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Knowledge Co-Creation - Profiles of researchers

Frontier of attosecond science:
Measuring electron dynamics in molecules

Hiromichi Niikura
Associate Professor, Faculty of Science and Engineering, Waseda University

Before the era of attosecond

My research interests are developing new lasers and experimental methodology to measure ultra-fast structural changes in molecules, as well as the behavior of electrons or electron wavefunctions in molecules in the attosecond time range. Molecules rotate and vibrate about its center of mass. The average rotational period of a molecule is on the picoseconds time scale (1 picosecond=10^(-12) of a second), while the average vibrational period is approximately several dozen femtoseconds (1 femtosecond=10^(-15) of a second). By achieving a time resolution of attoseconds (one-quintillionth of a second; 1 attosecond=10^(-18) of a second), it is possible to get images of dynamical changes in electron distribution (wave function) inside of molecules, whose motion is faster than the vibrational or rotational motion of molecules.

Laser pulses have been utilized to measure phenomena which occur on a fast time scale. The laser was invented in the 1960s for the first time. A pulse laser is referred to as a laser which emits photons at instantaneous time only, similar to the flash of a camera. Shorter shutter speeds for cameras make it possible to take an unblurred image of fast-moving objects. In the same way, it is necessary to shorten the pulse width of a laser in order to measure ultra-fast phenomena like electron motion.

In the 1980s, the width of the shortest laser pulse had been decreased to less than 10 femtoseconds. Femtosecond lasers have been used to many researches such as measuring chemical reaction dynamics as it occurs. However, for the next 15 years, it had not been possible to break through the 1 femtosecond barrier and achieve a time resolution in the attosecond range.

In 1989, it has been reported that focusing intense, infrared laser pulses on gas-phase atoms can generate a light in the extreme ultra-violet range (EUV). The wavelength in the EUV range is much shorter than that of the infrared, fundamental laser pulse. This process is referred to as "high-harmonic generation (HHG) or high-harmonic emission." The high-harmonic generation process is different from conventional non-linear processes. Therefore, developing a physical model to explain the generation mechanism drew much attention.

In 1993, Dr. Paul Corkum at the National Research Council of Canada (NRC) proposed a semi-classical model called a three-step model to clarify the generation mechanism of high-harmonic emission. In addition, he also proposed that using the high-harmonic generation, we can create attosecond laser pulses. In 1997, Dr. Corkum proposed an "attosecond electron streak camera" to measure the duration of attosecond pulses. By those proposals, the expectations to break one-femtosecond barrier and to achieve attosecond time-resolution were increased.

Interest in light and quantum mechanics

At my undergraduate university, I majored in Polymer Science. In the first year of the undergraduate school, I learned quantum mechanics in the class of Physical Chemistry. I was interested in the physics to describe microscopic behavior of molecules, especially for the light-matter interactions in quantum systems, which is different from the physics for describing macroscopic objects. At my 4th year of the undergraduate school and Master's Program, I joined the laboratory of Physical Chemistry and Photo-Chemistry. I studied the photo-excitation process of anthracene molecules in the ultra-violet region. In the laboratory, post-doctoral fellows from foreign countries stayed constantly. Likewise, the professor recommended me to obtain a Ph.D. as early as possible and to work at foreign universities or research institutes of other countries as a post-doctoral fellow. Therefore, when the time was approaching for me to get a Ph.D., I wrote letters to professors in several countries such as Germany, the United States or Canada to inquire whether they would hire me.

After a while, I received a reply from Dr. Corkum at National Research Council of Canada in Ottawa. Since 2000, I had worked at the laboratory of Dr. Corkum. Initially, I studied the electron re-collision process which is known as the key mechanism for generating high harmonic emission. When an intense, short laser pulse is irradiated into a gaseous molecule, a part of the electron wavefunction is pulled out and goes away from the molecule with accelerated by the laser fields. After approximately two-thirds of the laser period (in case of the 800 nm laser pulse, it is approximately 1.7 femtoseconds), the electron returns to the molecule. Then the electron collides with the parent molecule, followed by the generation of high-harmonic emission. I studied this process using hydrogen molecules instead of atoms. In this case, I observed a new phenomenon which is different from the re-collision process in atoms.

In 2001, the research groups in Europe have reported that they succeeded in measuring the width of an attosecond pulse train for the first time. I wondered what we should do, but looking at my own experimental data, I found that the re-collision electron itself has a pulse width in attosecond time-range. Thus, we have started to develop a new approach in attosecond measurements using the re-colliding electron, that is significantly different from the methods which use the high-harmonic generation as a probe pulse.

Breakthrough in attoseconds

First, in 2002, I experimentally and theoretically demonstrated that the re-colliding electron allows us to measure attosecond dynamics in molecules. By measuring phenomena which occur as a result of re-collision, for instance, the high-harmonic generation or the inelastic scattering process is one of the phenomena, it is possible to obtain information of molecular vibrational motion and electrons with attosecond time-resolution. The results has been published in Nature. Next, in 2003, I measured the vibrational wave packet motion of deuterium molecules with a minimum time-resolution of 700 attoseconds (Figure 1). This has been the world's best time-resolution of the structural changes in a matter and has been published in Nature again.

