“…Impulsive stimulated Raman scattering study [27] confirmed this fact: the relaxation time of soft mode, τ, which shows critical slowing down, i.e., τ = τ 0 T c /(T -T c ), has a value of τ 0 about eight times higher for DKDP than for KDP. Second, in contrast to the Raman scattering spectra of transverse modes [20], but in accordance with the Raman scattering spectra of longitudinal modes [21], the S-peak was found in the infrared spectra of the FE phase of DKDP at a frequency higher than in KDP [31]. Third, while the S-mode in KDP, even if it exists in the PE phase, contributes to the CP in the infrared spectra, the S-peak in DKDP is clearly visible in the infrared spectra of the PE phase, where it is well resolved from the CP [31].…”