Resonance-Enhanced Raman Scattering of Ring-Involved Vibrational Modes in the 1B2u Absorption Band of Benzene, Including the Kekule Vibrational Modes ν9 and ν10

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• 作者：Adam H. Willitsford ; C. Todd Chadwick ; Stewart Kurtz ; C. Russell Philbrick ; Hans Hallen
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：February 4, 2016
• 年：2016
• 卷：120
• 期：4
• 页码：503-506
• 全文大小：244K
• ISSN：1520-5215

文摘

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.