How are photons generated?
What can the energy of these photons tell us about how they were created?
What is the difference between an emission and absorption spectrum?
How are they generated?
These are critical tools that are used to analyse the light we receive from stars and
galaxies but as you will see there are several mechanisms that can change the spectrum.
Connect to this is how matter can interact with photons but this will be covered in the
next part.
Emission Spectrum It is well known that an atom can be characterized
by a unique set of discrete energy states.
When excited through heating or electron bombardment in a discharge tube, the atom makes transitions
between these quantized states and emits light.
When an electron makes a transition from a high energy state to a lower energy state
it emits a spectrum of frequencies of electromagnetic radiation.
The energy of the emitted photon is equal to the energy difference between the two states.
If we examine the electron configurations it can be seen that there are many possible
electron transitions for each atom, and each transition has a specific energy difference.
The emission spectrum is made of up a collection of different transitions, which generate different
radiated wavelengths.
This means that each element's emission spectrum is unique.
Forbidden lines emissions Bound electrons in atoms can be excited and
de-excited in two ways.
By photons resulting in absorption and emission or by collisions between atoms and electrons
resulting in non-radiative processes.
The Balmer and Lyman transitions in hydrogen are caused by the absorption and emission
of photons.
These will create what are called allowed transitions.
The de-excitation of the electrons within the atom occurs spontaneously with a very
high probability.
Some energy levels in metals are split into sub-levels or fine structure states.
Downward transitions amongst these low-lying energy levels can have a very low probability
of occurring spontaneously.
This typically can take 10-100 seconds.
In Earth's atmosphere, the excited atom would collide with other atoms or free electrons
and lose energy in the collisions long before it could radiate the energy away.
However, in the low densities of interstellar space, these collisions are extremely rare
and there is enough time for spontaneous decay to occur.
These transitions when they occur are called forbidden lines.
Because photons emitted in the forbidden line transition have only a very weak probability
of being absorbed the emitted photon easily escapes nebula plasmas, carrying away the
photon energy and this cools the gas or plasma.
This process can therefore remove kinetic energy from the gas or plasma and transform
it into photon energy.
But there are other mechanisms that can create forbidden lines like the Zeeman effect which
we will cover in a bit.
Absorption Spectrum If a continuous spectrum of light is passed
through a substance parts of the continuous spectrum will be absorbed by the substance
creating specific black bands which are missing.
Atoms and molecules may change their atomic or molecular state when they absorb specific
amounts of energy.
Atomic states are defined by the arrangement of electrons in the atomic orbitals.
So an electron may be excited to a more energetic orbital by absorbing a photon which has exactly
the energy equal to the energy difference of the two orbitals.
Molecular states are defined by the molecule's modes of vibration and rotation.
These are also quantised, just like the electron orbitals, meaning specific photon energies
are required to change its state.
The excited states for both molecular and atomic do not last long.
After a random amount of time, the atoms and molecules revert back to their original, lower
energy state.
In both cases, this will cause them to emit a photon but not necessarily in the same direction
as the original photon.
The most common angle has been shown to be about 45 degrees from the original photon.
This applies to any situation where gases lie between a light source and an observer:
the observer will see gaps in the spectrum of light corresponding to the wavelength of
the photon which was absorbed even though re-emission of the photon occurs as they tend
not to be emitted in the direction of the observer leaving a gap.
Rotational lines occur when the rotational state of a molecule changes.
These are typically found in the microwave spectrum.
Vibrational lines correspond to changes in the vibrational state of the molecule and
are typically found in the infrared region.
Electronic lines correspond to a change in the electronic state of an atom or molecule
and are typically found in the visible and ultraviolet region.
X-ray absorptions are associated with the excitation of inner shell electrons in atoms.
These changes can be combined, leading to new absorption lines at the combined energy
of the two changes.
Electric and magnetic fields can cause a shift.
Interactions with neighbouring molecules can cause shifts as well.
Stimulated emission This is the process by which an incoming photon
of a specific frequency can interact with an excited atomic electron causing it to drop
to a lower energy level.
The liberated energy transfers to the electromagnetic field, creating a new photon with a frequency,
polarization, and direction of travel that are all identical to the photons of the incident
wave.
This process is identical in form to the process of atomic absorption we just covered.
In most media, there are a higher proportion of electrons in a lower energy state so absorption
will dominate.
If however there are more atoms with electrons in an excited state stimulated emission will
dominate.
