Results 1 - 10 of 15
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[en] We present the detailed analysis of a new two-pulse orientation scheme that achieves macroscopic field-free orientation at the high particle densities required for attosecond and high-harmonic spectroscopies (Kraus et al 2013 arXiv:1311.3923). Carbon monoxide molecules are oriented by combining one-colour and delayed two-colour non-resonant femtosecond laser pulses. High-harmonic generation is used to probe the oriented wave-packet dynamics and reveals that a very high degree of orientation (Nup/Ntotal = 0.73–0.82) is achieved. We further extend this approach to orienting carbonyl sulphide molecules. We show that the present two-pulse scheme selectively enhances orientation created by the hyperpolarizability interaction whereas the ionization-depletion mechanism plays no role. We further control and optimize orientation through the delay between the one- and two-colour pump pulses. Finally, we demonstrate a complementary encoding of electronic-structure features, such as shape resonances, in the even- and odd-harmonic spectrum. The achieved progress makes two-pulse field-free orientation an attractive tool for a broad class of time-resolved measurements. (paper)
[en] The hyperfine structure of ns and nd Rydberg states of 83Kr has been measured in the range n=30-190 below the 2P3/2 ionization threshold by pulsed-field ionization following single-photon excitation from the 1S0 ground state using a narrow-bandwidth vacuum-ultraviolet laser system. A multichannel quantum defect theory (MQDT) treatment of the hyperfine structure in Rydberg states of the rare-gas atoms has been developed that quantitatively accounts for the effects of the nuclear spin on the spectral structures over the entire range of principal quantum number investigated. The model allows the parametrization of the hyperfine structure of the Rydberg states in terms of the ionic hyperfine structure and relies on the assumption that the interaction with the nuclear spin is negligible in the close-coupling region of the electron-ion collision, an assumption that is also expected to be valid in other atomic and molecular systems. Improved eigen quantum defects for the ns and nd Rydberg series with J=1 and 2 have been derived from the MQDT analysis, and the hyperfine structure of the two 2P3/2 and 2P1/2 spin-orbit components of the ground state of 83Kr+ has been determined
[en] We present a new design of a time-preserving extreme-ultraviolet (XUV) monochromator using a semi-infinite gas cell as a source. The performance of this beamline in the photon-energy range of 20 eV–42 eV has been characterized. We have measured the order-dependent XUV pulse durations as well as the flux and the spectral contrast. XUV pulse durations of ≤40 fs using 32 fs, 800 nm driving pulses were measured on the target. The spectral contrast was better than 100 over the entire energy range. A simple model based on the strong-field approximation is presented to estimate different contributions to the measured XUV pulse duration. On-axis phase-matching calculations are used to rationalize the variation of the photon flux with pressure and intensity.
[en] A combined experimental and theoretical investigation of the role of electronic and nuclear spins in molecular photoionization is reported. Photoionization spectra of autoionizing p Rydberg states belonging to series converging on the X 2Σg+(v+=0,N+=3) level of ortho-H2+ have been measured in the range of principal quantum number n=50-200 from the long-lived H 1Σg+(v=0,J=3) level. The use of a pulsed near-Fourier-transform-limited laser with a bandwidth of less than 10 MHz resulted in a Doppler-limited linewidth of 25 MHz which sufficed to partially resolve the hyperfine structure of the Rydberg states. Below n≅70, the exchange interaction between the ion core and Rydberg electrons is larger than the hyperfine interactions in the ion core and the observed levels can be understood in terms of Hund's angular momentum coupling case (d). With increasing value of n, the hyperfine interactions in the core lead to a mixing of singlet and triplet characters of the Rydberg states and eventually to a complete decoupling of the Rydberg electron spin from the core spins that results in distinct series converging on the hyperfine components of the ion. Several intermediate coupling cases have been identified and two of them completely characterized. Most interestingly, the total spin angular momentum has been found to be a good quantum number up to n≅150 at least, i.e., far beyond the region where the ionic hyperfine structure starts dominating the coupling hierarchy. Multichannel quantum defect theory including nuclear and electron spins has been extended to treat autoionization and predict spectral intensities. The comparison with the experimental spectra has revealed a satisfactory agreement between calculated and measured line positions, linewidths, and intensities and has enabled us to extract, by extrapolation, a more accurate term value for the H 1Σg+(v=0,J=3) level. The calculations have been used to characterize the role of hyperfine, spin-rotational, and pf interactions in rotational autoionization and have revealed a very strong dependence of the autoionization lifetimes of high Rydberg states on the value of the total angular momentum quantum number F
