Question

what is raman spectra

Answer 1

Raman spectroscopy is one of the vibrational spectroscopic techniques used to provide information on molecular vibrations and crystal structures.

Answer 2

Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system.[1] Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.
 is the inelastic scattering of a photon by molecules which are excited to higher vibrational or rotational energy levels. It was discovered by C. V. Raman and K. S. Krishnan (who was a student of C.V. Raman) in liquids,[1] and independently by Grigory Landsberg and Leonid Mandelstam in crystals.[2] The effect had been predicted theoretically by Adolf Smekal in 1923.[3]

When photons are scattered from an atom or molecule, most of them are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. A small fraction of the scattered photons (approximately 1 in 10 million) are scattered inelastically by an excitation, with the scattered photons having a frequency and energy different from, and usually lower than, those of the incident photons.[4] In a gas, Raman scattering can occur with a change in energy of a molecule due to a transition to another (usually higher) energy level. Chemists are primarily concerned with this "transitional" Raman effect.