For light to be absorbed by an atom, certain conditions must be satisfied. A fundamental condition is that the energy carried by one photon of light should be equal to the energy required to bring about a change, or transition, in the state of the atom.
The energy carried by a photon of light is equal to the product hv, in which h is a constant known as Planck's constant and v is the frequency of the light. Planck's constant represents a very small quantity. It has the value:
h = 6.626 x 10-34J s (joule second).
It is because of the smallness of Planck's constant that quantum mechanical effects are not more noticeable in the behavior of large, or macroscopic, objects. For example, for visible light with a wavelength of 500 nm (nanometer) in the green region of the spectrum, the frequency v has the approximate value:
v = 6 x 1014 s-1, or 6 x 1014 Hertz (Hz)
(Note that the symbol v is the Greek letter nu and not the letter vee.) The energy of such a photon is calculated as:
hv = (6.626 x 10-34)x(6 x 1014)
= 3.98 x 10-19J (joule)
The very small value for the energy of a photon has led to it being specified in another, more convenient, unit of energy; the electron volt or eV.
1 eV = 1.602 x 10-19 J
so that, 3.98 x 10-19J = 2.48
eV
A condition that has to be satisfied for such a photon to be absorbed by an atom is that the difference in energy between the two atomic energy levels involved in the process of absorption should be 2.48 eV. This particular case is illustrated in the diagram below.
In the general case, for a photon of any frequency, the condition is:
hv = E1 - E0
When the frequency of the light incident upon an atom satifies this condition, the light is said to be resonant with that transition in the atom, and the absorption process is sometimes referred to as resonant absorption.
(The pictorial representation of a photon by the green wavepacket shown in the diagram is for illustrative purposes only. We do not know what a photon 'looks like' and we cannot represent a photon accurately in a pictorial way.)
The processes of spontaneous and stimulated emission both result in the emission of photons as a result of a transition from an energy level to one of lower value. In terms of the quantities referred to in the above diagram, the transition could be considered to be from the level of energy E1 to the level of energy E0. In these cases, the frequency of the emitted radiation would again be found from the relationship:
hv = E1 - E0
When light of very high intensity is incident on an assembly of atoms, it is possible that two or more photons will be simultaneously absorbed by an atom. Multiphoton absorption is an improbable process for light of low intensity but it can be a significant, and even dominant, effect with the high intensity light produced by high power pulsed lasers. Two-photon absorption involves the simultaneous absorption of two photons by a single atom. For the two-photon case, the frequency of the absorbed photons is given by:
2hv = E1 - E0