Enhanced and stimulated multiphoton processes in free atoms

An enhanced two-photon emission has been observed between the 6S and the 4S levels of potassium. The priming field is the stimulated electronic Raman emission at 2720 cm
^-1
due to scattering of ruby laser radiation from the
4P_{3/2}
state in potassium. The intense emission at the complementary frequency ω
2
is close to the 5P → 4S resonance lines. As a result, the coupling coefficient has a resonance character that is favorable for the enhancement. The observed results are compared with a simple analysis. The relation of the enhanced emission with two-quantum absorption, stimulated Raman effect, and parametric four-photon processes is discussed.