  The central star and its energetic radiation fields play a vital role in
setting the vertical and radial chemical structure of planet-forming disks. We
present observations that, for the first time, clearly reveal the UV-irradiated
surface of a protoplanetary disk. Specifically, we spatially resolve the
atomic-to-molecular (C I-to-CO) transition in the IM Lup disk with ALMA
archival observations of [C I] $^3$P$_1$-$^3$P$_0$. We derive a C I emitting
height of z/r $\gtrsim$ 0.5 with emission detected out to a radius of
${\approx}$600 au. Compared to other systems with C I heights inferred from
unresolved observations or models, the C I layer in the IM Lup disk is at scale
heights almost double that of other disks, confirming its highly flared nature.
C I arises from a narrow, optically-thin layer that is substantially more
elevated than that of $^{12}$CO (z/r $\approx$ 0.3-0.4), which allows us to
directly constrain the physical gas conditions across the C I-to-CO transition
zone. We also compute a radially-resolved C I column density profile and find a
disk-averaged C I column density of 2$\times10^{16}$ cm$^{-2}$, which is
${\approx}$3-20$\times$ lower than that of other disks with spatially-resolved
C I detections. We do not find evidence for vertical substructures or
spatially-localized deviations in C I due, e.g., to either an embedded giant
planet or a photoevaporative wind that have been proposed in the IM Lup disk,
but emphasize that deeper observations are required for robust constraints.
