What happens when a molecule “takes a selfie”: exploring attosecond physics

We asked the 2018 winner of the IOP’s Isaac Newton Medal and Prize – Professor Paul Corkum –some questions about his research. Here he explains his work in attosecond physics in his own words

Does your work in femtosecond physics or in attosecond physics mean that a human observer can actually look at images of atoms involved in a reaction or at electrons? In an interview you were quoted as saying “you can’t really take a picture of an electron – it’s a blur. We take pictures up to the limits of what quantum mechanics says you could ever possibly see”.” So what can be observed?

Let me begin by explaining what an attosecond is. One way to understand an attosecond is to consider its representation in its “fractional format”. An attosecond is 1/1,000,000,000,000,000,000 of a second – or, in words, a billionth, of a billionth, of a second. By comparison, a nanometre is only a billionth of a metre. Even a proton, which most people will agree must be very, very small, has a diameter of a millionth of a billionth of a metre – 1000 times larger relative to a metre than an attosecond is relative to a second.

Here is another valid way to understand how short an attosecond is: an attosecond is to a second as a second is to the age of the universe.

You might say: “But that is way too fast for anything interesting.” I would answer: “But electrons don’t weigh much, and so they move quickly when they are excited, and they move between atoms that are extremely close together. Small distances and high speeds mean that important things happen very, very quickly. Therefore, electrons in atoms, molecules or solids are in the domain of attosecond science.”

You ask about an image. Normal light has too large a wavelength to form an image of an electron. It is like a large ocean wave washing over a speck of sand, so a normal image is impossible. There is, however, another way to get an image, but to understand the second way we would need to know how attosecond pulses are made, and why Newton’s laws of motion become influential.

The making of an attosecond pulse

First, we need a material. For attosecond pulses, that material could be atoms, molecules or a transparent solid. We would, however, need to concentrate on a small molecule like H2 or N2. We would also need an intense light pulse. So, let’s set the stage!

Light is actually a wave of force on any charged particle. If the light is intense, then the force is large. It can be so large that a molecule’s most weakly bound electron is pulled free from its parent ion. Then, Newton’s law of motion, (F = ma) takes over.

The electrical force of the light wave pulls the electron away from its parent ion, but when the wave reverses sign (that is, when the force pushes the electron in the other direction), the electron is driven back to its parent ion, just as Newton’s law predicts. There, it can do one of three things: it can knock a second of the molecule’s free electrons; it can elastically scatter from its parent ion; or it can recombine with its parent, returning the molecule to its original state while creating an attosecond pulse.

But, if we could measure the electron after making choice #2, or if we could measure the attosecond pulse after the electron makes its choice #3, we would see both the position of its atoms and an image of the electron from which the electron was originally taken. (I sometimes call this “a molecule taking a selfie”.) Here is how the analogy works:

Step 1 –- Me taking a selfie: I pull a camera from my pocket and extend my arm to about 10 times the diameter of my head.

Step 1 -– The molecule taking a selfie: Light shining on an atom pulls an electron from the molecule with some probability and moves it about 10 molecular diameters away but leaves some of itself behind to be photographed. (Quantum mechanics corrected Newton, stating that an electron can do two or more things at once, each with some probability, as long as we do not measure that it is doing so.)

Step 2 –- Me taking a selfie: I push the trigger and the camera flashes, irradiating my face with light waves.

Step 2 – The molecule taking a selfie: Light sends the electron probability wave back, irradiating the molecule with electron waves.

Step 3 –- Me taking a selfie: The light waves scatter from my face towards a Charge-Coupled Device (CCD), which will transfer the light into an electrical signal.

Step 3 – The molecule taking a selfie: The two parts of the electron probability waves interfere, creating light that scatters towards a CCD detector, which will transfer the light to an electrical signal.

Step 4 – for both the molecule and me: A computer synthesises the signals into an image that we observe.

