Probing collective multi-electron dynamics in xenon with high-harmonic spectroscopy

Abstract

High-harmonic spectroscopy provides a unique insight into the electronic structure of atoms and molecules 1-5. Although attosecond science holds the promise of accessing the timescale of electron-electron interactions, until now, their signature has not been seen in high-harmonic spectroscopy. We have recorded high-harmonic spectra of atoms to beyond 160 eV, using a new, almost ideal laser source with a wavelength of 1.8 µm and a pulse duration of less than two optical cycles. We show that we can relate these spectra to differential photoionization cross-sections measured with synchrotron sources. In addition, we show that the highharmonic spectra contain features due to collective multielectron effects involving inner-shell electrons, in particular the giant resonance in xenon. We develop a new theoretical model based on the strong-field approximation and show that it is in agreement with the experimental observations. Measuring and understanding the electronic structure and correlated dynamics of matter on its natural timescale represents the main thrust of ultrafast laser science. Electron correlations affect essential properties of complex systems ranging from configuration interactions in molecules to cooperative phenomena in solids, such as superconductivity. Our knowledge of the electronic structure of matter originates from several decades of research on photoionization and photoelectron spectroscopy 6-8 , mainly driven by the development of synchrotron-based sources. Recent advances in strong-field physics have opened an alternative approach to probing both the electronic structure 1,9 and the dynamics 10-12 of molecules using table-top laser sources. These new methods rely on the recollision of an electron, removed from the molecule by a strong laser field, with its parent ion 13 , as illustrated in Fig. 1a. The electronic structure of the molecule is encoded in the emitted high-harmonic spectrum through the amplitude and phase of the photorecombination matrix elements 4,11,14,15. We use high-harmonic spectroscopy to investigate a new class of collective electronic dynamics-induced and probed by the recombining electron. The kinetic energy of the returning electron is usually much larger than the difference between electronic energy levels of the parent ion. Consequently, inelastic scattering followed by recombination is energetically possible, as illustrated in Fig. 1b. Using the xenon atom as an example, we demonstrate that such processes indeed occur and that they can locally enhance the efficiency of high-harmonic generation (HHG) by more than one order of magnitude. We show that such a seemingly complex pathway contributes significantly to the phase-matched process. This observation uncovers a new

High-Harmonic Spectroscopy: From Femtosecond to Attosecond Molecular Dynamics

“…The last step, photorecombination, is now well understood in terms of quantum scattering calculations developed in the context of photoionization. [15,16,19] Most remarkably, the strong laser field does not lead to a measurable distortion of the electronic wave functions within the accuracy of current experiments, even in polarizable systems like xenon [20] or in molecules with appreciable extension along the direction of the laser field like CO 2 . [4,5] The second step, propagation of the electron wavepacket, is largely dominated by the laser field and is well described by classical equations as evidenced by, e.g.…”

Theory of Nonlinear Propagation of High Harmonics Generated in a Gaseous Medium

“…1.2.1), the electronic structure of molecular orbitals, or electronelectron interaction is imprinted on the HHG spectrum. In a recent experiment Shiner et al [21] recorded high-harmonic spectra of several atoms (Kr, Xe and Ar) for the photon energy up to 160 eV using a few-cycle 1.8-µm laser. They showed that these spectra can be related to differential photoionization cross sections measured with synchrotron sources, and these spectra contain features due to collective multi-electron effects involving inner-shell electrons, in particular the giant resonance in Xe.…”

Harmonic generation spectroscopy with a two-colour laser field having orthogonal linear polarizations

“…Their interpretation is often based on the idea that the frequencydomain HHG yield can be factorized in terms of an electron wave packet (EWP) that depends on the parameters of the strong laser field driving the harmonic emission and the (field-free) photorecombination cross section (PRCS) of the atomic or molecular target [3]. This assumption has been successfully used to retrieve the PRCS (or, equivalently, the photoionization cross section, which can be related to the PRCS using the principle of detailed balance) from measured HHG spectra (see, e.g., [2]). For a linearly polarized field, this factorization was proposed phenomenologically, based on numerical solutions of the time-dependent Schrödinger equation [3,4].…”

“…In addition to its important technological applications, such as the production of attosecond pulses, high-order harmonic generation (HHG) is now widely used in spectroscopic applications, allowing one to image atomic and molecular dynamics [1,2]. Most experiments on HHG spectroscopy have been performed with a linearly polarized laser field.…”

“…While HHG spectroscopy can measure distinct features in the photorecombination of an electron to the outer p-subshells of rare gas atoms, such as the Cooper minimum in Ar [11,12] and the giant dipole resonance in Xe [2], HHG measurements for linear polarization are fundamentally limited. Specifically, they can only give access to the energy dependence of the differential PRCS σ (E, α) (where α is the angle between the photon polarization axis and the recombining electron's momentum).…”