[en] Using sequential strong-field double ionization in a pump-probe scheme we show through calculations how electronic dynamics can be prepared and imaged. Electronic dynamics may arise whenever multiple states of the ion are accessed in the ionization step. The dynamics in the cation influence the rate of the second ionization step and the momentum distribution of the ejected electron, allowing their detailed characterization. We show how the probe step is controlled through spatial propensities of the ionizing orbitals and the energy level structure of the dication. Both the final electronic state of the dication and the spin state of the ejected electron pair can be controlled through the time delay between the two ionizing pulses. We discuss how our results will extend to the preparation and measurement of attosecond electron dynamics. (fast track communication)
[en] High harmonic spectroscopy relies on high harmonic generation (HHG) in aligned molecules. The first step of HHG is the ionization of the molecule in the intense femtosecond laser field. Here we present measurements of both ionization yield and high harmonic yield as a function of molecular angle in N2 and CO2 molecules. Measurements were done at two wavelengths, 800 and 1200 nm, and for a range of laser intensities, to study the sensitivity of laser conditions on both processes. The behavior of N2 was relatively insensitive to laser conditions. However in CO2, a minimum in high harmonic emission was observed that was sensitive to both laser intensity and wavelength, and was attributed to interference in emission from the HOMO and HOMO-2 orbitals. (paper)
[en] We measured the relative ionization delays into the two spin-orbit components of the electronic ground states of Xe"+ and Kr"+ under the inuence of auto-ionizing intermediate and final states. The results are in good agreement with state-of-the-art calculations based on the time-dependent configuration-interaction singles (TDCIS) approach. (paper)
[en] We demonstrate a new method to investigate the origin of spectral structures in high-harmonic generation. We report detailed measurements of high-harmonic spectra in aligned nitrogen and carbon dioxide molecules. Varying the wavelength and intensity of the generating laser field, we show that the minimum in aligned N2 molecules is nearly unaffected, whereas the minimum in aligned CO2 molecules shifts over more than 15 eV. Our quantitative analysis shows that both the interference of multiple orbitals and their structural characteristics affect the position of the minimum. Our method provides a simple approach to the investigation of the high-harmonic generation process in more complex molecules.
[en] Attosecond Pulse Trains (APT) generated by high-harmonic generation (HHG) of high-intensity near-infrared (IR) laser pulses have proven valuable for studying the electronic dynamics of atomic and molecular species. However, the high intensities required for high-photon-energy, high-flux HHG usually limit the class of adequate laser systems to repetition rates below 10 kHz. Here, APT’s generated from the 100 kHz, 160 W, 40 fs laser system (HR-1) currently under commissioning at the extreme light infrastructure attosecond light pulse source (ELI-ALPS) are reconstructed using the reconstruction of attosecond beating by interference of two-photon Transitions (RABBIT) technique. These experiments constitute the first attosecond time-resolved photoelectron spectroscopy measurements with attosecond pulses performed at 100 kHz repetition rate and one of the first experiments performed at ELI-ALPS in the framework of projects commissioning its newly installed technologies. These RABBIT measurements were taken with an additional IR field temporally locked to the extreme-ultraviolet APT, resulting in an atypical ω beating. We show that the phase of the 2ω beating recorded under these conditions is strictly identical to that observed in standard RABBIT measurements within second-order perturbation theory. This work highlights an experimental simplification for future experiments based on attosecond interferometry (or RABBIT), which is particularly useful when lasers with high average powers are used. (letter)