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Register a new userAttosecond Transient Absorption Spectroscopy without Inversion Symmetry
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Transient absorption is a very powerful observable in attosecond experiments on atoms, molecules and solids and is frequently used in experiments employing phase-locked few-cycle infrared and XUV laser pulses derived from high harmonic generation. We show numerically and analytically that in non-centrosymmetric systems, such as many polyatomic molecules, which-way interference enabled by the lack of parity conservation leads to new spectral absorption features, which directly reveal the laser electric field. The extension of Attosecond Transient Absorption Spectroscopy (ATAS) to such targets hence becomes sensitive to global and local inversion symmetry. We anticipate that ATAS will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed.
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The recent demonstration of isolated attosecond pulses from an X-ray free-electron laser (XFEL) opens the possibility for probing ultrafast electron dynamics at X-ray wavelengths. An established experimental method for probing ultrafast dynamics is X-ray transient absorption spectroscopy, where the X-ray absorption spectrum is measured by scanning the central photon energy and recording the resultant photoproducts. The spectral bandwidth inherent to attosecond pulses is wide compared to the resonant features typically probed, which generally precludes the application of this technique in the attosecond regime. In this paper we propose and demonstrate a new technique to conduct transient absorption spectroscopy with broad bandwidth attosecond pulses with the aid of ghost imaging, recovering sub-bandwidth resolution in photoproduct-based absorption measurements.
We simulate the transient absorption of attosecond pulses of infrared-laser-dressed atoms by considering a three-level system with the adiabatic approximation. The delay-dependent interference features are investigated from the perspective of the coherent interaction processes between the attosecond pulse and the quasi-harmonics. We find that many features of the interference fringes in the absorption spectra of the attosecond pulse can be attributed to the coherence phase difference. However, the modulation signals of laser-induced sidebands of the dark state is found related to the population dynamics of the dark state by the dressing field.
We investigate theoretically charge migration following prompt double ionization of a polyatomic molecule (C$_2$H$_4$BrI) and find that for double ionization, correlation-driven charge migration appears to be particularly prominent, i.e., we observe exceptionally rich dynamics solely driven by the electron-electron interaction even in the situation when the electrons are emitted from outer-valence orbitals. These strongly correlated electron dynamics are witnessed in the theoretically determined time-resolved transient absorption cross section. Strikingly, features in the cross section can be traced back to electron hole populations and time-dependent partial charges and hence, can be interpreted with surprising ease. Remarkably, by taking advantage of element specific core-to-valence transitions, the hole population dynamics can be followed both in time and space. With this, not only do we report the high relevance of correlation-driven charge migration following double ionization but our findings also highlight the outstanding role of attosecond transient absorption spectroscopy (ATAS) as one of the most promising techniques for monitoring ultrafast electron dynamics in complex molecular systems.
The evolution of electron wavepackets determines the course of many physical and chemical phenomena and attosecond spectroscopy aims to measure and control such dynamics in real-time. Here, we investigate radial electron wavepacket motion in Helium by using an XUV attosecond pulse train to prepare a coherent superposition of excited states and a delayed femtosecond IR pulse to ionize them. Quantum beat signals observed in the high resolution photoelectron spectrogram allow us to follow the field-free evolution of the bound electron wavepacket and determine the time-dependent ionization dynamics of the low-lying 2p state.
We describe a numerical method that simulates the interaction of the helium atom with sequences of femtosecond and attosecond light pulses. The method, which is based on the close-coupling expansion of the electronic configuration space in a B-spline bipolar spherical harmonic basis, can accurately reproduce the excitation and single ionization of the atom, within the electrostatic approximation. The time dependent Schrodinger equation is integrated with a sequence of second-order split-exponential unitary propagators. The asymptotic channel-, energy- and angularly-resolved photoelectron distributions are computed by projecting the wavepacket at the end of the simulation on the multichannel scattering states of the atom, which are separately computed within the same close-coupling basis. This method is applied to simulate the pump-probe ionization of helium in the vicinity of the $2s/2p$ excitation threshold of the He$^+$ ion. This work confirms the qualitative conclusions of one of our earliest publications [L Argenti and E Lindroth, Phys. Rev. Lett. {bf 105}, 53002 (2010)], in which we demonstrated the control of the $2s/2p$ ionization branching-ratio. Here, we take those calculations to convergence and show how correlation brings the periodic modulation of the branching ratios in almost phase opposition. The residual total ionization probability to the $2s+2p$ channels is dominated by the beating between the $sp_{2,3}^+$ and the $sp_{2,4}^+$ doubly excited states, which is consistent with the modulation of the complementary signal in the $1s$ channel, measured in 2010 by Chang and co-workers~[S Gilbertson~emph{et al.}, Phys. Rev. Lett. {bf 105}, 263003 (2010)].
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