  Spin excitations stemming from the metallic phase of the ferrochalcogenide
superconductor K(0.77)Fe(1.85)Se(2) (T_c=32 K) were mapped out in the ab plane
by means of the time-of-flight neutron spectroscopy. We observed a magnetic
resonant mode at Q_res=(1/2 1/4), whose energy and in-plane shape are almost
identical to those in the related compound Rb(0.8)Fe(1.6)Se(2). This lets us
infer that there is a unique underlying electronic structure of the bulk
superconducting phase K(x)Fe(2)Se(2), which is universal for all alkali-metal
iron selenide superconductors and stands in contrast to the doping-tunable
phase diagrams of the related iron pnictides. Furthermore, the spectral weight
of the resonance on the absolute scale, normalized to the volume fraction of
the superconducting phase, is several times larger than in optimally doped
BaFe(2-x)Co(x)As(2). We also found no evidence for any additional low-energy
branches of spin excitations away from Q_res. Our results provide new input for
theoretical models of the spin dynamics in iron based superconductors.
