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Theory of resonance Raman scattering by molecules subject to twisting with an application to ethylene
Author(s) -
Siebrand Willem,
Zgierski Marek Z.
Publication year - 1990
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250210409
Subject(s) - raman spectroscopy , rydberg formula , resonance (particle physics) , excitation , coherent anti stokes raman spectroscopy , molecular physics , chemistry , raman scattering , x ray raman scattering , atomic physics , spectral line , absorption spectroscopy , molecular vibration , physics , optics , quantum mechanics , ionization , ion , organic chemistry
Model calculations are reported on molecules such as ethylene that twist on electronic excitation. Resonance Raman spectra and excitation profiles are calculated for several pairs of torsional potentials. For a 90° twist, the resonance Raman spectra show double‐quantum torsional progressions that tend to be very long for resonance in the region of strong absorption. The Raman excitation profiles of the torsional‐mode bands resemble the absorption spectrum in that most of the intensity is found in the region of near‐vertical transitions, where the upper‐state torsional levels change from vibrational to rotational. This picture remains valid if the upper state has minima at angles other than 90° relative to the ground‐state minimum, provided that the point group symmetry of the molecule is not lower than D 2 at these points; this is the case for ethylene. If, however, the minimum corresponds to a lower point group, as for asymmetrically substituted ethylenes, Raman transitions involving an odd number of quanta become allowed. For very small angles of twist (≲ 15°) such systems resemble displaced harmonic oscillators. These results are compared with recently reported resonance Raman spectra in the UV absorption region of ethylene where both π→π* and Rydberg transitions are active. The analysis suggests a new potential for torsion in the first Rydberg state.