Isolated high-harmonic XUV photon absorption and NIR strong-field tunnel ionization

Abstract

Extreme ultraviolet (XUV) pulses with a duration of tens of femtoseconds initiate 4s −1 or 4p −1 photoionization of krypton, which populates highly excited satellite states through the electron correlation. The excited ions are then tunnel ionized to Kr 2+ 4s −1 4p −1 or 4p −2 by a strong-field nearinfrared (NIR) pulse of a similar duration. The XUV pulses are produced by high harmonic generation in a gas jet and we employ a state-of-the-art timepreserving monochromator to isolate individual XUV harmonic orders. An enhancement of the Kr 2+ yield as a function of harmonic photon energy and XUV-pump NIR-probe delay is observed and compared with a two-step model, which allows the population of the satellite states to be inferred. Furthermore, relative 4s and 4p satellite excitation cross-sections are predicted at the photon energies studied. This proof-of-principle experiment demonstrates that isolated harmonics can be employed to pump specific electronic states, which will be highly complementary to synchrotron, attosecond and x-ray free-electron laser studies of complex systems.In the solid state, ferromagnetism, magnetoresistance and superconductivity are mediated by electron correlation dynamics; on the atomic scale, this coupling is of vital significance to scattering or absorption, many chemical processes and fullerene formation. Narrowband synchrotron radiation is used to measure photon cross-sections as a function of energy from 10 eV to several keV with a typical resolution of a few thousand, allowing the influence of electron correlations to be observed. The Fourier analogue, time-resolved photoexcitation and detection, is now capable of tracking electron dynamics on the attosecond timescale [1,2]. Resolving transient states is the key characteristic, rather than absolute resolution, typically t/ t 5 fs/100 as. In this work, we undertake a novel investigation of atomic ionization dynamics using monochromated high harmonic extreme ultraviolet (XUV) radiation and ultrafast strong-field near-infrared (NIR) pulses. Krypton is 4s −1 or 4p −1 photoionized, populating satellite states via the electron correlation [3][4][5][6][7][8]. Subsequent tunnel ionization [9, 10] of the satellites allows the relative photoexcitation populations and relative cross-sections to be recovered in a pump-probe configuration. We show this unique simultaneous time-and energy-dependent measurement allows weak electron dynamics to be quantified, and is highly complementary to the studies of complex systems with synchrotron, attosecond and x-ray freeelectron laser (XFEL) sources [11][12][13][14][15].Depending on photon energy and flux, radiation absorption processes can be roughly categorized as linear (single photon) or nonlinear multiphoton. Synchrotron sources with a photon energy of tens to thousands of eV are an excellent probe of linear processes [8]. On the other hand, intense ultrafast lasers operating in the visible and NIR spectral regions can initiate nonlinear processes such as multiphoton and tunnel ionization, first theor...

Electron correlation dynamics in atomic Kr excited by XUV pulses and controlled by NIR laser pulses of variable intensity

“…Our present time-dependent investigation of the Kr + satellite states is made using HH19 (29.6 eV), HH21 (32.8 eV), and HH23 (35.9 eV) as the XUV-pump, and a NIR probe (1.56 eV) in a range of intensities between 0.3 × 10 13 to 2.6 × 10 13 W cm −2 . In this NIR intensity range only a small amount of Kr + in the lowest lying electronic states can be produced [17], which is also confirmed in our Kr 2+ ion yield measurements. At intensities up to about 2 × 10 13 W cm −2 , direct double ionization is about seven orders of magnitude lower than single ionization with the NIR alone [17].…”

“…electric and magnetic fields in the interaction region. Selected examples of phenomena probed by ion charge state spectroscopy on atoms are observation of strong-field induced tunnel ionization [12], time-domain observation of the dynamics of Auger decay [12][13][14], study of Rabi oscillations, excited by the resonant coupling of ionic ground to ionic excited states [15], as well as determination of lifetimes and cross-sections of atomic excited states, populated by the absorption of XUV photons [16,17]. Additionally, ion spectroscopy has developed into a practical and very robust diagnostic technique to determine time-zero and temporal resolution in XUV and near-infrared (NIR) pump-probe experiments [13].…”

VUV excitation of a vibrational wavepacket in D2measured through strong-field dissociative ionization

“…The TOFMS has previously been employed to observe vibrational and rotational wavepackets in few cycle strong-field near infrared (NIR) pulse interactions with H 2 , HD and D 2 [7,31,32]. Based on previous measurements the duration of the monochromated VUV pulse is approximately 25 fs [21]. The isolated harmonic is the pump pulse for the experiment and has a photon flux at the interaction region of approximately 10 7 photons per pulse as measured with a calibrated channel electron multiplier and is focused to a spot size of ∼140 μm.…”

Population trapping in bound states during IR-assisted ultrafast photoionization ofNe+

“…These variations in the measured double ionization signal can then be associated with the interplay between the excited Ne + ion and the IR light field. The same process has also been studied in Kr atoms with longer excitation pulses, with an estimated duration of 24-28 fs [7]. The difference in XUV pulse length leads to subtle differences in the ionization yield.…”