Laser Clock With Quantum Hands

Electrons move extremely fast. But atomic cores move slower. The difference in their speed makes it hard to study their movement at the same time.

Image by Ralf Vetterle from Pixabay

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What is happening when a molecule falls apart? That is something you can study using short laser impulses. But it is not as easy because you need to measure two different time scales at the same time. The rearrangement of the electrons happens so fast that you need the attosecond scale (one attosecond is one-billionth of a billionth of a second). On the other hand, cores of atoms barely move in an attosecond. This is why observing the disintegration of a molecule that happens when the distance between the cores increases can be observed only in longer time scales.

Scientists from the Technical University in Vienna developed a method that allows us to observe both of the time scales during a single measurement. This method uses elliptically polarized laser impulses. In such an elliptically polarized impulse the electric field rotates similarly to the hands-on a clock. While the duration of the impulse is comparable to the amount of time it takes the molecule to fall apart the electric field rotation is fast enough to observe the ultra-fast movement of the electrons.

Breaking Up Hydrogen Molecules

The scientists were breaking up molecules of hydrogen with laser pulses. A molecule of hydrogen is made from two hydrogen atoms – two protons and two electrons. The laser's electric field rips an electron that flies away from the molecules in a matter of just a few attoseconds. But the fact that an electron is missing starts a chain reaction. The distance between the protons increases and if an electric field rips out a second electron the protons start repulsing each other and the molecules explode.

But a proton is roughly two thousand times more massive than an electron and because of that, it moves slower. The disintegration of a hydrogen molecule takes a few femtoseconds and in the case of heavier molecules it can take up to a few picoseconds. While this time scale is still incredibly fast it was hard finding a device capable of measuring the fast movement of electrons and the comparably slow movement of the rest of the atom.

Polarization Is The Fast Hand

The key is to put two processes – each having a different speed – together. The faster process is the laser's electric field rotation. Sort of like you can have a fast hand that measures seconds on your watch. The only difference is that it takes the hand on your clock sixty seconds to run around the clock while the electric field rotates every 2.5 femtoseconds. This rotation can be used to research the extremely fast electron movement.

At the same time, the researchers showed that the protons' slower movement can be observed thanks to the energy of the protons after the molecules disintegrate. Markus Kitlzer-Zeiler said that they put the energy of the proton and the rotational movement of the laser's electric field into context. The exact time when the electron is removed from the molecules plays a key role. The movement of the electron depends on the orientation of the electric field and at that same time, the movement of the electron determines how the protons move. This allows us to identify how the protons and electrons move into molecules during it falling apart.

Detailed Display Of Quantum Waves

According to the principles of modern physics, each particle is a wave. This new method now allows us to measure the quantum waves of atoms with incredible precision. It allows us to measure the quantum wave of protons and determine the distance in hydrogen with a resolution of just one picometer – one-hundredth of a hydrogen's atom's diameter.

Sources:

• https://doi.org/10.1103/PhysRevLett.123.263201

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