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The Interaction of
Electromagnetic Radiation with Matter
(Emission and Absorption of
Radiation) |
نعلم أن الذرة تكتسب طاقة وتفقدها بصورة مستمرة
وإن انتقال الطاقة إلى الذرة يتم بواسطة طريقتين هما:
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Collisions
with other atoms, and the transfer of
kinetic energy as a result of the collision. This kinetic energy is
transferred into internal energy of the atom.
-
Absorption
and emission of electromagnetic radiation
وحيث أن عملية الليزر تعتمد على انتقال الطاقة من
خلال امتصاص
Absorption
الاشعاع الكهرومغناطيسي
ثم تكبيره وانبعاثه
emission على
شكل شعاع ليزر، لذا سندرس ظاهرة الامتصاص والانبعاث.
ارجع إلى موضوع Photons and the energy diagrams
تحت باب
Essential
The Interaction of
Electromagnetic Radiation with Matter
The interactions between
electromagnetic radiation and matter cause changes in
the energy states of the electrons in matter.
Electrons can be transferred
from one energy level to another, while absorbing or
emitting a certain amount of energy. This amount of energy is equal to
the energy difference between these two energy levels (E2-E1).
When this energy is absorbed
or emitted in a form of electromagnetic radiation,
the energy difference between these two energy levels (E2-E1)
determines uniquely the frequency (n) of the
electromagnetic radiation:
(DE)
= E2-E1 = hn
| Example
The visible spectrum wavelength range is: 0.4 - 0.7
[mm] (400-700 [nm]).
The wavelength of the violet light is the
shortest, and the wavelength of the
red light is the longest. Calculate:
a) What is the
frequency range of the visible spectrum.
b) What is the amount of the photon’s energy associated
with the violet light, compared to the photon energy of the red light.
Solution:
The frequency of violet light:
The frequency of red light:
The difference in frequencies:
The energy of a violet photon:
The energy of a red photon:
The difference in energies between the violet photon and
the red photon is:
2.15*10-19 [J]
This example shows how much more energy the violet
photon have compared to the red photon.
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Question:
Calculate in units of Nanometer, the
wavelength of light emitted by the transition from energy level E3
to energy level E2 in which:
E1 = 0 [eV]
E2 = 1.1 [eV]
E3 = 3.5 [eV]
Emission and
Absorption of Radiation
Every system in nature "prefers" to be
in the lowest energy state. This
state is called the Ground state.
When energy is applied to a system, The atoms in
the material are excited, and
raised to a higher energy level.
(The
terms "excited atoms", "excited
states", and "excited electrons" are used
here with no distinction)
These electrons will remain in the
excited state for a certain period of time, and then will return to lower
energy states while emitting energy in the exact amount of the difference
between the energy levels (DE).
If this energy is transmitted as electromagnetic
energy, it is called photon.
The emission of the individual photon is random,
being done individually by each excited atom, with no relation to photons
emitted by other atoms.
When photons are randomly emitted from different
atoms at different times, the process is called
Spontaneous Emission. Since this emission is independent of
external influence, there is no preferred direction
for different photons, and there is no phase relation between photons emitted
by different atoms.
Spontaneous emission is one of a
family of processes, called relaxation
processes, by which the excited atoms
return to equilibrium (ground state).
This "classic" explanation assumes that the specific frequencies
emitted by an excited atom are the same as the characteristic frequencies of
the atom, which means that the
emission spectrum is identical to the absorption spectrum.
Possible Processes Between
Photons and Atoms
three possible processes
between photons and atoms: absorption, spontaneous emission, and stimulated
emission.
Photon Absorption:
A photon with frequency n12 hits an atom
at rest (left), and excites it to higher energy level (E2) while
the photon is absorbed.
Spontaneous emission of a
photon: An atom in an excited state (left) emits a photon with
frequency n12 and goes to a lower energy
level (E1).
Stimulated emission of a
photon: A photon with frequency n12
hit an excited atom (left), and cause emission of two photons with frequency
n12 while the atom goes to a lower
energy level (E1).
We saw that the
process of photon absorption
by the atom is a process of raising the atom (electron) from a lower
energy level into a higher energy level (excited state), by an amount of
energy which is equivalent to the energy of the absorbed photon.
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Absorption
&
Spontaneous
emission |
Remember that two photons with the same
wavelength (frequency) have the same energy:
E = hn
= hc/l
The incoming photon does not change at
all as a result of the stimulated emission process.
As a result of the stimulated emission process, we have
two identical photons created from one photon and one excited
state. Thus we have amplification in
the sense that the number of photons has increased.
This is the process that was explained in the introduction:
Light Amplification by
Stimulated emission of Radiation = LASER
ِAverage
Lifetime
Atoms stay in an excited
level only for a short time (about 10-8 [sec]), and then they
return to a lower energy level by spontaneous emission.
Every energy level has a
characteristic average lifetime,
which is the the average time the electron exists in the excited excited state
before making a spontaneous transition. Thus, this is the time in which the
excited atoms returned to a lower energy level.
According to the quantum theory,
the transition from one energy level to another is described by statistical
probability.
The probability of transition from higher energy
level to a lower one is inversely proportional to the lifetime of the higher
energy level.
In reality, the probability for different transitions is a characteristic of
each transition, according to selection rules.
When the transition probability is low for a
specific transition, the lifetime of this energy level is longer (about 10-3
[sec]), and this level becomes a "meta-stable"
level. In this meta-stable level a large population of atoms can assembled. As
we shall see, this level can be a candidate for lasing process.
When the population number of a higher energy
level is bigger than the population number of a lower energy level, a
condition of "population inversion"
is established.
If a population inversion exists between two
energy levels, the probability is high that an incoming photon will
stimulate an excited atom to return to a
lower state, while emitting another photon of light. The probability for this
process depend on the match between the energy of the incoming photon and the
energy difference between these two levels
Lecture (1)
