Polarization dependent two photon absorption (2PA) is a powerful spectroscopic tool for complete characterization of the excited electronic states of molecules since it is complementary to the traditional one-photon absorption (1PA) spectroscopy. On the other hand, lack of complete spectroscopic information of a species in bulk hinders full characterization of surface specific E-SFG (electronic sum frequency generation) spectra. An active electronic transition in E-SFG requires it to be active in both 1PA and 2PA. This relation between 1PA, 2PA and E-SFG spectra can help decipher unresolved key features and reveal new details about the excited molecular electronic structure. Thus, we applied these spectroscopic techniques to study the electronic structure of phenol, a prototypical aromatic biochromophore. Also, as a model aromatic amphiphile, the difference in phenol’s spectroscopy at air/water interface and in bulk will be valuable in studying organic aerosol particles and their formation.<br/>Moreover, the combination of different symmetry selection rules for 1PA and 2PA along with the experimental selection of different relative polarizations of the 2 photons absorbed in 2PA can be powerfully used to infer the symmetry of excited electronic states of a molecular species. Electronic structure calculations and theoretical polarization ratios of phenol in gas phase and in water, combined with bulk-specific and surface- specific experimental spectra unravel the complex, multiple electronic excited states of phenol and their vibronic coupling (e.g., a low lying ππ*/πσ* conical intersection). This study also reports on phenol-water hydrogen bonding patterns and interactions and how they contrast in the bulk and at the aqueous interface.

Excited electronic states of phenol: A combined polarization dependent 2PA and E-SFG study

ACS Spring 2022

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