- Students & Postdocs
- Education & Outreach
- About JILA
Manipulating Atoms and Molecules with Ultrafast Light
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.
Henry Kapteyn & Margaret Murnane run a joint group whose research in strong-field science spans from physics and chemistry to nanoscience (see Kapteyn-Murnane Group). The atomic and molecular research topics they explore with their students, postdocs, and collaborators include:
- the control of quantum systems with strong laser fields.
- the visualization of molecular dynamics at the electron level.
- the exploration of attosecond science to generate coherent X-rays and electrons.
The Kapteyn/Murnane group harnesses attosecond laser pulses to manipulate electrons and generate bright beams of coherent X-rays. By focusing an intense femtosecond laser into a gas, an electron can literally be ripped from an atom or molecule by suppressing the Coulomb barrier that binds the electron to the ion. 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 then emitted as an X-ray photon. This process is called high harmonic generation (HHG). It is the most extreme nonlinear process observed to date. It results in the generation of coherent, laser-like beams of X-rays that emerge in bursts lasting from a few femtoseconds (10-15 s) to <100 attoseconds (10-18 s). This process can really be thought of as a coherent version of the original Rontgen X-ray tube. The X-ray beams it produces are fast enough to capture dynamics inside atoms and molecules. The beams can also be used for a variety of applications in nanoscale imaging.
Kapteyn, Murnane, and their group have developed two approaches for manipulating electrons as they cause the emission coherent bursts of X-rays. The first approach uses a tabletop femtosecond laser to generate coherent X-ray beams at higher photon energies than ever before possible. This advance solved a grand challenge in the field of extreme nonlinear optics of high harmonic generation: Since the generation of higher-order harmonics is accompanied by ionization of the gas medium, the resulting free-electron plasma causes laser light waves to outrun the generated X-ray waves. This phase-velocity mismatch limits the useful X-ray flux at high photon energies because of the destructive interference experienced by the generated x-ray waves as they travel through the gas. In a breakthrough series of experiments, Kapteyn and Murnane’s students used a light pattern to adjust the phase of the radiating electrons, so that the generated X-rays all interfered constructively throughout the medium. This process is known as quasi-phase matching. Using this process, the researchers selectively enhanced the X-ray flux (at 70 eV and 140 eV) by 200-fold to 700-fold. In theory, this technique should work from the deep UV to the keV region.
In the second approach, Kapteyn, Murnane, and their students showed that by using mid-infrared (IR) laser pulses to generate high harmonics, there is an experimentally feasible and straightforward route for generating bright, fully coherent beams up to several keV photon energies. These new ultrafast light sources based on strong field atomic physics are opening the door to many new scientific explorations and technological applications. The new HHG methods for generating beams of X-rays are also making possible some exciting research projects in the Kapteyn/Murnane group, including studies of how electrons rearrange as molecules change their shape.
Murnane and Kapteyn have come up with innovative ways of looking at how electrons move around as a molecule changes its shape. This knowledge will help them better understand how bonds are formed or broken between atoms in a molecule during chemical reactions. In many ways, molecules act like tiny masses connected by springs of differing strengths. The springs in this case are chemical bonds, made up of shared electrons, which hold matter together. Unlike real springs, however, in molecules the properties of the springs can change – particularly if the molecule changes shape. To observe these changes, ultrafast light pulses are needed to capture even the most dizzying dance of electrons as they swarm around atoms in a molecule.
In an experiment done by Wen Li in Kapteyn and Murnane’s group in collaboration with Albert Stolow and his group (Queens University), a dinitrogen tetroxide (N2O4) molecule was first hit by a short burst of laser light. The N2O4 molecule is composed of two v-shaped nitrogen dioxide (NO2) molecules that have a weak, floppy bond between the two nitrogen atoms. The laser field distorts this weak bond so that the molecule begins to vibrate. A second laser pulse plucked an electron from the molecule, accelerated it and smashed it back into the same molecule at different times during the vibration. The researchers found that when the N2O4 molecule was stretched, the electron was able to easily recombine with the ion and release its energy as an X-ray. When the molecule was compressed, however, the electron could not recombine with the ion, so no X-rays were emitted.
