Determine the angular frequency \(\omega_{k}\) for the Brillouin peaks in water for \(90^{\circ}\) laser scattering, using a He-Ne laser with \(\lambda=632.8 \mathrm{~nm}\). Determine the width of the Rayleigh peak and show that the Brillouin peaks are well-separated from the Rayleigh peak. The thermal diffusivity of water is \(D_{T}=1.4 \times 10^{-7} \mathrm{~m}^{2} / \mathrm{s}\).