Interference effects in high-order harmonic generation with molecules
TL;DR: In this article, high-order harmonic generation for molecules in linearly polarized laser pulses by numerical solution of the Schr\"odinger equation was studied and maxima and minima due to intramolecular interference were found in the dependence of the harmonic intensities on the internuclear distance and on the orientation of the molecules.
Abstract: We study high-order harmonic generation for ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{2}$ model molecules in linearly polarized laser pulses by numerical solution of the Schr\"odinger equation. Maxima and minima due to intramolecular interference are found in the dependence of the harmonic intensities on the internuclear distance and on the orientation of the molecules. These extrema can be approximately predicted by regarding them as the result of interference between two radiating point sources located at the positions of the nuclei.
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TL;DR: The authors show the double-slit interference effect in the strong-field ionization of neon dimers by employing COLTRIMS method to record the momentum distribution of the photoelectrons in the molecular frame.
Abstract: Wave-particle duality is an inherent peculiarity of the quantum world. The double-slit experiment has been frequently used for understanding different aspects of this fundamental concept. The occurrence of interference rests on the lack of which-way information and on the absence of decoherence mechanisms, which could scramble the wave fronts. Here, we report on the observation of two-center interference in the molecular-frame photoelectron momentum distribution upon ionization of the neon dimer by a strong laser field. Postselection of ions, which are measured in coincidence with electrons, allows choosing the symmetry of the residual ion, leading to observation of both, gerade and ungerade, types of interference.
7,160 citations
TL;DR: In this article, a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a sub-femtosecond time scale was demonstrated.
Abstract: We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization.
664 citations
TL;DR: Evidence for quantum interference of electron de Broglie waves in the recombination process of HHG from aligned CO2 molecules is reported and it is proposed that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.
Abstract: High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions1,2,3. In the three-step model4,5,6 of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules7,8,9,10,11 can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves12,13,14 in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump–probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.
540 citations
TL;DR: Measurements of the HHG spectrum of N2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half-revival, which indicates the influence of electrons occupying the orbital just below the N2 HOMO, referred to as the HomO-1.
Abstract: Molecular electronic states energetically below the highest occupied molecular orbital (HOMO) should contribute to laser-driven high harmonic generation (HHG), but this behavior has not been observed previously. Our measurements of the HHG spectrum of N2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half-revival. This feature indicates the influence of electrons occupying the orbital just below the N2 HOMO, referred to as the HOMO-1. Such observations of lower-lying orbitals are essential to understanding subfemtosecond/subangstrom electronic motion in laser-excited molecules.
353 citations
TL;DR: Theoretical avenues of attack have emerged in two categories, which are strikingly different as mentioned in this paper, which can be labeled as "all-at-once" and "step-by-step" approaches.
Abstract: Experimental advances with laser intensities above1 TW/cm(2), with pulse durations between roughly 50 and 5 fs, have led to the discovery of new atomic effects that include examples of startlingly high electron correlation. These phenomena have presented an unexpected theoretical challenge as they lie outside the domains of both of the nominally applicable theories, namely, straightforward perturbative radiation theory and quasistatic tunneling theory. The two liberated electrons present a new few-body collective effect. When they are not released independently, one by one, the term nonsequential double ionization has been adopted. Theoretical avenues of attack have emerged in two categories, which are strikingly different. They can be labeled as "all-at-once" and "step-by-step" approaches. Although different, even conceptually opposite in some ways, both approaches have been successful in confronting substantial parts of the experimental data. These approaches are examined and compared with their results in addressing key experimental data obtained over the past decade.
336 citations