You ask about the image. It is not possible to see an electron distinctly, according to quantum mechanics – the mechanics that extended Newton’s mechanics. Instead, all we can know is the probability of finding the electron in a given place. That probability image is the best that can ever be measured (or known), no matter how we make the measurement. The reason is that whatever we do to measure such a small particle must inevitably affect the particle being measured, thereby blurring the image. One of the iconic images of attosecond science is the blurred image of the most weakly bound electron of N2 that we made here in Ottawa.

Since the time-intervals involved are so tiny, is it possible for a human observer to see that change has taken place between images – obviously not at the level of one femtosecond or one attosecond, but over a certain period? Do you ever turn a set of images into a “movie”, either for research purposes or for demonstrating the effect to an audience?

It is not possible with our naked eye to directly see an electron image of the kind we took of N2. However, as I described above, we can make a molecule take a selfie and report the image to us in the scattering pattern of the electron if it elastically scatters, or in the spectrum of the attosecond pulse it creates. That image can be snapped in a few attoseconds.

We have made a movie of electrons moving in an atom, but the challenge of taking a movie of electrons changing during a photochemical reaction is harder than we initially thought. The problem is not the movie itself. Instead, the biggest problem is that the most weakly bound electron of a molecule can change as a photochemical reaction progresses.  It is like your friend uses your camera and takes a selfie. Then, the selfies would sometimes be of you, sometimes of your friend and even sometimes a multiple exposure of both of you.

However, although taking a movie of a molecule’s electrons has some problems, taking movies of electronic circuits as they operate is certainly possible, and we think it will be possible to photograph electrons trapped in solids.

Your citation for the Isaac Newton Medal says: “These techniques are key to understanding photochemical processes such as photosynthesis, the mechanism of vision, and the photostability of DNA.” Are you involved in any of these areas of research yourself, or are these being taken forward by others, or in collaboration with you?

I am not involved in this area of research. However, I can briefly describe the issues for you. All of the experiments that you mention share one basic feature – light is absorbed, giving its energy to a molecule.

  • In the first two cases, the energy in the light must be captured by a molecule and that energy must be channelled to the chemical reaction underlying vision (or photosynthesis) without much, if any, of the energy leaking away. If absorbing light caused the molecule to vibrate even a bit, for example, the light’s energy would be wasted and photosynthesis or vision would be impossible.
  • In the case of the photostability of DNA the idea is that early precursors of DNA had to survive in a hostile environment with a lot of ultraviolet radiation. Almost all molecules have a small probability of being damaged (getting a sun-burn) each time a UV photon is absorbed. The DNA that survives to eventually be incorporated into us must have been the most stable form possible. That is, the DNA in biological life had to have a robust mechanism for dissipating the unwanted energy from the absorption of damaging ultraviolet light.

What are you working on now, and what will you be doing next?

Attosecond pulses are made of short wavelength light. Often this light is in the vacuum ultraviolet, but it can be even shorter wavelength, such as X-rays.  X-rays give us new tools for following the energy flow in molecules or solids because when X-ray light is absorbed an electron is promoted from the very core of an atom within the molecule. Each kind of atom has a unique frequency. Therefore, X-ray absorption tells us about the electronic environment at a particular place in the molecule. This is an invaluable tool for probing how electronic energy moves around within a molecule or solid. Combining X-ray spectroscopy with attosecond time resolution promises to be a very important new direction. It is a major motivation for every attosecond scientist. We will receive a new laser system in Ottawa that should allow us to produce attosecond pulses in the soft X-rays.

There is a second forefront in which I am deeply involved. The types of materials that can produce attosecond pulses keep expanding. First it was atoms, then molecules. Now it seems that any transparent material will produce high harmonic radiation and attosecond pulses. In each of these materials the characteristics of the attosecond we can produce provide us with fresh insight into the material itself.

Heather Pinnell

Heather Pinnell

Heather is the IOP news editor, writing news for the Institute’s website. Before joining the IOP in 2005 she was a journalist specialising in education. She has a BA in English and a BSc in physics.
Heather Pinnell

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