  We have performed Raman-scattering measurements on high-quality single
crystals of the superconductors K$_{0.8}$Fe$_{1.6}$Se$_2$ ($T_c$ = 32 K),
Tl$_{0.5}$K$_{0.3}$Fe$_{1.6}$Se$_2$ ($T_c$ = 29 K), and
Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.6}$Se$_2$ ($T_c$ = 31 K), as well as of the
insulating compound KFe$_{1.5}$Se$_2$. To interpret our results, we have made
first-principles calculations for the phonon modes in the ordered iron-vacancy
structure of K$_{0.8}$Fe$_{1.6}$Se$_2$. The modes we observe can be assigned
very well from our symmetry analysis and calculations, allowing us to compare
Raman-active phonons in the AFeSe compounds. We find a clear frequency
difference in most phonon modes between the superconducting and
non-superconducting potassium crystals, indicating the fundamental influence of
iron content. By contrast, substitution of K by Tl or Rb in
A$_{0.8}$Fe$_{1.6}$Se$_2$ causes no substantial frequency shift for any modes
above 60 cm$^{-1}$, demonstrating that the alkali-type metal has little effect
on the microstructure of the FeSe layer. Several additional modes appear below
60 cm$^{-1}$ in Tl- and Rb-substituted samples, which are vibrations of heavier
Tl and Rb ions. Finally, our calculations reveal the presence of "chiral"
phonon modes, whose origin lies in the chiral nature of the
K$_{0.8}$Fe$_{1.6}$Se$_2$ structure.
