  We report on observing photon recoil effects in the absorption of a single
monochromatic light at 689~nm through an ultracold $^{88}$Sr gas, where the
recoil frequency is comparable to natural linewidth of the narrow-line
transition $5\mathrm{s^2}\, ^1\textrm{S}_0 - 5\mathrm{s}5\mathrm{p}\,
^3\textrm{P}_1$ in strontium. In the regime of high-saturation, the absorption
profile becomes asymmetric due to the photon-recoil shift, which is of the same
order as the natural linewidth. The lineshape is described by an extension of
the optical Bloch equations including the momentum transfers to atoms during
emission and absorption of photons. Our work reveals the photon recoil effects
in a simplest single-beam absorption setting, which is of significant relevance
to other applications such as saturation spectroscopy, Ramsey interferometry,
and absorption imaging.
