A molecular emission spectrum for a specific molecule describes how that molecule interacts with electromagnetic radiation. It is important to understand why only some molecules interact with (far) infrared radiation while others do not, to understand the greenhouse effect.
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An important concept in this context is that of a dipole.
Dipole transitions are by far the most dominant transitions for the interaction of molecules and (ultraviolett, visible and infrared) light, at least under the circumstances found in the Earth’s atmosphere today. In this paragraph we will take a look at the quantum mechanical description to get an explanation for this.
In a first approximation we are going to neglect the spins of all elementary particles involved. Taking these into account is usually called fine structure (e.g. spin-orbital interactions for electrons) and hyper fine structure (electron-nucleus interactions).
As a further simplification, we will consider a single particle in a potential, which can be applied to a single electron in a molecule, for example.
In the Schrödinger picture the impulse operator of a particle in an external electromagnetic field is
where $e$ is the electric charge and $\vec{A}$ is the vector potential of the field.
Writing
and using the Coulomb gauge
we get for the Hamiltonian
Remember that we are looking at a single particle of mass $m$ and electric charge $e$. So the interaction term $U$ of electromagnetic field and particle in the Hamiltonian is proportional to $\vec{A} \cdot \vec{p}$.
In order to describe a plane wave we use for the vector potential
with some unit vector $\vec{e}$.
If we define a transition operator
then we get for the interaction term
where $T_0$ is an abbreviation for the constant amplitude:
The wavelength of ultraviolett to infrared light is rather long compared to the molecules that are present in Earth’s atmosphere: In this case we have
and can therefore approximate the exponential function in the definition of $T$ by the first terms:
Peter W. Atkins, Ronald S. Friedman (Author): “Molecular Quantum Mechanics”, Oxford University Press, USA; 5 edition (December 30, 2010)
Ingolf V. Hertel, C.P. Schulz: “Atome, Moleküle und optische Physik 1: Atomphysik und Grundlagen der Spektroskopie”, Springer, Berlin, 1st edition (Dezember 2007)
Ingolf V. Hertel, C.P. Schulz: “Atome, Moleküle und optische Physik 2: Moleküle und Photonen - Spektroskopie und Streuphysik”, Springer, Berlin, 1st Edition (May 2011)“