LINE-SHAPE AND FREQUENCY-SHIFT OF LAMB DIP AND CROSSOVER-RESONANCE DIP IN CLOSELY SPACED TRANSITIONS

The line shape of a pair of Doppler-broadened closely spaced transitions sharing a common lower level and under the effect of a standing-wave monochromatic radiation field is studied. The eight steady-state optical Bloch equations are solved analytically to obtain the polarization by retaining terms up to the third power in the electric-field amplitude. The absorption coefficient derived for the standing-wave case consists of two Lamb dips for the two allowed transitions at omega(1) and omega(2) and a crossover resonance dip at the intermediate frequency omega(c) = (omega(1) + omega(2))/2. The Lamp-dip peak frequencies show intensity-dependent shifts. Numerical computations are presented for different values of Rabi frequencies and relaxation linewidth parameters, which are small compared to the frequency difference DELTA = omega(2)-omega(1). The computed line shape and frequency shift are compared with the observed results in the Balmer H(alpha) line of hydrogen and the infrared Zeeman spectra of methane.