Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi’s Golden Rule Analysis

The vibrational relaxation dynamics following an ultrafast nitrile stretching (ν₃) excitation of thiocyanate anions dissolved in light and heavy water have been studied over a wide temperature and density range corresponding to the aqueous liquid up to the supercritical phase. In both solvents, the relaxation of the ν₃ = 1 state of the anion leads to a direct recovery of the vibrational ground state and involves the resonant transfer of the excess vibrational energy onto the solvent. In light water, the energy-accepting states are provided by the bending–librational combination band (ν_b + ν_L), while in heavy water, the relaxation is thermally assisted by virtual acceptor states derived from the stretching–librational/restricted translational hot band (ν_S – ν_L,T). The relaxation rate is found to strictly obey Fermi’s Golden Rule when the density of resonant solvent states is estimated from the linear infrared spectra of the solute and the pure solvents.