Knowledge Bank

The one and two phonon Raman bands of $NH4Cl$ and $ND4Cl$ have been observed at about $15\circ K$, using polarization. Measurements of the splitting and relative intensities of the transverse and longitudinal components of infrared allowed $(F2)$ fundamentals and using the known value of the nonlinear susceptibility in principle permits the evaluation of both the infrared $(\partial \mu \partial q)$ and Raman $(\partial \alpha \partial q)$ molecular intensity parameters as originally pointed out by Poulet. Then, by further simple relative intensity measurements of the $A1$ and E modes compared with the $F2$ modes, all molecular intensity parameters of the ammonium ion can be determined without any absolute cross section measurements. The resultant dipole derivatives are analyzed in terms of effective atomic charges. Several two phonon bands have been studied, notably $2\nu 6$, $\nu 4+\nu 6$, $2\nu 4$ and $\nu 1+\nu 5$. Those which are expected to contain an $A1$ symmetry component at $k=0$, i.e, $2\nu 6$ and $2\nu 4$, exhibit a sharp, partly separated component at the low frequency edge which is $A1$ polarized. In such cases, an-harmonicity has apparently separated the $\Delta \nu =2$ one phonon transition from the rest of the two-phonon band. The observed frequencies for $2\nu 6$ are in excellent agreement with an anharmonic potential function proposed by H""{u}ller and Kane.