Two-photon processes with chirped pulses

Two-photon excitation processes are studied with visible, chirped, picosecond, laser pulses. Two classes of processes are distinguished: 1) the situation where intermediate resonances at the one-photon level form a wavelength selection and therefore, due to the chirp of the optical pulse, also a time selection and 2) the non-resonant processes which are described by interference of many excitation paths. This interference has a close analogy with Fresnel diffraction. For all cases experiments have been performed with two-photon excitation in rubidium atoms. In the non-resonant situation also second-harmonic generation in thin KDP crystals is used. Relevance of these processes for the vibrational excitation and dissociation of molecules is discussed.