Apparatus for transient XUV absorption experiments. Red and blue colors represent the NIR and XUV beams, respectively. BS: beam splitter, HHG: high harmonic generation cell, TM: toroidal mirror, HWP: half wave plate, RM: recombination mirror, and MCP: microchannel plates.
(a) Absorption peaks due to transitions to different core-excited states of neutral xenon. The vertical bars correspond to the experimental data with error bars. The solid black line represents the simulated absorption profile using a natural linewidth of 0.11 eV (Refs. 27,28,30) convoluted with the instrumental energy resolution of 0.37 eV. The broad thin line represents a non-resonant background due to ionization. Its contribution to the spectrum in (a) is estimated by a broad Gaussian peak that is centered at the maximum of the Xe+(4d −1) giant resonance and for which the ratio of OD's at 64.4 eV and 67.8 eV is the same as the ratio between the ionization cross sections at these energies.28,30 (b) Emergence of xenon ions monitored by the transient absorption signal at 55.4 eV corresponding to the (5p 3/2 −1) 2P3/2 →(4d 5/2 −1) 2D5/2 transition in Xe+. The solid line is the result of a fit to an error function, giving a rise time of 70 ± 10 fs (FWHM). See text for details.
(a) Static absorption spectrum of neutral Xe atoms from a previous study,29 containing two different spin-orbit series of 4d −1→ np inner shell excitations.27,28,30 (b) Transient XUV absorption spectra of neutral and ionized xenon atoms. The time- and energy-resolved change in the XUV optical density (ΔOD) reveals that the NIR pump pulse simultaneously induces both enhanced absorption (ΔOD > 0) and enhanced transmission (ΔOD < 0) in the vicinity of 65.1 eV and 67.0 eV. (c) Static photoionization yield spectrum of Xe+ ions in the range of 4d −1→ 5p, 4f inner-shell excitations.32
Xe+ ion absorption spectra for different relative polarizations of the NIR and XUV pulses. The spectra are averaged over pump-probe delays between 25 fs and 250 fs. (The ion signals in this energy range are constant for delays beyond 25 fs.) (a) Perpendicular polarization of XUV and NIR laser field. The underlying spectra contain contributions from singly and doubly charged xenon ions. Black vertical bars represent the experimental data with error bars. The dashed red line is the sum of all individually fitted ionic species shown as black solid lines. (b) Polarization dependence of xenon ion absorption spectra. Red dashed and black dashed lines are measured with parallel and perpendicular polarizations of the XUV and NIR fields, respectively. (c) Difference of spectra in (b). Note the opposite signs at 55.4 eV and 57.3 eV.
Laser-induced transmission of neutral xenon at 65.1 eV and 67.0 eV after correcting for the depletion in the pump-probe interaction volume due to strong-field ionization. (a) 4d −1(2D5/2)6p(2P3/2) core-excited state at 65.1 eV. (b) 4d −1(2D3/2)6p(2P1/2) core-excited state at 67.0 eV. The transmission far from time zero is described by the optical density of xenon atoms in a field-free environment. While approaching the maximum of the NIR envelope, the transmission increases gradually by a factor of three.
Reduced level scheme for strong-field coupling. The left side of the figure shows the levels that are most important for the coupling of the 4d −1(2D5/2)6p(2P3/2) core-excited states. On the right, the coupling levels for the 4d −1(2D3/2)6p(2P1/2) core-hole state are shown. The vertical widths of the horizontal black and red bars represent the natural and effective linewidths, respectively. The numbers on the levels represent excitation energies in eV.
Comparison of the experimental change in optical density ΔOD with simulations at a pump-probe delay of 0 fs. In (a) and (b), the top figures represent the field-free neutral xenon absorption spectrum from Ref. 29. In the bottom of (a) and (b), the black solid curve shows the experimental result where the statistical error is indicated by the grey area (2σ). (a) The simulations with and without the RWA are shown as blue and red curves, respectively, without the inclusion of ionization broadening. (b) The green and red curves show the simulation with and without ionization broadening, both without the assumption of the RWA. The simulated curves are convoluted with the experimental energy resolution of 0.37 eV.
Hole-alignment in xenon ions. Comparison of quantum state distributions retrieved from the measured polarization anisotropies at 55.4 eV and 57.3 eV, results from Loh. et al. at 55.4 eV and the theoretical calculation in Ref. 19. The error bars in this work are the statistical errors of 95% confidence intervals after propagating through Eq. (4).
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