Since the brightness of the X-ray beam changes as the molecule vibrates, the emitted X-rays could be used to map energy levels in the molecule – and, most importantly, to understand how these levels rearrange as the molecule changes its shape. In the future, the Kapteyn/Murnane team hopes to capture an image of an electron cloud in real time to understand how electrons influence each other and other atoms during chemical reactions.
X-ray Driven Molecular Dynamics
In photoionization, a single photon impinging on a molecule can lead to the loss of two electrons. If a photon knocks out an electron from deep inside the molecule, an outer electron can fall into the hole. When that happens, a second outer electron gets ejected, carrying away excess energy, and the molecule falls apart. These reactions occur on attosecond time scales and can be observed in the laboratory, according to theorist Andreas Becker, who studies the process. Becker is interested in understanding exactly how photoionization occurs in specific molecules. He wants to answer such questions as (1) How does this happen? (2) Do the electrons "talk" to each other during the process? (3) How long does photoionization take? (4) How much energy is released? Becker is working with the Kapteyn/Murnane group to answer these questions.
In one photoionization experiment, the Kapteyn/Murnane group used an ultrafast femtosecond X-ray pulse to knock out an electron from an oxygen molecule (O2) and superexcite it to observe photoionization in real time. Surprisingly, the group found that in this molecule, the second electron took an unusually long time — up to 300 fs — to get ejected before the molecule finally fell apart.
The group determined that the molecule could not fall apart until the molecular fragments were separated by at least 30 Å. In essence, the second electron felt an attraction to each of the O fragments that kept the electron bound between the two ions until they were so far apart, they hardly interacted. Eventually, the electron transformed into a "Feshbach resonance" state in which it remained bound, albeit with a negative binding energy. The existence of the negative-binding energy state was completely unexpected. It is actually a new "superheated" phase of atomic matter. The researchers not only detected the Feshbach resonance states experimentally, but also used their results to test and refine computer models of the structure of atoms excited far from their ground state.
The Kapteyn/Murnane group is currently collaborating with Becker on a new experiment to image the complex photodissociation of bromine molecules (Br2).
Coherent Control with Attosecond Pulses
One of theorist Andreas Becker’s main areas of interest is ultrafast laser theory because such pulses have the potential to probe electron dynamics inside atoms and molecules. In one long-term effort, he has been investigating the dynamics of two electron ionization processes in atoms. He wants to answer such questions as (1) Do both electrons behave individually ? (2) What is the role of coulomb interactions in two-electron dynamics? and (3) What is the time scale on which a two-electron ionization takes place? A full numerical treatment of this problem requires an understanding of the evolution of both particles in six spatial dimensions and one time dimension. It is still unsolved, though Becker has created a model in reduced dimensions that approximates the interactions of two electrons with an ultrafast laser field. He anticipates spending 2–3 years using supercomputers to completely describe the two-electron problem.
Becker is also interested in determining whether ultrafast lasers can image the internal dynamics of such molecules as N2, O2, CO2, and fullerenes with 60 to 180 carbon atoms. For example, if the high-harmonic generation (HHG) technique developed in the Kapteyn/Murnane lab were used to probe a C60 fullerene, interference patterns could form an electron wave distribution over all 60 of the carbon atoms, yielding invaluable information about the molecule’s structure. Becker believes it should be possible to determine the C60 radius from the positions of the minima of such an interference pattern.
Becker’s theory suggests a method for observing the contraction and extension of the C60 molecule while it is breathing during an interaction with an intense laser pulse. Reading HHG spectra should yield information about how the size of the molecule’s radius is changing. The next step would be actual imaging of the breathing molecule.
In related work, Becker is exploring the use of ultrashort laser pulses to direct electron dynamics in the hydrogen molecular ion (H2+) as it dissociates. Preliminary work on this problem has already shown that an electron’s pathway during dissociation is not as simple as previously thought. It does not simply follow the electric field; interference effects must also be taken into account. Eventually, Becker wants to provide a new theory to guide laboratory experiments in the use of ultrashort lasers to direct electron behavior in H2+ as it falls apart. He believes it should be possible to use an ultrafast laser to control the exact portion of the electron associated with each proton in the H2+ molecule. This control is the first step to being able to manipulate electron movement between different chemical bonds, in essence controlling chemical reactions. At present, the calculations needed to develop these schemes are daunting, even in small molecules. Even so, Becker is optimistic about applying the main ideas he discovers to larger molecules.