* It is a non-linear molecule: While Br₂ is a diatomic molecule, the two bromine atoms are connected by a single bond, making the molecule linear. This means the molecule has a non-zero polarizability tensor.
* It undergoes rotational transitions: When a Br₂ molecule absorbs light, it can transition to a higher rotational energy level. This transition is accompanied by a change in the molecule's rotational energy, which in turn affects its polarizability.
* The change in polarizability is anisotropic: The polarizability of a Br₂ molecule is not the same in all directions. This means the molecule's polarizability changes as it rotates. This anisotropy is key to Raman scattering.
How Raman Scattering Works:
In Raman scattering, light interacts with a molecule, causing it to undergo a vibrational or rotational transition. This interaction can either increase (Stokes scattering) or decrease (anti-Stokes scattering) the energy of the scattered light.
* For rotational Raman scattering, the change in the molecule's rotational energy leads to a shift in the frequency of the scattered light. This shift is called the Raman shift.
* The Raman shift is proportional to the change in rotational energy, which is determined by the molecule's rotational constant and the change in rotational quantum number.
In summary: Because Br₂ is a linear molecule with a non-zero polarizability tensor and exhibits rotational transitions that change its polarizability anisotropically, it is Raman active. This means it can undergo rotational Raman scattering, which leads to a characteristic Raman spectrum.
