The time-dependent population dynamics of hyperfine (HF) sublevels of n²p_3/2 atomic states upon laser excitation in a cold medium of alkali atoms is examined. We demonstrate some peculiarities of the absorption HF multiplet formation in D2-line resulting from a long interaction time (~200 μs) interaction between light and Na (n = 3) and Cs (n = 6) atoms in a cold and slow sub-thermal (T ~ 1K) beam. We analytically describe a number of D2-line-shape effects that are of interest in spectroscopic studies of cold dusty white dwarfs: broadening by optical pumping, intensity redistribution within components of D2-line HF multiplet for partially closed transitions and asymmetry of absorption lines induced by AC Stark shifts for cyclic transitions.