The superposition of quantum states drives motion on the atomic and subatomic scales, with the energy spacing of the states dictating the speed of the motion. In the case of electrons residing in the outer (valence) shells of atoms and molecules which are separated by electronvolt energies, this means that valence electron motion occurs on a subfemtosecond to few-femtosecond timescale (1 fs = 10−15 s). In the absence of complete measurements, the motion can be characterized in terms of a complex quantity, the density matrix. Here we report an attosecond pump–probe measurement of the density matrix of valence electrons in atomic krypton ions1. We generate the ions with a controlled few-cycle laser field2 and then probe them through the spectrally resolved absorption of an attosecond extreme-ultraviolet pulse3, which allows us to observe in real time the subfemtosecond motion of valence electrons over a multifemtosecond time span. We are able to completely characterize the quantum mechanical electron motion and determine its degree of coherence in the specimen of the ensemble. Although the present study uses a simple, prototypical open system, attosecond transient absorption spectroscopy should be applicable to molecules and solid-state materials to reveal the elementary electron motions that control physical, chemical and biological properties and processes.
At a glance
- Multichannel coherence in strong-field ionization. Phys. Rev. A 79, 053402 (2009) &
- Attosecond control of electronic processes by intense light fields. Nature 421, 611–615 (2003) et al.
- Attosecond metrology. Nature 414, 509–513 (2001) et al.
- Femtochemistry: atomic-scale dynamics of the chemical bond. J. Phys. Chem. A 104, 5660–5694 (2000)
- Ultrafast probing of core hole localization in N2 . Science 320, 920–923 (2008) et al.
- Sub-laser-cycle electron pulses for probing molecular dynamics. Nature 417, 917–922 (2002) et al.
- Probing molecular dynamics with attosecond resolution using correlated wave packet pairs. Nature 421, 826–829 (2002) et al.
- Mapping attosecond electron wave packet motion. Phys. Rev. Lett. 94, 083003 (2005) , &
- Probing proton dynamics in molecules on an attosecond time scale. Science 312, 424–427 (2006) et al.
- High harmonic interferometry of multi-electron dynamics in molecules. Nature 460, 972–977 (2009) et al.
- Atomic transient recorder. Nature 427, 817–821 (2004) et al.
- Isolated single-cycle attosecond pulses. Science 314, 443–446 (2006) et al.
- Single-cycle nonlinear optics. Science 320, 1614–1617 (2008) et al.
- 2008) Strong Field Laser Physics (Springer,
- Attosecond photoionization of coherently coupled electronic states. Phys. Rev. A 72, 051401 (2005) et al.
- Wave packet theory of dynamic absorption spectra in femtosecond pump-probe experiments. J. Chem. Phys. 92, 4012–4029 (1990) , &
- Direct observation of the femtosecond excited-state cis-trans isomerization in bacteriorhodopsin. Science 240, 777–779 (1988) et al.
- Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy. Phys. Rev. Lett. 98, 143601 (2007) et al.
- Ultrafast strong-field dissociative ionization dynamics of CH2Br2 probed by femtosecond soft X-ray transient absorption spectroscopy. J. Chem. Phys. 128, 204302 (2008) &
- K-edge X-ray-absorption spectroscopy of laser-generated Kr+ and Kr2+ . Phys. Rev. A 76, 043421 (2007) et al.
- Inherent lifetime widths of Ar 2p −1, Kr 3d −1, Xe 3d −1, and Xe 4d −1 states. Phys. Rev. A 64, 012502 (2001) , &
- Time-resolved atomic inner-shell spectroscopy. Nature 419, 803–807 (2002) et al.
- ‘Short’ pulse MPI of xenon: the 2 P 1/2 ionization channel. J. Phys. B 29, 1479 (1996) , &
- Core relaxation in atomic ultrastrong laser field ionization. Phys. Rev. Lett. 94, 053602 (2005) et al.
- X-ray microprobe of orbital alignment in strong-field ionized atoms. Phys. Rev. Lett. 97, 083601 (2006) et al.
- Energy levels and observed spectral lines of krypton, Kr I through Kr XXXVI. J. Phys. Chem. Ref. Data 36, 215–386 (2007)
- Spin-orbit effect on strong-field ionization of krypton. Phys. Rev. A 74, 043403 (2006) , &
- Electronic wavepackets. Adv. At. Mol. Opt. Phys. 38, 1–38 (1998) &
- The future of attosecond spectroscopy. Science 317, 766–769 (2007)
- Attosecond physics. Rev. Mod. Phys. 81, 163–234 (2009) &
- Supplementary Information (219K)
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