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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 will also make it possible to understand the complex interactions of atomic nuclei and electrons as well as control matter in dynamic states far from equilibrium.

Henry Kapteyn & Margaret Murnane run a joint group whose research in molecular dynamics and imaging spans the disciplines of physics, chemistry, materials science, and nanoscience. The research topics they explore with their students, postdocs, and collaborators include:

  • Probing the behavior of excited molecules with x-ray laser fields.
  • Visualizing molecular dynamics at the electron level.
  • Using soft x-ray spectroscopies to explore electronics states, bond breaking, and charge transfer in molecules.

The Kapteyn/Murnane group collaborates with the Andreas Becker theory group to better understand their experimental work on x-ray-driven molecular dynamics and coherent control with attosecond pulses.

Radiation Driven Molecular Dynamics

The Kapteyn/Murnane group is investigating how molecules with three or more atoms breakup after being exposed to ultrafast x-ray pulses. This process, known as photoionization, begins with the removal of an electron from the molecule. The sudden disappearance of the electron initiates rapid-fire changes in the behavior of the remaining electrons and the atomic nuclei, which make up the now super-excited molecule. The behavior of the super-excited molecules is studied with advanced molecular imaging techniques.

These studies grew out of early work on photoionization in simple diatomic molecules such as oxygen (O2) and bromine (Br2).  Early work revealed that a single photon impinging on a molecule could lead to the loss of two electrons. For instance, if a photon knocked out an electron from deep inside the molecule, an outer electron could fall into the hole. When that happened, a second outer electron was ejected, carrying away excess energy, and the molecule fell apart. These reactions occurred on attosecond time scales such as those that occur in the larger molecules currently under study. Molecular dynamics studies enjoy theory support from Andreas Becker.

Andreas Becker is interested in understanding exactly how photoionization occurs in specific molecules. He wants to answer such questions as  

  • How does this happen?
  • Do the electrons "talk" to each other during the process?
  • How long does photoionization take?
  • How much energy is released? Becker collaborates with the Kapteyn/Murnane group to answer these questions.

Advanced Molecular Imaging

The Kapteyn/Murnane group is working to develop both strong field ionization and high-harmonic generation (HHG) as powerful tools to capture the behavior of electrons and the evolution of molecular structure when molecules are exposed to strong laser fields. The group has observed that atoms and molecules have a universal response to strong laser fields. Part of the electron wave function undergoes tunnel ionization, a quantum mechanical phenomenon in which an electron passes through an energy barrier it otherwise could not surmount. The electron can later recombine with the parent ion. By monitoring this process, the group hopes to probe the evolution of electron orbitals and changes in the structure of the atomic nuclei inside a molecule.

The group is developing HHG for this and other projects. In HHG, the researchers upconvert infrared (IR) laser pulses to generate generate bright beams of coherent laser-like x-rays. The process begins with the focusing of an intense IR femtosecond laser into a gas where an electron can literally be ripped from an atom or molecule as the laser supresses 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 extreme nonlinear process 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). HHG can be thought of as a coherent version of the original Roentgen x-ray tube.

X-ray Spectroscopy of Molecules

The Kapteyn/Murnane group is developing innovative ways to use HHG for soft x-ray spectroscopies to probe the internal dynamics of molecules. The group uses mid-IR-wavelength lasers to drive the HHG process, producing bright, high-energy, and coherent soft x-rays that are perfectly synchronized to the driving laser. The HHG spectrum is actually an x-ray super continuum spanning photon energies from the extreme ultraviolet into the soft x-ray region. The broad range of frequencies allows the researchers to simultaneously investigate different elements and their characteristics.

The new spectroscopies will allow the researchers to “watch” 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. 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.

Recently, the group learned how to make a multicolor light field consisting of finely tuned attosecond bursts of high-energy ultraviolet (UV) laser-like light of carefully selected wavelengths. Usin this optimized light field, the researchers were able to simultaneously control both the electrons and atoms in a deuterium (D2) molecule. This exquisite control allowed them to dictate the exact pathway by which the molecule loses an electron (ionizes), regulate the way the molecule vibrates, and manipulate the way in which the molecule falls apart. In short, they figured out how to use quantum physics to control chemical reactions in a simple molecule!

Observation and Control of Electron Behavior with Attosecond Pulses

JILA’s ultrafast AMO theory group, led by Andreas Becker and Agnieszka Jaroń-Becker, explores methods for observing, understanding, and controlling the dynamics (behavior) of electrons and nuclei in atoms and molecules with ultrashort laser pulses. The theorists want to answer such fundamental questions as

  • On which time scale do electrons and nuclei in an atom or molecule influence each other and exchange energy?
  • How can we use laser light ot control a chemical reaction by influencing the behavior of electrons in a molecule?

JILA’s culture of collaboration encourages theorists and experimentalists to work together to find answers to these questions. For instance, recent advances in laser technology make it possible to create light pulses with durations of less than 100 as, or 100 x 10-18 s; intense laser sources can now generate x-ray and infrared laser wavelengths in addition to optical wavelengths. This array of laser technology is helping to uncover the inner dynamics of atoms and molecules. The ultrafast theory group works side by side with the Kapteyn/Murnane group in explaining experimental findings and predicting new avenues to explore.

The ultrafast theory group recently investigated the attosecond electron dynamics of the simplest molecule found in nature, the hydrogen molecular ion, or H2+. The investigation led to surprising insights into how and when an electron becomes separated, or ionized, from a molecule during a laser pulse. Numerical simulations and theoretical analysis revealed the details of the underlying electron dynamics on the attosecond timescale. The group is now studying whether similar electron dynamics can be found in more complex molecules. The researchers anticipate developing a revised picture of molecular ionization in an intense laser field.

In a longer-term project, the group is investigating the ways in which electrons exchange energy in an atom or molecule. Using the JANUS supercomputer at the University of Colorado, the theoreticians have identified how the energy of a single photon is shared between two electrons over a distance of several Angstroms in a helium dimer (two weakly bound helium atoms) before both electrons leave the dimer. This investigation should make it possible for the theorists to propose future experiments aimed at observing the energy exchange and measuring the time required to balance the photon energy between the electrons in the dimer. 

The ultrafast theory group is also interested in exploring the challenges raised by technological developments that have made it possible to generate intense laser light from x-ray to infrared wavelengths. Thus far, initial observations and theoretical predictions have shown that a molecule can be dissociated without ionization within tens of femtoseconds of experiencing infrared laser pulses. The group now wants to provide a unified theoretical picture of how atoms and molecules respond to intense laser light over the whole range of frequencies experimentally accessible now—and in the near future.