Interaction of atomic and molecular systems with high-intensity ultraviolet radiation

The interaction of atomic and molecular species with picosecond ArF* laser radiation is studied at intensities up to 1015 W/cm2. Anomalously strong, collision-free multiple ionization is observed. Standard theoretical models of stepwise ionization fail to describe the results. The experimental findings point to a collective response of the atom. At intensities of ~1013 W/cm2, selective multiquantum excitation of autoionizing states in Kr, followed by stimulated emission at wavelengths as short as 91.6 and 93 nm, is observed. The 93-nm radiation is tunable over a 600-cm−1 interval, whereas the 91.6-nm frequency is fixed. It appears that electron collisions redistribute energy among excited states.