SME
Physical Chemistry II: Quantum & Spectroscopy · Problem type
Allowed or forbidden transition?
Use this for “Does N₂ absorb infrared light at its vibrational frequency? Does HCl show a line for J = 0 → 2?”
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Is this transition allowed or forbidden for one photon?
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Check two things: does the dipole couple to the light at all, and does the jump obey Δ = ±1 for its quantum number?
Assumptions
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Absorbing or giving out one photon needs the molecule’s dipole to couple to the light, and the photon carries one unit of angular momentum.
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Rotation: a permanent dipole and ΔJ = ±1. Vibration: a dipole that changes as the bond stretches, and Δv = ±1 for a harmonic oscillator.
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Hydrogen’s electron: Δl = ±1, while n may change by any amount.
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Population ratio of two levels (Boltzmann)
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