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Physical Review A

Deviation from the Fano profile in resonance-enhanced multiphoton autoionization

Lipika Adhya, Krishna Rai Dastidar

Published Oct 1, 1994• 11 citations
Publisher

Abstract

We know that the Fano profile is obtained for single-photon absorption to the ionization continuum in the presence of an autoionizing state. We have shown that, in the case of (1+1)-photon absorption to an autoionizing state embedded in the ionization continuum, the absorption profile changes radically to a mirror image of the Fano profile for the value of q=\ifmmode\pm\else\textpm\fi{}1, q being the Fano q parameter. Moreover, for higher values of q, the (1+1)-photon absorption profile exhibits a broad minimum, while a distinct interference pattern is obtained in the profile of single-photon absorption. The absorption line shape has been found to oscillate between the Fano and a modified Fano profile for the autoionization through odd and even number of steps, respectively, via resonances.

DOI
10.1103/physreva.50.3537

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We know that the Fano profile is obtained for single-photon absorption to the ionization continuum in the presence of an autoionizing state. We have shown that, in the case of (1+1)-photon absorption to an autoionizing state embedded in the ionization continuum, the absorption profile changes radically to a mirror image of the Fano profile for the value of q=\ifmmode\pm\else\textpm\fi{}1, q being the Fano q parameter. Moreover, for higher values of q, the (1+1)-photon absorption profile exhibits a broad minimum, while a distinct interference pattern is obtained in the profile of single-photon absorption. The absorption line shape has been found to oscillate between the Fano and a modified Fano profile for the autoionization through odd and even number of steps, respectively, via resonances.
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.
Ott et al. (Science (340, 716 (2013)) successfully transferred Fano profile into Lorentzian lineshape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward of 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 perturbed by the laser fields. Our result implies that excitation of the state through different paths can lead to different Fano profiles for the same resonant state. This allows more options for the combination of laser fields and lead to more opportunities for quantum phase control. Our result also indicates the breakdown of the classical two step picture for dielectronic recombination.
We have studied the effect of above-threshold transitions on the (1+1)-photon resonance-enhanced ionization and autoionization processes. We have shown that the effect of the photoionization decay of the autoionizing (AI) state to the higher continuum becomes dominant when the photoionization width of the AI state is comparable to or larger than the autoionization width to within an order of magnitude, i.e., for the intermediate range of laser intensity. The continuum-continuum coupling and the higher-order transitions become effective only for much higher values of the above-threshold transition strengths, i.e., for much higher values of the laser intensity. \textcopyright{} 1996 The American Physical Society.
Fano profiles are caused by the interference between the discrete states and continuum. It is a very common phenomenon in multi-electron Rydberg states, such as the Rydberg states of group II elements in the periodic table. It often leads to autoionization due to the fact that a discrete state couples with an autoionizing state. Unlike the typical asymmetric line shape, here we report a symmetric unusual Fano line shape that has been predicted. In addition, this profile is caused by molecular states or polymer states instead of multi-electron atomic states. Moreover, it is shown that the fano profile caused by the multi-electron polymer states does not necessarily lead to autoionization. We propose that the Fano profile studied is caused by the interference between discrete levels and the multipole-multipole coupled continuum, or energy band.
The ordinary Fano effect occurs in many-electron atoms and requires an autoionizing state. With such a state, photo-ionization may proceed via pathways that interfere, and the characteristic asymmetric resonance structures appear in the continuum. Here we demonstrate that Fano structure may also be induced without need of auto-ionization, by dressing the continuum with an ordinary bound state in any atom by a coupling laser. Using multi-photon processes gives complete, ultra-fast control over the interference. We show that a line-shape index q near unity (maximum asymmetry) may be produced in hydrogenic silicon donors with a relatively weak beam. Since the Fano lineshape has both constructive and destructive interference, the laser control opens the possibility of state-selective detection with enhancement on one side of resonance and invisibility on the other. We discuss a variety of atomic and molecular spectroscopies, and in the case of silicon donors we provide a calculation for a qubit readout application.