Critical Harmonic Scattering in NH4Cl

Observations of harmonic scattering (second-harmonic generation) by fluctuations in angular orientation are reported for polycrystalline N${\mathrm{H}}_4$Cl for temperatures in the immediate vicinity of the second-order phase transition at ∼242.4°K. Harmonic intensities were found to decrease exponentially within the range $2\times {10}^{-4\mathrm{}}<~\epsilon <2\mathrm{}\times {10}^{-2\mathrm{}}$, where $\epsilon =\frac{T}{T_c}-1\mathrm{}$. The results, together with suitable assumptions, are used to obtain an estimate of the variation of the long-range correlation parameter ${\kappa }^{-1\mathrm{}}$ with $\epsilon$, with results different from those expected.