Symmetry selection of IR and Raman modes
A transition matrix element ⟨initial|O|final⟩ can be nonzero only when Γinitial*⊗Γoperator⊗Γfinal contains the totally symmetric representation.
Key knowledge
A transition matrix element ⟨initial|O|final⟩ can be nonzero only when Γinitial*⊗Γoperator⊗Γfinal contains the totally symmetric representation. The bra transforms in the complex-conjugate representation of the initial ket; this distinction leaves real character-table examples unchanged. Electric-dipole IR operators transform as the Cartesian vector components; Raman polarizability components transform as quadratic Cartesian functions. Character tables therefore distinguish the symmetry conditions for IR and Raman activity.
Conditions and scope
- A symmetry-allowed transition can still be weak or have zero intensity for additional dynamical reasons.
- Use the actual molecular symmetry and appropriate operator; degeneracy is not a separate frequency for each component.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Massachusetts Institute of Technology2 sources
- Lecture 30: Character Tables and Spectroscopic Selection, pp.1–5Robert W. Field — MIT OpenCourseWare, Small-Molecule Spectroscopy and Dynamics (Fall 2008)Textbook chapter · PDF
- Lectures 30–31: Molecular Spectroscopy, pp.2–5Robert W. Field — MIT OpenCourseWare, Physical Chemistry (Fall 2017)Textbook chapter · PDF
Evidence and review
Read matrix-element/triple-product condition, Cartesian/quadratic operator columns and real D3h BCl3 displacement-reduction example on pp.1–5. Explicit complex-conjugate bra is inferred from the displayed bra/ket matrix-element definition; the teaching example itself uses real representations. Excluded universal intensity and all-selection-rules-known language.
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