Nina
c100
Xie Yiling
A21
Topic electron transition and emission spectral lines/hf=E1-E2
Light quanta and the photoelectric effect
The photoelectric effect occurs when a light beam is incident on a metal
surface causing electrons to be ejected from the surface. The long held wave
theory of light; even for light of high intensity, was inadequate to explain the
phenomenon. In 1905 Albert Einstein, a little known patent office clerk,
published a paper on the photoelectric effect postulating a “quantum”
explanation in which light can be considered as a stream of particles called
photons. This extended Planck’s quantization of energy hypothesis, which
he had proposed for understanding and interpreting the experimentally
obtained blackbody spectral-distribution curves of radiation power versus
wavelength.
These photons were to be understood as individually interacting with the
electrons of the metal surface of incidence and having energy, proportional
to their frequency given by Planck’s constant h and times the frequency f,
(hf). With enough photon energy, that is high enough frequency to remove
electrons from the metal—energy exceeding the metal’s work function; the
photoelectric effect occurs with electrons being ejected from the metal.
The source of photons analyzed in this experiment are emitted by light
emitting diodes (LEDs), which are devices made from a class of solids
called semiconductors—with electrical conductivity between that of a metal
and an insulator. These light emitting diodes have p-n junctions which
involve energy bands and gaps in solids. The carriers (electrons and holes)
are injected across the junctions producing the light. Rather than light
commonly produced from thermal excitation, the photons are emitted
having rather specific energies corresponding quite closely to the energy
difference, Egap, involved between the higher to lower energy bands of
transition for the particular LED.
Theoretically, the photoelectric effect with light incident on a metal surface
and electrons ejected can be expressed by
Ek=hf-W
where Ek is the kinetic energy of the electrons, e is the electron unit charge,
V0 is the stopping potential or voltage, h is Planck’s constant, f is the light
photon frequency, and W0 is the metal work function.
Energy level diagrams are a means of analyzing the energies electrons can
accept and release as they transition from one accepted orbital to another.
These energies differences correspond to the wavelengths of light in
the discreet spectral lines emitted by an atom as it goes through
de-excitation or by the wavelengths absorbed in an absorption spectrum.
Notice how each energy level closer and closer to the nucleus is more and
more negative. This signifies that the electron is trapped in an "energy well."
To ionize a ground-state electron [to take it from -122.4 eV to 0 eV in our
example], you would have to irradiate the gas with photons having energies
of 122.4 eV or greater. This is the only instance where the incident energy
does not have to exactly match the difference in two energy levels. Any
excess energy would remain in the form of the ionized electron's kinetic
energy.
Max Planck had already determined that the energy levels in an oscillating
system were quantized and followed the relationship
E = hf
where f represented the frequency of oscillation and h is Planck's constant,
6.63 x 10-34 J sec. In our case, f is the frequency of the emitted photon in
accordance with c = fλ. If we replace f with c/λ, we have the formula
E = h(c/λ)
The value of E, or energy, represents the difference in the energies of two
energy levels (ΔE) when an electron goes through de-excitation. The value
of λ corresponds to the wavelength of the emitted spectral line.
λ = hc/ΔE
Application of electron transiton
Nuclear Excitation by Electron Transition and Its Application to Uranium
235 Separation
A new mechanism for nuclear excitation is studied theoretically by
considering the deexcitation of electronic states of atom. When an electron
of inner shells is kicked off by the bombarded electron or X ray, an electron
of the adjacent shells immediately jumps into the vacancy. The energy
corresponding to the difference of the binding energies for these two shells,
E_1-E_2, is usually carried away by the emitted characteristic X ray or the
Auger electron which is ionized from an outer shell. There is, however,
another possibility of this energy release by exciting a nuclear state. That is,
the nuclear ground state is excited to the higher energy level by receiving
the excess energy of the electronic state by means of electromagnetic
interactions between nucleus and electrons. A theory is made for this
process, and it is found that the probability of this process is extremely small
in most of the cases, but it will be appreciable, if the interaction energy is
approximately equal to the energy mismatch E_N-(E_1-E_2), where E_N is
the nuclear excitation energy. An example of 235U is discussed in connection
with a possible separation of 235U from uranium isotopes.
Application of photoelectric effect
Applications of the photoelectric effect
A photocell is a vacuum tube with a concave cathode called an emitter, made
from a material that emits electrons easily. When light of a frequency greater
than the threshold frequency, shines on the emitter, electrons are released. The
concave emitter focuses electrons on a thin wire anode called a collector. The
thin anode does not block light. When the collector is at a positve potential,
electrons are attracted to it.
The photocell is at the centre of the many applications of the
photoelectric effect. It consists of a curved emitter and a rod as
collector, so as not to inhibit light from reaching the emitter.
The flash of a camera uses the photoelectric effect
(for the source of this image and for more information click here)
Photocells are used in garage door openers.
Photocells are used in movie film. An example is shown in the diagram
below: