Photon Basics - chemed.net

Unable to get host : RADIANT ENERGY AND ATOMIC ELECTRONS
Light detected by our eyes is called 'visible' and the range of colors is called the 'visible spectrum'. Light in this range, and in the invisible-to-our-eyes ranges called ultraviolet and X-ray, is associated with atomic electrons outside of the nucleus. How the electrons are related to light is the subject of this article.

To anticipate what is presented here, consider....

What properties of an electron change when it changes orbit?
Could the properties of a photon be derived from the change in electron properties?
What is there about the motion of an electron from one orbit to another that spawns a photon?

An electron moves from one orbit to another with a change in its energy and there are two general mechanisms by which electron energy changes -

thermal energy transfer results from a non-reactive collision between the atom in question and another atom at an inter-phase surface or in the same phase,
otherwise the transfer is radiative. It is here that we are to encounter the concept of "photon" (Gk. phos- light) as the individual radiative event for a single atom/molecule.
Keep in mind that the event can be either emissive or absorptive, in which an electron either "ejects" or "ingests" the photon; in a sense, the photon is 'birthed' or 'anhilated' by an electron which changes its orbit. Thus, the photon owes its properties to the changes in the electron properties.

Changing Orbit Properties

By changing orbit the electron changes its :

total energy

velocity, speed ( kinetic energy and momentum) {compare orbit kinetic properties here}
distance from the nucleus (electrostatic potential energy)

the electromagnetic force(s) acting upon it
In light of the conservation of energy and momentum, a photon can possess properties of energy and momentum (both linear and angular) and electromagnetic force - and the magnitudes of these properties are those of the change in electron orbit properties.

Study Question: From this animation ( go there) estimate the relative changes in orbit speeds for the three electrons depicted.
Which inter-orbit jump (orbits 3rd/2nd or orbits 2nd/1st) shows the larger speed change?
Which the larger driving force?
Do you imagine the force to be a single, discontinuous 'spike' in time or a dual pulse of continuous speeding-up followed by slowing-down?

The Roots of it All

The seminal experiment by Heinrick Hertz , to become the basis for today's fiber-optic and wireless (satellite positioning, microwave, radio/television/cellular, radar, laser, night vision ...) technologies, came 120 years ago.
It was James Clerk Maxwell^1,2 who established the theory that charged objects radiate energy and electromagnetic force during times when they change speed - experience acceleration. It is important to the photon story that Maxwell found that the speed of propagation of this force/energy is the same as that of light! Furthermore, the energy and force experienced by a remote charged object is 'full strength' - even when the distance of the remote object is so great that the through-space (static) force acting upon it is vanishingly small. Also interesting is that the force/energy tranverses even solid matter (like the bell jar in Hertz' experiment) without the atomic properties necessary to absorb it.
But Maxwell did not approach radiative energy from the perspective of an individual atom's electron orbit properties and so did not conceive the photon idea. Those ideas had to wait until the early 20th century.

... the important concept is ..... charged particles (here electrons) undergoing acceleration (slowing down or speeding up) experience radiant energy transfer with their 'surroundings'. Slowing down and speeding up are NOT to be exclusively associated with radiative energy loss and energy uptake, respectively. Each can entail either energy loss or gain for an electron. ???? .

Old- and New-Rules Electrons

Under classical laws (Newton) there are no special restrictions (beyond those of conservation of energy and momentum) imposed upon the change in the electon's velocity and distance from the nucleus. Those rules permit a continuum of electron speeds and distances from the nucleus for the atomic electron, and predict that an atomic electron would quickly fall into the nucleus! While the latter is observed in special cases of radioactive nuclear decay, it is a prediction in general disagreement with observation.

Bohr's early planetary model of atomic electrons postulated stable - persistent - orbits for the electrons, without explaning 'why' such orbits exist. His postulate was a response to the very interesting situations which arise when radiation encounters an atom; it interacts with the atomic electrons to be scattered with either unchanged energy or with a loss of energy.
In the former instance the electron changes velocity as it is driven by the accelerating force of the light (recall Hertz' experiment), only to return to its initial orbit without keeping any of the light's energy - this is simple light scattering and is not in conflict with Newtonian concepts.
The really novel, at the time, situation is that the electron can, even if only for a short period of time, "keep" some of the light energy, but not all.
A common example of this is the passing of sunlight through a stained-glass window. The light emerging from the window is of different colors (reds, yellows, greens, blues, ...) than that entering. The atom electrons are keeping some of the energy, but not all!

For "new-rules" (quantized) particles within atoms and molecules most of the speeds/velocities/distances of the continuum are NOT really forbidden, just unobservable (another way of saying they are transient, not persistent). Having reminded ourselves of that, it is still instructive to investigate by classical laws the description of the effect of a close encounter of a photon with an atomic electron. An aside...

