Raman scattering
= Raman scattering
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= Raman effect
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<Raman scattering> is <inelastic scattering> of <light> in which the outgoing <photon> has a different <frequency> because matter gains or loses internal <energy>. <Molecular vibration> is a particularly useful recipient: a visible <photon> can reveal an internal <energy> spacing normally associated with <infrared> <frequencies>. <Rayleigh scattering> supplies the much stronger, approximately unchanged-<frequency> reference. The distinction concerns the transferred <energy>, rather than merely a change of direction or the colour of the illuminating beam.
The elementary bookkeeping is <conservation of energy>:
$$
h\nu_0+E_i=h\nu_s+E_f,\qquad h(\nu_0-\nu_s)=E_f-E_i.
$$
Here $h$ is the <Planck constant>, $\nu_0$ and $\nu_s$ are the incident and scattered <photon> <frequencies>, and $E_i,E_f$ are initial and final internal molecular <energies>. This expression neglects translational recoil and <Doppler broadening>; those can matter in sufficiently precise gas measurements. A positive internal <energy> change gives <Stokes Raman scattering>, and a negative one gives <Anti-Stokes Raman scattering>. The <energy> difference determines a possible line position, but it does not by itself establish that a transition is optically allowed or predict its strength.
The spontaneous molecular process developed here has a connection to the <Raman transition> used in <atomic measurement methods>: two <photon> interactions transfer an internal <energy> difference. Coherently driving that process with two applied <light> fields is a different experimental regime from collecting weak spontaneous <Raman scattering> beside an illuminating beam.