Dictionary Definition
diatomic adj : of or relating to a molecule made
up of two atoms; "a diatomic molelcule"
User Contributed Dictionary
Extensive Definition

 I \, is the moment of inertia of the molecule
For microscopic, atomiclevel systems like a
molecule, angular momentum can only have specific discrete values
given by

 L^2 = l(l+1) \hbar^2 \,
 where l is a positive integer and \hbar is Planck's reduced constant.
Also, for a diatomic molecule the moment of
inertia is

 I = \mu r_^2 \,
 where

 \mu \, is the reduced mass of the molecule and
 r_ \, is the average distance between the two atoms in the molecule.
So, substituting the angular momentum and moment
of inertia into Erot, the rotational energy levels of a diatomic
molecule are given by:

 E_ = \frac \ \ \ \ \ l=0,1,2,... \,
Vibrational energies
Another way a diatomic molecule can move is to
have each atom oscillate  or vibrate  along a
line (the bond) connecting the two atoms. The vibrational energy is
approximately that of a
quantum harmonic oscillator:

 E_ = \left(n+\frac \right)hf \ \ \ \ \ n=0,1,2,... \,
 where

 n is an integer
 h is Planck's constant and
 f is the frequency of the vibration.
Comparison between rotational and vibrational energy spacings
The lowest rotational energy level of a diatomic
molecule occurs for l=0 and gives Erot = 0. For O2, the next
highest quantum level (l=1) has an energy of roughly:
This spacing between the lowest two rotational
energy levels of O2 is comparable to that of a photon in the microwave region of the
electromagnetic spectrum.
The lowest vibrational energy level occurs for
n=0, and a typical vibration frequency is 5 x 1013 Hz. Doing a
calculation similar to that above gives:

 E_ \approx 3 \times 10^ \ \mathrm \,.
So the spacing, and the energy of a typical
spectroscopic transition, between vibrational energy levels is
about 100 times greater than that of a typical transition between
rotational
energy levels.
Further reading
 Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules
 Physics For Scientists and Engineers : Vol. 1 (4th ed.)
See also
Notes and references
External links
 Hyperphysics  Rotational Spectra of Rigid Rotor Molecules
 Hyperphysics  Quantum Harmonic Oscillator
 3D Chem  Chemistry, Structures, and 3D Molecules
 IUMSC  Indiana University Molecular Structure Center
diatomic in Arabic: ثنائي الذرة
diatomic in Catalan: Molècula diatòmica
diatomic in Spanish: Diatómico
diatomic in Malayalam: ദ്വയാണുതന്മാത്ര
diatomic in Japanese: 二原子分子
diatomic in Russian: Двухатомная молекула
diatomic in Chinese: 双原子分子