Q1(m). Raman effect against fluorescence and phosphorescence

Answer in brief

Raman effect: inelastic scattering of monochromatic light, giving weak lines at ν₀ ± νm; it is instantaneous scattering through a virtual state, not absorption followed by emission from a real excited state.

Key points

• Monochromatic light ν₀ falls on molecules; most is scattered unchanged (Rayleigh, ν₀).
• About 10⁻⁶ to 10⁻⁷ of it appears at ν₀ − νm (Stokes, stronger) and ν₀ + νm (anti-Stokes, weaker).
• Rule: the polarisability must change during the rotation or vibration.
• Raman takes ∼10⁻¹⁴ s; fluorescence (S₁ → S₀, spin-allowed) 10⁻⁹ to 10⁻⁷ s; phosphorescence (T₁ → S₀, spin-forbidden) 10⁻⁴ s to minutes.
• Raman lines move with ν₀ with fixed shifts on both sides; fluorescence and phosphorescence are shifted to longer λ only.