82
4.
PHOTOELECTRIC EMÏSSÏON
4—11.
The Work Function and the Potential Barrier.——The
energy required by an electron, in order that it may escape from
the surface of a body, is called the work function qä of that surface.
it is possible to determine a value of from the intercept of the
straight line with the voltage axis in gure 4—9. This method is
inaccurate since the extrapolation to zero frequency is very large.
On the other hand, qä may be determined with reasonable accuracy
by use of the intercept of this straight line with the frequency
axis.
If each of the incident light quanta
contains an amount of
energy }lVO which is barely sufcient to remove an electron from
the surface, then the emergent electron will have zero kinetic
energy.
The retarding potential VO then required to stop this
electron will likewise be zero.
lt follows, from equation 4—11, that
@ = i…. = fic/rx…
(4—13)
where 5 is the velocity of light. The critical frequency v,,, or the
long-wave—length limit )… is referred to as the photoelectric
threshold.
It, and hence @, may be determined not only from the
retarding curves, as above, but also from the spectral distribution
curves, as in gure 4—5.
_
The work function is expressed in a number of different units.
We shall use @ to represent this quantity in general, irrespective of
the units, and du to represent its value when measured in ergs per
electron.
Then, the number of ergs required to remove one unit
of electrostatic charge will be (bg = cin/e, where 6 is the charge of
the electron in e.s.u.
Further, since 300 volts is the equivalent of
one
e.s.u.
of potential difference, the number of volts (energy
units) required to remove an e.s.u. of charge is
3 =
volts/e.s.u.
(4—14)
This latter is the unit most commonly used in practice. If numerical values are substituted in equations 4—13 and 4—14, it Will be
found that
12336
d>3 =
T
volts/e.s.u.,
(4—15)
in WhÏCh )\0 is measured in angstrom units.
4.
PHOTOELECTRIC EMÏSSÏON
4—11.
The Work Function and the Potential Barrier.——The
energy required by an electron, in order that it may escape from
the surface of a body, is called the work function qä of that surface.
it is possible to determine a value of from the intercept of the
straight line with the voltage axis in gure 4—9. This method is
inaccurate since the extrapolation to zero frequency is very large.
On the other hand, qä may be determined with reasonable accuracy
by use of the intercept of this straight line with the frequency
axis.
If each of the incident light quanta
contains an amount of
energy }lVO which is barely sufcient to remove an electron from
the surface, then the emergent electron will have zero kinetic
energy.
The retarding potential VO then required to stop this
electron will likewise be zero.
lt follows, from equation 4—11, that
@ = i…. = fic/rx…
(4—13)
where 5 is the velocity of light. The critical frequency v,,, or the
long-wave—length limit )… is referred to as the photoelectric
threshold.
It, and hence @, may be determined not only from the
retarding curves, as above, but also from the spectral distribution
curves, as in gure 4—5.
_
The work function is expressed in a number of different units.
We shall use @ to represent this quantity in general, irrespective of
the units, and du to represent its value when measured in ergs per
electron.
Then, the number of ergs required to remove one unit
of electrostatic charge will be (bg = cin/e, where 6 is the charge of
the electron in e.s.u.
Further, since 300 volts is the equivalent of
one
e.s.u.
of potential difference, the number of volts (energy
units) required to remove an e.s.u. of charge is
3 =
volts/e.s.u.
(4—14)
This latter is the unit most commonly used in practice. If numerical values are substituted in equations 4—13 and 4—14, it Will be
found that
12336
d>3 =
T
volts/e.s.u.,
(4—15)
in WhÏCh )\0 is measured in angstrom units.