  A set of 464-min high-resolution high-cadence observations were acquired for
a region near the Sun's disk center using the Interferometric BI-dimensional
Spectrometer (IBIS) installed at the Dunn Solar Telescope. Ten sets of
Dopplergrams are derived from the bisector of the spectral line corresponding
approximately to different atmospheric heights, and two sets of Dopplergrams
are derived using MDI-like algorithm and center-of-gravity method. These data
are then filtered to keep only acoustic modes, and phase shifts are calculated
between Doppler velocities of different atmospheric heights as a function of
acoustic frequency. The analysis of the frequency- and height-dependent phase
shifts shows that for evanescent acoustic waves, oscillations in the higher
atmosphere lead those in the lower atmosphere by an order of 1 s when their
frequencies are below about 3.0 mHz, and lags behind by about 1 s when their
frequencies are above 3.0 mHz. Non-negligible phase shifts are also found in
areas with systematic upward or downward flows. All these frequency-dependent
phase shifts cannot be explained by vertical flows or convective blueshifts,
but are likely due to complicated hydrodynamics and radiative transfer in the
non-adiabatic atmosphere in and above the photosphere. These phase shifts in
the evanescent waves pose great challenges to the interpretation of some local
helioseismic measurements that involve data acquired at different atmospheric
heights or in regions with systematic vertical flows. More quantitative
characterization of these phase shifts is needed so that they can either be
removed during measuring processes or be accounted for in helioseismic
inversions.
