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We present deep spectroscopic observations of a Lyman-alpha emitter (LAE)candidate at z ~ 7.7 using the infrared spectrograph LUCI on the 2 x 8.4m LargeBinocular Telescope (LBT). The candidate is the brightest among the four z ~7.7 LAE candidates found in a narrow-band imaging survey by Krug et al. 2012.Our spectroscopic data include a total of 7.5 hours of integration withLBT/LUCI and are deep enough to significantly (3.2-4.9 sigma) detect theLyman-alpha emission line of this candidate, based on its Lyman-alpha flux 1.2x 10^{-17} erg s^{-1} cm^{-2} estimated from the narrow-band photometry.However, we do not find any convincing signal at the expected position of itsLyman-alpha emission line, suggesting that this source is not an LAE at z ~7.7. The non-detection in this work, together with the previous studies of z ~7.7 LAEs, puts a strong constraint on the bright-end Lyman-alpha luminosityfunction (LF) at z ~ 7.7. We find a rapid evolution of the Lyman-alpha LF fromz ~ 6.5 to 7.7: the upper limit of the z ~ 7.7 LF is more than 5 times lowerthan the z ~ 6.5 LF at the bright end (f > 1.0 x 10^{-17} erg s^{-1} cm^{-2},or L > 6.9 x 10^{42} erg s^{-1}). This is likely caused by an increasingneutral fraction in the IGM that substantially attenuates Lyman-alpha emissionat z ~ 7.7.

Deep LBT/LUCI Spectroscopy of a Lyman-alpha Emitter Candidate at z ~ 7.7