Evidence for resonant effects in high-order ATI spectra

Abstract

We have observed resonant phenomena in the intense-field ionization of argon that do not easily fit into the standard models of above-threshold ionization. High-resolution angleresolved measurements of the energy region around 6-10 times the ponderomotive potential U P in the ATI electron spectrum show at least three separate series of peaks, each with a distinct intensity threshold for their onset. These series do not shift in energy and have characteristic values of intensity for their appearance, pointing to an intensity-selective resonant enhancement effect. In order to further investigate this phenomenon, we have observed VUV fluorescence photons from the focal region following ionization. Current theories for the creation of high-energy electrons do not seem to explain these phenomena without significant modifications. We discuss possible physical mechanisms and implications for extending the standard model.

Threshold effects in strong-field ionization: Energy shifts and Rydberg structures

“…This agrees with the collision theory predictions [35,37], and may be related to the experimentally observed ATI plateau enhancements in rare-gas atoms [2,[17][18][19][20][21]. Specifically, when one partial cross section becomes closed, the remaining open partial cross sections increase so that the total cross section is continuous.…”

“…Note that our analysis is particularly relevant to experimental observations of striking ATI plateau enhancements for rare-gas atoms [2,[17][18][19][20][21], i.e., that small changes in laser intensity can result in an order of magnitude increase of ATI electrons having a range of energies at the lower end of the ATI plateau. These experiments have stimulated a number of theoretical efforts aimed at identifying the origin of this effect.…”

Strong-Field Double Ionization of Ar below the Recollision Threshold

“…They are discussed in some detail in [13] but are not the focus of this Letter. At higher energies, beyond the classical momentum limit of 2 U p 1=2 in the so-called plateau regime, a series of highly intensity-sensitive structures arises in both experiments, which have been investigated in utmost detail in several previous experiments [20] for Ar and Xe. There is a large body of theoretical literature on the origin of these peaks (again beyond the scope of this Letter), discussing the relevance of doubly excited states [21] or of several mechanisms within the SAE.…”

“…There is a large body of theoretical literature on the origin of these peaks (again beyond the scope of this Letter), discussing the relevance of doubly excited states [21] or of several mechanisms within the SAE. These include interference resonances at channel closings [20,22], multiphoton resonances involving excited states [20], the involvement of trapped dressed states [23], or of so-called ''laser-induced states'' close to the continuum [24]. The common bottom line, important for the discussion of the double ionization data presented here, may be summarized as follows: (i) multiple ionization or excitation plays little or no role for single ionization, and (ii) multiple elastic rescattering of electronic wave packets launched at different maxima of the pulse and interfering with each other are responsible for the observed effects.…”

Intrinsic channel closing in strong-field single ionization ofH2

“…A striking phenomenon arises when high-order ATI is studied with respect to its dependence on laser intensity. For the rescattering plateau between 2U p and 10U p , it was found in experiment [6,7] and calculations [8,9,10,11,12,13,14], that a slight change in laser intensity can lead to order-of-magnitude changes in yield for groups of peaks within the plateau. Explanations were found in terms of multiphoton resonances with Rydberg states [8,9,15] and within the framework of quantum paths [10,11,16].…”