Observation of photon recoil effects in single-beam absorption spectroscopy with an ultracold strontium gas

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 frequency is comparable to natural linewidth narrow-line transition $5\mathrm{s^2}\, ^1\textrm{S}_0 - 5\mathrm{s}5\mathrm{p}\, ^3\textrm{P}_1$ strontium. In regime high-saturation, profile becomes asymmetric due photon-recoil shift, which same order as linewidth. The lineshape described by extension optical Bloch equations including momentum transfers atoms during emission and photons. Our work reveals simplest single-beam setting, significant relevance other applications such saturation spectroscopy, Ramsey interferometry, imaging.