  A measurement of the energy dependence of antineutrino disappearance at the
Daya Bay Reactor Neutrino Experiment is reported. Electron antineutrinos
($\overline{\nu}_{e}$) from six $2.9$ GW$_{\rm th}$ reactors were detected with
six detectors deployed in two near (effective baselines 512 m and 561 m) and
one far (1579 m) underground experimental halls. Using 217 days of data, 41589
(203809 and 92912) antineutrino candidates were detected in the far hall (near
halls). An improved measurement of the oscillation amplitude
$\sin^{2}2\theta_{13} = 0.090^{+0.008}_{-0.009} $ and the first direct
measurement of the $\overline{\nu}_{e}$ mass-squared difference $|\Delta
m^{2}_{ee}|= (2.59_{-0.20}^{+0.19}) \times 10^{-3}\ {\rm eV}^2 $ is obtained
using the observed $\overline{\nu}_{e}$ rates and energy spectra in a
three-neutrino framework.
  This value of $|\Delta m^{2}_{ee}|$ is consistent with $|\Delta
m^{2}_{\mu\mu}|$ measured by muon neutrino disappearance, supporting the
three-flavor oscillation model.
