  We investigate the thermal and kinematic configuration of a sunspot penumbra
using very high spectral and spatial resolution intensity profiles of the
non-magnetic Fe I 557.6 nm line. The dataset was acquired with the 2D solar
spectrometer TESOS. The profiles are inverted using a one-component model
atmosphere with gradients of the physical quantities. From this inversion we
obtain the stratification with depth of temperature, line-of-sight velocity,
and microturbulence across the penumbra. Our results suggest that the physical
mechanism(s) responsible for the penumbral filaments operate preferentially in
the lower photosphere. We confirm the existence of a thermal asymmetry between
the center and limb-side penumbra, the former being hotter by 100-150 K on
average. We also investigate the nature of the bright ring that appears in the
inner penumbra when sunspots are observed in the wing of spectral lines. The
line-of-sight velocities retrieved from the inversion are used to determine the
flow speed and flow angle at different heights in the photosphere. Both the
flow speed and flow angle increase with optical depth and radial distance.
Downflows are detected in the mid and outer penumbra, but only in deep layers
(log tau_{500} < -1.4). We demonstrate that the velocity stratifications
retrieved from the inversion are consistent with the idea of penumbral flux
tubes channeling the Evershed flow. Finally, we show that larger Evershed flows
are associated with brighter continuum intensities in the inner center-side
penumbra. Dark structures, however, are also associated with significant
Evershed flows. This leads us to suggest that the bright and dark filaments
seen at 0.5" resolution are not individual flow channels, but a collection of
them.
