Review Article | Published:

Attosecond imaging of molecules using high harmonic spectroscopy

Abstract

The availability of attosecond-duration extreme ultraviolet or soft X-ray light sources has opened up new fields of research in atomic and molecular physics. These pulses can be as short as 50 as, fast enough to freeze the motion of electrons within molecules, to resolve how electrons rearrange themselves after the removal of an electron and to study electron–electron correlations. Gas-phase molecules can be aligned in space using short laser pulses, permitting the measurement of molecular parameters in the molecular frame. Aligned molecules can be photoionized using a train of attosecond pulses, enabling the complete characterization of the partial waves making up the photoelectron angular distributions. Using a recolliding electron in the high harmonic process allows complex transition dipole matrix elements to be recorded (including their amplitude and phase) in the molecular frame. High harmonic spectroscopy makes it possible to image molecular orbitals and for unimolecular chemical reactions to be followed with femtosecond resolution. For example, the behaviour around conical intersections can be probed. Charge migration within molecules can be observed with sub-femtosecond resolution.

Key points

  • High harmonic spectroscopy uses femtosecond lasers to probe the valence electrons in gas-phase molecules. It records the transition dipole matrix elements upon recombination from a set of continuum wavefunctions. This is effectively time-reversed photoionization in which the highest occupied molecular orbitals are isolated.

  • A weaker laser can create rotational revivals that lead to an ensemble of molecules that are aligned in space and are field-free. This allows measuring of the dipole matrix elements in the molecular frame.

  • At the sub-optical-cycle level, on the attosecond timescale, electron–electron correlations can be revealed.

  • Simple chemical reactions such as unimolecular dissociation and the behaviour around conical intersections can be followed on a femtosecond timescale using pump–probe techniques.

  • Charge migration following removal of an electron from a molecule can be visualized with sub-femtosecond time resolution.

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Correspondence to David M. Villeneuve.

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Fig. 1: High harmonic spectra from atoms and molecules reveal details of the electronic structure of valence orbitals.
Fig. 2: Tomographic imaging of molecular orbitals reveal the structure of a single orbital wavefunction.
Fig. 3: High harmonic spectra from aligned CO2 molecules driven by an infrared laser.
Fig. 4: High harmonic spectroscopy using a transient excitation grating reveals nuclear dynamics.
Fig. 5: Reconstructed electron dynamics following ionization of iodoacetylene.