Tunable diode laser absorption spectroscopy (TDLAS) is used to measure the 6s 2 1 S-5d6p 3 D 0 absorption line profile of a Ba atomic beam produced in a molecular beam epitaxy (MBE) reactor. Despite the noisy MBE environment, a signal-to-noise ratio up to 100 is obtained thanks to a thorough optimization of the measurement setup. A model that realistically describes this absorption profile is presented, taking into account the angular distribution of atomic concentration in the atomic beam as well as the reactor and setup geometry. Our results highlight the influence of the latter on the Doppler line broadening, and allows to estimate the angular distribution of concentration in the beam emitted by the effusion cell. The model can be adapted to estimate the absorption profile for any element in any reactor geometry, making it a useful tool to design MBE dedicated atomic absorption sensors such as optical flux monitoring sensors, or to characterize the beam emitted by any effusion cell.