Light Emission

Basic Quantum Model of Light Emission:

The process by which light is absorbed by and emitted from an atom lies at the very
foundations of Quantum Physics.

Generally, the quantum model states that light is absorbed and emitted by an atom
in distinct packages of energy called "quanta". Such packaged quanta are often
referred to as a "photons".

When an atom absorbs a photon of sufficient energy, its orbiting electron "jumps"
to a higher energy level--to an "electron shell" further from the nucleus. Then,
after a brief period, the electron jumps back down (like a spring) to its previous
energy level. During this "jump down" process, a photon is emitted from the atom.
"The energy of the emitted photon is equal to the difference in the two energy
levels, obeying the quanta relationship: E=hv." (where v is the frequency of the
emitted light energy; and h is Plank's constant=6.625x10^-34 J-s)

So, the measured values of absorbed or emitted quanta are always restricted to
some multiple of Plank's constant.

The basic quantum model also states that the energy emitted by the atom is equal
to the energy that has been absorbed by the atom...adhering to energy
conservation laws. [See applet of light emission at this Physics 2000 site.]

The question, at hand, revolves around the mechanism by which this model
operates. Whereas most physicists seem content to accept that the process occurs
generally in an atom, others specify that the atom's electron actually emits a
photon when it accelerates in the "jump down". (Click here and here to see two
external links that illustrate this notion.)* We will argue here that this notion is
incorrect--that an electron cannot emit a photon; because, it cannot absorb a
photon without an unacceptable mass increase of approximately 0.00039%. For
a slightly more detailed treatment of this argument, please see this short essay.

Instead we will suggest an alternative mechanism--explaining how light is
absorbed and emitted by an atom--in which the energy of the photon is absorbed
by the electromagnetic (binding energy) field within the atom.

Keeping in mind the cautions of the two eminent physicists quoted above, we will
visualize this version of light emission with pictorial graphics; using (what we will call)
an "ideal atom". This is merely a simplified Bohr Model, consisting of a nucleus and one
orbiting electron. (This model is highly unrealistic; but is widely employed in similar
demonstrations.) Though, in our ideal atom, we must add the wave property of a
moving electron (often called DeBroglie waves); for it is this wave property that is
responsible for other important physical properties of atoms; such as the electron
"shell" (also called "electron cloud".) DeBroglie waves also account for the quantum
restrictions of an electron's energy state--and, hence, for the electron's quantum
"jump" from one energy level to another. So the wave-property of an electron is too
important to ignore in this ideal atom. In fact, ...it is essential.

Let us also be reminded that this is not (I repeat "not") real physics, but only my own
'cartoon physics' version of how the process of light emission occurs. We will look at
this process in super-slow motion--step-by-step; with graphic images 1 through 6:

Fig. 3. The increased outward EM
pressure causes the electron to jump
(away from the nucleus)--to a higher
energy level (e-2 shell); in accordance
with the quantum restrictions governed
by Plank's constant. However...the EM
field that once 'saturated' the smaller
electron shell, e-1 (see figure 2.)
suddenly occupies a much larger volume
of space--(the volume enclosed by the
larger e-2 electron shell.) Therefore the
density of that EM field is greatly
decreased; (as illustrated by the
decreased density of the yellow grid.)

We will say that the once-saturated EM
field is now "rarefied".

With decreased EM field density, comes
proportionally decreased EM pressure on
the electron--outward (from the nucleus).

Fig.5. During the jump down, the
electron and nucleus re-establish
equilibrium and a new ground state
electron shell begins to form. (e-1
shell). The density of the EM field
within the new e-1 shell is limited to
that which is necessary to bind the
electron to the nucleus in the ground
state energy level; as in figure 1.
(Hence, energy is conserved.)

Consequently, there is an area of
excess EM field that is virtually
'abandoned' within the area of space
of the (now empty) e-2 shell. The
energy of this "abandoned" field is
equal to the energy of the photon
that was originally absorbed.

This abandoned EM field
autonomously condenses and
becomes its own physical entity--
independent of the atomic system.

Some Problems with this Model:

1). Problems with he photon: This is not the perfect model. Not only is our "ideal atom"
a gross departure from the real thing, but the photon, as well, is misrepresented. Here the
photon is portrayed as a "particle" whizzing through space; when the actuality is likely to
be quite different. More than likely, a photon is more like a disturbance in a much larger
continuous field of energy. It might best be thought of as an impulse of energy in that
larger field. However, when we think of the model of light emission above in these terms,
the model makes even more sense. (See 2....below.)