Figure 1: Attosecond vibrational motion (changes in the internuclear distance) in deuterium molecules in the range from 1.7 fs to 4.2 fs. 1000 attoseconds = 1 femtosecond (fs); 1 Å = 10^(-10)m

The re-collision approach is similar to taking a "self-portrait" - measuring a molecule by a part of the molecule itself. Quantum mechanically, the phase of the probe pulse (the re-colliding electron) is correlated with the phase of the measured molecular wavefunction. In 2004, we applied this approach to image the spatial distribution of the amplitude and the phase of the highest occupied molecular orbital (electron wavefunction) of a nitrogen molecule (N2) by measuring the high-harmonic generation spectra from aligned N2. This result has been published in Nature and drew many attentions. This is because while the square of a wavefunction, which is related to the probability density of finding an electron in a space, is an observable, the wavefunction itself hasn't been recognized as a measurable physical quantity. Measuring the phase of a wavefunction is particularly important on the selectivity or reactivity of chemical reaction.

As I have noted above, another approach to attosecond measurements is to utilize high-harmonic generation as a probe pulse. Research groups in Europe or other countries have developed experimental apparatus including the attosecond electron streak camera, which has been proposed by Dr. Corkum. Nowadays, attosecond science is recognized as one of the new advances in the research field of photonics. For instance, Nature magazine has a special feature named Nature Milestones. They select important research topics which have been recognized as millstones in science. Attosecond science is selected as the 22nd milestone in Photonics. My research article published in 2003 is included in the reference list as one of the millstone articles in developing attosecond science.

Attosecond electron dynamics in molecules

One of the major goals of attosecond science is to measure electron motion in atoms or molecules. Since an electron is represented by a wavefunction, it is necessary to find a method of measuring dynamics of an electron wavefunction or an electron wave packet with attosecond time-resolution. Around 2004, when I did experiments on controlling chemical reaction dynamics using few-cycle femtosecond laser pulses, Dr. Corkum advised me to find a new approach to measure electron motion in a molecule with attosecond time-scale. This is because there had been no clear and general idea to measure the motion in that time. Then using quantum mechanical calculations, I found the new principle that "if an electron wave packet moves in a molecule or an atom, then the information is recorded in the spectrum of the high-harmonics emitted from the molecule as a result of electron re-collision."

This principle was a theoretical prediction, and experimental realization was no easy task. Recently, I had solved this problem by measuring polarization direction of the high-harmonic emission as well as the spectral intensity. In this case, changes in the spatial distribution of the electron wavefunction or wave packet in the molecule is mapped onto a two-dimensional high-harmonic generation spectrum emitted from molecules as shown in Figure 2. Figure 2 indicates that the spatial distribution of the electron wave packet in ethane (C2H6) changes drastically in the time from 800 to 1,200 attosecond after the motion is initiated. This is the first experiment to measure dynamical changes in the electron wavefunction (wave packet) in the valence state of molecules with attosecond time-resolution (Reference Article #1, Reference Article #2). This result was introduced in the Asahi Shimbun Newspaper and the Nikkei Sangyo Shimbun.

Figure 2: Attosecond electron wave packet motion in ethane (C2H6) mapped onto a two-dimensional high-harmonic generation spectrum.

Laboratory at Waseda University. Associate Professor Niikura carefully examples the equipments which were elaborately assembled from power sources to wiring and piping.

I will continue to make advances in attosecond technology and methodology. Now I am developing new experimental and theoretical approaches for simultaneous measurements of structural changes in molecules and the electron wave packet motion during chemical reaction dynamics. Such experiments have a potential to alter theories for chemical reaction dynamics in the research area of both Organic Chemistry and Physical Chemistry. Furthermore, I will contribute in advancing photon science, including an approach to zepto-seoconds (1 zepto-secpond = 10^(-21) of a second, a time range shorter than attosecond.

Hiromichi Niikura
Associate Professor, Faculty of Science and Engineering, Waseda University

In 1995, graduated from the Department of Polymer Science and Engineering at Faculty of Textile Science, Kyoto Institute of Technology. In 1997, completed the Master's Program in Polymer Science and Engineering at the Graduate School of Science and Technology, Kyoto Institute of Technology. In March 2000, completed the Doctoral Program in the Department of Structural Molecular Science at the School of Mathematical and Physical Science, The Graduate University for Advanced Studies. Holds a Ph.D. in physical science. Held positions including Research Fellow (DC2) at the Japan Society for the Promotion of Science, NSERC post-Doctoral Researcher at the National Research Council (NRC) of Canada, Overseas Research Fellow at the Japan Society for the Promotion of Science, and as a PRESTO Researcher at the Japan Science and Technology Agency. Assumed his current position in 2010. Has won the following awards: the 2002 Award for A Scientific Breakthrough or Technical Innovation from the National Research Council (NRC) of Canada, the 5th Prize of Encouragement for Young Researchers (2004) from the Society for Atomic Collision Research, the 2004 Award for A Scientific Breakthrough from the National Research Council (NRC) of Canada, the 2012 Commendation by the Minister of Education, Culture, Sports, Science and Technology (Scientific Technology Award, Research Division), and the 9th (2012) Japan Society for the Promotion of Science Award.