Signatures of electronic structure in bi-circular high-harmonic spectroscopy

High-harmonic spectroscopy driven by circularly-polarized laser pulses and their counter-rotating second harmonic is a new branch of attosecond science which currently lacks quantitative interpretations. We extend this technique to the mid-infrared regime and record detailed high-harmonic spectra of several rare-gas atoms. These results are compared with the solution of the Schrodinger equation in three dimensions and calculations based on the strong-field approximation that incorporate accurate scattering-wave recombination matrix elements. A quantum-orbit analysis of these results provides a transparent interpretation of the measured intensity ratios of symmetry-allowed neighboring harmonics in terms of (i) a set of propensity rules related to the angular momentum of the atomic orbitals, (ii) atom-specific matrix elements related to their electronic structure and (iii) the interference of the emissions associated with electrons in orbitals co- or counter-rotating with the laser fields. These results provide the foundation for a quantitative understanding of bi-circular high-harmonic spectroscopy.Comment: Accepted in Physical Review Letter

High-order harmonic generation in solids: A unifying approach

ISSN:1098-0121ISSN:0163-1829ISSN:1550-235XISSN:0556-2805ISSN:2469-9969ISSN:1095-3795ISSN:2469-995

ATTOSECOND TIME-RESOLVED MOLECULAR SPECTROSCOPY

Attosecond time-resolved spectroscopy is beginning to provide experimental access to the most fundamental time scales of molecules, on which the electronic dynamics take place. A few recent experiments that access purely electronic dynamics, as well as coupled electronic and nuclear dynamics in molecules will be discussed. The theoretical developments that accompanied the experimental work will also be presented. The ionization of most molecules on the sub-femtosecond time scale prepares the molecular cation in a superposition of several electronic states that supports charge migration. Detailed measurements of the phase and amplitude of high-harmonic emission from spatially oriented iodoacetylene molecules have enabled the reconstruction of sub-femtosecond charge migration in the iodoacetylene cation (see figure)\footnote{P. M. Kraus \textit{et al.}, \textit{Science} \bf{350}, 790 (2015)}. \begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.8]{HCCI.eps} \end{wrapfigure} The ionization of molecules by attosecond pulses and a synchronized infrared field was used to measure photoionization time delays between the two highest-lying occupied valence orbitals of H$_2$O and N$_2$O. These measurements revealed delays of up to $\sim$160 as in the case of N$_2$O, which are characteristic of the transient trapping of the photoelectron by shape resonances\footnote{M. Huppert \textit{et al.}, \textit{Phys. Rev. Lett.} \bf{117}, 093001 (2016)}. Finally, the extension of attosecond spectroscopy to the soft-X-ray domain (water window) will be discussed. The broad spectral bandwidth available in this domain has been exploited to synthesize one of the shortest attosecond pulses to date (43 as)\footnote{T. Gaumnitz \textit{et al.}, \textit{Opt. Exp.} \bf{25}, 27506 (2017)}. Transient absorption spectroscopy at the carbon K-edge has been used to study the photodissociation dynamics of CF$_4^+$, revealing the rearrangement of the electronic structure during this ultrafast ($\sim$40 fs) process\footnote{Y. Pertot \textit{et al.}, \textit{Science} \bf{355}, 264 (2017)}. An outlook on attosecond spectroscopy of both isolated and solvated molecules will be given

Preparing attosecond coherences by strong-field ionization

Strong-field ionization (SFI) has been shown to prepare wave packets with few-femtosecond periods. Here, we explore whether this technique can be extended to the attosecond time scale. We introduce an intuitive model, which is based on the Fourier transform of the subcycle SFI rate, for predicting the bandwidth of ionic states that can be coherently prepared by SFI. The coherent bandwidth decreases considerably with increasing central wavelength of the ionizing pulse but it is much less sensitive to its intensity. Many-body calculations based on time-dependent configuration-interaction singles support these results. The influence of channel interactions and laser-induced dynamics within the ion is discussed. Our results further predict that multicycle femtosecond pulses can coherently prepare subfemtosecond wave packets with higher selectivity and versatility compared to single-cycle pulses with an additional sensitivity to the mutual parity of the prepared states. © 2016 American Physical Society.Alexander von Humboldt FoundationNSF/ITAMPHelmholtz associationDFGERC/307270-ATTOSCOP

Theoretical Study of Molecular Electronic and Rotational Coherences by High-Harmonic Generation

The detection of electron motion and electronic wavepacket dynamics is one of the core goals of attosecond science. Recently, choosing the nitric oxide (NO) molecule as an example, we have introduced and demonstrated a new experimental approach to measure coupled valence electronic and rotational wavepackets using high-harmonic generation (HHG) spectroscopy [Kraus et al., Phys. Rev. Lett. 111, 243005 (2013)]. A short outline of the theory to describe the combination of the pump and HHG probe process was published together with an extensive discussion of experimental results [Baykusheva et al., Faraday Discuss 171, 113 (2014)]. The comparison of theory and experiment showed good agreement on a quantitative level. Here, we present the generalized theory in detail, which is based on a generalized density matrix approach that describes the pump process and the subsequent probing of the wavepackets by a semiclassical quantitative rescattering approach. An in-depth analysis of the different Raman scattering contributions to the creation of the coupled rotational and electronic spin-orbit wavepackets is made. We present results for parallel and perpendicular linear polarizations of the pump and probe laser pulses. Furthermore, an analysis of the combined rotational-electronic density matrix in terms of irreducible components is presented, that facilitates interpretation of the results.Comment: 14 figure

Effect of nuclear motion on tunneling ionization rates of molecules

ISSN:1094-1622ISSN:0556-2791ISSN:1050-294