2). Problems with the EM field: In figure 5 (above), we show the "free" electromagnetic
field "collapsing" and "condensing". This is an arbitrary notion. However, since we are
treating the photon as something of a 'particle', this seemed the best way to portray this
stage of the process. Perhaps a more accurate description of the entire absorption and
emission process would be: An "impulse" in the exterior electromagnetic field (a photon)
is introduced into and released from the interior electromagnetic field (binding energy) of
the atomic cloud.

Though, the important point of the model featured above is: A photon (impulse) is
absorbed and emitted by the electromagnetic field within the inner space of the atom
(and not by the electron.)

3) Problems with the electron's "jump": Quantum physics poses a paradox, where it is
determined that a electron of an atom can exist in one energy state (orbit) or another;
but it can not exist anywhere in-between. This premise begs the question: "Is the
electron non-existent during the jump?" For the most part, we will leave this question
open. But my guess, since there is no huge energy release to indicate it, that it exists
throughout the entire process.

The primary objective of this pictorial demonstration was to illustrate a conceivable
mechanism for the process of light emission. Specifically: energy of the introduced
quanta is absorbed by the electromagnetic field that serves as the binding energy
between the electron and the nucleus. Sudden changes in density and pressure in
this electromagnetic field, in conjunction with certain electron properties, govern the
"jumps" towards the ionization threshold and back to ground state. Surplus EM field
separates from the atomic system and is perceived as light emission.

The model presented here, however, has some problems. Let us take a look at
some of those. After that, let's look at some positive aspects of this model; to see
where it conforms to established ideas of quantum and field physics.

Some attributes of this model:

1) It works and it fits: Aside from the three problems above, this model gives a complete
account of the process of light emission. There is nothing missing in this scenario.
Absorbed energy is accounted for throughout the entire process--from beginning to end.
The model also agrees with observation.

2) Physical consistency: In our model, we have an 'electromagnetic field energy' (the
binding energy of an atom) absorbing another 'electromagnetic field energy' (a photon).
(As apposed to one kind of thing [an electron] absorbing another kind of thing [a photon].)
This notion seems conducive to natural intuition. That is, we could compare the
absorption/emission scenario of this model to following familiar scenario of every-day
experience:

Start with a brim-full glass of water. if we add an ounce of water to this brim-full glass, an
ounce of water will overflow, spilling over the brim. Though, the displaced water is not
necessarily the 'same water' that was added; but it is...'water'--the same substance--of
equal volume (or energy) to that which was added. (This is not a bad analogy, if I might
award myself with a small complement.)

3) Measurement and theory: From spectroscopic analysis we find: "analyzing the
kinetic energy of the electrons gives the binding energy of the state, where the
electron was exited from." (Link to this quote.)

[I am not sure what to make of this analysis...but it does sound hopeful.]

Also, check out the applet of the Schrodinger atomic model on this "Physics 2000" site. It is
a great animated visualization; and might easily be interpreted as:'the binding energy of
the atom is absorbing and emitting the photon.'

4) Unification: We notice a striking conformity between the model of light emission,
above, and processes involved in electric and thermal energy.

In other words, conduction of heat and electricity are the same basic process as light
emission. [Please see unification of electric and thermal conduction here] In each and
every one of these cases, the basic process might be described as:

a) electromagnetic-field energy that is 'bound up' in the job of holding some material
together, is saturated by an influx of energy--(thermal, electrical, or light energy);

b) due to the saturation of this EM field, electrons are dislodged from their bound
positions; thus freeing the electromagnetic field energy associated with binding them to
those positions;

c) the 'freed' EM energy is either radiated from or conducted through the material.

5) Insight: This model gives us an insight to the fundamental nature of energy. It tells us,
simply, that within the binding energy of every atom, there lies--"trapped"-- the makings of
a photon; and that all that is necessary to free that "trapped photon", into open space, is
to displace the electron from the atom (simultaneously releasing the field energy that
binds it to the nucleus). This suggested phenomenon might be re-stated from a different
perspective: Light cannot escape from the event horizon of a non-energized atom.

Addendum: Note, as an example of some common misconceptions in optics, the second post in this
on-line physics forum. "Integral" states: "The electron can only emit a photon when it is
accelerated. Nothing happens to the (EM) field due to the electron itself." We will take issue
with every aspect of Integral's statement:

A) Electrons in atoms are always accelerating, so why aren't they always emitting photons? [This
is a commonly asked question..and a good one.] B) An electron cannot emit a photon because it
cannot absorb a photon. [See attached essay on this topic.] C) A lot happens to the EM field in
the process of light emission. [This...according to the model presented below].