Abstract.A theoretical study of the nonlinearity reduction in a four-level system of potassium, atom in the presence of a strong nanosecond laser field, which excites the transition 1/2 1/2 4S 6S ↔ with two photons, is presented. It is shown, that the destructive quantum interference between the laser field and the internally generated radiations results in a linear response of the atomic path-1 ( 1/2

The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P–4S transition under the two-photon 4S1/2 –6S1/2 excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.

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Potassium 4S1/2–6S1/2 two-photon excitation initiates the emission of several internally generated photons. For the first time two emission lines, one close to and one below the potassium 4P3/2 level, are reported for low pumping intensity. Radiation emitted below the 4P3/2 level is due to a parametric four-wave mixing process that uses the photons emitted at the 5P3/2–4S1/2 transition and a two-step four-wave mixing process generates the line emitted close to the 4P3/2 level.

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Results related to the energy transfer in the rare gas mixtures are reported for an electrical discharge excitation with plasma expansion and a heavy-ion-beam excitation. Energy transfer from the lighter rare gases to the heavier ones was observed in a very efficient way for both excitation mechanisms. Strong emission from excited atomic states, which are possible candidates for new lasers, was observed for the electrical discharge case. The broad-band emission of the discharge sources makes them appropriate for strong lamps in the VUV. Some comparisons of the two excitation mechanisms were discussed with respect to the energy deposition, the density of dimers and their vibrational relaxation.