During the 1980s the duration of the shortest pulses available from lasers got dramatically shorter - from nanoseconds (10-9 s) at the start of the decade to femtoseconds (10-15 s) at the end. However, it has proved very difficult to generate pulses shorter than this because the duration of a laser pulse cannot be shorter than the oscillation period of the electromagnetic field. As a result, a visible laser pulse cannot break the femtosecond threshold. Indeed, the shortest pulse ever generated by a laser operating at 800 nm is about 4-5 femtoseconds.
Such femtosecond pulses can be used in time-resolved molecular-spectroscopy experiments to monitor molecular dynamics with unprecedented temporal resolution. However, tightly bound electrons in atoms cannot be observed directly because most of the relevant dynamics occurs on the attosecond (10-18 s) timescale. Such ultrashort pulse durations require the use of coherent sources of radiation in the ultraviolet and soft X-ray region (see From femtochemistry to attophysics Physics World September 2001 pp41-46).
Now Ferenc Krausz at the Technical University of Vienna and co-workers at the Steacie Institute of Molecular Sciences in Canada and Bielefeld University in Germany have made the first time-resolved attosecond-spectroscopy measurement. In the experiment, they studied the ionization of krypton gas and the kinetic-energy spectrum of the resulting photoelectrons by simultaneously irradiating the atoms with a soft X-ray pulse lasting 650 ± 150 as and an infrared light pulse (M Hentschel et al. 2001 Nature 414 509).
In the January issue of Physics World, Maciej Lewenstein and Anna Sanpera of the University of Hannover, Germany, describe how a technique known as 'harmonic generation' has allowed physicists to overcome the fundamental limit on pulse length.