Credit: Mark Leffingwell

Studies of the interaction of light with matter in the 20th century led first to the development of quantum theory and then to tremendous progress in our understanding of atoms, molecules, and materials. Today, armed with a fundamental understanding of atoms, molecules, and optics, physicists stand ready to tackle research that just a few decades ago seemed impossible: the precise control of atoms, molecules, and electrons using light fields. The ability to control quantum systems with light becomes even more exciting in the strong-field regime. In this regime, electrons can literally be ripped from an atom or molecule by a light field. Once released, the electrons oscillate in the light field, and some of them can coherently recombine with the same atom or molecule. The excess energy of the recombining electron is emitted as an X-ray photon. This process is called high harmonic generation and results in the generation of coherent laserlike beams of X-rays.

The Henry Kapteyn & Margaret Murnane group has performed two experiments that highlight the exquisite control over quantum systems that is possible with strong light fields. These experiments also show that such control has useful outcomes. Recently, the group used electrons plucked from a molecule to demonstrate a new, ultrasensitive probe of the dynamic position of atoms inside the same molecule. In contrast to optical laser pulses, which are only indirectly sensitive to atomic motion and unable to visualize the changes in a molecule that are important for function in a chemical reaction, X-rays allow direct tracking of atomic positions. Electrons can also be used to explore the nanoworld since electrons have a wave-particle duality that allows them to diffract from small objects. For these reasons, an external source of electrons or X-rays is normally used to monitor a fast process.

In their experiment, however, the Kapteyn/Murnane group used the electric field from an intense optical laser pulse to pluck electrons away from a molecule and then accelerate them back toward the same molecule. Rather than measuring the scattered electrons, as might be done in an electron diffraction experiment, the new method took advantage of the fact that when electrons recollide with the molecule, they emit X-rays (a process known as high-harmonic generation). The wavelength of the recolliding electrons is comparable to distances between atoms in the molecule. Thus, the strength of the emitted X-rays is sensitive to the minute changes in atomic position within the molecule. Using this internal probe, the group was able to observe complex motions inside a molecule that had been excited by a separate laser beam. The method shows great promise for imaging ultrafast molecular structural transformations, including the fundamental action of all of chemistry, the making and breaking of chemical bonds.

In a related experiment, the Kapteyn/Murnane group finely tuned the light pulse used to generate the coherent X-ray beams in such a way as to force an atom to emit only a single X-ray wavelength. A shaped light pulse was used to rip electrons from atoms, while the magnitude of the X-ray emission was measured simultaneously. In general, if a laser pulse is not carefully shaped, X-ray bursts from adjacent cycles of the laser pulse will not necessarily add completely in phase, leading to a partial destructive interference that limits the intensity of the X-ray emission. In contrast, in an optimally shaped laser pulse, X-ray bursts emitted from adjacent cycles add constructively, leading to stronger X-ray emission. This experiment required that the laser pulse be shaped to a precision of ~ 150 attoseconds, while the X-ray phase was made constant to a precision of 10-20 attoseconds. This work is one of the first results in the new field of "attosecond science" and demonstrates control of the shape of a radiating electron wave function on subangstrom length scales and subfemtosecond time scales.

For additional information please see http://jilawww.colorado.edu/kmgroup