PHYSICAL REVIEW A VOLUME 50, NUMBER 5 NOVEMBER 1994 Adiabatic stabilization against photoionization: An experimental study M. P. de Boer, J. H. Hoogenraad, R. B. Vrijen, R. C. Constantinescu, L. D. Noordam, and H. G. Muller FOM I— nstitute for Atomic and Molecular Physics, Kruislaan 407, I098 SJ Amsterdam, The ¹therlands (Received 19 January 1994) 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 circu- lar Sg state in neon. A picosecond pump laser was used to prepare this Rydberg state. Initially, the pop- ulation 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 I. OVERVIEW II. INTRODUCTION Since the invention of the laser, photoionization studies on atoms have been performed at ever increasing intensi- ties. Numerous aspects of high-intensity photoionization could be accounted for by using elementary theories such as lowest-order perturbation theory (LOPT), over-the- barrier ionization, or quantum tunneling theory [1]. LOPT, however, cannot account for an effect such as above threshold ionization (ATI). Recent theories do ac- count for light intensities around 1 a.u. (3. 5X10' W/cm ) [2], with several groups working on nonpertur- bative approaches to the calculation of photoionization. High-frequency theory is an example of such a nonper- turbative theory [3]. In this theory the expansion param- eter is not the intensity of the light field but rather the in- verse frequency. The zero-order approximation is that the light frequency is infinitely high and successive ap- proximations can be made by including more orders of the inverse frequency. One of the predictions of high-frequency theory is an efFect called stabilization [4]. In the calculations the ion- ization rate decreases once the intensity rises above a crit- ical intensity. This very counterintuitive prediction was originally made for the ground state of atomic hydrogen subjected to high-energy photons at intensities in excess of 1 a.u. This paper describes in detail experimental evidence for stabilization [5]. In Sec. II we discuss the physical mechanism that is responsible for stabilization and we distinguish two possible kinds. High-lying Rydberg states also show stabilization and are experimentally much more feasible, as discussed in Sec. III. A detailed account of the considerations that eventually led to the actual experimental configuration is given in Sec. IV. Section V describes the experimental preparation of the 5g Rydberg state, while Sec. VI describes the ionization step that actually demonstrated stabilization. A. Photoionization at low intensities For low intensities the theory of photoionization is well developed. It was initiated by Einstein's explanation in 1905 of the photoelectric efFect [6]. If the photon energy of the ionizing radiation is high enough, there is a finite probability for a photon to be absorbed. The energy of the photon is partly used to overcome the ionization threshold and the excess energy is transformed into kinet- ic energy of the emitted electron. The ionization rate I of an atom can be calculated us- ing Fermi's "golden rule" [7]: where ~i ) is the initial state, ~f ) the final state, and p(Ek ) the density of states in the continuum, at the ener- gy of the emitted electron. Since the interaction Hamiltonian H is proportional to the electric-field strength, the ionization rate increases with intensity and depends linearly on the flux of pho- tons. The total ionization yield depends on the fluence (time-integrated intensity). A trivial exception to the linear dependence on intensity is depletion of initial-state atoms. Fermi's "golden rule" also no longer applies if resonances play a role. In this paper we are concerned with single-photon ionization, where resonances do not play a role as long as there is no structure in the continu- um. B. Adiabatic stabilization Once the intensity passes a certain critical threshold, the single-photon ionization rate no longer follows Fermi's "golden rule, " but actually decreases with in- creasing intensity. This effect is called adiabatic stabiliza- tion, because the stabilized state is reached by adiabatic 1050-2947/94/50(5)/4085(14)/$06. 00 Qc1994 The American Physical Society