Solid-state attosecond metrology

Solid-state attosecond metrology. Chemistry takes place at the femtosecond speed of atomic motions. In contrast, electrons within atoms, or traveling between neighboring atoms in a molecule or solid, zip along at an attosecond pace (1 as = 10⁻¹⁸ s). A European collaboration of physicists, led by Ferenc Krausz at the Max Planck Institute for Quantum Optics in Munich, Germany, has now timed the transport of photoelectrons prior to their emission from tungsten. To do that, the researchers sent through a tube of neon gas an intense femtosecond laser pulse of near-IR light comprising a few well-controlled field oscillations (see Physics Today, April 2003, page 27 ). Through the magic of high-harmonic generation, a single, isolated 300-as coherent extreme UV pulse in a selected spectral band (90–100 eV) was produced. Next, the XUV pulse was directed at a tungsten target where it energized electrons lying close to the sample’s surface. Affected electrons included both the delocalized conduction-band variety and those tightly bound in the tungsten atoms’ 4f orbital. Electrons from those two populations were cleverly distinguished: Part of the original NIR laser beam remained coherently linked to the XUV pulse and was engineered to selectively accelerate the conduction-band electrons as they sprang out of the sample’s surface ahead of the 4f electrons. The detected time delay of 110 ± 70 as (see the figure) between the two types of photoelectrons was due entirely to propagation differences of the electron wavepackets in the sample. (A. L. Cavalieri et al., Nature 449 , 1029, 2007 http://dx.doi.org/10.1038/nature06229 . )