21 coulombs _
per
Since the charge of each electron is 1.59 X 10*19
'
coulombs, the quantum yield y is 000977 electrons per photon.
Approximately, then, one hundred photons were absorbed for
each electron emitted; the efciency was about one per
cent of
.
quantum equivalence; an unusually favorable case. As a further
'
example: in the selective region (4360A) of a potassium surface, a
sensitivity of 00345 coulombs per
calorie was observed.
Then,
fty photons were absorbed for each electron ejected. Again, in ’
the case of a platinum surface, at 250,up.(= 2500 angstroms), a
Ü
sensitivity of 1.5><10—4 coulombs per calorie corresponds to
.
about 5600 photons per electron. Values of _y = 10“4 are common.
.
,
If the intensity of the light is increased, more electrons will be
_
-
emittedbut the ratio of the number of electrons ejected to the
number of photons absorbed remains the same for a given surface
.
.
and wave-length. The eiciency is a property of the surface.
Ï_Ï
4—10. Einstein’s Photoelectc Equation.—In 1905, Einstein
_predicted that the energy of the fastest photo-electrons would be
direCtly proportionalto the frequency of the incident light; He
stated this in the
equation,
'
.
%m02max : ]ZV '_ ÇÔ,
'
(4—10)
where 772» is
the electron, vmax is the velocity of the
fastest electron,
the frequency of the light, & is Planck’s constant
'
(= 6547 X 10Ç\2îx erg—sec.) and q5 is the Energy required for an ,
electron to escape fr0m the surface of the emitting
,j‘}
,
_(çbis analogOus to the action of gravity in preventing the escape
;
95 the atmosphere of the earth.) The magnitude 1111 of each
or photon of radiant energy depends dnectly
frequency;
the higher the frequency, the greater the
°V€rtoaneïe°tmn> S€fVeS to remove it from the surface (45 e1gS
arereq“1œd)afterWhlch the
energy is in the kmetœ
velocmeS°ftheeleth0nS>,ÿWçrè
the frequency®f