Energy States (Levels)
Every atom or molecule in nature has a specific structure for its energy levels.
The lowest energy level is called the ground state, which is the naturally preferred energy state. As long as no energy is added to the atom, the electron will remain in the ground state.
When the atom receives energy (electrical energy, optical energy, or any form of energy), this energy is transferred to the electron, and raises it to a higher energy level (in our model further away from the nucleus).
The atom is then considered to be in an excited state.
The electron can stay only at the specific energy states (levels) which are unique for each specific atom. The electron can not be in between these “allowed energy states”, but it can “jump” from one energy level to another, while receiving or emitting specific amounts of energy.
These specific amounts of energy are equal to the difference between energy levels within the atom.
Each amount of energy is called a “Quantum” of energy (The name “Quantum Theory” comes from these discrete amounts of energy).
Energy transfer to and from the atom
Energy transfer to and from the atom can be performed in two different ways:
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.
Since we are now interested in the lasing process, we shall concentrate on the second mechanism of energy transfer to and from the atom (The first excitation mechanism is used in certain lasers, like Helium-Neon, as a way to put energy into the laser, and will be discussed in chapter 6 about the different kinds of lasers).
Photons and the energy diagrams
Electromagnetic radiation has, in addition to its wave nature, some aspects of “particle like behavior“.
In certain cases, the electromagnetic radiation behaves as an ensemble of discrete units of energy that have momentum. These discrete units (quanta) of electromagnetic radiation are called “Photons“.
The relation between the amount of energy (E) carried by the photon, and its frequency (ν), is determined by the formula (first given by Einstein):
E = hν
The proportionality constant in this formula is Planck’s constant (h):
h = 6.626*10-34 [Joule-sec]
Sometimes angular frequency (ω) is used instead of frequency (n), so a corrected constant h(bar) is used:
h(bar) = h/2π = 1.054*10-34 [Joule-sec]
The energy is given by:
E = hν = h(bar)ω
This formula shows that the frequency of the radiation (ν), uniquely determines the energy of each photon in this radiation.
This formula can be expressed in different form, by using the relation between the frequency (n) and the wavelength: c = λ*ν
E = h * c/λ
This formula shows that the energy of each photon is inversely proportional to its wavelength. This means that each photon of shorter wavelength (such as violet light) carries more energy than a photon of longer wavelength (such as red light).
Since h and c are universal constants, so either wavelength or frequency is enough to fully describe the photon.