Resonance enhanced Raman scatter in liquid benzene at vapor-phase absorption peaks

Abstract

The resonance enhanced Raman spectra in the 1B2u mode of the forbidden benzene electronic transition band, ~230-270 nm, has been investigated. Resonance enhanced Raman scattering in both liquid benzene and liquid toluene exhibit the greatest enhancement when the wavelength of excitation is tuned to the vapor-phase absorption peaks; even though the sample volume is in a liquid state. Raman signals for the symmetric breathing mode of the carbon ring are found to be resonantly enhanced by several orders of magnitude (>500X) with deep UV excitation compared to non-resonant visible excitation. Since the benzene absorbs near this resonant wavelength, its effect on the sampled volume cannot be neglected in determining the resonance gain, as we discuss in detail. Large resonant gains correspond with excitation at the 247, 253, and 259 nm absorption peaks in the benzene vapor spectrum. The narrow region of resonance gain is investigated in detail around the absorption peak located at 259 nm using 0.25 nm steps in the excitation wavelength. We observe the resonance gain tracking the vapor phase absorption peaks and valleys within this narrow range. Results are interpreted in terms of the coherence forced by the use of a forbidden transition for resonance excitation.

DNA Methylation Detection Using Resonance and Nanobowtie-Antenna-Enhanced Raman Spectroscopy

“…9 a shows the spectra normalized by the usual method, and the sharp increase is evident. We note that although the maximal relative scale factor of 15 in the data within the figure is not as large as the resonance gain of $500 in the phonon-allowed absorption resonance, as shown in our previous measurements (33), all spectra in Fig. 9 are at least partially resonant, so the resonance gain here is larger than 15.…”

“…Therefore, many Raman-based methylation assays read not the intrinsic signal but rather that of a Raman-active label that can be resonantly excited, thus losing the inherent advantage of Raman scattering (28,30,32). We utilize resonance excitation at wavelengths in the ultraviolet region to tune excitation wavelength to an absorption resonance of the targeted 5mC or nonmethylated cytosine at the ring absorption resonance (33)(34)(35). In this way, the Raman signal is enhanced without the need for a label.…”

UV–Vis spectroscopy of tyrosine side-groups in studies of protein structure. Part 1: basic principles and properties of tyrosine chromophore

“…Figure 9 shows Raman spectra of liquid benzene for several excitation wavelengths near 259 nm, separated by 0.25 nm (Willitsford et al 2013 ). These excitation frequencies belong to the region of the first excited state of this molecule ( 1 L b ).…”

“…9 Resonance Raman spectra of liquid benzene plotted for several excitation wavelengths (each spectrum is offset). Taken from Willitsford et al ( 2013 ) …”

“…The frequency 992 cm −1 corresponds to the totally symmetric benzene ring breathing mode (Willitsford et al 2013 ; Ziegler and Hudson 1981 ; Asher and Johnson 1985 ), labeled by using the Wilson convention (Wilson 1934 ) with the ν 1 symbol. The second resonance-enhanced Raman peak at ∼1200 cm −1 was interpreted as the strengthened v 15 (1150 cm −1 ) vibrational mode according to the Wilson convention (Willitsford et al 2013 ). Graphic representation of the two normal modes of benzene assigned to the enhanced Raman bands observed in the excitation with 259-nm radiation is shown in Fig.…”