The problem of correctly constructing atomic kinetics in plasma under the action of laser pulses of various durations is considered. Based on the use of the generalized probability of temporal evolution of the photoprocess, calculations of laser fluorescence signals on helium lines in plasma with temperature ~10 eV and density ~ 10¹² cm^-3 were performed. The laser pumping and fluorescence scheme was modeled for the E-1 setup plasma (prototype of an electrode-less plasma rocket engine) for different numbers of modes in the pulse. Data on the temporal evolution of the populations of levels responsible for the fluorescence signal under the action of laser pulses are presented. Keywords: laser pulse, plasma, atomic kinetics, perturbation theory.