Autler-Townes dressed states in Fraunhofer-type absorption of Pb laser-induced plasma

Systematic studies were conducted to elucidate the mechanism of splitting and polarization of the Pb Fraunhofer-type absorption (FTA) lines at 363.95 nm and 283.3 nm. These two lines share the same upper absorption level. It was found that the line splittings ΔU_obs, expressed in wavenumber units, are proportional to the square root of the second laser pulse intensity.The two lines show similar splittings in wavenumber units, approximately 26.4 and 24.9 cm⁻¹, and the wavelength separation Δλ differs, as expected, proportionally to the square of the central wavelength of the line (Δλ ∝ λ₀²⁾. These results are characteristic of the Autler–Townes (AT) dressed-state mechanism, indicating that this mechanism is responsible for the FTA line splitting and polarization. A simplified AT model is presented. Based on this model, the effective dipole moment of the dressing transition, d_eff, was estimated. The polarization of the split lines is explained by considering magnetic sublevels m, selection rules, and pump-induced alignment of these sublevels. A strongly polarized absorption line at 285.57 nm, proportional to the second-laser intensity, was also observed. The 285.57 nm line is not listed in spectral databases. We propose that this line may appear due to the resonantly enhanced two-photon absorption (TPA) in Pb I atoms. This transition may be driven coherently by one UV continuum photon and one 1064-nm pump photon in DP LIP. Transitions are driven coherently by one continuum UV photon and one 1064 nm pump photon in Double-Pulse Laser–Induced Plasma (DP LIP). This hypothesis needs additional experimental and theoretical investigations.