Earlier we realized that to learn about a photon's properties we examine the change in properties of its parner atomic electron... again:
That the electron changes speed as it changes orbit means that the electron experiences an acceleration, which requires the presence of a force and it is the photon which bears this electrodynamic force.
It is also inescapable that, since the electron moves in response to the force, work ( energy) is involved as the electron alters its speed ( kinetic) and distance from the nucleus ( potential) energies. Accordingly, the photon bears this energy.
Since the partner electron changes its rotational (angular) momentum, the photon must possess momentum of this type.

Visualizing Photon Emission and Absorbtion

Emitter ......... Absorber

Energy vs time
Photon
Energy
property

Force Property

Energy

Lower

Higher

Text

So the physical entity called the photon, a word arising from the studies by Einstein and Planck nearly a century ago, can exert force on a charged particle and it possesses energy and momentum which can be transferred to such a particle during the 'ingestion' event. Notice that this necessarily means the photon is anhilated ONLY when the energy-absorbing electron passes from one persistent state (orbit) to another. Conversely, the force and momentum properties of the photon arise during photon creation by changes in force and momentum of the electron as it passes between persistent orbits. [???? Why must the transition be between persistent orbits ????]
It is the expansion/contraction motion of extranuclear electrons within the isolated atom/molecule that provides a radiant energy transfer mechanism - as the alternate to thermal transfer from collision of atoms.

Thinking of flame test colors for atoms and of neon lights, we find atoms in high energy collisions, with some of that energy absorbed by the electrons of the atoms, and the subsequent radiation of that energy. The energy the nerves in your skin detect when near an open flame or electric burner (or from the Sun) is called infrared radiation and originates from oscillations of atoms within molecules; skin nerves have molecules which absorb that radiation.
The electomagnetic field provides a means for the transfer of energy/momentum between atoms/molecules NOT in physical contact. It is a 'through-space' mechanism for energy transfer in nature, also called radiant energy transfer.
On yet a lower electron-energy scale, electron shuttling is at work within FM, microwave and radar antemmae ...
[Alternating voltage (a low frequency force field) is applied to the electrons in an FM antenna so as to accelerate/decelerate them up-and-down the antenna; these oscillating electrons radiate their force field into their surroundings, where electrons in receiver antenna circuits begin to oscillate. In this process radiant energy is transferred from source to absorber - transmitter to receiver. The energy is provided to transmitter electrons by electrical generators so as to move electrons in the antenna to high energy (by crowding them together), after which they lose their energy radiatively (the electrons return to their original energy by separating - accelerating and deaccelerating). The process is repeated over and over, at a frequency called the broadcast frequency of the FM station.]

Chiral Photons
Animation
From the earlier discussions of changes in electron orbit properties appearing as photon properties, you may have already surmized that photons have angular momentum properties. The origin of photon momentum derives directly from the change in electron angular momentum as it jumps orbits; but since the electron has a signed momentum in a particuar orbit you might surmise that the photon's angular momentum is signed, as the electron increases or decreases its momentum with the orbit change. In fact, the oscillating force field vector of the previous section is actually the resultant of two oppositely rotating force components. It is the intent of the animation for this section to depict for you these components and their resultant.
In the animation snapshot just above you see on the left a view of the yellow, clockwise ('dextro' or 'right') rotating component and of the blue, counterclockwise ('levo' or 'left') components as they advance away from you. These two components are near the end of a full cycle of rotation. On the right is a 'side-view'near the end of two full rotation cycles. The side view shows the helical paths traced by the tips of the dextro and levo force vectors. The animation will also show you the vertical and horizontal components of the individual force vectors and for the option of both simultaneously (for the both-at-the-same-time option, the horizontal components offset each other and only the vertical composite remains - and this was what was depticted in the animation of previous section).
It may have occurred to you that the oppositely-rotating components are mirror images of each other - like your right and left hands. It is this analogy that leads to the non-mathematical description of 'right' and 'left' photon components. This is also the chemist's language for chiral ('handed') molecules: such a molecule can have its atoms distributed in space in two ways - each being the mirror image of the other. These molecules are 'isomers' - specifically, stereoisomers (notice the similar use of the prefix 'stereo' when discussing stereophonic systems).
When a photon encounters either isomer of such a compound, the two photon-components interact differently with the molecule's electron cloud; this difference can be quantitatively measured and becomes a 'signature' for each of its isomers.

¹ S. R. Paglia, Introductory Quantum Chemistry, Harper Row (1971), pp-21-22.
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Friday, 21-Sep-2007 14:22:47 CDT