Two- and one-photon absorption spectra of aqueous thiocyanate anion highlight the role of symmetry in condensed phase

Calculation of the two-photon (2PA) absorption spectrum of aqueous thiocyanate using high-level quantum-chemistry methods is presented. The 2PA spectrum is compared to the 1PA spectrum computed using the same computational protocol. Although the two spectra probe the same set of electronic states, the intensity patterns are different, leading to an apparent red-shift of the 2PA spectrum relative to the 1PA spectrum. The presented analysis explains the intensity patterns and attributes the differences between 1PA and 2PA to the native symmetry of isolated SCN-, which influences the spectra in the low-symmetry solution environment. The native symmetry also manifests itself in variations in the polarization ratio (e.g., parallel versus perpendicular cross sections) across the spectrum. The presented results highlight the potential of the 2PA spectroscopy and high-level quantum-chemistry methods in studies of condensed phase phenomena.