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Resonance-enhanced Raman scattering of ring-involved vibrational modes in the ¹B_2u absorption band of benzene, including the Kekule Vibrational Modes ν₉ and ν₁₀

Document ID: 219

Willitsford, Adam H.¹
Chadwick, Christopher Todd²
Kurtz, Stewart¹
Philbrick, C. Russell^1,2
Hallen, Hans D.²

¹ The Pennsylvania State University, School of Electrical Engineering and Computer Science, University Park, PA, U.S.A.
² North Carolina State University, Department of Physics, Raleigh, NC, U.S.A.
 

Abstract

Resonance Raman spectroscopy provides much stronger Raman signal levels than its off-resonant counterpart and adds selectivity by excitation tuning. Raman preresonance of benzene has been well studied. On-resonance studies, especially at phonon-allowed absorptions, have received less attention. In this case, we observe resonance of many of the vibration modes associated motion of the carbons in the ring while tuning over the ¹B_2u absorption, including the related ν₉ (CC stretch Herzberg notation, ν₁₄ Wilson notation) and ν₁₀ (CH-parallel bend Herzberg notation, ν₁₅ Wilson notation) vibrational modes along with the ν₂ (CC-stretch or ring-breathing Herzberg notation, ν₁ Wilson notation) mode and multiples of the ν₁₈ (CCC-parallel bend Herzberg notation, ν₆ Wilson notation) vibrational mode. The ring-breathing mode is found to mix with the b_2u modes creating higher frequency composites. Through the use of an optical parametric oscillator (OPO) to tune through the ¹B_2u absorption band of liquid benzene, a stiffening (increase in energy) of the vibrational modes is observed as the excitation wavelength nears the ¹B_2u absorption peak of the isolated molecule (vapor) phase. The strongest resonance amplitude observed is in the 2 × ν₁₈ (e_2g) mode, with nearly twice the intensity of the ring-breathing mode, ν₂. Several overtones and combination modes, especially with ν₂ (a_1g), are also observed to resonate. Raman resonances on phonon-allowed excitations are narrow and permit the measurement of vibrations not Raman-active in the ground state.

 

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Keywords: resonance Raman, Raman scatter, benzene

Citation:

"Resonance-enhanced Raman scattering of ring-involved vibrational modes in the 1B2u absorption band of benzene, including the Kekule Vibrational Modes ν9 and ν10", Willitsford, A. H., C. T. Chadwick, S. Kurtz, C. R. Philbrick, H. D. Hallen, Journal of Physical Chemistry, American Chemical Society, 2016, pp. 1 - 4, DOI: 10.1021/acs.jpca.5b08159