The feat of imaging electrons

This year’s Physics Nobel Prize winners have excelled in experimentally observing electrons by creating attosecond pulses and viewing them in attosecond spectroscopy. It will have tremendous scope in further advancements in electronics and medicines and open the floodgates of advanced studies in Chemistry and Physics.

Anne L’Huillier, a professor at Sweden’s Lund University, Ferenc Krausz, Director at Max Plank Institute of Quantum Optics, Munich (Germany) and Pierre Agostini, a Professor at Ohio State University (USA) will get the Physics Nobel Prize this year.

With the emergence of Quantum Physics, the curtain has been put on the era of classical Physics. From classical physics to Quantum Physics, we have travelled quite far in understanding atoms, molecules and compounds. Experimental Physicists are exploring the sub-atomic realm and have reached a level where electrons are detected. Days are not far when even protons are broken up by accelerating them nearly to the speed of light.

Atoms move very fast and are invisible to the naked eye. This is why we cannot detect what happens inside the atom. Consider the beating of a hummingbird’s wing, a horse in gallop or the whirring of a helicopter’s blade. The rotation or vibrations of these objects are so fast that their cycles are invisible to our naked eye.

Yet, we can see them in photographs. It all depends upon the speed of the camera’s shutter exposed to capture the rotation/ vibration. From the principle of photography, we understand that the length of exposure determines the camera’s shutter speed. The length of the exposure needs to be adjusted, and the shorter it is, the faster it moves.

Billionth of a billionth second!

If the exposure is shorter than one cycle of the rotating/ vibrating object, it can be easily photographed. The shutter speed of 1/1000 or 1/2000 of a second of the camera is more than the periodic time of a rotating object like a horse in gallop or the whirring of a helicopter’s blade or the beating of a hummingbird’s wing, and that is how these can be photographed.  

The same principle applies to the photography of atoms as well. But the challenge is that the atom or its constituent, like an electron, moves so fast that we needed advancement in knowledge to produce the shortest pulse of it. Physicists successfully created femtosecond (millionth of a billionth second) light pulses, and now they have succeeded in creating attosecond (billionth of a billionth second) pulses.

Light is made up of waves, and a single period of the light wave moves from the mean position to the peak and returns to the trough. It moves again to the peak in the reverse phase and returns to the mean position. The position and energy of electrons change between one and a few hundred attoseconds. Therefore, an attosecond pulse can detect an electron wave.

So far, femtosecond light pulses were useless, as the speed of the shutter is less than that of light waves. When dealing with light waves, the periodic time of pulses that can be generated will be about 1/1000 of a femtosecond. It should be of the order of attosecond.

Infrared laser to the rescue

Regular laser systems could not generate pulses shorter than femtosecond in the 1980s. Nevertheless, scientists knew that light of any wavelength can be created by the interference of light having two or more different wavelengths. If a laser beam is passed through a gas, the overtones created can complete several cycles, while the original laser wave completes just a single cycle.

Anne L’Huilllier and her colleagues in France produced overtones by passing an infrared laser beam through noble gas in 1987. Infrared laser changed the game by helping create several overtones. By the 1990s, physicists knew that when the laser is passed, it shakes the atom so that the electrons from the gas atoms escape.

However, under the influence of an electric field, the electrons escaping the atoms are rushed back towards their nuclei. Thus, the electrons undergo to-and-fro motion and gain energy. This energy is finally lost in the form of a pulse of light. The laser beam, interacting with various gas atoms, creates light waves of various wavelengths. These are the overtones of incident infrared beams that interfere with each other and create pulses of ultraviolet light.

Pierre Agostini in France also produced and studied consecutive light pulses and found that each pulse lasted 250 attoseconds. Ferenc Krausz and his colleagues isolated a pulse that lasted 650 attoseconds in Austria. The achievement won them a combined Nobel Prize in Physics. These scientists are now on course to break more barriers to understanding the constituents of atoms. 

(The author is a retired PCCF (Head of Forest Force) Karnataka and a postgraduate in Physics)