Abstract
The article presents the density matrix theory describing a three-level system in single- or multi-photon resonant interaction with two finite-bandwidth lasers and subject to high pressure. Under the steady-state and the weak-probe approximations, analytic solutions for the excitation spectra and the conditions for resolving the ac Stark splitting (also known as the Autler-Townes effect) are given. Calculations show that the resolution of the ac Stark doublet, observed in the two-color, three-photon 1s→3p excitation spectra of atomic hydrogen produced in H 2/air atmospheric-pressure flames, is greatly improved by eliminating the Doppler contribution of the 1s→2s two-photon transition. It is found that as pressure is increased, the asymmetry parameter of the doublet decreases; and at high enough pressure (e.g. several atm), the doublet becomes one broad resonance.
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