In this case, a net optical amplification can be achieved.
This can occur with either light or microwave photons and are termed LASER or MASER.
These can be manmade but both have also been detected from various astronomical objects
Zeeman Effect In the presence of a magnetic field, the energy
levels of the emitted photons can shift.
In an atomic energy state, an electron orbits around the nucleus of the atom and has a magnetic
dipole moment associated with its angular momentum.
In a magnetic field, it acquires additional energy just as a bar magnet does and consequently
the original energy level is shifted.
The energy shift may be positive, zero, or even negative, depending on the angle between
the electron magnetic dipole moment and the field.
Due to the Zeeman effect, some degenerate energy levels will split into several non-degenerate
energy levels with different energies.
This allows for new transitions which can be observed as new spectral lines in the atomic
spectrum
This can create lines which are called forbidden lines.
These are spectral lines associated with absorption or emission of photons by atomic nuclei, atoms
or molecules which undergo a transition that is not allowed by a particular transition
rule.
Stark Effect The Stark effect is the shifting and splitting
of spectral lines of atoms and molecules due to the presence of an external electric field.
Although initially defined for a static electric field it is used in a wider context to describe
the effect of time-dependent electric fields.
It is responsible for the pressure broadening of spectral lines by charged particles in
plasmas.
The stark effect can be observed both for emission and absorption lines.
An electric field point from left to right tends to pull the nuclei to the right and
the electrons to the left.
If an electronic state has its electron disproportionately to the left its energy is lowered, while if
it has the electrons disproportionately to the right, its energy is raided.
This effect is greater for outer electron shells.
The splitting of the energy levels by an electric field first requires that the field polarizes
the atom and then interacts with the resulting electric dipole moment.
Unlike the Zeeman effect, the splitting of a line is not symmetric beyond hydrogen.
This can cause complicated blending with new lines which occasionally appear very close
to parent lines.
New lines appear in the spectrum which are normally forbidden by the selection principle.
Stark's data suggests that the blended line He 4472 for a field of 30kV per centimetre
is displaced towards the red by about 0.5 Angstroms.
For a field of 10kV per centimetre, the red shift would amount to only 0.02Angstroms.
One important property is that the degree of Stark broadening depends on the atomic
mass of the affected gas.
The lines of heavy elements are only slightly broadened, whereas those of lighter atoms
and ions are smeared to a much greater extent.
This does not happen in the Zeeman effect.
Bremsstrahlung The word literally means brake radiation.
This is electromagnetic radiation produced by the deceleration of a charged particle
when deflected by another charged particle.
The moving particle loses kinetic energy, which is converted into radiation.
Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak
intensity shifts toward higher frequencies as the change of energy of the decelerated
particles increases.
Bremsstrahlung induced by low-energy electrons is emitted over a relatively broad range of
directions around the direction of the incident electron.
As the electron energy increases, the direction of the peak intensity shifts increasingly
towards the forward direction, until for electrons above a few MeV, it is confined to a very
narrow forward beam
Broadly speaking, bremsstrahlung is any radiation produced due to the deceleration of a charged
particle, which includes synchrotron radiation, cyclotron radiation and the emission of electrons
and positrons during beta decay.
Synchrotron radiation is the electromagnetic radiation emitted when
relativistic charged particles are subject to an acceleration perpendicular to their
velocity.
Most known cosmic radio sources emit synchrotron radiation.
Cyclotron radiation Radiation emitted by charge particles moving
non-relativistic in a magnetic field is called cyclotron emission.
Free-Free radiation Bremsstrahlung emitted from plasma is sometimes
referred to as free-free radiation.
This refers to the fact that the radiation in this case is created by electrons that
are free before, and remain free after the emission of a photon.
Bound-bound radiation refers to discrete spectral lines when an electron jumps between two bound
states
Free-bound radiation refers to the radiative combination process, in which a free electron
recombines with an ion.
Gamma Emissions It is not possible to generate a gamma ray
from a simple electron transition.
The highest energy photon that this can produce is X-ray.
Instead, changes in the nucleus can cause the emission of a gamma ray from a few keV
to 8 MeV.
These are associated with specific types of decay.
This can allow for the identification of the decay radionuclides.
Very high energy gamma rays in the 100-1000 TeV range have been observed from sources
such as the Cygnus X-3 microquasar
Beta plus decay When an electron and positron collide it results
in the annihilation of both particles and the creation of two gamma-ray photons directed
180 degrees from each other.