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We study the stabilization behavior of the circular 5g state in neon in a two-pulse experiment. A first pulse prepares the state. Comparison of the single-photon ionization yield, due to a second laser pulse for both short (0.1 ps) intense and long (1 ps) less intense pulses, shows an intensity-dependent suppression of ionization. The smaller yield due to the short pulses, at intensities of several times 10 13 W/cm 2 , is in accordance with recent predictions of stabilization.PACS numbers: 32.80.Rm, 42.50.HzThe photoionization rate of an atom will usually increase when the atom is exposed to increasing intensities. Recently, however, different mechanisms have been proposed that could prevent photoionization of an atom, even at the highest intensities. Collectively these are called stabilization. One such mechanism is dynamic or transient stabilization, in which part of the population is temporarily inaccessible for ionization [l].Adiabatic stabilization [2-5] is a different mechanism that suppresses photoionization. More precisely, it corresponds to an ionization rate which decreases with increasing intensity. This type of stabilization is predicted to occur in the high-frequency regime, where the photon energy is much larger than the binding energy. Classically, this means that the electron motion due to the light field is much faster than its motion in the Coulomb potential. Quantum mechanically, it means that the electron wave function cannot adiabatically respond to the oscillating light field. As a result, in the high-frequency regime, the wave function of the electron will be driven in an oscillatory motion by the light field.Adiabatic stabilization may occur once the amplitude of the oscillatory motion becomes comparable to the size of the unperturbed wave function, which usually implies a high intensity [6]. In the rest frame of the oscillating electron cloud (the Kramers-Henneberger frame), the atomic core will appear to oscillate. The electron wave function is determined by the potential of the atomic core, which, in this rest frame, is time dependent. However, in the high-frequency regime, it is valid to time average the motion of the atomic core. The potential which results is no longer Coulombic, but is deepest at the outer turning points of the oscillation, where the atomic core spends most of its time. In response the probability of finding the electron is largest at these points. One result of the electron spending more of its time further away from the nucleus is that ionization is suppressed and stabilization occurs. Ionization depends on transfer of momentum from the core to the emitted electron and once the wave function is far away from the core this becomes impossible [7], Originally, stabilization was predicted for the ground state of atomic hydrogen [2], and was later confirmed by other calculations [3], However, with present-day laser technology, the stabilization regime can only be reached when the initial state is a Rydberg state. For that case, different theoretical groups [4,5] give s...

We observe new effects in frequency doubling of colliding-pulse, mode-locked dye-amplified pulses (300 fsec, 620 nm, up to 1 mJ of energy) due to phase mismatch. If the second-harmonic generation in the nonlinear crystal (30-mm KDP crystal) is phase matched, the output is a square pulse. In contrast, when a phase mismatch is introduced, the generated pulse contains two peaks. We observe that the time profile is affected by depletion and chirp of the fundamental. The experimental results agree well with our numerical calculations.

We describe a detailed account of an experiment demonstrating light-induced stabilization against photoionization. The choice of initial state and atom is discussed in relation to the laser wavelength and laser pulse duration. In combination with a 100-fs, 620-nm probe pulse, the optimum choice is the circular Sg state in neon. A picosecond pump laser was used to prepare this Rydberg state. Initially, the population in this state was probed with a nanosecond laser pulse. Subsequently, the nanosecond probe pulse was replaced by an intense, (sub)picosecond pulse and the photoionization signa1 was studied.When the probe intensity is several times 10" W/cm' a decrease in yield with respect to a less intense pulse with the same fiuence is observed, which indicates stabilization. The results are in accordance with recent theoretical predictions.PACS number(s): 32.80.Rm, 42.50.Hz

Resonant and nonresonant multiphoton ionization of xenon is studied using short, circularly polarized light pulses (100 fs, 597 nm, 22 TW/cm ). A pump-probe measurement shows that, although bound states are substantially populated, they do not enhance the ionization signal. The bound states do not ionize because their high angular momentum repels the wave functions from the nucleus. Ionization does occur through intermediate states in the continuum, in spite of a large energy mismatch, because these states have more energy and therefore suffer less from the centrifugal barrier. PACS number(s): 32.80.Rm Recently several situations have been discovered in which an atom subjected to intense electromagnetic radiation is unexpectedly resistant to photoionization [1 -3].This "stabilization" occurs because, under the inAuence