Quantum Interference in Two-Photon Absorption: Non - Level Crossing Studies in the Atomic Spectroscopy of Strontium.

Quantum interferences are found when an atom is irradiated by two collinearly propagating light beams whose sum frequency corresponds to the frequency difference between two ('1)S states. If the polarization of the two light beams are perpendicular to each other, no radiation is absorbed. When a magnetic field is applied in the propagation direction of the light beams, two-photon excitation occurs. This effect is interpreted as an interference phenomenon, and can be considered a non-linear analogue of a zero field level crossing (the Hanle effect). As in level crossing studies, unique polarization and field dependences are predicted. Constructive and destructive interferences are expected depending on the polarization of the two exciting light beams. The field dependence gives partially constructive interferences for any arbitrary polarization case. These effects are reported here for the (7s)('1)S (<---) (5(,s)('2))('1)S transition of atomic Sr, when excitation is nearly resonant with the lowest lying ('1)P state. An atomic beam of Sr is studied using two dye lasers operating at different wavelengths. The two-photon excitation rates are detected with laser induced fluorescence, or multiphoton ionization. As with the Hanle effect, the two-photon interference described here provides the basis for measuring Lande factors of intermediate states.