LINE-SHAPE, FREQUENCY-SHIFT, RABI SPLITTING, AND 2-PHOTON RESONANCES IN 4-LEVEL DOUBLE-RESONANCE SPECTROSCOPY WITH CLOSELY SPACED INTERMEDIATE LEVELS

The interaction of a four-level atomic system with two electromagnetic radiations has been studied. The two intermediate levels are closely spaced such that a single radiation frequency can induce transitions f rom the ground state to both of them. In a cascade-type double resonan ce, the pump electric-field amplitude is much stronger than the amplit ude of the signal field, which can induce transitions from both of the intermediate levels to the upper state. The 15 optical Bloch equation s obtained from the Liouville equations are solved analytically to obt ain the Doppler-free signal line shape. The computed signals, exhibiti ng two Lorentzian line profiles, are presented for different pump freq uencies. In addition to these one-photon transitions, we obtain a weak two-photon resonance peak. The two-photon resonance becomes weaker wh en the detuning of the pump frequency from a resonance frequency is in creased. When the pump frequency OMEGA(p) is intermediate between the pump resonance frequencies omega1 and omega2, the two-photon resonance is very weak and almost vanishes when OMEGA(p) = (omega1 + omega2)/2. When the pump frequency is on resonance with one of the transition fr equencies, the Rabi splitting of the signal shows asymmetry because of the presence of a close-lying transition which is pumped off resonanc e. The intensity-dependent shifts of the transitions are such that the y repel each other more and more as the intensity of the pump field is increased. These shifts arise from the nonresonant interaction betwee n the atom and the field.