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Physical Review Letters

Observation of Continuum-Continuum Autler-Townes Splitting

Barry C. Walker, Matjaž Kaluža, B. Sheehy, Pierre Agostini, L. F. DiMauro

Published Jul 24, 1995• 72 citations
Publisher

Abstract

Photoelectron energy spectra resulting from two-photon ionization of calcium atoms by 180 fs pulses have been studied as a function of wavelength (380--405 nm) and intensity ( $9\ensuremath{-}900\phantom{\rule{0ex}{0ex}}\mathrm{GW}/{\mathrm{cm}}^{2}$). When the wavelength is tuned near the $4s\ensuremath{\rightarrow}4p$ or $4s\ensuremath{\rightarrow}5s$ ionic core transition, the photoelectron peaks display a characteristic splitting proportional to the field strength and assignable to a two-electron continuum-continuum Autler-Townes effect. Spectra obtained by an essential-state model involving three coupled continua are compared to the experiment.

DOI
10.1103/physrevlett.75.633

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Europhysics Letters (EPL)•Apr 10, 1995•17 citations
Photoelectron energy spectra resulting from two-photon ionization of calcium atoms by 180 fs pulses have been studied as a function of wavelength (380--405 nm) and intensity ( 9\ensuremath−900GW/cm29\ensuremath{-}900\phantom{\rule{0ex}{0ex}}\mathrm{GW}/{\mathrm{cm}}^{2}9\ensuremath−900GW/cm2). When the wavelength is tuned near the 4s\ensuremath→4p4s\ensuremath{\rightarrow}4p4s\ensuremath→4p or 4s\ensuremath→5s4s\ensuremath{\rightarrow}5s4s\ensuremath→5s ionic core transition, the photoelectron peaks display a characteristic splitting proportional to the field strength and assignable to a two-electron continuum-continuum Autler-Townes effect. Spectra obtained by an essential-state model involving three coupled continua are compared to the experiment.
A two-photon resonance in the sodium atom $3s\text{\ensuremath{-}}4s$ transition is investigated in the strong coupling regime with photoelectron momentum imaging techniques. The kinetic energies of the ionized electron map out the eigenenergies of the dressed states and emerge as the Autler-Townes (AT) splitting, which is proportional to the laser intensity due to the two-photon nature. The partial wave analysis of the angular distribution of the photoelectrons reveals the relative phase shift from dressed states which is related to the attosecond ionization time delays. The phase shift between the two electron wave packets produced from AT splitting are attributed to the combination of the Coulomb phase and the quantum defect phase.
We study the photoelectron spectra of hydrogen and helium ionized by intense ultrafast two-color XUV laser pulses by numerically solving the time-dependent Schrödinger equation.The photon energy of the pump pulse is tuned to the 1s-2p excitation energy, inducing Rabi oscillations between these bound states and forming an Autler-Townes (AT) doublet of dressed states.When ionized by a time-delayed ultrafast XUV pulse with duration shorter than the Rabi oscillation period, we find novel and drastically enhanced AT splittings in the photoionization spectrum.We identify their origin in terms of the temporal interference of two attosecond wave packets whose spectral distribution is controlled by the interplay between Rabi frequency and bandwidth of the probe pulse.These time-dependent features in the spectra can be much larger than the standard AT spacing induced by the pump pulse alone and can be controlled by the time delay and the duration of the probe pulse.
We present a bound-continuum ionization model combined with numerical simulations to investigate the asymmetric features of Autler-Townes (AT) splitting in the ionization spectra of hydrogen atoms, induced by a spectrally phase-modulated ultrashort pulse. This model incorporates a resonance-enhanced two-photon ionization process, enabling transitions from the ground electronic state to the continuum state via the first excited electronic state through the absorption of two photons. By comparing our model predictions with exact numerical simulations based on time-dependent electronic wave-packet calculations, we demonstrate that the characteristics of the AT doublets, shaped by the spectral phase, emerge from the competing effects of selective population of dressed states and the transition amplitude to the continuum. In addition, through a perturbative analysis of the photoelectronic spectrum and the application of the Magnus expansion, we reveal a correlation between the number of AT fringes, resonant states, and pulse area, while drawing insights from bound states. This work enhances our understanding of how the spectral phase can be utilized to explore bound-state effects in ultrafast ionization dynamics.
Abstract Based on numerical solutions of the time-dependent Schrödinger equation, we theoretically investigate the photoelectron spectrum of hydrogen atoms ionized by a pair of ultrashort, intense, and orthogonally polarized laser pulses with a relative time delay in a pump–probe configuration. The pump pulse resonantly excites electrons from the 1s and 2p levels, inducing Rabi oscillations. The resulting dynamically enhanced Autler–Townes (AT) splitting is observed in the photoelectron energy spectrum upon interaction with the second probe pulse. In contrast to the previous parallel-polarization scheme, the proposed orthogonal-polarization configuration enables the resolution of dynamically enhanced AT splitting over a considerably wider range of probe photon energies.
We theoretically investigated the Autler-Townes (AT) splitting in the photoelectron spectra of four-level ladder K-2 molecules in the presence of two control pump fields and one probe field by employing the time-dependent wave packet approach. The wavelength dependence of AT splitting on three laser pulses was studied in detail. The photoelectron spectra show triple splitting with symmetric profiles, but with asymmetric profiles when the wavelength is changed. The shifts of the absorption peaks are predicted by using an analytical fitting function when the wavelength of one of the three fields is changed, and the functional dependence appears to be linear. These novel results are of importance for the molecular spectroscopy, and may stimulate further theoretical studies analytically from the first principles. Copyright (C) EPLA, 2014