The 2023 Nobel Prize in Physics recognizes three researchers for their work using attosecond light pulses, which has revolutionized the study of the rapid motion of electrons and broadened understanding in various fields of physics and chemistry. A team of three researchers has won the 2023 Nobel Prize in Physics for work that revolutionized the way scientists study electrons - illuminating molecules with attosecond-long flashes. But just how long is an attosecond, and what can these infinitesimally short pulses tell researchers about the properties of matter?
I first learned about this field of research when I was a graduate student in physical chemistry. My doctoral supervisor's research group has a project that uses attosecond pulses to study chemical reactions. Before we understand why attosecond research has won the most prestigious award in science, let’s first understand what attosecond pulsed light is.
How long is an attosecond?
"Ato" is the prefix for scientific notation, which represents 10-18, that is, a decimal point followed by 17 zeros and a one. Therefore, a flash lasting one attosecond, or 0.00000000000001 seconds, is an extremely short pulse of light. In fact, the number of attoseconds in a second is equal to the number of seconds in the age of the universe.
Previously, scientists could study the motion of heavier, slower-moving nuclei using femtosecond (10-15) pulses of light. One thousand attoseconds equals one femtosecond. But until attosecond light pulses are generated, researchers cannot see motion on the electron scale—the electrons move too fast for scientists to accurately resolve their motion at the femtosecond level.
attosecond pulse
The rearrangement of electrons in atoms and molecules guides many processes in physics and is actually the basis of every part of chemistry. So researchers put a lot of effort into figuring out how electrons move and rearrange.
However, electrons move very quickly during physical and chemical processes, making them difficult to study. To study these processes, scientists use spectroscopy, a method of studying how matter absorbs or emits light. To track electrons in real time, researchers need a pulse of light that is shorter than the time it takes for the electrons to rearrange.
As an analogy, imagine a camera that can only take longer exposures (about 1 second). Objects in motion, such as people running towards the camera or birds flying across the sky, will appear blurry in the photos taken, making it difficult to see what is going on. Then, imagine a camera using a 1 millisecond exposure. Now, motion that was previously indistinct will be well resolved into clear and precise snapshots. This is how using the attosecond scale rather than the femtosecond scale sheds light on electron behavior.
Attosecond research
So what research questions can attosecond pulses help answer?
First, the breaking of chemical bonds is a fundamental process in nature in which electrons shared between two atoms separate into unbound atoms. During this process, previously shared electrons undergo ultrafast changes, and the attosecond pulses make it possible for researchers to track the breaking of chemical bonds in real time.
The ability to generate attosecond pulses - work for which three researchers were awarded the 2023 Nobel Prize in Physics - first became possible in the early 2000s, and the field has continued to develop rapidly ever since. By providing shorter snapshots of atoms and molecules, attosecond spectroscopy helps researchers understand the behavior of electrons in single molecules, such as how electron charges migrate and how chemical bonds between atoms break.
On a larger scale, attosecond techniques are also used to study the behavior of electrons in liquid water and electron transfer in solid-state semiconductors. As researchers continue to improve their ability to generate attosecond pulses of light, they will gain a deeper understanding of the fundamental particles that make up matter.
Author: Aaron W. Harrison, Assistant Professor of Chemistry at Austin College.