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Journal of Physics B Atomic Molecular and Optical Physics

Line shapes in photoexcitation of triply excited hollow states

Lars Bojer Madsen, P. Lambropoulos

Published May 2, 2001• 10 citations
Publisher

Abstract

Taking as the point of departure a frequently used experimental detection scheme, we present a derivation of the analytical expressions of the line shapes in partial decay channels pertaining to photoexcitation of triply excited hollow states in atomic lithium with subsequent autoionization into a specific doubly excited state in the singly charged ion. The line shapes represent useful generalizations of the Fano parametrization of the profile of an autoionizing resonance. The line shapes give the positions and widths of the triply excited autoionizing resonances, and carry information about specific partial autoionization widths, and partial asymmetry parameters. It is also shown that a laser-induced transition from the triply excited state to another lower-lying one can be used to gain direct information about partial widths. The schemes are readily implemented experimentally, and would allow for the first measurements of such quantities.

DOI
10.1088/0953-4075/34/10/301

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Taking as the point of departure a frequently used experimental detection scheme, we present a derivation of the analytical expressions of the line shapes in partial decay channels pertaining to photoexcitation of triply excited hollow states in atomic lithium with subsequent autoionization into a specific doubly excited state in the singly charged ion. The line shapes represent useful generalizations of the Fano parametrization of the profile of an autoionizing resonance. The line shapes give the positions and widths of the triply excited autoionizing resonances, and carry information about specific partial autoionization widths, and partial asymmetry parameters. It is also shown that a laser-induced transition from the triply excited state to another lower-lying one can be used to gain direct information about partial widths. The schemes are readily implemented experimentally, and would allow for the first measurements of such quantities.
Photon absorption spectroscopy is a powerful tool for uncovering the structure of atoms, molecules, and solids. Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures related to the structural and dynamical properties. Recently, Ott et al. [Science 340, 716 (2013)] successfully transferred Fano profile into Lorentzian line shape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward in quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e., electron ion recombination with photon emission, making sure the resonant autoionizing state is not modified or decorated by the laser fields. Our result implies that excitation of the state through different paths---for example, one photon versus multiphoton excitation, or even one step versus multistep excitation---can lead to different Fano profiles for the same resonant state. We also report an experimental determination of the energy shifts in the recombination photon-intensity peaks due to the interference between the resonant and nonresonant processes.
We present a theoretical study of laser-induced autoionization and laser-induced continuum structures, performed with the aim of deriving predictions on the effect of continuum-continuum transitions on the resonance line shape. The model, developed as an extension of models introduced for the description of laser-induced autoionization and laser-induced continuum structures, on one hand, and for above-threshold ionization, on the other hand, provides quantitative predictions. For specific atomic systems, the line shapes of the structures are shown not to be affected by continuum-continuum transitions, in contrast with previously reported results.
We derive a parametrized formula for the photoionization cross section in the vicinity of an autoionizing state (AIS) coupled with another AIS by a strong laser field. The rotating-wave approximation is used and adiabaticity is assumed. In the probe field spectrum two Fano resonances contribute to the cross section, while a single resonance occurs in the strong-field spectrum. The correlation between the resonance shape and the complex energies of the laser-driven two-level system is described. Calculations are performed for the and AIS pair in He and for the and pair in Mg. The results agree with those obtained using time-dependent approaches. The formula for the cross section describes the spectral features of the ion yield measured in the two-photon ionization of the Mg atom via the AIS coupled with the state. The appearance of laser-induced degenerate states in the cross section shape is studied.
Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures that quantify the structural and dynamical properties of nuclei, atoms, molecules, and solids. This study introduces a universal temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase φ of the time-dependent dipole response function. The formalism is confirmed experimentally by laser-transforming Fano absorption lines of autoionizing helium into Lorentzian lines after attosecond-pulsed excitation. We also demonstrate the inverse, the transformation of a naturally Lorentzian line into a Fano profile. A further application of this formalism uses quantum-phase control to amplify extreme-ultraviolet light resonantly interacting with He atoms. The quantum phase of excited states and its response to interactions can thus be extracted from line-shape analysis, with applications in many branches of spectroscopy.
We present a theory and calculations of transitions between autoionizing states in He. A 1P^{1}\mathrm{P}1P resonance is assumed excited by a weak source while a strong laser couples it to other autoionizing states. We calculate the total ionization probability, the line shape, and their dependence on the intensity of the laser. Possible experimental verification is also assessed.
We have observed ionization of Mg by both direct and stepwise two-photon excitation of the 3p2{p}^{2}p2 1{\mathrm{}}^{1}1S state. The line shape of the single-color direct process is strongly modified by the resonance denominator associated with the intermediate virtual state. The measured energy and width of this resonance as determined by the stepwise two-color technique agree well with previous determinations.