This work focuses on visualization of dark states arising in systems
of hyperfine states upon two-photon excitation. In our experiment, levels 3S 1/2 and 3P 1/2 of sodium atoms are coupled by a strong S-laser field forming the laser-dressed adiabatic states, which are then monitored by scanning a weak probe P-field across the 3P 1/2 → 7D 3/2 transition. The excitation spectrum of the 7D 3/2 state exhibits one main peak and several side peaks. The latter shift farther apart with increasing S-laser intensity, while the position of the main peak is hardly affected, which associates its origin with a class of dark states decoupled from laser interactions. The results of numerical simulation of the observed spectra based on the solution of the optical Bloch equations are in good agreement with the experimental data. The results are of interest for
atomic and laser physics.