Vibrational spectroscopy of aromatic compounds: Is hyper-Raman spectroscopy worth the effort?

摘要

Hyper-Raman spectroscopy is a nonlinear optical variant of vibrational spectroscopy to acquire information about molecular structures. Hyper-Raman spectroscopy complements existing infrared and Raman spectroscopy techniques due to differences in the selection rules. Performing hyper-Raman spectroscopy at 532 nm results in the signal emission in the UV spectral range (266 nm - 296 nm), which benefits from near-resonant conditions for many biomolecules. Even operating in the electronic resonant enhancement regime, hyper-Raman spectroscopy requires high average and peak power, picosecond laser systems to achieve reasonable collection times (1 minute - 30 minutes). In this report, we explore applications of hyper-Raman spectroscopy to aromatic structures (L-phenylalanine and imidazole) that experience significant two-photon absorption and two-photon fluorescence which can obstruct measurements of the hyper-Raman spectra of these molecules. Since competing two-photon processes could significantly limit future UV hyper-Raman applications, we explore mitigating strategies to circumvent the fluorescence background of L-phenylalanine and imidazole by applying a quenching agent (hydrogen peroxide). We also outline a more general solution to alleviate two-photon absorption and fluorescence by proposing tailored laser configurations where the excitation wavelength could be tuned to avoid two-photon absorption resonances while remaining in the UV regime.